Weather456's Tropical Weather Blog

Behind the Samoa Tsunami

By: Weather456, 10:07 AM GMT on September 30, 2009


Figure 1. The downtown of Fagatoga was flooded when a tsunami hit American Samoa early on Tuesday. Image credit: The Associated Press.

The Western Pacific Ocean has stolen the attention again but in a rather different form that we have not seen since 5 years ago. A tsunami generated by a powerful undersea earthquake inundated parts of Samoa, wiping out villages and killing scores of persons early on Tuesday.

The Earthquake

The earthquake originated along the fault line between the Australian and Pacific plates, just along the northern section of the Tonga Trench. The earthquake had a magnitude of 8.0, which is strong since the earthquake that generated the 2004 Indian Ocean Tsunami was around 9.0 on the Richter scale. It struck around 6:48 local time Tuesday (1:48 PM EDT), below the ocean about 120 miles southwest of American Samoa and 125 miles south of Samoa, and it was centred only 17 kilometres below the seabed, according to the United States Geological Survey.

The earthquake was caused by a rupture of the Pacific Plate. Now how this works, is that the Australian and Pacific Plates lay next to and is moving towards each other; and becomes separated by the Tonga Trench. The two plates have nowhere to go but down and thus the heaviest or more dense plate, slides under the lighter plate – subduction. In this case the Pacific plate is the heaviest and subducts westward under the Australian Plate. As this occurs, the plate is bent and pressure builds up until it ruptures, releasing a huge amount of energy in what we feel as earthquake.


Figure 2. A schematic diagram illustrating the forces behind the Samoa Earthquake on Tuesday 29 September 2009. The arrows show the plate direction, separated by the Tonga Trench which is clearly visible. The earthquake's epicenter is depicted by the yellow circle. Image credit: Google Earth.


Figure 3. USGS's seismicity cross section of the Samoa earthquake with the plan view (top) and the vertical cross section (below). The focus is where the earthquake originated while the epicenter is the region directly above the focus. The focus maybe located differently from this image. Image credit: USGS.

The Tsunami

The seabed shifts upward due to the rupture of the plate (because it was already forced downward in subduction) and this displaces a huge amount of water, which spread out in a wave – a tsunami. The tsunami waves have very large wavelengths and are subtle over deep open ocean, travelling at fast speeds. As they near the coast, they slow down as they interact with the sea floor but the waves behind continue at their respective speeds. This results in a pile-up of water that we see as a tsunami.


Figure 4. An animated illustration of the science behind the formation and propagation of a tsunami's wave.

The Damage/Death Toll

The quake generated three separate tsunami waves, the largest measuring 5.1 feet from sea level height, said Vindell Hsu, a geophysicist with the Pacific Tsunami Warning Center. Preliminary data had originally reported a larger tsunami.

The death toll from the tsunami killed at least 77 people thus far on Western Samoa (55) and American Samoa (22), which makes up the Samoa Islands. Entire villages lay flattened or submerged with one of the hardest hit areas near Leone Village. The walls of water were so strong that they twisted concrete beams and mangled cars. Roads, buildings and private homes were heavily damaged. The American Samoa government ranks this as “one of the worst” disasters in the island history.

The damage on neighbouring Samoa was extensive with heavy damage to roads, buildings and most importantly, infrastructure, which cut off all telephone signals to most of the island, hampering disaster efforts.

Tsunami awareness is relatively high in this earthquake-prone part of the world, particularly after the devastating earthquake and tsunami on Dec. 26, 2004, that killed 227,898 people around the Indian Ocean, according to the United States Geological Survey.


Figure 5. I-report images from Leone village located on the south side of American Samoa. Image credit: CNN I-report.

Meanwhile, in the Northern Hemisphere, Typhoon Ketsana killed over 30 persons as it made landfall in Vietnam yesterday bringing heavy rains, which triggered floods and landslides.

I had to delay my outlook until tomorrow.

Weather456

Typhoon Ketsana strikes Vietnam

By: Weather456, 10:03 AM GMT on September 29, 2009

Typhoon Ketsana roared ashore early this morning along the Central Vietnamese coast with winds of 100 mph. Satellite imagery and surface observations indicate the center cross the coast at about 2am this morning, EDT, bringing gusty winds, choppy seas and heavy rains to the region. The storm made landfall just north of Quang Ngai and the nearest observations came from Da Nang airport, which reported peak sustain winds of 55 mph with gusts up to 71 mph and 5.51 inches of rain. This is where Ketsana was originally supposed to make landfall but the storm unexpectedly turn towards the southwest, sparing the city. The storm is expected to dump an additional 3-5 inches across Vietnam, Thailand and the remainder of Southeastern Asia as it continues inland. Some flooding is possible.


Figure 1. GMS visible image of Typhoon Ketsana earlier this morning.


Figure 2. Windfield of Typhoon Ketsana nearing landfall at around 0600 UTC (2am EDT). Notice the core of strongest winds (category 2) was confined to a small region to the north, with predominately category 1 winds affecting nearby locations and tropical storm force elsewhere.

As many may have known, Ketsana dumped extremely large amount of rainfall across the Northern Philippines on Saturday causing major flooding and killing over 140 persons mainly near the capital of Manila. Why was this deluge so pronounce? The reason for the enhanced rainfall over on the Manila-side of the island as the storm approached was the interaction between Ketsana's low-level circulation and the seasonal southwest monsoon. The southwest monsoon comes about from the summertime heating of the Asian landmass. As warm air rises over the continent, it induces low pressure as the surface, which draws air in from surrounding regions. The southwest monsoon typically runs from June to September in the Philippines and draws warm, humid air up from the southwest across the South China Sea and into the islands where it can interact with the topography. Ketsana's counterclockwise circulation enhanced the effect, which resulted in the torrential rains.


Figure 3. This diagram is schematic overview of the surface features on Saturday 26 September 2009. The dashed line is the intertropical front, which is divided between the monsoon trough (between the NE Trades and Southwesterlies) and the ITCZ (between the NE and SE Trades). The low pressure area over the Philippines is Ketsana while the two others are 18W and 19W. Now Ketsana's circulation helped to enhanced the southwest monsoon which were previously freshen by a series of mid-latitude systems in the Southern Hemisphere. Combined with the terrain, this lead to the torrential rains that occurred in the Philippines.


Figure 4. The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center is used to monitor rainfall over the global Tropics. TMPA rainfall totals are shown here for the 7-day period 21 to 28 September 2009 for the northern Philippines and the surrounding region. The highest rainfall totals occur in an east-west band over central Luzon, including Manila, south of storm's track (indicated by the solid black line). Amounts in this region are on the order of 375 mm (~15 inches, shown in dark yellow) to over 475 mm (~19 inches, shown in orange). The highest recorded amount near Manila was 585.5 mm (almost 24 inches). Image credit: NASA.

Tropical Update

A tropical wave is approaching 40W south of 15N moving off towards west near 10-15 mph. This wave has nice inverted-V pattern seen on the first visible images of the day. Shower activity has significantly increased along this wave as it interacts with cyclonic flow between two upper ridges at 55W and 25W. Wind shear is about 10-20 knots and there is some cyclonic curvature in the lower levels. This feature was the large wave that emerged off the coast on Saturday and will be monitored for development. However, I’m not too optimistic about this one since there is a region of very high shear ahead that is forecast to remain in place for at least the next 5 days.

I will issue my outlook on the remainder of the season tomorrow morning.

Weather456

The power of rainfall

By: Weather456, 2:00 PM GMT on September 27, 2009

Flooding in the Philippines

Tropical Storm Ketsana delivered one of the worst flooding in 42 years to the Philippine capital of Manila. Heavy insistent rains from the tropical cyclone have killed as many as 70 people across the island nation with rainfall reaching 13-20 inches, just shy of the monthly average. Senior Meteorologist, Dave Samuhel at Accuweather reported rainfall amounts since Friday neared 20 inches across parts of Manila and over the past day has totalled more than 10 inches in parts of the Philippines. Diliman, in the Manila metropolitan area, received nearly 22 inches of rain since Friday. Tanay, also near Manila was soaked with 18 inches.

More than 250, 000 have been driven from their homes, officials reported, and up to 4000 people were rescued, many clinging on rooftops. TV footage even showed some survivors clinging to high-voltage power lines. There was not a state of emergency issued for the country, rather a “state of calamity” describing the size of the disaster.
The federal government began massive relief efforts to aid the local governments, and set up aid centres addressing pressing problems such as sanitation and water purification. President Gloria Macapagal-Arroyo has ordered pay parking lots at malls in Manila to be opened so motorists can leave their cars there without charge.

Since the storm made landfall, it has since then exited into the South China Sea, where it intensified slightly and is now forecast to become a typhoon later on Monday.


Figure 1. TRMM's estimated rainfall accumulation for the Western Pacific region ending 2AM this morning. The Northern Philippines received a total of 500 mm over the past week of which 300 mm was cyclone related.


Figure 2. Another product from TRMM which incorporates rainfall rates and geography to assess the likelihood of flooding and landslides. Floods kill more people each year around the globe than tropical cyclones.


Figure 3. Residents stand on electric wires to stay on high ground while others wade in neck-deep flood waters caused by Typhoon Ondoy in Cainta Rizal, east of Manila September 27, 2009. Image credit: REUTERS/Erik de Castro (PHILIPPINES ENVIRONMENT DISASTER IMAGES OF THE DAY).

Southeast Flooding

So far at least 6 people are dead and others are missing across the southeastern United States as a result of severe flooding brought about by several days worth of heavy showers and thundershowers. Atlanta, Georgia was especially hard hit with reports of over 15 inches of rain in the metro region. The culprit was a persistent area of low pressure located over the lower Mississippi River Valley that pumped copious amounts of moisture up from the Gulf of Mexico northeastward over the Southeast, providing fuel and a trigger for the numerous showers and thundershowers. A stagnant upper-air pattern allowed the area of low pressure to persist for several days in the same location.

Armed with both a passive microwave sensor and a space-borne precipitation radar, the primary objective of the Tropical Rainfall Measuring Mission satellite (better known as TRMM) is to measure rainfall from space. For increased coverage, TRMM can be used to calibrate rainfall estimates from other additional satellites.

The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center is used to monitor rainfall over the global Tropics. TMPA rainfall totals are shown here for the 8-day period 14 to 22 September 2009 for the southeastern US and the surrounding region. Nearly the entire southeastern US from the Gulf Coast to the Ohio Valley and from the southern Mississippi Valley to the southern Appalachians received at least 50 mm of rain (~2 inches, medium green) with a good portion receiving at least 100 to 150 mm (4 to 6 inches, shown in light green and yellow areas, respectively). Embedded within these regions are locally higher amounts exceeding 250 to 300 mm of rain (~10 to 12 inches, shown in orange and red, respectively). The highest TMPA rainfall totals for the Atlanta region are on the order of 350 mm (~14 inches).

Credit: NASA


Figure 4. TMPA rainfall totals for the 8-day period 14 to 22 September 2009 for the southeastern US and the surrounding region.


Figure 5. The Six Flags Over Georgia amusement park sits submerged in flood waters, in this aerial view taken September 23, 2009. Constant rains for nearly a week have saturated the metro Atlanta area, including the amusement park, bringing Chattahoochee River basin flood waters to residents' doors. Flooding in north Georgia that killed nine people caused $250 million worth of damage to property and tens of millions of dollars more damage to infrastructure, the state insurance commissioner said on Wednesday. Image credit: REUTERS/David Tulis

Tropical Update

Tropical Depression 8 disintegrated into an open wave yesterday due to the decoupling of the low level center and the heaviest convection from southwesterly shear. This morning satellite imagery and QuikSCAT showed little change and it is unlikely the system will regenerate today.

The area of showers in the Bay of Campeche did as expected and slide into the Gulf coast of Mexico.

I will have an update later this week on the outlook for the remainder of hurricane season and the September forecast verification.

Weather456

Tropical Depression 8

By: Weather456, 11:28 AM GMT on September 26, 2009

Tropical Depression 8 is located near 17.9N-33.8W or about 660 miles west of the Cape Verde Islands, moving off towards the west-northwest near 14 mph. Estimated surface winds are near 35 mph, with a minimum central pressure of 1008 mb. Satellite imagery revealed that the depression has been able to produce some substantial convection overnight and is somewhat organize but 4KM visible images revealed that much of this convection is towards the northeast of the low level closed circulation – albeit the center is not expose, rather the convection is lopsided and this due to southwesterly shear. There is still a brief period for this tropical depression to become a tropical storm today if it can organize some as intensity estimates currently support a tropical storm; but beyond 24-48 hrs, much of the models are showing the system encountering a band of higher shear and combined with the presence of dry air and cool sea surface temperatures, weakening is likely beyond 48 hrs.

The depression is moving west-northwest under the influence of a weakness in the subtropical ridge due to an advancing upper trough. This weakness is best illustrated on the GFS MSLP and 1000 mb winds. This depression will likely continue west-northwest then towards the northwest through the next 4 days but beyond then, if the system does not get absorbed or dissipates, a rebuilding ridge may take the system back west. What? Dejavu. It seems this year’s quick succession of troughs is causing systems to undergo stair step patterns. The system will be monitored over the next week.


Figure 1. Goes-12 visible image of Tropical Depression 8 with the official forecast track overlaid.


Figure 2. 00Z GFS mean sea level pressure and 1000 mb winds valid 12Z (8am EDT) today, showing the advancing upper trough creating a weakness within the high.

Showers and thunderstorms have become increasingly concentrated over the Bay of Campeche in association with the tail end of a stationary frontal boundary that cannot progress westward because of a mid-level ridge over Florida. Little development is expected due to proximity to land and the feature may eventually retrograde back over Mexico if the ridge shifts southwest.

Weather456

Watching two African waves

By: Weather456, 10:48 AM GMT on September 25, 2009

A tropical wave is located near 31W south of 20N moving off towards the west near 10-15 mph. QuikSCAT and shortwave infrared imagery revealed a low pressure area is attached along the axis of this well define wave near 15N. Shower activity has waned some of overnight but the feature remains a well organize entity with deep convection in around the center of circulation. Estimated surface winds are near 25 knots with a minimum central pressure of 1009 mb. Wind shear is around 5-10 knots over the disturbance and sea surface temperatures are around 2C higher than the threshold. If showers continue to persist an invest could form over the weekend. However, the future of this invest is uncertain as shear increases in about 72 hrs and the area might have to deal with some dry air to its north and west.

This feature is moving off towards the west-northwest under the influence of the Azores high-pressure ridge. This motion is expected through the next 3-5 days until a trough digs across the area and creates a weakness in the ridge, causing the storm to take a more northward motion. Track guidance beyond 5 days becomes uncertain due to the intensity uncertainty so the feature will be monitored.

A second, larger and more vigorous wave is just emerging off the coast of Africa accompanied by very strong convection in and around the axis. QuikSCAT already revealed this feature has a low-pressure area near 11N. This feature will be monitored for persistence as it merges as it well south of any shear or dry air and could potential go more westward.


Figure 1. QuikSCAT morning pass of the Eastern Tropical Atlantic.


Figure 2. Infrared satellite imagery of the Eastern Tropical Atlantic.

A weak non-tropical low-pressure area has developed south of Bermuda as result of the upper level circulation spinning over the area. This feature is showing little signs of development with barely any convection as few days ago. This feature will continue to meander over the area over the next 2-3 days until it becomes absorbed into a series of frontal troughs beyond 3 days. Until then, I will monitor the feature. If the low decides to deepen, then the pressure gradient between it and high-pressure could produce some gusty winds for Bermuda. This seems slim at the moment though.

A stationary frontal boundary over the Bay of Campeche showed up well on the last visible images of the 24/9 and this morning satellite imagery showed clusters of deep convection continue to develop across the Bay and the Yucatan Peninsula due to the aiding influence of a surface trough. This area is just one broad area of low pressure and will be monitored as it heads either into the Bay of Campeche or Eastern Pacific was upper winds are favourable in both regions.


Figure 3. Infrared satellite imagery with mean sea level pressure and surface analysis overlaid.

I will have update on all these features on Saturday.

Thanks,
Weather456

Tropical Update

By: Weather456, 9:59 AM GMT on September 24, 2009

A mid-upper low-level circulation south of Bermuda in conjunction with an associated surface trough continues to produce scattered showers and thunderstorms in around the area between 70W and 55W, north of 25N. Satellite imagery along with QuikSCAT winds continue to indicate that this surface trough is amplifying and a surface low may eventually form if shower activity persists as they have been over the past 3 days. Pressures around the area are slightly falling but remain relatively high, while upper winds have relax to about 15 knots, which is why the shower activity is seen wrapping westward on this morning satellite loops. Upper winds may continue get more conducive for development, but only briefly as a deep layer trough swings down in 3-5 days brining high westerlies and probably absorbing the feature. The feature will be monitored as it meanders within weak steering flow over the next 3 days.

The other area of interest is a surface trough right along the Yucatan Peninsula that is producing disorganize shower activity over the Northwestern Caribbean with the aid of upper diffluence. This feature will likely continue westward under deep layer ridging over the Gulf coast states and emerge over the Bay of Campeche in 2-3 days. Upper winds in the Bay will remain favourable for development so the feature will be watched, especially if it interacts with a stalled out frontal boundary. However, as it gets into the Bay, proximity to land will be its biggest problem since deep layer ridging will push it west to west-northwest into Mexico rather than northwest to north into the Gulf of Mexico.

There are few waves in the Central/Eastern Atlantic being affected by heavy shear/Saharan dust and thus development is not expected.



Weather456

Met Office GloSea Model Performance for 2009

By: Weather456, 10:54 AM GMT on September 23, 2009

The 2009 Atlantic Hurricane Season has been one of slowest seasons since the new active decadal cycle started in 1995. This season has accounted for the latest starting year since 1992 and the least amount of storms so far since that same year. The 1997 hurricane season was also an inactive year but like 1992, strong El Nino patterns were to blame. According to the latest statics, the 2009 El Nino event is weak in comparison to other years even 2006 and yet we are accounting for less storms than stronger El Nino years. A combination of a global tropical decline due to a cooling atmosphere and a more stable Atlantic contributed to this significant decline in tropical storm activity.

Was this predicted?

Figure 1 shows some of the seasonal and intra-seasonal outlooks for the 2009 Hurricane Season and both the UKMET and NOAA’s August 6 lowest extreme were fairly accurate with the current figures. However, I would discount NOAA’s outlook for the purpose of this blog and that the fact they have modest range, unlike other forecasts that a more precise on storm numbers.


Figure 1. A comparison showing the predictions of tropical activity in the 2009 season and what was actually observed so far. Source: Wikipedia, the Free Encyclopaedia.

Recent studies have shown that GloSea and other European dynamical models have considerable skill predicting the number of tropical storms - for example successfully predicting the change from the exceptionally active season of 2005 to the below-normal activity of the 2006 season. Last year the Met Office forecast was for 15 storms with a 70% probability range of 10–20; in the event, 15 storms were observed to occur.

At the start of each hurricane season, the Met Office predicts the number of storms between July and November using forecasts made using a dynamical global seasonal prediction model called GloSea, which simulates the ocean-atmosphere processes and interactions that determine tropical storm development. This model is different from other techniques since it predicts the ocean-atmosphere variables that affect tropical storms rather than the traditional forecasts that rely on pre-season sea surface temperature and mean sea level pressure patterns.

In late June, the Met Office released their 2009 Outlook predicting six named storms for the 2009 Hurricane Season, which is along the lines of what is being observed. Upon further analysis, it was revealed that the GloSea model output for April 2009, predicted above normal sea level pressures, a cooler troposphere, near normal sea surface temperatures, and below normal rains (stability) for the peak months of July, August and September. The model also predicted a 40-60% of a moderate El Nino, which is a factor of wind shear. All these conditions were observed in 2009, and many healthy tropical waves and storms struggled with dry air and wind shear despite warm sea surface temperatures. Even if we do end up with more storms, there is still some level of skill to this model since the range of the forecast was between 3-9.



Figure 2. Met Office GloSea model in April 2009 for the period of July, August and September of 2009. The top image shows the probability of above normal pressures which was largely 80% and above for the Atlantic while the below image shows the probability of above normal rains which was 20% and below for the Atlantic basin. Image credit: The Met Office.


Figure 3. Ensemble forecast of SST anomaly for region Nino 3.4 from May 1 2009. Notice the observed Nino forecast was along the lines of the ensemble consensus.

Now, there is nothing new to this model. I used the model along with other data to predict 15 named storms in my 2008 Hurricane Season Outlook in May of that season. That seasonal forecast had verified. However, I did not use the model this year, but rather a different approach of looking at current conditions. The 2009 Hurricane Seasonal Forecast back in May called for 12 named storms, twice what we have observed so far. It is safe to assume that if the GloSea model output had dominated my forecast for 2009 as it did in 2008, I would have predicted less named storms, resulting in higher verification points. This is probably the approach that I will use my 2010 Hurricane Season Outlook, next May.

How has these forecasts stood up in past seasons?

There is not much data available to verify the model over a long period since the forecasts only go back to 2005. However, recent upgrades in the model relative to pre-2005 have allowed the model to accurately predict the change activity from 2005-2006, a fairly accurate forecast in 2007, and probably, extremely accurate forecasts for the 2008 and 2009 seasons.

Met Office North Atlantic Forecast

ECMWF Seasonal Forecasts (which is like the GloSea but available for free)

Much thanks the Met Office for this information.

I am watching three areas of interest - a tropical wave just emerging off the coast of Africa; a surface trough in the NW Caribbean that is interacting with an upper low near the Florida Straits to produce showers and thunderstorms; and an upper low southeast of Bermuda that continues to produce persistent shower activity. Model support for these systems are lacking. I will have an update Thursday if anything changes.

Weather456

Texas Drought Situation

By: Weather456, 10:44 AM GMT on September 21, 2009

Tropical Update

The tropics remain quiet with both the remnants of Fred and 98L producing little shower activity. There is a surface trough to the northeast of 98L which is producing scattered showers but the feature is heavily sheared as much of the thunderstorms remain east of the trough axis. None of the global models are forecasting development over the next 7 days.

Texas Drought Situation


Figure 1. This image shows the impact of drought on vegetation in southern Texas. The image is a vegetation index, which depicts the relative health of plants in 2009 compared to average conditions observed between 2000 and 2008. The green areas are where vegetation growth has been better than the average while the brown areas are where vegetation growth has been worst the average. This image was taken by the Spectroradiometer onboard NASA’s Terra satellite.

Over past 24 months, parts of Texas have been stricken with prolong drought conditions, which has reduced water supply, and led to disastrous results. Today we are going to look at drought in general, particularly the exceptional drought conditions that have been affecting Southeastern Texas from since late 2007, one of the worst in history.


Figure 2. US Drought Monitor display showing exceptional and extreme drought conditions across Southeastern Texas. Image credit: Climate Prediction Center

What is a drought?

In meteorology, a drought or drought conditions is described as a period of prolong below average rainfall. A drought is basically caused by any weather or climatic pattern that reduces the amount of rainfall a region receives, well below the average for the region. Water is a very essential resource and thus any decrease or shortage can have disastrous results ranging from agriculture to social impacts.

The Cause

The drought along southeastern Texas was likely caused by three major factors – persistent high pressure ridging, the lack of tropical moisture and La Nina.

Persistent High Pressure Ridging

High pressure can sometimes work in your favour especially for a day at the beach or that summer time outdoor barbeque, but sometimes too much of a good thing is bad. Persistent high pressure causes a block in the normal west to east flow of storms and can result ongoing weather pattern of dry and warm conditions. Figure 4 illustrates how moisture flow is diverted around a high-pressure ridge and the regions under the high are subject to warm and dry weather for weeks. High-pressure systems are broader and last 2-10 times longer than low pressure systems which are more transient, thus have the ability to affect weather on seasonal time scales.


Figure 3. Upper level total wind (ms-1) from 12 June to 9 September 2009 showing the upper level high-pressure system and the resultant storm tracks. This is typical of the summertime and La Nina patterns but persisted longer than usual.


Figure 4. There was a very deep layer anticyclone located over Northern Texas, with anticyclonic flow dominating much of the Southern United States. Water vapour imagery showed most of the moisture remains on the periphery of the high with drier conditions at the heart. This dry and hot weather pattern was lingering for weeks with stations across the state reporting rainfall as low as 20% of average and temperatures 5F above average. No wonder they referred to it as the death ridge. The ridge was expected to linger over the area for atleast the next 4 days but shifting west between 5-7 days. Little moisture was also expected across the state for the next week, which was not good news, since drought was and is still exceptionally high across the state.

Lack of Tropical Disturbances

Well maybe I should re-phrase that heading, since southeastern Texas has had their fair share of tropical activity over the past 3 years. Since 2006, there were two tropical cyclones making landfall across southeastern Texas, that is, Tropical Storm Erin (2007) and Hurricane Dolly (2008). However, the nature of these storms did not allow the rainfall to fall in the areas experiencing drought. For example, Erin’s highest rains were recorded well inland over Texas while Dolly’s highest rainfall was recorded near the Texas/Mexican border. There were no tropical cyclones recorded in 2006 or 2009. In fact moisture off the Gulf of Mexico has been dry and diverging for much of the 2009 Hurricane Season.


Figure 5. Total rainfall reported by stations along 2007’s tropical Storm Erin’s track. Erin dumped up to 12 inches of rain across Texas and Oklahoma but barely any of this was recorded in Southeastern Texas.

La Nina

During La Nina years, the region just along the Tropic of Cancer from the Baja Peninsula to the Bahamas becomes drier than normal due to the anomalous northern shift in the subtropical and polar jet streams. Thus, the storm tracks and the associated precipitation shifts northward similar to the blocking effects of persistent ridging. Over the past 2 years, we have been experiencing a very persistent La Nina, which only recently ended in spring of this year. This La Nina peaked in early 2008 and was one of the strongest on records. Ironically, the Texas drought began round this period (late 2007) and it is likely that La Nina played a role in the severe drought conditions in southeastern Texas.

We have now entered an El Nino pattern, but the drought continues – why? Well just like any climatic pattern, the atmosphere may take time to respond and the lack of tropical activity along the Gulf coast of south-eastern Texas may not be helping.


Figure 6. Summary of La Nina conditions during the winter (above) and summer (below). Notice that the Southern United States is normally dry and warm during La Nina events. The La Nina of 2007-2009 is the main contributor of the Texas drought situation.

The Impacts and Response

The impacts of drought can be summarized into three main categories – agriculture, water and energy and social impacts.

Agriculture

Whenever you hear about drought, the first image that comes to mind is a parch field or agriculture and that is because drought’s first line of impact is the soil and soil moisture, which is directly related to crops. Reported topsoil conditions indicated that much of Texas reported very dry soil conditions for August 2009 relative to August 2008. Poor topsoil probably led to poor grass and crop quality and consequently, poor livestock.

Drought conditions caused almost $100 million in agricultural losses in Nueces County in 2009, according to the county extension agent with Texas A&M. Pastures are dead and offer nothing for the cattle. Eighty percent of the corn and fifty percent of the grain were not worth harvesting. The cotton crop was nearly completely devastated. In addition, thin cattle were bringing just twenty cents per pound in Atascosa County, according to the AgriLife Extension agent, and the rising cost of hay resulted between 12 and 18 percent of 2 million cattle head to being sold in 2009.

World food production plummeted between 20 and 40 percent in 2009 due to widespread drought and other stresses on agricultural production and Texas was included in that statistic since the lower Plains is a major global contributor.

The cotton gin in Woodsboro did not open this year because only 180 acres of cotton were harvested in the vicinity of Austwell, despite the nearly 27,000 acres that were planted with cotton. This couldn’t have proved well for jobs in such a downed economy.

In response to these adverse effects, farmers and ranchers in Texas who have lost livestock grazing due to drought or fire damage are eligible for federal assistance through the Livestock Forage Disaster Program.


Figure 7. USDA percentage of pasture and range land in poor or very poor conditions with Texas reporting 54%, only third to Arizona and California.

Water and Energy

Streamflow conditions reported by the US Geological Survey (USGS) were largely consistent with other drought indicators east of the Rockies. Texas gauges were near historic lows for the season and observed August precipitation for Texas was among the ten driest of the instrumental record (1895-2009).

As a result, water restrictions in Austin became more stringent on August 24 as lakes Travis and Buchanan continue to fall and were at 45 % of capacity as of August 15. Lawn watering may be done once weekly; water may be served in restaurants only if the customer asks for it; car washes for fund-raising purposes are prohibited; and decorative water fountains are not allowed. In the past, violators of the water restrictions were merely warned of the infraction, but starting on August 24, violators are being fined a minimum of $400. These water restrictions became mandatory as of September 1.

Elsewhere, a domestic well east of Austin went dry, although it has never before gone dry during the decades that the present owner has resided there.


Figure 8. Streamflow conditions reported by the US Geological Survey (USGS) for August 2009. Most of the gauges across Southeastern Texas have been reporting water levels well below average.

Social Impacts

The low water level of Lake Travis is exposing more archaeological sites from Native Americans and early European settlers and the remnants of the Anderson Mill, which was constructed in 1859. More divers have been exploring the area recently to see the newly revealed treasures, but the Lower Colorado River Authority reminded everyone that it is illegal to take artifacts. In addition, part of a German family's homestead has appeared in Lake Travis near Briarcliff as the water level continues to fall. The structure was last visible in 1955.

The low water levels in Lake Travis is also hurting business for boaters in the area. A Travis County man has been transporting water for his home since his well went dry in May and uses the water for his garden, for domestic uses, and to keep his pool liner wet.

The shallow end of Deep Eddy Pool closed on August 28 because the two wells that supply the pool water are producing insufficient amounts of water for the entire pool. The deep end will remain open as the wells permit.


Figure 9. A view of Lake Travis earlier this month as the water level continued to lower. The structure in the middle of the image was once covered with water but has now become exposed. German settlers built this homestead when the southern Colorado River was much narrower. This image is almost eerie and brings new meaning to the phrase “up from the depts.”

Relieve

A series of upper level disturbances and tropical moisture last week have combined to produce scattered showers and thunderstorms over drought affected areas helping to ease the drought situation over southeastern Texas. A drought information statement release by the National Weather Service in Brownville, Texas indicated that this rainfall has eased the drought impacts for agriculture interests, especially those in need for the autumn planting season. Water restrictions and conservation remain in place, as water levels have not return to normal yet. The long-term outlook for Texas looks promising as El Nino conditions will continue to linger into the fall and winter seasons, bringing a gradually increase in wetter conditions that should help ease the drought situation.


Figure 10. There is a 40% chance that Texas will receive above average rains over the next 3 months as El Nino conditions continue to take hold.

If nothing changes in the tropics, my next blog will be Wednesday, the first full day of fall.

Weather456

Saint Kitts/Nevis Today

By: Weather456, 12:48 PM GMT on September 20, 2009


Figure 1. A view of downtown Basseterre looking east along the Bay Road.

Despite seceding from Britain in 1983, the twin island federation retained much of the British way of life. Our education, political and judiciary system is British-based. For example, we use words like “favour” instead of “favor” and “colour” instead of “color”. Our measuring system is also metric based (cm, grams and Celsius rather than inches, pounds and Fahrenheit).

The two islands have a population of about 42000 persons as July 2005 and are governed under Parliamentary democracy and Federal constitutional monarchy system with our Monarch being Queen Elizabeth 2, a governor general who represents the queen and then our prime minister. Blacks mainly dominate the islands but there is a cultural diaspora including Chinese, Americans, East Indians and Spanish. We mainly speak English as the standard when communicating in a formal environment but retain a rich sense of colloquial English.

The capital of the Federation lies on Saint Kitts – Basseterre while the capital of Nevis is Charlestown. In Basseterre lies the administrative, financial, recreational and transportation centres – schools, banks, government headquarters, insurance companies, sporting complexes bus and ferry services. The airport lies just outside of the capital – the Robert L Bradshaw International Airport that was named after our greatest national hero who fought for the islands' independence.


Figure 2. A view of the western section of the Basseterre Habour with an overview of the Kittitian landscape.

Saint Kitts’s economy was once dominated by the agriculture sector led by sugar, especially in the 1990s. However, that industry was a failing one and sugar was discontinued in 2005. The economy was then diversified into the service sector where tourism has now become our main source of revenue. Despite this, you can still locals on the wayside selling local provisions, which are quite more healthy and tasteful than imported ones.


Figure 3. A view of a sugarcane field with the one and only sugarcane factory in the background. Around the island you can see the remains of old sugar mills, boil house and storage houses.

Saint Kitts/Nevis experiences and is prone to a good amount of natural disasters such as flash floods, hurricanes, volcanic eruptions, earthquakes and tsunamis. Hurricanes are probably the most often occurring of these disasters with about 65 cyclones of tropical storm intensity and above affecting the islands since 1851. The worst of these was Hurricane Georges, which struck tonight, 11 years ago.


Figure 4. Satellite imagery of Hurricane Georges has it approached the islands on September 19. Georges eye passed over Saint Kitts during the early morning hours 21 September.

Our climate is described as a tropical marine type, meaning it is moderated by sea during through the day and year. Our temperature average rarely changes from 27C as the cooling/warming effect of the sea prevents the islands from getting too hot/too cool in the summer/winter. Our rainy season is from June to November when there is increase in tropical wave and tradewind moisture activity.

The country has two main islands, Saint Kitts and Nevis. The highest peak, at 1,156 metres, is Mount Liamuiga.

The islands are of volcanic origin, with large central peaks covered in tropical rainforest; the steeper slopes leading to these peaks are mostly uninhabited. The majority of the population on both islands lives closer to the sea where the terrain flattens out. There are numerous rivers descending from the mountains of both islands, which provide fresh water to the local population. St. Kitts also has one small lake.


Figure 5. An aerial view of Mount Liamuiga, which currently a dormant but not extinct volcano.

So that’s my story, the story of two small islands in the Caribbean, ever changing through the course of history. In such a warm tropical climate of friendly persons, secluded from the terrors of the outer world, I would not trade it for anything in the world.

Saint Kitts Picture Gallery

Tropical Update

Tropical Invest 98L is located about mid-way between the Cape Verde Islands, moving off towards the west-northwest near 10 mph. Satellite imagery revealed that the disturbance has become less organize, the circulation remains broad and maybe trying to relocated so the exact center at this point is difficult to ascertain. The environment does appear conducive for the development of 98L – warm SSTs and low shear – but it is not doing much, and that is because the system is struggling with some mid-level dry air.

Intensity guidance continues to bring the storm to a weak tropical storm within 48-72 hrs before shear increases over the disturbance. I agree since the environment is conducive enough that the disturbance still has potential to become a depression. We cannot really depend on the track guidance at this point when we don’t have a well-define center. I don’t believe 98L will move directly northwestward but I do believe that some northward motion will hold true as the high breaks down briefly within 3-4 days. If the trough does not pick up 98L as some models indicate, it will likely take a more westward course.

Over the past 24 hrs ex-Fred slowed down under the influence of a passing frontal trough and now the trough has pass, it seems Fred has shifted back towards the west under the influence of high pressure ridging over the Atlantic States. Now I mentioned the northern motion as a possibility yesterday in my blog and now it seems the track guidance has shifted northward also. In addition, the models are showing that an advancing trough over the Rockies will push the high into the Atlantic and which would place ex-Fred on the SE portion of the high causing it to move between west-northwest and northwest. Now will the storm regenerate? Well the intensity guidance keeps it below 35 knots and shear is expected to remain marginal at best, but because sea surface temperatures are warm, there is possibility albeit, it is small.


Figure 6. Visible image of Ex-Fred and tropical Invest 98L.

Weather456

Photo Gallery of Saint Kitts

By: Weather456, 11:47 AM GMT on September 20, 2009


Figure 1. Welcome to Saint Kitts during the Norwegian Dawn cruise in October 2007. Image Credit: Panoramio user, Pianoman6582.


Figure 2. Overlooking cruise ships from Timothy Hill. Image Credit: Panoramio user, jiangliu.


Figure 3. A view of a 17 th century St Georges Anglican Church, Basseterre, St. Kitts. Image Credit: Panoramio user, jiangliu.


Figure 4. A view of the Saint Kitts coastline (Caribbean side) with Brimstone Hill in the far background. Image Credit: Panoramio user, leisureguy.


Figure 5. Brimstone Hill Fortress. Image Credit: Panoramio user, Ron Mullenax.


Figure 6. Twin cruise ships docked at Porte Zante.


Figure 7. A view of the Saint Kitts landscape as seen when approaching from the Caribbean Sea. Image Credit: Panoramio user, mysterybay.


Figure 8. This is a view of one of another major towns, Sandy Point which is 2nd to Basseterre on Saint Kitts. Image Credit: Panoramio user, FGP.


Figure 9. A view of the Southeastern Peninsula mariner area. Image Credit: Panoramio user, Burai István.


Figure 10. The Four Season's Hotel on Nevis. The hotel is closed at the moment due to the effects of Hurricane Omar. It recently re-open earlier this decade from the effects of Hurricane Lenny which coincidently took the same route as Lenny. Image Credit: Panoramio user, lynck.


Figure 11. A shipwreck off the coast of Nevis. Image Credit: Panoramio user, bshreffler.


Figure 12. The Botanical Gardens on Nevis.Image Credit: Panoramio user, gloney.


Figure 14. An aerial view of the Southeastern Peninsula which is located in the SE part of the island of Saint Kitts. Image credit: Google Earth.


Figure 15. An aerial view of the island of Nevis. Image credit: Google Earth.


Figure 16. An aerial view of both Saint Kitts and Nevis. Image credit: Google Earth.

Weather456

About Saint Kitts/Nevis: History

By: Weather456, 11:51 AM GMT on September 19, 2009


Today, my nation is celebrating 26 years of independence from Britain and I will like to take this time to tell you a little about by island nation in a 2-part blog commencing with History.

Saint Kitts and Nevis are two small islands in the Northeastern Caribbean about 200 miles south-southeast of San Juan Puerto Rico. The islands are relatively young geographically, forming several thousands of years ago as the Atlantic and Caribbean plates collided. These islands along with surrounding islands like Montserrat, Guadeloupe and Martinique are just of the tops of historic volcanoes that formed along subduction zones. The entire eastern Caribbean island chain lies along this line, just like the Aleutian Islands of Alaska and the Hawaiian Islands. There exceptions like Antigua, Barbuda, Anguilla and Barbados, which formed from ancient coral reefs.

The twin islands, as they are called, were first colonised by the Caribs or Kalinago Indians, 5000 years before the arrival of Christopher Columbus. The Indians called the island of Saint Kitts, the one where I live, “Liamuiga”, which means fertile land as the soil originated from volcanic rocks, which are the most fertile in the world. Nevis was called “Qualie” which translates to “land of beautiful waters".


Figure 1. Carib (Kalina or Galibi) indian family after a painting by John Gabriel Stedman. The Caribs were some of the fieriest groups in the Western Hemisphere.

Christopher Columbus first discovered the islands in 1498 on his second voyage and like many other islands, he named Saint Kitts after him – Saint Christopher and our sister island was called Nevis. These stand as the official names of the twin island federation but the name Saint Christopher was shorten to Saint Kitts and is more often used than the former. Both Europeans and the natives attempted to share the islands but differences led to the great Kalinago Genocide of 1626, when the tribe was wiped out.

The islands of Saint Kitts and Nevis are two of the Caribbean's oldest colonised territories. Saint Kitts became the first British colony in the West Indies in 1624 and then became the first French colony in the Caribbean in 1625, when both nations decided to partition the island. That also did not work out and the British eventually dominated the entire country.


Figure 2. A view of Brimstone Hill Fortress looking northward with the island of Saint Eustatius in the distance. It commands astounding views of the Caribbean, including Nevis, Montserrat, Saba, St. Martin and St. Barts. Brimstone Hill sprawls over 38 acres, and its massive Fort George citadel is defended by seven-foot-thick walls of black volcanic stone--then better known as brimstone. In 1782 Brimstone Hill had been under nearly continuous construction (by slave labor) for almost nine decades. The fort was used in the 18th century by the British who try to defend the island against the French. Eventually the French won and the British were driven out of Brimstone Hill. The fortress is also known as"The Gibraltar of the West Indies".

During slavery, a combination of favourable conditions for the growth of sugarcane on the island made Saint Kitts the richest British colony in the 18th century. Eventually, slavery was abolished in the British Empire in 1834. Saint Kitts/Nevis remained a British colony through a series of administrations and federations, including the Saint Kitts-Nevis-Anguilla administration, but Anguilla eventually seceded and Saint Kitts/Nevis gained independence on 19 September 1983.

Tomorrow I will discuss part 2 where I will discuss modern day livelihood of the islands. Hope you enjoyed.

Tropical Update

Tropical Invest 98L is located near 13.5N-42W or mid-way between the Lesser Antilles and the Cape Verde Islands, moving off towards the west near 10 mph. Estimated surface winds are near 30 mph with a minimum central pressure of 1009 mb. Satellite imagery this morning showed 98L continues gradually organize with deep convection sustaining near the center of circulation and pre-mature banding of convection taking place. QuikSCAT this morning revealed a fairly define low-level closed circulation and supports at 25 knot depression. I would look for persistence and if this continues, 98L could become Tropical Depression 8 later today or on Sunday.

Intensity estimates have trended upward since we last spoke in response to decreasing shear within 72 hrs. The intensity consensus takes it to a minimal tropical storm. Beyond 72 hrs, the models continue to show an increase in upper westerlies over the disturbance and weakening occurs. This is the intensity guidance that I will go with for now.

98L has been tracking westward over the past 24 hrs under a mid-low level ridge to the north. This motion is expected over the next 3-4 days with increasing northerly component as a weakness develops within the ridge. If 98L does not get pick up by the trough through 5 days, it could trend back west at the end of the forecast cycle – 7 days.


Figure 3. Infrared image of Tropical Invest 98L.

The remnants of Fred continue to hang on as it chugs westward towards the southeastern United States. Satellite imagery this morning showed a rather significant increase in convection associated with Fred, but it appears south of center of circulation based on shortwave infrared imagery. The circulation center has also gotten much better define overnight with winds that could support a tropical depression in the thunderstorm activity to the south. Much of intensity guidance and consensus does not develop Fred and keeps it below tropical storm strength. However, Fred is expected to slow down as it approaches most likely the Florida Peninsula, as it enters a weak steering environment. This could allow Fred to develop more quickly than previously thought IF the upper environment allows it since SSTs are very warm. For one, shear remains marginal and the atmosphere remains modestly dry ahead of Fred so this should moderate development. The disturbance is expected to cross Florida in about 2-3 days and enter of the Gulf of Mexico in about 4-5 days. Conditions in the Gulf of Mexico are also marginally favourable and thus will be watched for development. There is also the possibility that as Fred slows down, it could be pull more north than expected.

The hurricane hunters are scheduled to inspect the system later today.

You can see the high-pressure system and its associated cold airmass that is expected to move southeast near the Mid-Atlantic States and steer Fred’s remnants towards the west or west-northwest. Until this happen, the discontinuity between the high and current frontal trough will cause Fred to slow down.

Weather456

Tropical Invest 98L is born; Fred is being Fred

By: Weather456, 10:47 AM GMT on September 18, 2009

Tropical Invest 98L is located near 13.5N-38.7W or about 950 miles south-southwest of the Cape Verde Islands, moving off towards the west near 10-15 mph with estimated surface winds near 30 mph and central pressure of 1009 mb. The center of 98L was easily identified using a blend of shortwave infrared imagery, the first visible images and a QuikSCAT pass from this morning. Additional satellite imagery revealed that 98L has gotten somewhat better organize over night but the thunderstorms remain well east of the well-define circulation, indicating that some shearing is going on. Microwave imagery from the DMSP's SSM/I indicate winds are between 23-28 knots and Dvorak estimates stand at 1.5/25 knots. If 98L continues to organize, it would likely become a depression.

The future intensity of 98L is uncertain due to increasing upper winds to its north. Much of the intensity guidance keeps 98L below tropical storm intensity with the official consensus taking it to 40 mph, about the borderline of a tropical storm. This is due to an increasing band of westerly shear that 98L may encounter as it heads west-northwest. However, if 98L was to continue on its southerly track, it could miss this area of shear and organize much further. In addition, some models are showing that shear will lift out. I will go with the intensity guidance and a depression could form but its future is uncertain.

98L has been moving southwest over the past day or two due to high pressure ridging to the north. The disturbance has since then move westward in time. Expect this motion to continue over the next 24 hrs. A more northerly motion may take over as a weakness develops in about 4 days, which could pull 98L northwestward, into the high shear area. Beyond 5 days, if 98L isn’t pulled out to sea, the weakness leaves and the ridge rebuilds and force 98L back west beyond 5 days. Just like Fred, timing is crucial here so folks in the Lesser Antilles should check up on the progress of this disturbance.


Figure 1. Visible image of Tropical Invest 98L this morning, just south-southwest of the Cape Verde Islands.


Figure 2. 37GHz radiance is more sensitive to the lower levels of a storm...unlike the 85GHz channel which is sensitive to deep convection. This 37 GHz pass was taken by TRMM earlier this morning and the circulation 98L is clearly defined.

The remnants of Fred are being monitored for signs of regeneration and even though the circulation has gotten a bit more define; it remains void of any deep convection. The upper environment seems marginal for now but could become increasingly hostile it nears the southeastern United States. The future of Fred does not look promising but I will continue to monitor the situation. The system could be approaching the Bahamas or Southeastern United States in about 3 days.


Weather456

Tropical Update

By: Weather456, 11:50 AM GMT on September 16, 2009

The remnants of Fred are producing disorganize shower activity over the Central Atlantic near 45-55W, north of 20N as moves off towards the west. This morning QuikSCAT pass, satellite imagery and low-level infrared winds revealed Fred has open up to a wave whose axis is located along 53W, with the greatest curvature near 20.5N. This is a sign that Fred has not shown any significant signs of development overnight as it lost its low-level closed circulation. Estimated surface winds are near 25 knots with a minimum central pressure of 1008 mb. Water vapour imagery indicate that Fred remains enveloped within upper cyclonic flow which is helping to produce some low shear but at the same time, causing dry air entrainment which is slowing development. I do not expect much development from Fred until that upper low weakens which is about 48 hrs. Thereafter, the environment may become a bit more favourable for development and Fred could redevelop into a tropical depression over the next several days.

Fred motion is being governed by deep layer ridging to the north causing him to track west to west-northwest. If you remember, this ridge was expected to rebuild after the first trough turn Fred towards the north. However, the unexpected timing of Fred caused it to miss the second trough and it will likely miss the third trough, currently near the Bahamas. Expected this westward motion to continue through the next 5 days with some gradual increase in latitude. Most of the models show Fred nearing the Bahamas next week and threatening the Southeastern United States thereafter. The exact strength is difficult to predict since models differ from a tropical wave to a category 1 hurricane. The islands should be spared from the remnants, even though the rain is welcomed, but we cannot rule out a lingering shower or two as it passes. It is basically a watch and see situation at this point.


Figure 1. Infrared image of Ex-Fred over the Central Atlantic Ocean.

Elsewhere, a well-define upper low is spinning across the Northeastern Caribbean Sea producing scattered showers across the area from 60-70W south of 20N. Expect overcast skies with periodic showers and thunderstorms across the Lesser Antilles throughout the day. This area will be monitored for any signs of development as it shifts west.

A non-tropical low is developing just north of the Bahamas and even through the cold front south of the system disintegrated into a surface trough, this feature is still expected to develop as a non-tropical system. However, the southern part will be watch, especially if both systems (the other being Fred) meets.

Some of the models are showing development in the Eastern Atlantic, and even though the wave that is expected to develop has a well-define close circulation, it still is having trouble to produce sustainable convection.

There is another wave that is about to emerge over the Eastern Atlantic that could develop, as it remains vigorous.


Figure 2. Infrared imagery of the entire North Atlantic Ocean showing 5 of the features discussed above.

Weather456

Structure of a tropical cyclone: Microwave Imagery of Choi-Wan

By: Weather456, 10:51 AM GMT on September 15, 2009


Figure 1. Infrared loop of Typhoon Choi-Wan as it rapidly intensified into a category 4 hurricane. Dvorak pressure estimates fell 77 mb from 2030 UTC 13/9 to 2030 UTC 14/9.

Most of us have seen an intense tropical cyclone on conventional satellite imagery, that is, visible, infrared and water vapour imagery, and can easily identify several features such as the eye, rainbands, central dense overcast and upper outflow. However, sometimes, a forecaster needs to assess the internal structure of these systems and their intensity ironically does not allow this. As the cyclone intensify, the thunderstorms grow higher and eventually hit the tropopause and spread out – most of the time in the form of cirrus clouds. Cirrus clouds form the outflow that emanates from these thunderstorms and they can cover the entire tropical cyclone, hiding much of the internal structure, especially if an eye exists but is clouded-in. Another advantage of seeing into the internal structure of the cyclone is to assess whether a tropical cyclone has hot towers, which more or less results in rapid intensification. Hot towers are intense towering cumulonimbus that forms the focal point of intense updrafts in a cyclone and is normally found in the eyewall or rainbands. The more intense the updraft, the more the air is moving against gravity and the more the pressure heights will fall to balance this.

Before we go any further, lets define some simple terms regarding tropical cyclones structure.


Figure 2. The advantage of microwave imagery is that they can peer through the cirrus cloud canopy to reveal partiuclar feature of the internal structure of tropical cyclones. This AMSR-E overpass of Typhoon Choi-Wan reveals such features.


Figure 3. The main parts of a tropical cyclone are the rainbands, the eye, and the eyewall. Air spirals in toward the center in a counter-clockwise pattern in the norther hemisphere (clockwise in the southern hemisphere), and out the top in the opposite direction. In the very center of the storm, air sinks, forming an "eye" that is mostly cloud-free.

Eye - The relatively calm center in a hurricane that is more than one half surrounded by wall cloud. The winds are light, the skies are partly cloudy or even clear (the skies are usually free of rain) and radar depicts it as an echo-free area within the eye wall.

Eyewall - It is an organized band of cumuliform clouds that immediately surrounds the center (eye) of a hurricane. The fiercest winds and most intense rainfall typically occur near the eye wall. VIP levels 3 or greater are typical. Eye wall and wall cloud are used synonymously, but it should not be confused with a wall cloud of thunderstorm.

Rainband - Curved bands of clouds and thunderstorms that trail away from the eye wall in a spiral fashion. These bands are capable of producing heavy bursts of rain and wind, as well as tornadoes. There are sometimes gaps in between spiral rain bands where no rain or wind is found.

In fact, if one were to travel between the outer edge of a hurricane to its center, one would normally progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again with each period of rainfall and wind being more intense and lasting longer.

Central dense overcast - This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands.

Now let us indentify each feature using two microwave imagers – CLOUDSAT and the TRMM’s precipitation radar.

TRMM’s Precipitation Radar

The Tropical Rainfall Measuring Mission’s (TRMM) precipitation radar is design to give a three-dimnesatioal overview of a storm’s structure. These measurements yield invaluable information on the intensity and distribution of the rain, on the rain type, on the storm depth and on the height at which the snow melts into rain. The estimates of the heat released into the atmosphere at different heights based on these measurements can be used to improve models of the global atmospheric circulation.

Personally, I used several TRMM data obtain from a typhoon in 2007 to conduct a class presentation on tropical cyclone’s hot towers since the precipitation radar measures water distribution in a cyclone, it therefore can be used to measure the amount of latent heat release through condensation of this water.

In the 3-dimensional TRMM image below of Typhoon Choi-Wan, it clearly showers the features discussed above. There is basically no rain falling within the eye, with very heavy rainfall occurring at the base of the eyewall. In addition, notice that rainfall significantly decreases in vertical extent in the eyewall and this is because as one goes higher, the rainfall either is being condensed or stored as ice crystals in temperatures below 0C (look at the CloudSat pass). A secondary rainfall maxima occurs on the rainbands in the outer parts of the storm. You would not be able to see such detailed rainfall distribution of this cyclone was it far out in the Eastern Atlantic, where there are no radars, thus this TRMM image is beneficial to the work of a forecaster.


Figure 4. TRMM's Precipitation radar overpass of Typhoon Choi-Wan at 2334Z 13 September 2009.

CloudSat

The second instrument we are going to look at is recently launched CloudSat. CloudSat is an experimental satellite that uses radar to observe clouds and precipitation from space. CloudSat is part of the army of polar orbiting imagers such as MODIS’s Aqua and Terra, TRMM, AMSU and QuikSCAT, all providing invaluable data to the forecaster.

CloudSat capture "the image of the year" late last night when it perfectly dissected intense Typhoon Choi-Wan. I cannot begin to tell you how remarkable a pass like this comes along. Like the TRMM overpass, the CloudSat captured most of the features discussed above. It shows no reflectivity within the eye with intense reflectivity in the eyewall and rainbands. It also shows the tops of the cumulonimbus clouds very well. Also notice on the CloudSAT pass, are isotherms, showing the temperature at each level of the atmosphere. Rain exists below the 0C isotherm, which is about 5 kilometers, while ice exists from 5 to 20 kilometers (all relative to the freezing and melting point of water). I could remember that the hurricane hunters had to abort one of their missions into Hurricane Felix of 2007 due to ice accretion, which is the build-up of ice on parts of an aircraft.


Figure 5. CloudSat overpass of Typhoon Choi-Wan at 0357Z 15 September 2009.

Tropical Update

The remnants of Fred have really caught my attention this morning, are still struggling to produce thunderstorms near the center of circulation as it battles dry air, and shear. QuikSCAT revealed that the system has a well define low-level closed circulation and any substantial increase in shower activity could result in re-development. It will be monitored as most models show the storm missing the next trough and heading west under mid-level ridging.

Another tropical wave west of the Cape Verde Islands is also being watched as showers have significantly increased this morning in association with the feature.


Figure 6. Water vapor imagery of the Central Atlantic depicting the remnants of Fred (center) and the other tropical wave (right).

Weather456

Welcomed rains for Texas; Tropical Update

By: Weather456, 11:46 AM GMT on September 13, 2009

A non-tropical low-pressure system is dumping heavy rains across the South Central United States with most locations across Texas, coastal Louisiana, Arkansas and Oklahoma picking up at least 2 inches of precipitation over the last day or two. This heavy rainfall has caused for numerous flood watches and warnings with another 1-2 inches of rain expected. This expected rainfall is welcomed signs in a region plague by exceptional drought conditions for the past year. However, several lakes, reservoirs and rivers are still recording gauge heights well below the normal level, with one of the few rivers experiencing flooding in Texas around the Dallas-Fort Worth area, where 3-4 ft above flood stage has caused some minor flooding. This low pressure system is expected to head towards the northeast over the next 3 days with heavy rains spreading into the Southeast United States. Expected 1-2 inches of rain with higher local amounts across parts of Tennessee, Mississippi, Alabama, Georgia, the Carolinas and Florida.


Figure 1. Infrared satellite loop of the non tropical system over Texas.


Figure 2. 24-hr estimated rainfall accumulation for the South-Central United States.

There is another non-tropical low located just offshore Georgia, which brought heavy rains to the Florida Peninsula during the day on Saturday. Expect this low to continue off towards the east-northeast into the Southwest Atlantic where it will be monitored for subtropical development. However, most of the computer models keep this feature a non-tropical entity and that seems to be the official guidance.

A third non-tropical feature, which has caught the attention of some, is producing a region of moderate to strong convection near 30N/55W. This feature was associated with an area of low pressure but the first QuikSCAT pass this morning reveal that has opened up and signs are it’s being absorbed into frontal boundary. I will monitor the area for any signs of changes but I don’t expect much from it.


Figure 3. Visible image of a non-tropical feature in the central subtropical Atlantic.

Elsewhere, the low-level remnants of Fred are churning stationary in the Eastern Atlantic. This is expected to head west-northwest over the next few days under shallow layer flow where it will be monitored for development.

A tropical wave along the coast of Africa remains limited in shower activity but still has potential to develop over the next couple of days.


Figure 4. Visible image of the Eastern Tropical Atlantic showing the low level remnants of Fred and another tropical wave that has been mentioned in the NHC's tropical weather outlook.

Weather456

Did the NHC drop the ball on Subtropical Storm Grace?

By: Weather456, 12:17 PM GMT on September 12, 2009

Yesterday morning, a remarkable “hybrid” system made landfall along the coast of Southern New Jersey bringing with it heavy rains, gusty winds and heavy surf. According to radar estimates and actually observed rainfall, the storm dumped between 2-5 inches of rain, with isolated amounts up 6-10 inches across New Jersey and the Delmarva Peninsula in Delaware. There were reports of flooding as a result of this rainfall, which feel during the early morning hours of Friday. In addition, several coastal stations reported water rises as a high as 6 ft, which caused minor coastal flooding. Probably the most striking effects were the winds, which gusted to tropical storm forced, downing power lines and trees, and causing power outages which gradually were repaired during the day on Friday. One of the highest wind gusts reported was in Atlantic City, New Jersey where 51 mph were reported. Buoy ACMN4 reported winds of 57 mph while buoy 44009 reported winds of 48 mph, both located offshore the Mid-Atlantic States. Tornado watches were also issued for parts of New Jersey.


Figure 1. Observed rainfall amounts of the Mid-Atlantic States from the period of 1200 UTC 11/9 to 2200 UTC 11/9, with the track of the system overlaid.

Now there is evidence that suggest this was not any cold-core extratropical storm. For example, satellite appearance indicated organize convection near a center of circulation and it could of posed as a subtropical cyclone. Secondly, temperatures rose at several locations along the coast after remaining normal – no previous cold fronts. One of the largest temperature changes was reported by Wildwood, NJ with a temperature increase of 8F as the system centre’s made landfall. Upon further analysis using cyclone phase diagrams and the GFS 850 mb temperature product at Weatherunderground, it was determined that the feature had a warm-core in the lower levels. What was most striking is that only warm fronts were analysed by the Ocean Prediction Center. Another piece of evidence was the amount of rainfall was rather unusual for a cold core low since one of the factors determining rainfall in any cyclone is the upper level support; indicating there was a good amount of tropical air present in this system. The last piece of evidence was seen on QuikSCAT sea winds. Normally non tropical cyclone produce winds through the process of pressure gradient. They interact with a nearby high pressure system that produce winds well away from the center. However, in the QuikSCAT pass taken on the system, the winds were not only near the system but they were not being caused by pressure gradient, since there was no high south of the low to produce those offshore winds illustrated.

Now that we have looked at the warm-core characteristics, let us look at the features, which probably did not make it tropical or subtropical. First, frontal boundaries were analyses – straight off the back. Subtropical/tropical cyclones are non-frontal features and some of these were analyzed and appeared apparent on satellite imagery. Second, this feature was embedded within an upper trough – but wait. Andrea was embedded within an upper trough too. The biggest difference was that the upper environment near 200 mb was cooling rather than warming as the system passed as indicated by the sounding at Wallops Island, VA. Still this is quite a distance from where the system mad landfall but was embedded in the upper low. This could only indicate the feature was cold-core in the upper levels and is backed again by cyclone-phased diagrams.

This was not tropical nor was it truly extratropical, what could it be? My thinking is it was a weak warm seclusion. A warm seclusion is the mature phase of a cyclone with warm air wrapped all the way round the center as it matured. The cyclone may have done this when it was blocked and cut off from the colder air up north due the blocking high.

So in regards to the NHC, I wouldn’t blame them for not naming the feature but I wouldn’t be surprised if the feature was upgraded in post season analysis since it came close. This is the third system this year that raised some debate on whether the NHC is being too conservative, not trying to name too many subtropical cyclones or just doing their job. Many, including myself remember Tropical Invests 90L and 92L back in May and June, respectively which came pretty close to being upgraded.

Elsewhere in the tropics

Tropical Storm Fred continues to get blast from strong southwesterly winds and this morning's visible imagery shows the center has become completely exposed. It is likely that further weakening will continue today as Fred meanders over the area. Most of the models take what’s left of Fred back west under the influence of a rebuilding ridge that may eventually reach near 50W. Some of the intensity guidance thinks Fred will re-developed into at least a tropical storm, but the problem with them is that they end the weakening process today when Fred is getting hammered by strong wind shear. Thus, these models are being inconsistent for now and it is likely Fred will weaken to a tropical depression over the next couple of days. The remnants of Fred will be watched as I don’t think it may dissipate completely.


Figure 2. Visible image of Tropical Storm Fred showing the expose low level center as it battles with strong wind shear.

An area of disturbed weather located over the Gulf of Mexico is associated with a surface trough interacting with a nearby upper trough. Though wind shear has dropped a good clip, the area remains disorganize and not showing much signs of development. I will still continue to monitor the area but I'm not impressed thus far. Regardless of development, the system has already dumped 1-3 inches across parts of Texas and the Gulf coast to much relieve of the drought conditions.

The area that interests me is the disturbed weather in the Northeast Gulf which is associated with an area of low pressure over Florida separated by two surface troughs – one over the Gulf of Mexico and the other extending from the same hybrid system over the Northeast. This low is expected to emerge over the Southwest Atlantic over the next 24 hrs where it will be monitored for development. Remember I said yesterday that I would be watching that region for development. Anything that develops will likely track northeast but it’s too early to tell out to sea or along the US East Coast.


Figure 3. Infrared image of the Gulf of Mexico and Southwest North Atlantic, depicting several low level features.

There is one more area of interest far in the Eastern Atlantic associated with a tropical wave that could develop over the next day or so. Details on track will come later.


Figure 4. Infrared satellite imagery of the tropical Eastern Atlantic.

Weather456

Tropical Update

By: Weather456, 10:18 AM GMT on September 11, 2009

As of 5am EDT, Hurricane Fred was located near 17.7N-35.1W or about 745 miles west of the Cape Verde Islands moving towards the north near 3 mph with maximum sustain winds of 85 mph and a minimum central pressure of 980 mb. Fred’s satellite appearance continues to gradual deteriorate as westerly shear steadily increases and it moves over progressively cooler waters. This weakening trend was highly expected and should continue through the next day or two as Fred churns across the Eastern Atlantic.

Water vapour imagery shows Fred is moving along the north and maybe slightly east of due north as it continues under the influence of well-amplified upper trough. This is the same upper trough that is inducing strong westerlies over Fred, causing him to weaken. Expect this motion to continue over the next 24-48 hrs with little change in speed. Thereafter, models have become in agreement that a ridge will rebuild across the region, forcing Fred to move back towards the west through day 5. Fred will continue along the path until the next trough swings across the region and the timing of this feature will determine how far west Fred gets. Some models show Fred reaching as far west as 60W but he might already be a remnant low at that time.


Figure 1. Infrared imagery of weakening Hurricane Fred.

As expected a surface trough has developed across the Western Gulf of Mexico and is now interacting with an upper trough over the South Central United States to produce scattered showers and isolated thunderstorms along the Mexico, Texas and Louisiana coasts and adjacent waters. Satellite imagery shows showers remain disorganize in the diffluent flow ahead of the trough and there is no evidence of a surface low per QuikSCAT imagery and shortwave imagery over water yet - though there is on land. This feature is expected to remain stationary over the region over the next few days with the development of a surface low that is expected to move ashore over Southeast Texas or Louisiana. Some of the models show shear relaxing to marginal levels later this weekend and thus this may allow for some development, however by Monday, shear is reintroduced into the region and thus development becomes less likely. In addition, the low might already be heading ashore. I still give this feature a 30% chance of developing into a tropical depression. Regardless of development, locally heavy rainfall will spread across coastal regions of Mexico, Texas and Louisiana over the next few days.

I would like to add a thought that anything remaining stationary over the Gulf of Mexico is somewhat a benefit for development so the feature will be watched.


Figure 2. Water vapor depiction of the Western Gulf of Mexico region. The "L" over Texas is the center of the upper low/trough that is interacting the surface trough (dashed lines).

Elsewhere

A surface low is expected to develop in the vicinity of the SW Atlantic (Florida/the Bahamas area) over the next 3-4 days so it will be watched for any development. I will have more on these features on Saturday.

Looks like a tropical storm, behaves like one, but it’s not. A non tropical low located over the Northwestern Atlantic is dumping heavy rains and bringing gusty winds and choppy seas to the Mid-Atlantic coast as it moves ashore. If you member I had a blog on it about 3 days ago as this was the same low offshore that dumped heavy rains across the Carolinas early this week. It did as it was forecasted to do – head northeast, become stalled by a blocking high over Southeastern Canada and retrograde back over land. The system will be monitored for subtropical development but regardless of development, heavy showers area spreading over the region with several locations already picking up an inch for the morning.

I posted some radar and satellite images in the comment section.


Figure 3. Dover AFB, Base Reflectivity 0.50 Degree Elevation Range 124 NMI.

Weather456

Tropical Update

By: Weather456, 10:09 AM GMT on September 10, 2009


Figure 1. MODIS TERRA capture this image of Fred near peak intensity of 120 mph, right after rapid intensification over
the Eastern Atlantic.

Hurricane Fred was located at 5am EDT, near 15.9N-34.6W or 705 miles west-northwest of the Cape Verde Islands. Estimated surface winds are near 105 mph with a minimum central pressure of 970 millibars. Yesterday, Fred rapidly intensified into a major hurricane, the 2nd of the season but has since then gradually lose organization and has now weaken back to a category 2 hurricane. Satellite imagery this morning reveals Fred remains a decently impressive hurricane but the cloud canopy appears to becoming asymmetric with much of it to the north and east of the center. This is probably the start of the westerly shear that is predicted to affect the hurricane. Much of the intensity guidance foresee gradually weakening of Fred over the next 48 hrs with most showing Fred weakening to a minimal tropical storm in 72 hrs. The models then become in disagreement beyond 4 days as some re-strengthen Fred and others dissipate him. Wind shear is expected to increase over Fred up to 40 knots over the next 5 days and sea surface temperatures will be along the borderline of 26C. In addition, the NHC mentioned this morning that he would enter a very dry environment and further analysis reveals as dry as 39%. Thus, I will go with the models that foresee weakening with little room for reintensification.

Fred is now moving towards the northwest and almost looks like north-northwest under the influence of an advancing upper trough as seen on water vapour imagery and 200 mb vorticity charts. Expect Fred to continue towards the northwest, with a gradual turn towards the north and slightly east thereof as Fred interacts with the trough. This takes place over the next 2 days but beyond then, the trough exits the region and the ridge rebuilds causing Fred to shift back west – his intensity at this point will make it easier. Whatever is left of Fred tracks northwest or west-northwest until the next trough swings down in about 7 days. Some models are indicating the ridge will be strong enough to turn Fred back southwest but I’m not ready to go with solution yet.

Fred was a rare storm to become a major hurricane so far south, and east in the Atlantic basin and was also one of three major hurricanes noted east of 35W in the tropical Atlantic Ocean. Fred set the record on becoming the strongest hurricane so far south and east in the Atlantic ocean. However, this type of system would have been very difficult to accurately observe before satellite images began in the 1960s.


Figure 2. Infrared imagery of Hurricane Fred this morning. Notice how much the cloud canopy appears towards the north and east, indicating signs of increasing upper winds.

An area of low pressure over the South Central United is producing scattered showers and thunderstorms over parts of Texas and is expected to drift towards the Texas coast later this week into Saturday where it has potential to spawn a tropical system, not just from the low itself, but the associated surface trough that drapes across the Western Gulf of Mexico. Models are not really doing anything but keeping the low mostly onshore and non tropical with the NAM, never good with tropical forecasting, being the most vigorous solution with a developing tropical storm offshore. Despite this, any low-pressure area developing offshore could quickly develop under the right upper-air conditions so the area will be monitored. I still give the feature a 30% chance of becoming a tropical depression. Regardless of development, moisture will spread across coastal parts of Texas and Louisiana later this week into the weekend.


Figure 3. 00Z GFS MSLP/6-hr precipitation valid through 72 hrs with rainfall totals through 5 days overlaid.

Weather456

Fred becomes the season's 2nd hurricane; Gulf disturbance to bring rains

By: Weather456, 10:09 AM GMT on September 09, 2009

Hurricane Fred was located near 13.2N-31.7W or about 500 miles west-southwest of the southernmost Cape Verde Islands, moving off towards the west-northwest near 13 mph. Estimated surface winds have increased to 105 mph with a minimum central pressure of 970 millibars. Satellite imagery reveals Fred has taken advantage of warm sea surface temperatures and has rapidly organize into a category 2 hurricane with an eye-like feature embedded around a well-define central dense overcast (CDO). He has atleast 24 hrs more of ample conditions and as long as an eye wall replacement cycle does not commences, Fred could become a minimal category 3 hurricane today, the second major of the year. Much of the intensity guidance show Fred peaking over the next 24 hrs as mentioned before but then weakening after 48 hrs. I do not believe the intensity models, which weaken Fred rapidly after 12 hrs since shear does not increase until 48 hrs and it is rather hard for well form hurricanes to rapidly breakdown. That being said, I expect intensity to level off with weakening expected between 24-48 hrs due to an increase in westerly shear and cooler sea surface temperatures.

Water vapour imagery shows Fred has slowed some and has now begun that turn towards the northwest as the trough advances and continues to break down the ridge that was steering Fred west-northwest. I expect this motion to continue through the next 24 hrs with a turn towards the north-northwest through 48 hrs. Fred gets caught into the weakness through days 3-4 and begins a more northerly motion but as the trough passes, the ridge rebuilds briefly and the system resumes a more northwesterly motion and this maybe enhance by the weakening of the system through this time frame. By days 6-7, a more powerful trough swings across the Atlantic and either absorb Fred or finish re-curvature.

Interesting facts

Because of extensive destruction, the name Frederic (1979) was retired, and was never again used for an Atlantic hurricane. It was replaced with Fabian in the 1985 season, although Fabian itself was retired after 2003 and was replaced by Fred for 2009.


Figure 1. Central Atlantic water vapor imagery.

Gulf of Mexico development

I have analysed some model data and seems reasonable that a disturbance may form across the Gulf of Mexico later this weekend. Genesis originates from a short-wave that digs across the South-Central United States with the associate frontal boundary draped across the Gulf of Mexico. Meanwhile, a mid-level ridge extends across the Caribbean and Southwest United States and causes the front to become stationary which secludes into a low pressure feature over the SW Gulf of Mexico. Under favourable upper winds and warm sea surface temperatures, the feature gradually acquires some subtropical or tropical characteristics. Most of the models drag the feature towards the north, towards either Texas or Louisiana. However after looking at cyclone phase diagrams, it shows the feature quickly becoming a non-tropical feature especially if it slides along the coast and isn’t able to remain over water, thus proximity to land will the major inhibiting factor. Wind shear appears to relax some over the next couple of days and sea surface temperatures are very warm but the models don’t do much with it. I give this feature a 30% chance of becoming depression. Regardless of development, cloudiness and showers is expected to spread across coastal Texas ad Louisiana by the beginning of next week.

I will have more the feature as time goes on.


Figure 2. 00Z GFS 500 mb vorticity and height analysis, valid 126 hrs or 5 1/4 days.

Weather456

Hybrid System dumps heavy rains across the Eastern United States; Fred is born

By: Weather456, 10:03 AM GMT on September 08, 2009

A non-tropical low-pressure system continues slide slowly up the East Coast of the United States, dumping heavy rains across the region. Some analysis reveal this feature is developing downstream of a short-wave trough centred across the Ohio Valley. Though this feature is non-tropical in nature, it has warm-core characteristics such as organize convection due to the injection of warm tropical air; some coming from the remnants of Erika. This feature has already dump several inches of rain across the Carolinas with several stations reporting up to 4 inches and as high as 6 and 8 inches yesterday, causing some flooding. Expect this feature to slide towards the northeast through the next 24-48 hrs with some decrease in forward motion. This feature is expected to become stationary due to the progression of a strong high-pressure system over Eastern Canada and the pressure gradient between the two may produce moist, gale force onshore flow along the Mid-Atlantic States causing flooding and the threat for rip currents. Rainfall amounts could reach 3-4 inches across the region.

I posted some additional animations in the comment section.


Figure 1. Water vapor imagery of the Northeast United States.


Figure 2. Morehead City, Storm Total Surface Rainfall Accumulation Range 124 NMI.

Tropical Depression 7 formed yesterday afternoon, and quickly organized into Tropical Storm Fred by late last night. Satellite imagery overnight revealed Fred continues to undergo gradual organization and is now pose to strengthen into a hurricane later this week.

As of 5am EDT, Tropical Storm Fred was located near 11.8N-27.3W or about 285 miles southwest of the Cape Verde Islands, moving off towards the west near 15 mph. Estimated surface winds are near 50 mph with a central pressure of 1000 mb. Satellite imagery revealed Fred has taken on very tight curve banding features around a very define low level circulation, which is a sign of a healthy tropical cyclone ready to intensify further. The cyclone is moving under a vigorous upper anticyclone, which is aiding in the ventilation of the system. In addition, Fred has only now begun to enter a region of 28-29C waters and thus intensification is likely and much of the intensity guidance is going with a 65-knot hurricane in 36-48 hrs. There is a slim chance that Fred will become a major hurricane due to low-marginal ocean heat content. Beyond 48 hrs, the cyclone appears to encounter a band of upper level westerly winds and cooler waters and intensity guidance levels off and begin a gradual decline thereafter.

Fred has been in a general westward direction and at one point, south thereof, due to a high pressure over the Eastern Atlantic – the Azores High. I expect this motion to continue through the next 2-3 days. Afterwhich, Fred begins to slow down and turn more towards the north due an advancing trough that is expected to break down the ridge and create a weakness, and a northward motion is expected through day 5. If Fred does not recurve under this trough, a ridge rebuilds and forces Fred back towards the west-northwest or northwest. This is expected to last through day 7, at which point another vigorous trough digs across the region and affect Fred now located near 50W. If Fred does not recurve under this second trough then it could reach further west than expected.


Figure 3. Infrared imagery of Tropical Storm Fred, this morning.

Weather456

Tropical Update

By: Weather456, 12:12 PM GMT on September 07, 2009

Happy Labour Day to all Americans both at home and overseas.

Tropical Invest 96L is estimated to be located near 13.5N-23.5W or about 98 miles south of the Cape Verde Islands moving off towards the west near 15 mph. Estimated surface winds are near 25 knots with a minimum central pressure of 1006 mb. Satellite imagery revealed that 96L is only slowly gaining organization with QuikSCAT showing a well-define low-level center on the eastern edge of the main convective band. Still satellite presentations remain remains impressive and environmental conditions are conducive for the future development of the system.

Much of the intensity guidance indicates that 96L will continue to develop and become our next tropical depression sometime this week. Much of the guidance also rapidly develops 96L into a hurricane over 28C waters and low vertical wind shear. In about 96 hrs, the system begins to turn more towards the northwest and north and encounter cooler sea surface temperatures and possibly higher vertical shear and thus intensity levels off and begins a slow decline.

Tropical Invest 96L is moving off towards the west under the influence of the Azores side of the subtropical ridge and this motion is expected for at least the next 3 days. Through day 4, a deep frontal trough digs across the Atlantic and breaks down the ridge enough to create a weakness which pulls 96L more towards the northwest and then north-northwest. Some models are indicating that this trough is so trough, that 96L sharply turns towards due north just a day after moving west-northwest.

My thinking is that the storm will continue west and take a gradually turn towards the northwest through days 4-5. Afterwhich, the storm may fluctuate between west-northwest and north-northwest through days 5-7 due to successive trough and ridge flows. Full recurvature is expected near 50-55W beyond 1 week.


Figure 1. Visible satellite loop of Tropical Invest 96L.

Meanwhile, a non-tropical low-pressure system is developing offshore the Southeast United States brining showers and isolated thunderstorms to South and North Carolina. Expect this low-pressure area to slide north-northwest and move over the Outerbanks North Carolina over the next 24-36 hrs, bringing 1-4 inches of rain. The low should emerge east of the Mid-Atlantic States through 2-4 days and become stationary due to a high-pressure system over Nova Scotia Island. The pressure gradient between the two features may produce gusty onshore winds along the Mid-Atlantic coast.


Figure 2. Infrared depiction of the non tropical low pressure system


Figure 3. Current conditions across the Carolinas as of 7:59 EDT this morning.

Later this week or into next weekend, I will discuss some of the factors, which is contributing to a slow progressing season, especially during the peak of hurricane season.

Weather456

Tropical depression may form this week

By: Weather456, 12:09 PM GMT on September 06, 2009

Tropical Invest 95L

The circulation center of Tropical Invest is located near 19.5N-34.0W or 719 miles northwest of the Cape Verde Islands moving off towards the northwest near 10-15 mph. Estimated winds are near 30 mph with a minimum central pressure of 1008 mb. First satellite images revealed a sheared tropical disturbance with convection displaced northeast of the well-define low level closed circulation due to the influence of increasing southwesterly shear which you all may have known, was expected. Shear is expected to increase further over the next 48-72 hrs so little development is expected. However, some models are showing that shear will decrease between 72-120 hrs, but probably not enough to save the low pressure system since none the intensity guidance show much strengthening thereafter.

Tropical Invest 95L has been moving almost northwest to north-northwest over the past 24 hrs under the influence of mid-deep layer flow, which shows the disturbance moving towards a weakness between two high-pressure cells. As the weakness closes, 95L hits the high-pressure bridge and begin to move more towards the west-northwest and this is already indicated on the last few frames satellite imagery. This motion may deviate between west-northwest and northwest over the next 5 days but beyond then, 95L is completely absorbed by a second frontal trough and is taken into the North Atlantic.


Figure 1. Visible image of Tropical Invest 95L, revealing a define but expose low level circulation

African Wave

A vigorous tropical wave is now exiting the coast of Africa as I type. The wave is located along 16W south of 16N; moving off towards the west near 12 mph. Satellite imagery show a very vigorous mid-level circulation and QuikSCAT imagery indicate this may also be present at the surface. Further satellite imagery analysis indicate that this feature has everything it needs to quickly develop as it exits the coast with a vigorous upper anticyclone overhead and increasing low level vorticity and convergence. Sea surface temperatures right along the coast are near 29C and get as low as 27N south of 15N. The Saharan dust layer is almost non-existent at this moment. Most models also quickly develop this feature as it leaves the coast and rapidly develops the feature west of the Cape Verde Islands. Given the environmental conditions and model support, a depression will likely form this week.

Where will it go? Well most indications are it will travel west for some time after leaving the coast, up to 72 hrs (3 days) due the ridging provided by the Azores side of the subtropical ridge. Afterwhich, the disturbance is expected to turn more towards the west-northwest then northwest through day 5, while west of the Cape Verde Islands under the influence of pronounce weakness in the subtropical ridge due to a vigorous frontal trough. It is still too early to tell how far west the system gets but most models are indicating 50-60W, north of 30N.


Figure 2. Visible satellite loop of the new African disturbance that has the potential to develop into a tropical depression later this week. Notice the vigorous spin.

Elsewhere in the tropics remain quiet. There is an area of shower activity offshore the Southeastern United States that is associated with a stationary front that will be monitored for any signs of development, but nothing out there that could develop over the next day or two.

Weather456

The future of 95L uncertain

By: Weather456, 12:20 PM GMT on September 05, 2009

Tropical Invest 95L was estimated to be located near 15.5N-29.5W or 415 miles due west of the Cape Verde Islands moving off towards the west-northwest near 10-15 mph. Estimated minimum pressure is at 1008 mb with winds of 25 knots. This morning satellite imagery reveals a fairly organize tropical system with evidence of a closed low level circulation based on infrared satellite winds but located to the southwest of the main convective band. This may be due to some increasing effects of southwesterly shear. The disturbance is moving over 80-85F waters with 10 knots of shear so I expect some level of organization through 24 hrs but rather uncertain as to the long term faith of 95L.

Tropical Invest 95L is moving under the influence of high pressure ridging over the central and eastern Atlantic and this motion is expected through 3-4 days with some increase in northwesterly motion thereafter due to a weakness in the subtropical ridge. This weakness is both evident on Rainbow coloured imagery and water vapour imagery as a trough continues to amplify over the north Atlantic. This very same trough is also responsible for creating a band of high vertical shear that extends near 20N and thus much of the intensity guidance keeps 95L weak through day 5. The other solution is that 95L does not develop and heads further west in the shallow steering flow but it will result in the same thing – little development through day 5.

I will continue to monitor Tropical Invest 95L over the upcoming days as 95L may recover beyond day 5 when is expected to lessen some.

Meanwhile, the remnants of Erika are now interacting with an upper low south of Bermuda to produce numerous scattered showers across Santo Domingo and Puerto Rico. Little development expected.


Figure 1. GOES-12 water vapor depiction of the Tropical Atlantic. When this image is looped, it shows cold air advecting from the trough over the Eastern Seaboard is helping to amplify the other trough near 95L.

Weather456

Erika almost gone

By: Weather456, 10:07 AM GMT on September 04, 2009

Erika was downgraded to a tropical depression yesterday afternoon and then a remnant low late last night. Despite Erika remaining weak over the past 24 hrs, she dumped a good amount of rains across the islands with most locals picking up about an inch of rain. Most of the rain was recorded where the convection was displaced towards the southwest near the central Antilles with Dominica’s Melville Hall picking up 11.92 inches. Dominica is one of the most mountainous islands in the Eastern Caribbean and thus large amounts of rain can be recorded under weak disturbances.

Currently the remnant low is located south of Puerto Rico and most of the models are saying that it will likely move towards the north-northwest under the influence of a frontal trough draped across the Eastern Seaboard into the Northwest Caribbean for days now. The models appear to be hasty in bringing this trough east since water vapour imagery show little movement. Those being said, expect a general west-northwest to northwest motion over the next 3 days, passing between Puerto Rico and the Dominican Republic and emerging over the Southwest North Atlantic just to the east of the Turks and Caicos. Afterwhich, she is likely to make that NNW shift and become absorbed, that is if she doesn't completely dissipate before then. She may have become a remnant low but some of her energy will likely produce some showers across Puerto Rico and Hispaniola later today. She will be monitored for signs of regeneration but this is the last full update on her on my part.


Figure 1. Water vapor depiction of several features affecting Ex-Erika path over the next several days.

Erika may have gone but she provided us with some very good information on why tropical storms weaken.

One of the causes why Erika convection increased and then decrease but has led it to weaken was that mid-level dry air was entrained into the system. Below is an upper airtime cross section from Guadeloupe and the green represents moist air while the red represents dry air. Erika's convective bursts was likely due to warm air (from SSTs) below dry air which is an unstable situation but because this dry air was not in the upper levels but rather in the mid levels, evaporational cooling eventually cooled the mid-levels. The downdrafts may have hit the surface and aided in the further momentum of the low-level circulation (LLC).


Figure 2. Upper air cross section of relative humidity values at Guadeloupe. I choose a location closest to the heavy convection.

Vertical shear was one of the other causes. Below is the actual wind at Saint Maarten which shows a rather big difference between the 200 mb winds (upper level winds) and the near surface winds with a 10-20 knot bias on the near surface easterlies explaining why the low-level circulation (LLC) may have raced out ahead of the convection.


Figure 3. Upper air cross section of the actual wind at Saint Maarten. I choose a location that was further north where shear was the greatest over the low level center.

Elsewhere

Models continue to develop a rather robust tropical wave now located south of the Cape Verde Islands. While showers have increase in association with this feature, there appears to be no significant signs of organization. I will continue to monitor this feature but it still is likely to become our next invest later this weekend.


Figure 4. METEOSAT9 visible image of the area of disturbed weather that has potential to develop into a tropical depression over the next day or so.

Weather456

Erika dumping heavy rains across the Islands

By: Weather456, 9:56 AM GMT on September 03, 2009

As of 5am EDT this morning, Tropical Storm Erika was located near 16.5N-62.7W or about 85 miles west of Guadeloupe moving off towards the west-northwest near 7 mph. Hurricane Hunters managed to find winds of 40 mph with a minimum central pressure of 1008 mb. Satellite imagery continue to show Erika producing some vigorous thunderstorm actvity but remains east of center due to southwesterly vertical wind shear. These thunderstorms have managed to keep Erika a tropical storm as they are helping to produce storm-forced gusts. Water vapour imagery continue show Erika may encounter some southwesterly shear from the edge of the same upper anticyclone that is supposed to be protecting her; but the biggest problem was the upper anticyclone is not moving in tandem with her, in fact, it has shifted south to 11N. Intensity guidance also continues to decrease and it is likely that Erika will not intensify much as it heads west.

Erika has continued to move more west and south than previously thought due to a center relocation and ridging to her north. I am confident that Erika will move towards the west-northwest over the next 5 days under this ridging but also the fact that the ridge weakness under the influence of a broad area of low pressure from the Eastern Gulf to 70W. She will be like water flowing down a trough, following the path of least resistance. This motion should bring her close to Puerto Rico and the Dominican Republic in 24 hrs and over the southern Bahamas by 5 days at which point she should be a tropical wave, remnant low or surface trough. Afterwhich some models have her absorbed by a frontal through day 6 and move off towards the north. The exact faith of Erika is uncertain but this scenario along with dissipation seems plausible. However, she will be monitored for regeneration or interaction with any feature once over the SW Atlantic.


Figure 1. GOES-12 water vapor depiction of several features affecting Erika's intensity and future track.

Regardless of how strong she is or if she weakens, Erika is dumping heavy rains across the islands and producing gusty winds. Radar imagery along with surface reports is indicating that huge amounts of rain and 30 mph winds are spreading west across the islands and it is likely some tropical storm force wind gusts are embedded in those thunderstorms. Up to 5 inches can be expected from Erika as she moves slowly off towards the west. Some of this rainfall will spread across Puerto Rico and Hispaniola later on Friday.


Figure 2. 48 hr lead rainfall accumulation.

Elsewhere

A very healthy looking tropical wave along the coast of Africa is likely to develop into our next tropical invest, 95L and eventually the season's next depression. Most of the models develop this feature and take it towards the west-northwest with time. Since we have days to watch it, I will have more details as time goes on.


Figure 3. MTSAT infrared imagery of the next area of interest along the African coast.

Weather456

Tropical Storm Erika closes in on the Leeward Islands

By: Weather456, 9:46 AM GMT on September 02, 2009


Figure 1. GOES-12 infrared image of Tropical Storm Erika this morning.

As of 5am AST, Tropical Storm Erika was located near 17.0N-59.0W or about 280 miles, east-southeast of the Leeward Islands, moving off towards the west near 5 mph. Measured surface winds are near 50 mph with a minimum central pressure of 1005 mb.

Satellite imagery continue to show Erika has been putting on impressive rounds of thunderstorms development but hurricane hunter data along with shortwave infrared imagery indicate her center remains located near the northwest tip of the convective blob. Vertical shear is currently 10-15 knots above Erika and although it has delayed some organization of the system it appeared to help with some of the thunderstorm develop through upper diffluence. It is likely that Erika will intensify some over the next day or two as the upper level environment becomes a bit more favourable and she continues to move over warm waters.

Erika is currently slowly moving off towards the west to west-northwest under the influence of a mid-low level ridge over the Subtropical Atlantic. The storm is expected to continue on this west-northwest motion over the next 1-2 days, which should bring the system close to the islands by Thursday morning.

Thereafter, models become increasingly divergent through days 3-5 with some taking the system northwestward (ECMWF) while others (GFS) have the system going straight west into the Bahamas. Those two extremes are likely due to difference in intensity. The ECMWF deepens Erika enough to be steered by deep layer flow, which shows a more pronounce weakness in the subtropical ridge near 70W than the shallow layer as the GFS barely develops Erika. Despite this uncertainty, most models show one thing; Erika will pull more northwestward due that lingering trough draped across the Western Atlantic.

This could only mean that whether Erika deviates from the center of the forecast cone will depend on her intensity. Folks from South Carolina go north should monitor the situation of Erika since there is a 50/50 chance that the trough deflects Erika further up the coast. I am going with slightly left of the forecast track due to the uncertainty of Erika's intensity.

The effects of Erika will be immediately notice in the islands by tonight into early Thursday. Heavy thundershowers with gusty will be experience across the islands but it seems the core of 60 mph winds will remain to the northeast of the center. Still there is not much difference between 50 mph and 60 mph, thus the system should taken as it is. Expect 2-4 inches of rain with higher location amounts especially in the higher areas. According to how slow Erika moves across the area, some flooding is possible. These effects enter the Virgin Islands and Puerto to a lesser extent by Thursday as Erika moves more north.

Interest in the Turks and Caicos, Bahamas, and the Southeastern United States should continue to monitor the progress of Tropical Storm Erika.


Figure 2. Erika's forecasted windfield through 33 hrs.

Hurricane Jimena

The Baja Peninsula felt the brunt of Jimena winds yesterday as it passed Cabo San Lucas to the west and this morning as it makes landfall further north along the Baja Peninsula. Jimena brought strong winds and heavy rains to the Southwest tip of the peninsula yesterday knocking down trees and power lines; and creating some heavy surf along resort areas. The biggest threat now from Jimena is the heavy rains as she slows down and rainout over areas that are prone to flooding and landslides. Up to 10 inches of rain could fall over the region over the next day or two.


Figure 3. Tidal surge at Medano Beach (Playa el Medano), where popular on-the-sand-restaurants such as the Office and Mango Deck enjoy robust business on calmer days. Credit: Pete Thomas


Figure 4. Fallen trees as Hurricane Jimena passed Magdalena Bay early Wednesday morning. Credit: Pete Thomas

I will post an update from the islands at 2PM this afternoon.

Weather456

Baja braces for Jimena; California burns; The case of a curious invest

By: Weather456, 9:57 AM GMT on September 01, 2009


Figure 1. MODIS TERRA image of Hurricane Jimena's eye taken yesterday, 31 August as she intensified into a powerful category 5 hurricane.

Hurricane Jimena

Hurricane Jimena was located near 20.2N-110.1W or about 185 miles south of Cabo San Lucas, Mexico moving off towards the north-northwest at 12 mph. Estimated surface winds are near 155 mph, the border line of category 4/5 but I often ask myself, what the heck is a difference of 1 mph? Minimum pressure is down to 931 mb. Some fluctuation in intensity is expected as Jimena moves towards the Baja Peninsula and into slightly cooler waters.

This morning satellite imagery showed Jimena is less than 24 hrs away from the coast with the eye widening and the outer rainbands already along the southwest tip of the Baja Peninsula. Jimena is expected to pass west of Cabo San Lucas enough to keep the destructive winds offshore and it is likely the area will only experience tropical storm forced conditions. However, large waves and heavy rains will affect the area with some very gusty winds since they are expected to be located on the right side of the hurricane.

Areas further north may not be as lucky, where the storm is expected to make landfall as a category 2/3, likely causing considerable damage.

Jimena biggest threat would be the torrential that is expected to fall over areas that are prone to flooding due the mountainous terrain. Up to 4-8 inches of rain can be expected with localised higher amounts.

Jimena is also generating a 10-14 ft swell that is travelling up the coast towards the Baja. Some locations along the Mexican west coast could experience relatively high surf. Storm surge is a threat with 5-10 ft water rise expected along the peninsula. Fortunately, here, it is not like the Gulf of Mexico as the area not surrounded by a large continental shelf.


Figure 2. National Hurricane Center (NHC) 5-day forecast track for Hurricane Jimena.

Tropical Invest 94L

Tropical Invest 94L is located near 16.4N-55.9W or about 400 miles east southeast of Saint Johns, Antigua moving off towards the west-northwest near 10-15 mph. Estimated surface winds have increased to 30 knots with suspected gusts of 35 knots in the heaviest thunderstorms. Estimated minimum pressure remains at 1006 mb. Satellite imagery continues to show a burst of impressive convective activity this morning but it seems any low-level feature remains on the southwestern edge of this heavy convection due to southwesterly shear. However, most of the reliable models are showing a decrease in upper winds as the upper feature creating this shear shifts west over the next 3 days. This along with increase ocean heat should allow some gradual development and 94L could become a depression or storm later this week.

Satellite imagery showed 94L may have slowed and it could only mean the invest has entered the region of weak steering flow where the high-pressure ridge is interrupted. I expect 94L to continue to move off towards the west to northwest, reaching the islands or just north thereof by tomorrow night. It is likely the system will pass close enough to bring squally weather to the Leeward Islands and maybe the Virgin Islands.

The tricky part comes after the system passes the islands. Much of the steering flow that seems to be handling the system well, favours a mid-level ridge to build near Bermuda. This ridge is expected to support deep layer ridging that keeps a trough stalled across the Eastern United States allowing the system to move west-northwest to northwest through day 5. This is also supported on water vapour imagery as the mid-level flow continues out of the southeast (a northwest direction). The system may eventually be caught into the weakness of the trough to pull more north through day 6-7 similar to a path of Danny. I am still unsure of this scenario but for the time being, I will go with the global model consensus.

Hurricane Hunters will be out this afternoon, investigating 94L and should provide us with precious data not only on its organization and strength but where this baby is going. Remember the HHs data are placed into the model runs.

Interest in the Leeward Islands, Virgin Islands, Puerto Rico, Hispaniola and the Bahamas should continue to monitor the progress of 94L.


Figure 3. GOES-12 water vapor imagery showing an overview of several features affecting 94L's organization and movement.

California Wildfires

Very dry weather conditions across Southern California have pushed one of the most major wildfires seen in the Los Angeles area in more than 60 years. Officials stated on Monday that the wildfires increased from 45, 000 acres to more than 100, 000 acres in a matter of hours. The Station fire, the largest of the eleven fires burning across the state has forced thousands of evacuations and threatened thousands of structures but luckily, the lack of wind kept the fires from driving explosively into the hearts of the dense suburbs of northeast Los Angeles.

The 11 active large wildfires terrorizing California have killed at least two firefighters, of more than 6,400 battling the blazes; prompted a declaration of a state of emergency from Gov. Arnold Schwarzenegger in two counties, Los Angeles and Monterey; threatened communications towers critical not only for television and radio broadcasts, but also for national security; and threatened the Mount Wilson Observatory; and prompted the evacuation of more than 10,000 homes.

About 60,000 acres have burned in the last 10 days in California, and nine additional fires are burning throughout the West. Arizona, Colorado and Texas fighting one fire each, and Oregon, Utah and Washington are fighting two each.

In addition, the fires have created dangerous air quality issues for several counties across Southern California due to smoke. Forecasters are not seeing any change in the drought conditions for the next 5 days but I suspect the remnants of either Jimena and/or Kevin may help in the upcoming days.

It is fortunate that these wildfires are not being sparked by strong winds like the Santa Ana or they might have been much worst.


Figure 4. The entire state of California is experiencing drought, with a large portion classified as "severe drought" by the U.S. Drought Monitor.


Figure 5. NASA’s Terra satellite captured this true-color image around 11:45 a.m. local time (18:45 UTC) on August 30, 2009. Red outlines indicate hotspots of anomalously high surface temperatures associated with wildfires. In this image, the hotspots remain west of Mt. Wilson. The site of critical communication centers, Mt. Wilson had not burned but was threatened by the fire as of August 31, according to the Los Angeles Times. Smoke from the fire blows toward the northeast in this image. Clouds, perhaps mixed with some smoke, linger over Los Angeles.


Figure 6. A U.S. Forest Service air tanker drops fire retardant next to a line of fire as the Station fire burns in the hills above a home in Acton, Calif. The fire, so far, has destroyed 21 houses. Credit: Dan Steinberg-AP


Figure 7. Smokes rises high into the atmosphere as the Station Fire gradually marches west across southern California. The wildfire has claimed the lives of two firefighters and destroyed 21 homes. Credit: Kevork Djansezian-Getty Images

Thanks to TheDailyGreen for providing some of this information.

Weather456


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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About Weather456

With a Bachelors Degree in Environmental Sciences (2009), began tracking tropical storms in 2002 and is now a private forecaster.