Weather456's Tropical Weather Blog

Hurricane Alex chugs towards the Mexican Gulf Coast

By: Weather456, 10:41 AM GMT on June 30, 2010

Hurricane Alex in the last advisory, was located near 23.3N-95.1W or about 235 miles southeast of Brownsville, Texas moving off towards the west-northwest near 7 mph. Maximum sustains are near 80 mph with a category 2 central pressure of 961 millibars. Satellite imagery along with a microwave overpass from AMSR-E revealed a strengthening hurricane with the central dense overcast (CDO) becoming more define around a clouded-eye feature. Much of the banding is to the south and east of the storm, which is progressively increasing around the storm. A second convective band is found to the system’s north. The overall appearance of Alex signifies an intensifying system and with such a low central pressure, I suspect the intensity forecast given by the NHC is not aggressive enough. Alex could become a strong category 2 or possibly a major hurricane at this level of organization since the Dvorak reading given by SAB is already 4.5 or 90 mph. For now, my predictions of a storm around 100-105 mph during landfall remains the same since Alex will make landfall in the next 12-24 hrs.


Figure 1. AMSR-E 39GHz (low level channel) overpass of Hurricane Alex this morning around 3:44am EDT showing well define banding around an evident eye feature.

I think most people noticed that Alex slowed over the past day or two and we can thank its deepening for this. Had Alex been a much shallower system, it would have already been closer to the coast because the shallow layer steering does not possess such a weakness to the north. However, the deeper layer flow has a more pronounce weakness to the north between the exiting frontal trough and the replacing ridge, so effectively, the deeper Alex got, the more it was subject to slow down. In addition, the replacing ridge is not as progressive as previously forecasted and is not having much of an influence over the storm. However, this should change as the trough lifts out, and the ridge would have more control over Alex, bringing him more west to west-northwest into the coast and inland over Northern Mexico. We still cannot rule out a border landfall.

Rain, Wind and Storm Surge

Current radar imagery showed the outer bands have already begun to lash the Texas and even the Louisiana Gulf Coast with moderate to heavy rainfall, especially the front leading band to the north. Alex is expected to produce total rainfall accumulations of 6 to 12 inches over portions of northeastern Mexico and southern Texas with isolated maximum amounts of 20 inches. These rains could cause life-threatening flash floods and mudslides especially in mountainous terrain.


Figure 2. Current Nexrad regional radar of Eastern Texas.

Areas along the coast of Mexico should prepare for hurricane-force winds, with the areas of direct impact near Northern Mexico, category 2 storm force winds. Tropical storm force winds are expected to reach the coast later today and will cover a large area of the coast, as far north of Corpus Christi, Texas. A dangerous storm surge will raise water levels by as much as 3 to 5 feet above ground level along the immediate coast to the north of where the center makes landfall. The surge could penetrate inland as far as several miles from the shore with depth generally decreasing as the water moves inland. Near the coast, the surge will be accompanied by large and destructive waves.


Figure 3. Modelled wind field at 21 hours lead.

Interesting facts about Alex

Alex is the first June hurricane since the active period began 15 years ago in 1995 – Hurricane Allison. There were no hurricanes recorded in June 2005, but Arlene came very close, peaking at 70 mph. In an average season, the first hurricane normally forms by August 10. I suspect that the 18-20 named storms forecasted this season, along with the La Nina predicted makes this more of a 1995 active season.

Alex’s central pressure, from the time it emerged, remained a category higher than his current winds, and since winds normally lag behind the pressure readings, forecasting Alex’s intensity has become a bit easier than it otherwise would be.

Weather456

Alex nearing hurricane strength

By: Weather456, 9:56 AM GMT on June 29, 2010

Tropical Storm Alex was centered near 21.7N-91.9W or 460 miles southeast of Brownsville Texas, moving off towards the north-northwest near 8 mph. Maximum sustain winds have increased to 70 mph and minimum central pressure is down to 984 millibars. This morning satellite images of Alex showed a somewhat large but gradually organizing system with very deep convection banding in and around the center of circulation. The overall convective cover remains rather irregular but closer views revealed the system maybe forming a western eyewall, suggesting the storm is close to becoming a hurricane. Alex is currently moving under progressively low vertical shear and progressively warm sea surface temperatures and thus I suspect he will become a hurricane today. If you notice Alex’s central pressure is equivalent to a category 1 hurricane and this is because of the lower than normal pressures were are experiencing this hurricane season. This may be the story with most storms this year and it should not take long for the winds to follow. Environmental conditions appear conducive for further strengthening upon until landfall and I suspect an intensity forecast slightly higher than the official forecast because of its slow speed. This should bring Alex to a category 2 hurricane upon landfall with winds near 100-105 mph.


Figure 1. Current infrared image loop of Tropical Storm Alex over the Southwestern Gulf of Mexico.

Currently, Alex is being steered by a weakness between the subtropical ridge over the Atlantic and a ridge of high pressure over the Eastern Rockies. This has caused Alex to slow down considerably and turn towards the north-northwest. As the trough the gradually lifts out through day 2, deep layer ridging will turn Alex more towards the west but probably not as steeped as previously thought. The storm should make it ashore over Northern Mexico but a landfall near the Mexican/Texas border or southern Texas is becoming more possible as the likely scenario.

Storm Surge, Rainfall and Wind

A landfall over Northern Mexico from a slow moving category 2 hurricane could create serious water rises over Southern Texas. Located in the front right quadrant, water rises could reach 10 ft with waves reaching 20 ft, easily inundating coastal communities across extreme southern Texas with some level of coastal flooding. Rainfall amounts could reach as high as 8 inches and it all depends again on the forward speed of the storm and how much time it has to dump all its moisture overland. Expect, inland flooding to occur in the most low-lying regions of Texas and the mountainous areas of Eastern Mexico. Category 2 hurricanes are strong enough that they can lift a house, and inflict damage upon poorly constructed doors and windows. Vegetation, poorly constructed signs, and piers can receive considerable damage. Mobile homes, whether anchored or not, are typically damaged, and many manufactured homes also suffer structural damage. Small craft in unprotected anchorages may break their moorings.


Figure 2. Maximum storm tide (ft) of a category 2 hurricane hitting Brownsville, Texas at high tide. Image courtesy - Weatherunderground's Texas Storm Surge Models.

Weather456

Alex slows down as it exits the Yucatan

By: Weather456, 10:09 AM GMT on June 28, 2010

Tropical Storm Alex was last located near 19.7N-91.6W or about 75 miles west of Campeche, Mexico, moving off towards the northwest near 6 mph with maximum sustain winds near 50 mph and a minimum central pressure 990 millibars. Yesterday afternoon, hurricane hunter aircrafts flew into Alex as it emerged over the Bay of Campeche to find a deeper depression than anyone had expected. Winds normally lag behind pressure readings so the 35 mph depression downgraded at 11am on Sunday quickly regained tropical storm status. Currently, satellite imagery showed Alex has become better organized since it emerged with a relatively small area of deep convection near the center of circulation. Shortwave infrared imagery showed the circulation has gotten tighter, smaller, and spinning much faster indicating that Alex is strengthening. There is a hurricane hunter flight leaving the Northern Gulf Coast as we speak for flight 5 into Alex. I suspect they would find a bit deeper system. Alex is currently moving in tandem with an upper anticyclone and over sea surface temperatures of 29C so I expect gradual strengthening. The ocean heat content where Alex is located is very low so I do not expect any drastic change in intensity until he moves further northwest in about 12-36hrs. How strong will Alex become over the Western Gulf of Mexico is governed by the available energy, the conditions and the amount of time Alex has to use this energy. Alex should move over marginal heat content, under favourable conditions in about 3 days, with a maximum potential intensity of a category 3 hurricane. Because hurricanes rarely reach their maximum potential, intensity along the official forecast is the most feasible solution. However, the unexpected decrease in speed could allow Alex more time to intensify before landfall.


Figure 1. Ocean heat content and forecast track from 2am EDT this morning.

There is a wide division between the statistical model tracks and the global model tracks. Both set of models foresee a northwest motion through atleast 2 days but the statistical models turn Alex towards the north-northwest then north into Texas, while the majority global models foresee a turn towards the west into Mexico. A track more similar to the global guidances is likely. Why? One important factor we have to consider is that Alex has slowed. Now the trough pulling Alex towards the northwest is moving into the Great Lakes region and should be over the Eastern United States by Wednesday. In the wake of this trough, deep layer ridging should force Alex back towards the west. Given Alex has slowed gives the trough more time to clear the area before it gets too far northwest near Texas. However, it is rather bitter sweet since it also gives the second trough (6-8 days) more time to enter from the Western United States. However, I do not think that Alex will be around that long to witness the second trough and so a track between the official and GFDL is what I am going with. This should bring Alex to northern Mexico in about 3-4 days with some impacts in extreme southern Texas.


Figure 2. Steering flow for a moderately deep tropical cyclone valid 2am EDT this morning.

The impacts for now should be extremely heavy rains over Mexico as Alex slowly pulls away from the coast.


Figure 3. Current infrared image of Tropical Storm Alex taken 5:58am EDT this morning.

Weather456

Alex now moving over the Yucatan

By: Weather456, 10:54 AM GMT on June 27, 2010


Figure 1. Current infrared image of Alex taken 10:56am UTC 27 June 2010.

Tropical Storm Alex made landfall last night and is now moving over the Southern Yucatan Peninsula. The storm is located near 18.3N-89.4W, or 75 miles west of Chetumal, Mexico moving off towards the west-northwest at 12 mph. Maximum sustain winds are down to 40 mph with a minimum central pressure of 1000 millibars. Radar imagery from Belize revealed the center of Alex made landfall around 00:45 UTC (8:45pm EDT) last night just north of Belize City. Radar and satellite imagery clearly shows the center moving over the southern Yucatan Peninsula with a very pronounce circulation still evident both in the lower and upper levels. Despite this, friction over land will dissipate any winds associated with the cyclone, and I suspect Alex will be downgraded to a tropical depression later today. On the current path and speed of motion, the storm should emerge over the Bay of Campeche in the next 12-18 hrs and based on the current rate of organization, I expect Alex to regain tropical storm status on Monday.


Figure 2. Belize radar image of Tropical Storm Alex as the center of circulation crossed the coast last night.

Upon emerging over the Gulf of Mexico, the storm should enter an environment of low vertical shear below 10 knots and marginal sea surface temperatures. The storm will take some time to re-organize itself and since ocean heat content is not exceptionally high, I will go with a more gradual increase in intensity, based on the OFCI (NHC) and the LGEM. This should bring Alex to a category 1 hurricane in about 3 days. However, the intensity forecasts are widespread ranging from a minimal hurricane to a category 2 hurricane. The SHIPS, most aggressive on the intensity of Alex in the Gulf of Mexico, only accounts for marginal ocean heat content, thus the need to be more reasonable for now and go with a blend of the LGEM and the official intensity forecast.

Track guidance has been interesting none the less. Currently, the storm is being steered in the mid-low levels around the SW quadrant of the subtropical ridge. This is inducing more of a west-northwest motion, but I am thinking some northwest motion may evolve over the next day or two as a weak shortwave passes north of Alex. In the wake of this trough, deep layer ridging should force Alex back towards the west from days 2-3. Now models differ greatly on the track. For example, the COAMPS and GFS show the storm heading more north into Texas while the BAMM and BAMS show the storm taking a sharper curve towards the south and west, spending a maximum of 4 days of the southern Bay of Campeche. For some reason, there is a battle between the exiting trough and the entering ridge at this time. The first two models indicate that even though a ridge is over the Central CONUS, that Alex will still head north because of the influence of the exiting trough, while the last two think otherwise. Both ends of the spectrum are not entirely feasible and so I will go down the center and close to the official and HWRF tracks. That being said, folks in southern Texas should not discount this system.


Figure 3. This is one of the extremities in the track forecast of Alex. This is modeled basic radar reflectivity from 12Z June 26 out 5 days or 120 hrs. This is not entirely feasible since if you look closely, the model is already north of the actual motion of Alex, thus a landfall south of this seems more reasonable.

Winds reported during the passage of Alex were relatively light, but rain rates were pretty high. Belize City picked up 110 mm or 4 inches just yesterday and the latest satellite imagery revealed heavy rains are still falling over the region. Rainfall estimates indicate up to 6+ inches of rain over the next 6-12 hrs over parts of Southern Mexico and Guatemala as the storm crosses. Probably the most high-risked areas are the mountainous regions hard hit by Agatha last month. TRMM flood and landslide monitoring has indicated this risk as possible.

Elsewhere, tropical cyclone formation is not expected over the next 3 days. Tropical Invest 94L however, delivered a spectacular lightning show last night with heavy downpours across the Leeward Islands. Expect this activity to continue throughout the day.

There is a strong tropical wave over Western Africa that will be monitored for development over the next 5 days.

Weather456

Tropical Storm Alex bears down on Belize; his forecast

By: Weather456, 9:49 PM GMT on June 26, 2010


Figure 1. NASA's MODIS TERRA image of Tropical Storm Alex this afternoon showing a well organize and strong tropical storm.

Hurricane hunters found a stronger Alex with maximum sustain winds near 65 mph. According to the latest advisory the storm is located near 17.3N-87.8W or about 30 miles east south-east of Belize City, moving off towards the west near 12 mph. Maximum sustain winds are near 65 mph with a minimum central pressure of 996 millibars. Latest satellite and radar imagery showed the center of circulation just offshore the Belizean Coast, near the Turneffe islands and cays. This was also supported by microwave imagery and aircraft center fixes. The strongest winds were found in the southwest quadrant of the storm where the deepest thunderstorms are found. Overall satellite presentations have continued to gradually improve with the central dense overcast (CDO) more regular surrounded by well define banding resulting in satellite estimates as high as 61 knots. I suspect there should be steady strengthening of Alex until landfall over the next 12 hrs and it may come very close to hurricane status.

Based on the latest information from the hurricane hunters, tropical storm forced winds have already begun to affect the coast. Thus far, only light winds with showers are being reported indicating that much of the severe weather remains offshore. Weather should continue to directorate throughout the evening, with gusty winds, heavy rains and high surf later tonight. Isolated amounts could reach 8 inches with minor tree and infrastructure damage.


Figure 2. Current windfield of Tropical Storm Alex based on this afternoon's advisory at 5PM EDT.

Upon landfall, weakening is likely and depending on the angle of entry, angle of crossing and speed of motion it could take anywhere between 6-24 hrs overland. I will go with the NHC current forecast and foresee a re-entry into the Bay of Campeche by this time tomorrow, being that there are no center relocations. I suspect given the favourable conditions in the Gulf of Mexico over the next 4-5 days, that Alex will have minor troubles regaining tropical storm intensity and gradual, not rapid, re-intensification is likely. Reason being the ocean heat content in the path of the storm is not exceptionally high, but high enough to support a hurricane.


Figure 3. Latest intensity guidances at 18Z this afternoon.

Now the big question is where the storm will go. I expect a west-northwest to northwest motion over the next couple of days as the storm rounds the edge of the subtropical ridge to the north. This seems to be the consensus of the track guidance and is supported by the trough that will be moving over the Northern CONUS over the next 4 days. Meanwhile, deep layer ridging builds in the wake of this trough and forces Alex back towards west after such time once it is in the Western Gulf of Mexico. The exact timing of Alex and these features and re-entry location will determine how north it will get. In the meantime, the track guidance has landfall much further south along the central Mexican Gulf coast, which I am not entirely incline to believe until the system emerges by Monday.


Figure 4. Current water vapor imagery of the CONUS and tropics showing the general motions in the upper westerlies. If you notice, these features appears well north of Alex so motion will be mainly governed by the steering in the subtropics and tropics. For now, we will have to go with the model guidances until Alex emerges later this weekend.

I will have another update tomorrow morning.

Weather456

Tropical Storm Alex; Bonnie not too far

By: Weather456, 1:12 PM GMT on June 26, 2010

Tropical Storm Alex based on the latest advisory was located near 17.0N-85.3W, or about 200 miles east of Belize City moving off towards the west northwest at 8 mph. Maximum sustain winds are near 40 mph with a minimum central pressure of 1004 millibars. Latest satellite imagery revealed an extremely organized tropical storm with banding features around a solid cold dual central dense overcast. This is likely due to the extremely favorable environment with high ocean heat content below and a nice upper anticyclone above. The environment appears conducive for further intensification of Alex and the possibilities of a hurricane before landfall is increasing especially with the slow movement of the storm.


Figure 1. Infrared image of the first tropical storm of the 2010 Atlantic Hurricane Season. Our first tropical storm is pre-mature version of Hurricane Wilma (2005) which further illustrates the potential of this season.

Intensity guidances follow a typical pattern with some strengthening likely before landfall on the Yucatan Peninsula and then weakening as it crosses and re-strengthening over the Gulf of Mexico. The exact amount of time the system remains over the peninsula will determine the state of its condition once it emerges in about 3-4 days. Conditions in the Gulf of Mexico are expected extremely conducive for re-intensification and it should not take long for Alex to regain tropical storm status.

Currently, Alex is being steered by a sprawling high over the Atlantic. The steering pattern looks like August, rather than June with a large high extending from the Azores to Texas so a west-northwest to northwest motion will be expected over the next couple of days. Meanwhile, a trough in the upper westerlies should pass across the Great Lakes in about 2-4 days and “tug” and induce a more northerly motion. Now, not all models agree on the timing and effects of this trough. It is very northerly so might not have an effect at all. A suspect a northwest motion will occur through 96hrs, largely due to the beta effect and the motion relative to the edge of the ridge. This should bring the storm close to the Mexican/Texas coast by day 5-6.

Currently, the impacts from Alex will be the heavy rains within the rainbands over parts of Central America and the Cayman Islands. These areas can expect up to 3 inches before the initial landfall. Areas along the Yucatan and Belize coast can expect squally weather, gusty winds and heavy rains over the next couple of days. An additional 4 inches can be expected during landfall. All interest along the areas should prepare for the possibility of Alex becoming a hurricane before landfall. Rapid development in the Caribbean is all too common, most recently – Ida from November 2009.

Bonnie may not to be too far behind. A Bertha type wave is moving over West Africa and will emerge in a few days where most of the reliable models are indicating development will occur. The tropics have started.


Figure 2. Infrared image of the tropical wave that has the potential to develop into a tropical cyclone over the next 2-5 days as it emerges.

Expect another update later this afternoon.

I apologize for my leave of absence over the past week, but personally, I really could not be here, I had to be there for my son. Hopefully I can be here to continue to track these two systems – I will find a way.

Weather456

Tropical Update

By: Weather456, 12:08 PM GMT on June 20, 2010

A tropical wave along 72W and south of 22N continues to produce widespread cloudiness, showers and thunderstorms over the Lesser Antilles, Puerto Rico and Hispaniola as it interacts with diffluent flow between an upper level circulation at 25N and an upper ridge centered east of Barbados. NASA’s TRMM estimates indicate 4-8 inches of rain may have fell across the islands over the past week during the passage of this system. Another 10 inches, possibly isolated higher amounts may fall across Puerto Rico, Hispaniola, Jamaica and Eastern Cuba over the next 3-5 days, which has the potential for flooding. I expect this tropical wave to continue off towards the west-northwest across Cuba through day 2 and reaching the edges of the Gulf of Mexico by day 3. At this point, conditions maybe favourable enough for development, which I suspect, is the reason why the NHC is still following this system. Interests in the Western Caribbean and Gulf Region should pay attention to the progress of this system.

Elsewhere, showers and thunderstorms in the Southeast Caribbean is associated with a second tropical wave enhanced by the rather northerly ITCZ.


Figure 1. Water vapor depiction of the Caribbean region this morning of potential initiator(s) for development later this week.

I will have an update on Monday.

Weather456

Tracking Tropical Waves: Structure, Nature and Propagation

By: Weather456, 6:27 PM GMT on June 15, 2010

Since 92L is a few days away from land and we do seem to have any imminent areas of concern I will post a full tropical update at 5PM this afternoon after the full 12Z and 18Z models are updated. In the meantime, here is the blog on tropical waves that I had to postpone on Sunday. You can enjoy the read while you wait for 92L to do something. It is one my personal best researches all in the name of Tropical Meteorology and I assure you it will cover everything you need to understand about African Easterly Waves. Thanks for all the get well wishes, I am evidently getting there.

Tracking Tropical Waves: Structure, Nature and Propagation

Last week we looked at the conditions that set the stage for the development of the African Easterly Jet (AEJ) and found that mid-level African waves developed to the south of the jet and that low-level waves developed north of the jet. Today we are going to take a further look at African Easterly Waves (AEWs), particularly the mid-level waves south of the jet. Before we do so we are going look at an overview of low-level waves and the genesis discussed last week. Much of the information you see here was made possible by numerous studies done on African Easterly Waves, most importantly the African Monsoon Multidisciplinary Analyses Project (AMMA).

Low Level Waves

These waves develop to the north of the African Easterly Jet (AEJ) in a region of static instability associated with the low-level thermal trough below the Saharan High as suggested by Chang (1993) and Thorncroft (1995). These waves that develop north of the AEJ and lack any convective activity due to the availability of moisture. They often head westward with uncertainty as to where they will go. Some may move towards the southwest and eventually are located below the AEJ, while others continue northwards neutral Rossby waves. For this reason, we will stress more on the classic mid-level waves that we recognise over tropical North Africa.

Overview of Mid-Level African Easterly Waves (AEWs)

For these tropical waves to develop, you need instability within a fast flowing stream of air at the mid-levels and a rapid reversal of the potential vorticity (PV) gradient as suggested by Charney and Stern in 1969. The African Easterly Jet (AEJ) satisfies these conditions. But how? Well the AEJ is a fast flowing stream of air typically near 600mb over Northern Africa. The flow near the jet axis is going faster than that of flow further way in the surroundings. What happens, is that eddies begin to develop due to differences in speed of the flow within the AEJ (barotropic instability/horizontal shear). A cyclonic eddy develops south of the jet and in the northern hemisphere, this is positive vorticity (+PVU). Anticyclonic eddies develop north of the wave and in the northern hemisphere, this is negative vorticity (-PVU). In the natural world, positive vorticity is stronger and thus will bulge the axis of the AEJ northwards resulting in a kink or wave. The wave continues to amplify until the jet weakens and an AEW is born. Now if we look closely, we see the PV gradients as the positive eddy to the south and the negative to the north. This is the rapid change in PV gradient that we often discuss. Figure 1 summarizes the stages of development and figure 2 shows the vertical profile of this PV gradient.


Figure 1. Schematic diagram of the stages of tropical wave development. From the upper left the AEJ is established as we looked at last week, vortexes begin to develop within the fast flowing stream which leads to a kink in the AEJ axis. The last stage of development is the initiation of convection due divergence aloft near 700-600 mb.


Figure 2. Vertical cross section of the PV gradient associated with tropical waves. Notice the +PVU is south of the -PVU as described by Charney and Stern in 1969.

Development of Mesoscale Convective Systems (MSCs)

These can develop along wave disturbances by a combination of factors. First, the supply of moisture is provided by the moist monsoon southwesterlies from the Gulf of Guinea. Daytime heating and the topography aids in the spatial and temporal scale of these MSCs. The last and most important factor is divergence ahead of the 700 mb wave as described in figure 1. MCSs reach maximum intensity usually in the late afternoon, evening and early morning local time, and die during the day due to diurnal influences over land. These MCSs are important features of the rainy season over Africa. Figure 3 shows the propagation of a strong AEW in September 2006 that became Hurricane Helene. If you notice, the convection normally is initiated by a developing disturbance in the AEJ over Eastern Africa, most commonly known as the Ethiopian Highlands and propagates westward with diurnal variations and much of the convection ahead of the wave axis.


Figure 3. METEOSAT infrared image loop of Northern Africa between September 05 and September 13 2006. The wave is initiated over the Eastern Ethiopian Highlands and propagates westward with diurnal variations and convection mainly ahead of the axis.

Propagation

African easterly waves normally have a wavelength of 2500 km and transit period of 3-5 days. They tend to develop the most intense convection during the late afternoon, night and early morning local time. It takes about 6.5 days for a wave to move from Eastern Africa to the Eastern Atlantic going at a speed of 8 m/s or 20 mph. They often reach maximum amplitude just before exiting the coast. They propagate westward much slower than the environmental flow, which they were first created in. The main steering factor is the mid-level Saharan High to the north.

Horizontal and Vertical Structure

Horizontally, AEWs are characterized by kinks in the mid-low level wind field with a sharp PV gradient along the axis. As we discussed before, much of the bad weather occurs west of the axis where divergence aloft near 600 mb is greatest. Strong tropical waves often amplified north into the Saharan Thermal Low giving them twin vorticity centers north and south along the axis. Winds flowing along this kink are normally from the northeast (ahead of the axis) to southeast (behind the axis). At the surface, this may not be as pronounce (see the section: Weather).


Figure 4. GFS analysis of West Africa around 2:00pm June 5 showing a strong tropical wave about to emerge off the coast of Africa. The top left image shows the 650 mb winds with the African Jet axis nicely illustrated with the wave-like disturbance within the jet axis. The top right image shows the potential vorticity showing the greatest PV is often found south of the jet axis as we discussed. The bottom image is another illustration of the moisture gradient over Northern Africa and the position of the jet relative to this gradient. Without these gradients, there would be no jets and consequently no African waves.

Vertical – Winds

Typically, the greatest change in winds will occur near 600 mb and this is from northeast (ahead of the axis) to southeast (behind the axis). Below the waves are the monsoon southwesterlies that extend to about 900 mb and above the waves the prevailing westerlies from 300 mb-150mb. The African Easterly Jet (AEJ) is often found at the core of each wave as an easterly jet max that accompanies. Figure 5 shows the vertical structure of a wave that passed Dakar, Senegal on Friday.


Figure 5. Upper air time cross section of Dakar, Senegal showing the passage of two African waves on the 8 of June and 12 of June 2010. The waves normally have their greatest wind shifts near 700 mb with southwesterly dominating the surface and the AEJ above at 600 mb and normal westerlies much higher up.

Vertical - Potential Vorticity

Figure 2 shows a simplistic overview of the vertical profile of the PV gradient found along tropical waves. The positive vorticity is found south of the wave axis and the negative vorticity is found north of the wave axis.

Weather

The changes in surface conditions over Africa are much more different from the changes in weather over the Atlantic and Caribbean. First, there are no interruptions of the trades since the prevailing low-level winds over Africa are southwesterlies. Before the passage of the wave, the temperature is normally high, the dew points are low and the wind is out if the southwest. As the wave approaches, cloudiness increases, the winds turn easterly and the temperature drops causing relative humidity to increase. As the axis approaches, maximum bad weather is experience. Behind the axis, pressure begins to drop and the winds turn southeasterly. After the wave passes, weather returns to normal and southwesterly prevailing winds return.


Figure 6. Meteogram of Ouagadougou, Burkina Faso in Central West Africa showing the passage of an African Easterly Wave on 10 June 2010. A shows that air temperature decreases as the wave approaches causing relative humidity to rise. B shows the weather which is characterized by thundershowers ahead of the axis as appose to behind. C shows the cloud cover and winds shifting from the normal southwesterly flow so southeasterly. D shows the visibility decreases and finally E shows the increase in pressure ahead (likely due to the mesoscale highs behind outflow boundaries ahead of the thunderstorms) leading to a drop in pressure behind the axis.

Beyond Africa

After emerging off the coast of West Africa, tropical waves go through a number of influences and changes but the original trough axis is maintained. The changes are induced by sea surface temperatures, the tradewind inversion, the inter-tropical convergence zone (ITCZ), the tropical upper tropospheric trough (TUTT), among others.

The Saharan Air Layer

In May of 2009, I posted a blog, which discussed the influences of AEWs and the Saharan Air Layer (SAL). Numerous studies done on the relationship of these two phenomenons revealed the negative vorticity north of the AEWs is what disturbs the dust particles over the Saharan Desert. This is why emerging SAL has that unique anticyclonic signature. Excellent examples of these are shown in video 1 and figure 11, which show a very large dust event succeeding a tropical wave in July 2005 and July 2007.

Dust is often entrained into the convection associated with tropical waves.


Figure 7. SAL conceptual model illustrating the following properties: geographic location of the African continent, ICTZ, and dust plume, surface flow (solid yellow arrows), particle trajectories (dashed yellow arrows), mid-level easterly jet (thick red arrow), 700 hPa wave axis (thin red arrow), regions of convection, and the rise of the SAL base to the west (Karyampudi et al. 1999).


Video 1. Large African dust plume from July 2005 showing the anticyclonic rotation of SAL caused by the negative vorticity north of AEWs. There is always a tropical wave at the leading edge of such intense outbreaks.

The Oceanic Environment

Lapse rates over the ocean are much gentler than those over land and this is because of the properties of water - it absorbs, and releases heat much slower than rocky land. Over the Eastern Atlantic, there are two regions of extreme tropical sea surface temperatures. Just off the coast of Africa, there is a region of high sea surface temperatures due to the downwelling of monsoon southwesterlies. Further west near 30W-50W, the ocean temperatures drop sharply due to 1) cooler air coming from the Canary Current and 2) surface divergence south of the Azores High. This cooler ocean cools the air above it and is capped by an inversion due to 1) the subsidence warming from the Azores High and 2) dry stable air from SAL. This is called the Tradewind Inversion. Consequently, the lapse rate will support deep convection once the wave emerges (warm air near the sea surface below cooler mid-level air). However, as the wave tracks further west, increase stability results in loss of convection. Sometimes a wave will not generate any convection between 30W and the Caribbean. Thus, we see warm sea surface temperatures are important for the sustainability of convection along waves but the tradewind inversion more than often results in the loss of this convection.


Figure 8. Sea surface temperatures taken June 11 2010 of the Eastern Atlantic overlaid in Google Earth showing the monsoon southwesterlies, NE trades, ITCZ/Monsoon Trough, the cold Canary Current and two major areas of SST extremes.


Figure 9. GFS soundings taken at two locations (A and B) in figure 8. Notice the differences in stability and winds between each location. The left is region A and the right is region B.

The Inverted-V Pattern

Easterly waves in the tropical Atlantic have been found to be associated with a characteristic cloud pattern, which has the appearance of an “Inverted V.” Cloud bands are aligned approximately parallel to the lower tropospheric winds and change orientation along the wave axis. We have already established how the wind changes but unlike over Africa, maximum convergence and band weather is behind the axis. This is because the wind behind the axis slows down to compensate for the sub-geotropic flow. When air slows down, the flow behinds piles up and causes convergence. As the air flows around the axis to other side, it speeds up and diverges. Lower level convergence promotes rising air and cloudiness, while lower level divergence promotes subsidence and fair weather.


Figure 10. Location of convergence and divergence in an easterly wave in relation to the trough axis.

This wave model was developed by Frank et al in 1969-1970 and thus is dubbed the Frank’s Inverted V Wave Model. They have average wavelengths of 2000-2500 km and mean speed of 10-15 knots, much slower than AEWs. The reduce speed is the result of the lack of the AEJ further west. The deep easterlies are much slower. It takes about 6-7 days for a wave to travel between the West Coast of Africa to the Caribbean Islands.

Diurnal variations in convection are opposite to over land and the waves reach convective maxes during the early morning and convective minimums during the late afternoon.


Figure 11. GOES-12 visible image of the Tropical Atlantic taken July 17 2007 of a large Inverted-V wave approaching the islands at the leading edge of a large African Dust Plume. This is one of the largest dust outbreaks I have ever observed so it was easier to relocate the exact date.

Riehl's Easterly Wave Model

This is one of the first wave models develop for the Eastern Caribbean and it largely involves interaction with the tropical upper tropospheric trough (TUTT). As waves transit the tradewind inversion with little or no convection, they suddenly explode in convective activity as we saw last week when they approach the Eastern Caribbean. This is due to the interaction with the TUTT among other factors like warmer sea surface temperatures and a deeper moisture field. Westerly upper winds keep the wave vertically tilted and much of the convection remains east of axis. As the wave passes a station in the Eastern Caribbean, the northeast trades veer easterly then south easterly. Relative humidity increases, pressure drops and maximum cloudiness is found along and behind the wave axis. This is quite the opposite of what we see at Dakar further east across the ocean.


Figure 12. GOES-13 water vapor image taken of the Caribbean on June 9 2010 showing the interaction of a tropical wave and the TUTT. Notice how the ITCZ is pulled northward by the wave as discussed in the next section.


Figure 13. Meteogram of Point, Grenada from June 8-9 showing the passage of the wave. It is more or less the same as over Africa except that the bad weather lies behind the axis (5).

Interaction with the ITCZ, Lake Maracaibo, Tropical South America and the Colombian Low

As waves interact with the TUTT they amplify northwards and with that, the ITCZ follows. Strong tropical waves can pull the ITCZ northwards. It has happen before and has been proven in studies. This mostly happens in the Southeastern Caribbean. Early seasonal tropical waves are often embedded within the circulation of the ITCZ and they create classic inverted-V patterns in the otherwise linear/flat cloud band.

As waves across over into South America, they often revert to diurnal forces they were once under over Africa. Tropical waves will enhance afternoon thunderstorms over northern South America and the availability of moisture from Lake Maracaibo and the northern portions of the Amazon aides in the development of these thunderstorms.

The third point of interactions is the Panama Low or Colombian Low, which enhances cloudiness along wave just like the monsoon environment over Africa.

Here we see there are positive interactions between tropical waves and the ITCZ, Amazon, Colombian Low and small water bodies.


Figure 14. Visible image of the Caribbean and Northern South America showing the major contributors of tropical wave's positive interactions. This was taken 3 days after the wave in figure 11.

Development into Tropical Cyclones

Under favourable environmental conditions, (warm ssts, deep layer of moisture, low vertical shear and enough positive vorticity). MCSs along tropical waves can develop into tropical cyclones through a process called CISK. CISK, or "Convective Instability of the Second Kind", is a popular theory that explains how thunderstorms can evolve and organize into hurricanes. CISK is a positive feedback mechanism, meaning that once a process starts, it causes events, which enhance the original process, and the whole cycle repeats itself over and over.

The theory of CISK states that as thunderstorms develop, condensation within the thunderstorm releases heat (heat of condensation). This warms the air and causes it to rise. As air rises, the pressure heights lower and a low-pressure area forms. Air rushes into this low-pressure area while picking up moisture from the sea, converges and rises. Rising air condenses and releases heat of condensation and the process continues repeatedly. If conditions are favourable, then air rises higher and higher with each cycle and pressure lowers and lowers. Thus, we see a thunderstorm develops into an organized storm.

While only about 60% of the Atlantic tropical storms and minor hurricanes ( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995).

Personal Insights

The study of weather was my first drive but began to pay more attention to Tropical Meteorology due the fact that I reside in the Caribbean. My desire and longing to learn about tropical cyclones drove me years ago to explain where they came from and it led me back to Africa. This is where my interest in tropical waves sparked. I have studied more on tropical waves than any other synoptic feature in the tropics, with tropical cyclones and the ITCZ close behind. I have been look at synoptic maps of tropical waves and the associated weather since the summer of 1998 when I was only 9 years old, but the peak of actually getting into where they came from occurred from 2004-to the present. No tropical met book is completely without ever mentioning tropical waves, these fascinating creatures of nature.

References

African Monsoon Multidisciplinary Analyses Project (AMMA)

Hurricane Research Division FAQ – Easterly Waves

African Easterly Jet: Structure and Maintenance, Journal of Climate, Sep 1, 2009 by Wu, Man-Li C, Reale, Oreste, Schubert, Siegfried D, Suarez, Max J, Koster, Randy D, Pegion, Philip J

Mesoscale Convective Systems and African Easterly Waves, Doug Paker, 2007

Three-Dimensional Structure and Dynamics of African Easterly Waves. Part I: Observations GEORGE N. KILADIS Earth System Research Laboratory, NOAA, Boulder, Colorado CHRIS D. THORNCROFT Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York NICHOLAS M. J. HALL Laboratoire d’e´tude des Transferts en Hydrologie et Environment, Grenoble, France, 2005.

Generation of African Easterly Wave Disturbances: Relationship to the African Easterly Jet; JEN-SHAN HSIEH AND KERRY H. COOK; Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, 2004.

Energetics of Easterly Waves; M.A. Estogue and M.S. Lin; Rosentiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, Florida, 1977.

Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000 TSING-CHANG CHEN Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, 2006.

Characteristics of North African Easterly Waves During the Summers 1968 and 1969, Robert W. Burpee, Laboratory of Atmospheric Research, University of Illinois, Urbana, 1973.

Three-dimensional structure of easterly wave disturbances over Africa and the tropical north Atlantic, George N. Kiladis and Chris D. Thorncroft, NOAA Aeronomy Laboratory, Boulder, Colorado Department of Earth and Atmospheric Sciences, SUNY Albany, NY, Unknown.

The Low-Level Structure of African Easterly Waves in 1995 Ioannis Pytharoulis and Chris Thorncroft, Department of Meteorology, University of Reading, Reading, United Kingdom, 1998.

Weather456

Tropical Update

By: Weather456, 9:49 AM GMT on June 15, 2010

I am really under the weather right now guys that I am not able to effectively do a tropical update (since yesterday afternoon). However, due to my need to don't leave readers in the dark here's a short take on 92L. It encountered less favourable upper winds yesterday which entrained some dry into the system causing it to lose organization. This was rather unexpected but likely due to the unexpected northerly motion of 92L into the MDR. Satellite images this morning showed the system continues to hang on with some improving organization and will likely have window of opportunity of 2 days to develop. My joints are hurting me so typing is like hell.

I would also watch the Bay of Campeche as it has more climo going for it than 92L.

000
ABNT20 KNHC 150513
TWOAT
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 AM EDT TUE JUN 15 2010

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

A LOW PRESSURE SYSTEM LOCATED ABOUT 1150 MILES EAST OF THE LESSER
ANTILLES HAS BECOME A LITTLE BETTER ORGANIZED DURING THE PAST
SEVERAL HOURS. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO REMAIN
MARGINALLY FAVORABLE FOR THIS SYSTEM TO BECOME A TROPICAL
DEPRESSION TODAY BEFORE UPPER-LEVEL WINDS BECOME LESS FAVORABLE ON
WEDNESDAY. THERE IS A MEDIUM CHANCE...50 PERCENT...OF THIS SYSTEM
BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES
WEST-NORTHWESTWARD TO NORTHWESTWARD AT ABOUT 15 MPH.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$
FORECASTER STEWART/BLAKE


I will try and post a full update this evening if I'm feeling better, just need some rest. Thanks for understanding.


Weather456

Tropical Invest 92L may become depression later today

By: Weather456, 10:22 AM GMT on June 14, 2010

Tropical Invest 92L is estimated be located near 9.0N-38W, moving between west-northwest and northwest near 10-15 mph. Estimated surface winds are near 30 mph with a pressure of 1010 millibars. Satellite imagery and microwave data continues to show considerable organization of the cloud clusters associated with 92L. The circulation appears to have continued to get better define with the convection banding around the western semicircle due to the superimposition of the upper anticyclone over the system. I am convinced that there is a high chance that the first tropical depression will be declared today based on the improving organization of 92L, being that the convection does not wane significantly like yesterday. A tropical cyclone formation alert (TCFA) has been issued so as along as organization continues, then a tropical depression isn’t very far.

The environment around 92L continues to favour gradual development over the next couple of days with low vertical shear in the path over the next 3 days. After three days, modest shear begins to increase over the disturbance with gradually weakening expected. Much of the intensity guidances reflect this and I am going with the LGEM in particular, as I found this handles vertical shear the best. Thus, I expect moderate Tropical Storm Alex in about 3 days with gradually weakening thereafter. The GFS , however implies the upper anticyclone over 92L will reduce shear along the path and so we must consider this as a possible outcome.

A large mid-latitude trough continues to move north of the tropical Atlantic and with the increase speed of 92L it is likely the system is feeling some weaknesses near 40W-50W thus the need for a more northerly motion atleast for the next 2 days. However, this jump northward over the past 24 hrs could be 1) a center relocation or 2) independence of the ITCZ. We have to consider what will happen when 92L gets into the MDR and be embedded within the deep easterlies. For now I will continue to go with the consensus of the models which has not change much from yesterday’s update – a west-northwest motion with deviations west and northwest due to changes in the pattern to the north. This should bring 92L closer to the Central Antilles in 5-6 days or by weekend. Interests in the islands should monitor the progress of 92L.

Regardless of development, heavy rains and gusty winds will spread from the Windwards to Hispaniola during the next weekend from Friday to Tuesday.

Behind 92L is a large African wave that is far south near 5N, has no model support and not in a climalogically favoured area. Despite this, I will continue to monitor this feature as we have seen anything is possible at this moment.


Figure 1. Infrared imagery of Invest 92L showing nice consolidation of cloud clusters around a defining circulation. The circulation in the first visible images have appeared to relocated further northwest and is better define, taking advantage of the earth's rotation.

Weather456

92L: History in the making

By: Weather456, 9:43 AM GMT on June 13, 2010

In utter shock, the tropical wave that I was tracking over Africa last week and expected to develop in the Caribbean has defy all known fact of Atlantic Tropical Climatology and is now nearing depression status in the Eastern Atlantic. Satellite images taken of the Atlantic shows a rather disturbing scene at the start of what is expected to be "one hell of a season."

Satellite imagery showed a bone chilling image of an organizing cyclone near 8N-33W moving west-northwest near 10-15 knots. Banding continues to increase around a possible but broad closed low-level circulation and I suspect that we may have Tropical Depression 1 before the day’s end and Alex by Monday. There is still some vertical shear affecting the system from the east but that is likely due to the upper anticyclone superimposition over the low. Further analysis of the circulation revealed it continues to get stronger as it nears 10N. Environmental conditions are favourable for continued development with very warm sea surface temperatures, upper anticyclonic outflow, deep moisture field and low vertical shear. Much of the intensity guidances keep shear very low as 92L heads west, reaching moderate to strong tropical storm intensity by 2-4 days. Intensity begins to drop afterwards due to increasing shear as the system nears the Caribbean in about 5-7 days. I suspect that gradual intensification may lead to a moderate tropical storm in about 2-3 based with the consensus of the intensity models and the environmental conditions such as the availability of ocean heat content. I suspect if the system stays below 15-20N, that intensification will occur until reaching the SE Caribbean where high levels of shear may weaken the system.


Figure 1. Water vapor imagery of the Central Atlantic taken this morning at 4:45am EDT.

Track guidance favours a west-northwest motion over the next couple of days with a bit more northerly component as it nears 50W. However, I think some of the models are a bit north for my fancy. For one, there is a large mid-latitude trough passing through near 50W and as it does so should give way to ridging. This should favour a more west-northwest track than northwest track bringing the system near the islands in about 5 days. Folks in the Lesser Antilles should monitor the progress of this system.

Now there is an upper trough guarding the western Tropical Atlantic but because it is located in the upper levels, it should have little effect on steering since the low-mid level flow is westerly. This is the TUTT and has more say over intensity than track of tropical cyclones. In addition, this trough served to weaken the steering flow below it and systems tend to move west-northwest to northwest in the absent of steering due to the beta effect. My current thinking is a track towards the west-northwest, following the southern models.

The significance of 92L is great as there has been no recorded cyclones this far east in June. This would become the only cyclone to develop in the Eastern Atlantic in June and if it does become a hurricane, it would shattered the record for earliest CV Hurricane by 2008's Bertha.

Early seasonal development like this does not bode well for the season ahead.


Figure 2. 92L's position relative to June climatological tracks between 1851 and 2006.

Weather456

CATL wave shows some signs of organization; significant development not expected

By: Weather456, 11:52 AM GMT on June 12, 2010

Satellite imagery revealed a broad area of low pressure associated a tropical wave near 33W has shown some signs of organization with clear cyclonic turning and consolidation of clusters of deep convection in and around the low pressure area. An ASCAT pass from last night revealed a much-amplified surface circulation but with winds less than 25 knots, which is supported by buoy observations. The circulation associated with this wave is also evident at 850 mb. Currently much of the circulation remains in a favourable zone south of 10N with anticyclonic outflow aloft creating 5-10 knots of shear, deep moist field of upward motion and sea surface temperatures of 28-30C. It however remains broad because of its proximity to the ITCZ and the equator – less Coriolis force. Water vapour imagery indicated this might change as the mid-upper flow near 40W should allow the wave to amplified northwards to about 15N which would be detrimental to the development of the wave as a band of 30 knot shear lies just north of 10N. Most of the reliable models – GFS, ECMWF and UKMET deepens the feature some but keeps it an open wave once it gets into the hostile conditions of the tropical Atlantic with only the CMC showing a closed feature going to the islands. The only how I see this feature defying climatology and following the CMC’s solution is if the ridge over this disturbance moves west-northwest with it. All of the models are in such agreement that this wave will reach the Leeward Islands and Puerto Rico in 1 week and the Bahamas later; yes, the wave may reach the Bahamas, where it will have to be watched. Currently I give this a low chance of becoming a depression over the next few days but a slightly higher chance of being tagged an invest. Regardless of development, this feature will bring weather to the islands next weekend.

This is the strong wave that emerged earlier this week. It is the second of three strong tropical waves that I tracked and studied over the past 2 weeks. The first wave blew up as interacted with the tropical upper tropospheric trough (TUTT) near the Eastern Caribbean earlier on Wednesday and the third wave just exited this morning with little or no shower activity.



Figure 1. MSG-2 visible imagery of the Tropical Atlantic taken this morning at 06:45am EDT.

Elsewhere, a broad area of low pressure apparently did moved across Central America as expected and surface speed convergence is helping to generate strong thunderstorm activity over the Western Caribbean. I will continue to monitor this area but models have not been doing well predicting development, which is why I will continue to give a low chance of anything developing here.

My next blog will be on Sunday and will discuss every other aspect of tropical waves. I promise it will be better than the first.

Weather456

Tropical Update

By: Weather456, 10:40 AM GMT on June 10, 2010

Broad scale upper troughing over the Caribbean continues to enhance numerous scattered showers and thunderstorms across the Southern Caribbean from Central America to the Windward Islands. Deep convection increased in intensity and spatial scale along a tropical wave nearing the southeastern Caribbean yesterday due to upper divergence ahead of the upper trough. Activity has since decreased but the wave axis remains active. I do not expected any development for the meantime due to strong upper winds, but the wave will have to be watched when it enters of the SW Caribbean where an area disturbed weather associated a broad area of low pressure continues to be energized by passing tropical waves. Models continue to hint that this feature will move off towards the northwest and may have a chance to develop in either the Southwest Caribbean (NOGAPS) or the Gulf of Honduras/Bay of Campeche region (GFS) later next week. I will continue to give this a low chance of development.

Regardless of development, close to 5-10 inches may fall over parts of South and Central America over the next 4-5 days.


Figure 1. Infrared imagery of the Western North Atlantic with mean sea level pressure overlaid.

A monstrous wave is over West Africa near 8W, south of 20N moving off towards the west near 10 knots. The location of the wave coincides with a 700 mb vort max on GFS analysis with satellite imagery showing a very large mesoscale convective system (MCS) accompanying this wave. The wave is expected to emerge over the next 2-3 days as a highly amplified feature. In response, models have been indicating that some deepening may occur in the eastern Atlantic due to relatively favourable conditions – very warm sea surface temperatures, reduced African dust and low vertical shear. There is still no guarantee that development will occur, as this is not a climatologically favoured area and not all models are being consistent. In addition, a band of westerlies is situated in the central and western tropical Atlantic and this may reduce any chances of development, or further development. Once the feature gets to the Caribbean by mid-late next week, then the chances of development may increase. I give this a low chance of becoming a tropical depression in the eastern Atlantic. This of course can change when we see the wave emerging. Regardless of development, any feature will be steered due west.


Figure 2. MSG-2 infrared imagery of Western Africa and the Eastern Atlantic taken this morning at 5:30am EDT.

Weather456

Little change to the tropics

By: Weather456, 10:26 AM GMT on June 09, 2010

I continue to monitor the southern Caribbean this morning where a broad area of low pressure enhanced by a series of tropical waves continues to produce scattered showers and thunderstorms over the Southwest Caribbean, Central America, Colombia and Venezuela. Satellite imagery showed some level of turning in this region indicating a broad area of low pressure. Most of the reliable models continue to hint that an area of low pressure will develop across this area over the next 2-4 days, just west of Colombia and move off towards the west to west-northwest. Despite very favourable environmental conditions, much of the models show not enough deepening occurring before the disturbance heads into Central America. I suspect an area of low pressure will develop in this broad zone of disturbed weather but proximity to land will be its biggest problem. I will continue to give this a low chance of development. Regardless of development, showers and thunderstorms will continue over the region throughout the week.

The feature will be watched as both the ECMWF and GFS takes energy into the Gulf of Honduras later next week.

Elsewhere, an upper trough extending into the central Caribbean continues to enhance numerous scattered showers and thunderstorms over a wave approaching 60W. Expect numerous scattered showers and thunderstorms over Barbados, the Windward Islands and Trinidad.

A tropical wave approaching 5W over Africa is expected to emerge later this weekend and reach the Caribbean 7 days later around the 19-20th June. This wave will be very vigorous when it emerges but due to the differences in ocean and land lapse rates, convection will not be sustained very long along the axis, typical with June waves. In addition, a band of westerlies will dominate the MDR for most of next week, lessening the chances of development. It seems climatology should be favoured here over what the GFS was predicting yesterday. This is not the end of the wave as much of the reliable models show this wave keeping its low-level flow as it heads west. Such as vigorous system, entering the Caribbean could easily spark development. Thus, the period for a named storm looks to be late this month into early July.



Weather456

Watching the SW Caribbean

By: Weather456, 10:55 AM GMT on June 08, 2010

I continue to monitor the Southern Caribbean this morning where a broad area of low pressure continues to produce widely scattered showers and thunderstorms over the Southwest Caribbean, Eastern Pacific and the adjacent landmasses of Central America. Satellite imagery revealed two pieces of energy near or over Panama on the Caribbean side and just west of Costa Rica on the Pacific side. Further analysis using the TPC’s 06Z surface maps, 06Z GFS analysis and 09Z 925 and 850 mb vorticity reveals these two areas of energy centered roughly on 1010 mb lows with dominance over the Pacific. Most models show that vorticity on the Caribbean side will remain weak as it heads west over the next few days and development will take place in the Eastern Pacific. This is being supported by the low-level steering flow, which favours the advection of vorticity into the Eastern Pacific. Nevertheless, before development takes place in the Pacific, models show some weak disturbance developing in the southwest Caribbean over the next 3-6 days due to interaction from a series of tropical waves. The GFS, ECMWF, and NOGAPS all have a low-pressure area north of Panama in 144 hrs. This consistency leads me to believe that one will develop but likely remain a weak entity despite favourable conditions. I will continue to give this a low chance of becoming a tropical depression over the next week since none of the models keeps this a closed low. Regardless of development, close 1-2 inches of rain will fall over Colombia, Panama, Costa Rica and Nicaragua over the next 2 days.

Broad upper troughing over the Atlantic is enhancing scattered and isolated showers and thunderstorms over Hispaniola, Puerto Rico and the Lesser Antilles. We here in Saint Kitts are experiencing periods of moderate to heavy showers with San Juan radar showing isolated shower activity. Expect showers to continue over the next day and half as the upper trough slowly clears the area. There is a surface trough east of the Bahamas that will be monitored as it interacts with the tail end of an advancing frontal system.

Meanwhile, divergence in the right exit quadrant (SE) of the upper trough axis is enhancing very heavy convection along a tropical wave nearing South America/SE Caribbean. I suspect heavy shower and thunderstorm activity will pull across Guyana, Venezuela and Trinidad later this morning and for much of the day. Expect 1-2 inches.

The MJO is currently in a downward phase over the Western Atlantic but the region remains rather convectively active.

A few tropical waves will be monitored as they head west with a very strong wave expected to emerge this weekend.


Figure 1. Water vapor depiction of the Western Atlantic at 5:15am EDT this morning.

Weather456

Tropical Update

By: Weather456, 10:08 AM GMT on June 07, 2010

The tropical Atlantic remains quiet this morning but there are few areas that need to be watch as they threaten this tranquillity. First, a very broad upper level circulation, adjacent to an upper ridge protruding into the Gulf of Mexico is enhancing showers and thunderstorms over SE Mexico, up along the Northern Gulf Coast and into the Florida Peninsula. I do not expect any development along this feature since there is no evidence of a surface reflection. Expect the upper low to migrate westward into Mexico/Texas as upper ridging takes over.

Further south, showers and thunderstorms have developed over the Southwest Caribbean and the adjacent landmasses of Colombia, Panama and Costa Rico in associated with the monsoon trough and a broad area of low pressure. The low pressure is relatively low compared to climatology and the GFS and NOGAPS continue to hint that a weak depression may develop here and head west-northwest to northwest over the next day or two. The ECMWF keeps the feature weak and open but brings it all the way across Central America and into the Bay of Campeche later this week. This area will have to be watched closely as a series of tropical waves will continue to energize the area and broad scale upper ridging is going to provide low vertical shear over the Western Caribbean and Gulf south of 25N over the next 3-5 days. Its proximity to land however, gives me reason to believe this has a small window of opportunity to develop. I will give it a low chance for now. Regardless of development, showers and thunderstorms will continue to spread across Central during the week.

A tropical wave is nearing 40W south of 10N at about 10-15 knots. Satellite imagery continue to show turning in the low level moisture field with disorganize showers and thunderstorms over the Central Atlantic between 40W and 25W, south of 10N mainly along the ITCZ. I do not expect development over the next 24 hrs due to the presence of shear and dry air over the tropical Atlantic; and proximity to the ITCZ. However, this wave will need to be watched very closely as it heads west over the next week and half since for the last 2 runs on the GFS have hinted that this feature develops near the Yucatan Peninsula and heads into the Gulf. It actually interacts with energy from the low in the SW Caribbean that headed northwards and lingered over the NW Caribbean Sea. This has move from a time frame of 348 hrs to 240 hrs so the model is hinting at something. However, long-range models can be unreliable and I would look for two things – consistency of the GFS and consensus of other models before paying much attention.

The wave is expected to reach the Caribbean by Thursday-Friday, bringing showers and thunderstorms to the Leeward Islands then Puerto Rico and Hispaniola over the weekend.

A mid-upper level circulation north of the islands continues to enhance showers and thunderstorms along a surface trough of low pressure near 25N-60W. The area has improved somewhat in organization and will be monitored for subtropical development. However, an advancing frontal system will absorbed this feature in 2-3 days, so there is a small window of opportunity here.


Figure 1. Water vapor depiction of the Tropical Atlantic this morning at 4:45am EDT.

If anyone missed it on the weekend, I posted a blog on the genesis of tropical waves.

Weather456

Tracking African Easterly Waves: Genesis

By: Weather456, 12:57 PM GMT on June 06, 2010

African Easterly Waves (AEWs) are warm season type disturbances, which are unique to the tropical North Atlantic Ocean. These disturbances form over Northern Africa and propagate westward at a mean speed of 15 knots. African easterly waves have wavelengths typical near 2500 km or 3-5 days. The Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) was conducted in 1974. Our understanding of the structure and dynamics of AEWs was enhanced by analyses using GATE observations. However, GATE experiments where located at the coast of Africa, when waves had already matured and exited the coast. Today, I am going to give a description of the genesis of AEWs using a collection of post-Gate researches, focusing on the one-month period between June 15-July 15 2008 and the two-week period between August 1-15 2007, as synoptic overviews, both periods of active wave development, which led to Hurricanes Bertha and Dean respectively.

Synoptic Overview of Western Africa

Before we continue, we have to understand the synoptic patterns over Northern Africa. The Northern section of Africa is described as having a desert climate of low precipitation and soil moisture and intense heating during the summer. As a result of intense heating, air rises in convection and forms a thermal low. This is the first feature we come across – The Saharan Thermal Low. Now this is not like any low-pressure system as it is very shallow, only extending to about 800 mb. Above the thermal low, air expands like an inflated balloon and cause pressure heights to shift upwards creating a high aloft. This is the second feature we will come across – the Saharan High. A basic representation of the vertical profile of the Saharan Desert is given in figure 1.


Figure 1. Vertical profile of a thermal low and thermal high as over the Saharan Desert. This profile also similar to the thermal low over Southern Asia, Australia and US 4-corners/Mexico during the summer months. Tropical cyclones also have this profile.

Further south near 0-10N, we have the Gulf of Guinea, which like any body of water during summer, will be relatively cooler and moister than the adjacent landmass. In addition to being relatively cooler, the pressures are also relatively higher near the near surface. This results in a temperature, moisture and pressure gradient from the Gulf of Guinea northward to the Saharan Desert. This pressure gradient gives rise to wind flow, which you would expect to be from south to north, but because of the earth’s rotation, this flow is deflected towards the right of motion and become southwesterly. Here we see our third feature – the monsoon southwesterlies.

The southwest winds continue to blow ashore but they do not always reach the thermal low over the desert. Friction slows down some of the wind before ever reaching the desert and rises near sub-Saharan Africa in a region we call the ITCZ, our fourth feature. Another reason why the flow does not always converge over the desert is the northeast winds called Harmattan.


Figure 2. NCEP/NCAR reanalysis of Northern Africa during August 1-15 2007 showing the temperature (left) and moisture (right) gradients between the cool moist Gulf of Guinea and hot dry Saharan desert. This gradient sets the stage for baroclinic instability for the development of African waves.

Air flowing and rising either in the ITCZ or Saharan thermal low, rises to about 600 mb then is expelled southwards due to the relatively higher heights over the desert as compared to the Gulf of Guinea. Thus, the flow is reversed in the upper levels. As the air flows southward, the earth’s rotation again, deflects the winds to right of motion to become easterly – the African Easterly Jet (AEJ).


Figure 3. NCEP/NCAR reanalysis of Northern Africa during the period June15-July 15 2008 showing the surface pressure (left) and 500 mb height (right) gradients which drives the southwesterlies and African easterly jet. The turning of the low level winds (left) is barotropic instability while the difference between southwesterlies and jet easterlies is baroclinic instability.

This is the general circulation of the African Monsoon System, which only extends to the mid-levels. There is a larger thermal high above the African monsoon system called the Asian Monsoon High and south of it is the Tropical Easterly Jet. Both features play a role in the vertical development of AEWs.


Figure 4. Vertical cross sections of Northern Africa at 5E during the ECMWF reanalysis project between 1991–2000 showing the general circulation of the African and Asian monsoon (left). The right image now shows the difference between waves that form near 10N as appose to waves that form near 20N. H is the Asian Monsoon High and H2 is the Saharan High.

Vegetation Influence

There is a fourth factor which enhances temperature and moisture gradients over the African continent, that is, vegetation. Vegetation has the power to create a moister atmosphere above heavily forested areas through transpiration. As plants transpire water through their leaves, it evaporates and adds moisture to the air above it, creating clouds and a moister environment above the canopy. This is called evapo-transpiration. Vegetation also shadows some solar radiation and cause the area to be cooler than it otherwise would be. Over Africa, there is basically no vegetation over the Saharan Desert and thus would expect it to remain dry and hot. Further south, we have the Guinean Forest of West Africa which borders the Guinea Gulf coast and receives much of its rainfall from the ITCZ. Here, temperatures are cooler and the air is moister.


Figure 5. NASA Earth Observations of vegetation index over Africa during May 2010. Dark green areas show where there was a lot green leaf growth; light greens show where there was some green leaf growth; and tan areas show little or no growth. Black means "no data."

Genesis of African Waves


Figure 6. Points of genesis (dots) of two types of easterly waves with the left panel - waves that form north of the AEJ and right panel - waves that form south of the AEJ. The top panel is the 600 mb streamflow with the AEJ and the bottom panel is the 925 mb streamflow with the Saharan Thermal Trough.

It was thought that there was one single means by which African waves were born – that is – instability within the AEJ. For tropical waves to develop you, need instability within a fast flowing stream of air at the mid-levels and a rapid reversal of the potential vorticity (PV) gradient as suggested by Charney and Stern in 1969. I will not bore you with the technicalities. The AEJ, which flows south of the Saharan High, satisfies these conditions. Instability south of the jet creates wave-like features, which grow to become African waves. However, AEWs were also generated north of AEJ over the Saharan Desert. These synoptic disturbances do not satisfy the Charney-Stern instability theory. It was suggested by Chang (1993) and Thorncroft (1995) that this mechanism is the static instability within the Saharan Thermal Low underneath the mid-tropospheric Saharan High, generated waves north of the AEJ. The exact explanation is very technical so I will not venture further. If you would like me to explain it further, I will be more than welcome.

Effectively, the greater the moisture, temperature and pressure gradient, the stronger the African Easterly Jet and the subsequent waves. This can be achieved by anomalous cooling of the Gulf of Guinea.

Now, the greatest amplitude of waves south of the AEJ is found at 600 mb and the greatest amplitude of waves north of the AEJ is found at 925 mb. Both wave-types eventually propagate westward and converge near the African coast.

Both wave types only exists in the mid-lower troposphere, but why is there such a contrast between AEWs north of the AEJ (925 mb) and south of the AEJ (600 mb)? In addition, why is that both are found lower than 600 mb? To answer these questions we must first go back to figure 4. If you notice, the Saharan High is found almost at 800 mb and upwards. Thus, vertical development of waves near 20N is hindered at a much lower altitude. In contrast, waves that develop south of the AEJ can grow higher because they are capped by the Asian Monsoon High which is much higher than the Saharan High.

Relationship between African Easterly Waves (AEWs) and the African Easterly Jet (AEJ)

It was suggest by Burpee (1972) that the formation and growth of African Easterly waves were energetically supported by the AEJ. Even if the wave formed from the second type north of the jet, they still emerged near African coast and become accentuated by the jet. The maximum east–west extent of the AEJ (from North Africa to the North Atlantic) is about 120 degrees in longitude, while the AEW wavelength is roughly 2000–4000 km. Length scales of the AEJ and AEW are equivalent wavenumbers 3 and 9–18, respectively, at 15 degrees north. African waves are normally found south of the jet axis where PV changes sign (positive to negative). Thus, the position of the jet dictates the points of genesis and propagation of waves. In April, May and June, the pre-summer patterns cause the jet to be near 10N. During the peak summer months of July, August and September, the jet shifts northward as the surface gradients intensify. This is why AEWs emerges at low latitudes early in the hurricane season and higher latitudes as the season progresses.

Barotropic-Baroclinic Instability

It was also suggested that African waves develop in the barotropic -baroclinic instability of the AEJ. However, what does that mean? Well barotropic instability is associated with horizontal shear produce by deep convection near the AEJ. Baroclinic instability is associated with wind shear in the vertical (monsoon southwesterlies over easterly jet) and the moisture, temperature and pressure gradients. These are all found near the AEJ and satisfies the Charney-Stern instability theory (figures 2 and 3).

Analysis of African Easterly Wave May 27-June 5

A strong tropical developed over the Ethiopian Highlands in Eastern Africa around May 27, the wave developed in the instability of the African Easterly Jet per GFS analysis of the Western African continent. The wave is now tracking near 10W with exceptional well-define amplitude in the African Easterly jet and a steep PV gradient along the axis. Thus, this type of wave fits the Charney-Stern instability theory with its greatest amplitude south of the AEJ.

During the period May 27-June 3, synoptic patterns over West Africa was very similar to those that produce the strong waves of June-July 2008 and August 2007. Steep pressure gradients gave rise to monsoon southwesterlies, which converged, into the thermal trough and ITCZ. Rising upwards and expelled outwards the Saharan High to become the African Easterly Jet. We can see the maximum amplitude of the AEJ is around 10N, with the maximum vorticity just south thereof.


Figure 7. NCEP/NCAR daily composite of Northern Africa for the period May 27-June 3 2010.


Figure 8. GFS analysis of West Africa around 2:00pm June 5 showing a strong tropical wave about to emerge off the coast of Africa. The top left image shows the 650 mb winds with the African Jet axis nicely illustrated with the wave-like disturbance within the jet axis. The top right image shows the potential vorticity showing the greatest PV is often found south of the jet axis as we discussed. The bottom image is another illustration of the moisture gradient over Northern Africa and the position of the jet relative to this gradient. Without these gradients, there would be no jets and consequently no African waves.

Conclusion

It was found that the synoptic scale environment over Northern Africa gives rise to a number of general and secondary circulations, which aid in the genesis of disturbances that travel eastward hence their name African Easterly Waves. Such a combination of conditions is only found in the tropical and subtropical areas of North Africa and these AEWs are unique to this area of the word. While the exact formation of African easterly waves is well studied it is rather a technical process but in simplified terms, the disturbances can form south of the African Easterly Jet (AEJ) or north thereof by two different processes. As a result, of the differences in point of formation, the waves have differing vertical dimensions but they all end up near the West African coast where their intensity is governed by the strength of the AEJ.

The next topic will discuss the growth, structure and propagation of AEWs across the North African landscape.

This is an expansion of Tracking Tropical Waves, a blog entry from June 2008.

References:

Generation of African Easterly Wave Disturbances: Relationship to the African Easterly Jet; JEN-SHAN HSIEH AND KERRY H. COOK; Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York (2004).

Energetics of Easterly Waves; M.A. Estogue and M.S. Lin; Rosentiel School of Marine and Atmospheric Science, University of Miami, Coral Gables, Florida (1977).

Characteristics of African Easterly Waves Depicted by ECMWF Reanalyses for 1991–2000 TSING-CHANG CHEN Atmospheric Science Program, Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa (2006).

Tropical Update

Now we have looked at how tropical waves form, let us look at how to identify one over the continent using the available tools such as satellite imagery, surface and upper air observations and numerical models.

Satellite imagery and its derivatives

The satellite structure of AEWs over land differs greatly from those tropical waves over the ocean. First, the heaviest convection is west of the wave axis because of easterly shear in the AEJ. Second, waves over land have convective maxes opposite to the diurnal cycles over water, that is, diurnal maximum is late afternoon and diurnal minimum is early morning, except for waves nearing the coast.


Figure 9. Infrared image of an African wave early this morning about to emerge as schedule. Also look at the genesis of the other wave over Nigeria. Also notice these waves form south of the jet axis.

Hovmöller diagrams are useful tools for tracing tropical waves back to their origin by monitoring each convective cycle as they head west.

Using the 850 mb vorticity maps from the CIMSS, there is usually a vort-max along the wave axis but sometimes you will encounter twin vorticity centers north and south of the axis. The north vort-max is the Saharan Thermal Low and the south vort-max is the ITCZ.

The lower level wind product will identify where the greatest cyclonic turning is occurring. We should focus on the 600-950 mb winds and this assists in identifying where the wave was born – either north or south of the AEJ as discussed above.

Another useful tool is imagery available at NOAA's Satellite Service Division (SSD), particularly the rainbow enhanced infrared. Here you can see differences in surface temperature by the amount of heat radiating from the land surface. Intense heat would radiate from the Saharan Desert where it is cloud-free and lack vegetation, while less heat would radiate from the areas further south where cloudiness and vegetation cover is high. Here, the temperature gradient is well illustrated over Northern Africa.

Surface and upper air observations

Usually the passage of a tropical wave will cause a wind shift in the southwesterlies over Africa as oppose to the northeast trades over the Atlantic. The passage of a tropical wave will cause wind to shift from the southwest to the southeast. However, AEWs over land has little surface reflection until they near the coast.

Aloft, the disturbances have greater effect where anomalous northerly winds are found ahead of the wave axis and anomalous southerly winds are found behind the wave axis.

Numerical Models

Numerical models such as the GFS can model the atmosphere. There are three levels that I monitor; the 500 mb vorticity chart – since AEWs are mid-level disturbances they are often accompanied by mid-level circulations or curvature; 700 mb heights, winds, relative humidity and omega chart – AEWs have their greatest amplitude at this level thus the kinks and curvature in the heights and winds will be identifiers of wave axis while omega (lifting index) will show the greatest lifting and moisture ahead of the wave axis; and 850 mb heights, vorticity and winds chart is similar to the 500 mb analysis done on waves and the CIMSS 850 mb products.

The GFS is also capable at modeling the AEJ and conditions over Africa, which is illustrated in figure 8.

Another Tropical Wave over the Atlantic

Tropical waves over the ocean exhibit inverted V cloud patterns with more than often the strongest convection occurring east of the wave axis due to westerly shear after leaving the presence of the AEJ. Maximum cyclonic turning can picked up by looping the images or using tropical precipitable water loops by the CIMSS. Much of the turning is northeast (ahead of the axis) to southeast (behind the wave axis) which will be noticed on satellite derived winds and vorticity charts, scatterometer and buoys.

Numerical models can also model waves as with over Africa and the same rules apply.

Links to follow African Waves

CIMSS Satellite Analysis of Africa and the North Atlantic
Satellite Imagery of Northern Africa and East Atlantic
NCEP GFS Modeling of Africa and Atlantic
Albany Real-Time Weather Plots for the Northern Hemisphere Tropics
Upper Air Time Charts
Hovmöller Diagrams
Tropical Precipitable Water Loop
ASCAT and Windsat Scatterometer
National Data Buoy Center
Hovmöller Diagram

Elsewhere...

A tropical wave nearing 35W south of 10N has increased in shower activity this morning as it heads off towards west. No development expected due to high upper winds north of the wave and its proximity to the ITCZ. I will monitor this wave as it has the potential to develop in the Western Caribbean later this month.

An area of disturbed weather northeast of the islands will be monitored for subtropical development as it heads northward over the next few days.

Models are hinting at development in the SW Caribbean this week. I will have a full update on the tropics on Monday.

Weather456

A Book Review: Divine Wind by Kerry Emanuel

By: Weather456, 1:38 PM GMT on June 05, 2010


No tropical meteorology library is completed without having a copy of Kerry Emanuel’s Divine Wind. The book dubbed “History and Science of Hurricanes” captures a perfect combination of hard science and hurricane folklore. Kerry Emanuel is professor of Earth, Atmospheric, and Planetary Science at the Massachusetts Institute of Technology (MIT). His writings have appeared in America Scientist and New Scientist, among other publications.

Kerry Emanuel has gone as deep into this subject as it’s possible to go, emphasizing the scientific understanding of hurricanes and considers them from a historical and artistic perspective. I have read the book and found that it covers three broad headings of genesis, maturity and demise of tropical storms but requires readers to have prior understanding of tropical meteorology to better cope with the charts and graphs. The book applies this scientific data to past storms such as the loop current and Hurricane Camille; and tropical cyclone track forecast and the Galveston Hurricane of 1900.

After reading each chapter, you are left with a better understanding of how hurricanes work but it is even more fascinating after reading each of the 15 past stories. My favorite story is Chapter 5 – Columbus’ Hurricane, which reveals a tale of the meteorological aspect of the New World era. In this chapter, Columbus is face with mockery after predicting the arrival of a hurricane in Hispaniola in 1502 as indicated by warning signs such as cloud and wind patterns. The hurricane came with such force that there was no re-colonisation for more than a year. This is just one of the many tales, which are discussed in chronological order from 1502 to 1992.

The book was also reviewed by some infamous names in the meteorological community such as Max Mayfield, former director of the National Hurricane Center (2000-2007); Dr. Steve Lyons, former Hurricane Expert at The Weather Channel and Neil L. Frank, former director of the National Hurricane Center (1973-1987).

“Until I read Divine Wind I had never found a book unique enough to contain the science and the history of hurricanes accented with the prose, songs and art about them. Both meteorologist and non-meteorologist will be captivated with it. Divine wind is a must read for everyone interested in hurricanes.” – Dr. Steve Lyons, former Hurricane Expert at the Weather Channel.

The book can be bought at Amazon for 40 dollars but I actually got it for 3 dollars from bookconsignmentsllc, one of the sellers listed on Amazon. It was a used book but in band-new condition.

Why didn't Phet reach it's maximum potential?

During peak intensity of Phet earlier this week, the storm was forecasted to become a category 5 hurricane over very warm sea surface temperatures and under low vertical shear. However, the system never surpassed category 4 status due to a combination of track and environmental conditions. Initially, Phet was forecasted to recurve abruptly well east of the Arabian Peninsula, but due to timing differences of the upper trough over the Middle East, Phet was able to make it very close to the Arabian Peninsula, actually, it made its first landfall there. As a result, proximity to the Arabian Peninsula caused the storm to entrain huge amounts of dry air into the circulation close to the middle layers of the atmosphere, since dust here is similar to the layer of the Saharan Layer. Microwave imagery loops from CIMSS showed how dry became entrained into the system and resulted in the lost of thunderstorms south of the system. Further analysis of a dry slot using GFS soundings revealed mid-level dry air intrusion at 700 mb, which is the level dust is found. Another piece of evidence, which suggest the storm entrained dry air, was the presence of outflow boundaries to the north. These are signs that moisture is evaporating inside the storm, which cools the air and cause it to sink. The sinking air hits the surface and spreads out as an outflow boundary.


Figure 1. Visible satellite image of tropical cyclone Phet on June 2 2010 around 2:00am EDT showing the entrainment of dry air, outflow boundaries and the location of the GFS sounding in the dry slot.


Figure 2. NASA's MODIS TERRA captured this image of Tropical Cyclone Phet, a powerful category 3 equivalent storm in the Arabian Sea. It is so rare to see tropical cyclones right next to a desert which was the source of the dry air entrained into Phet.


Figure 3. Morphed Integrated Microwave Imagery at CIMSS (MIMIC-TC) on 3 June at 12:30am EDT (left) and 1:15pm later that day (right), showing the lost of convection south of the system due to dry air entrainment.

TRMM reveals Agatha’s Deluge

The 2010 East Pacific hurricane season began pretty much on schedule. While the season officially begins on May 15 and runs through November 30, in an average year, the first named storm of the season forms around June 10th; this year the first storm of the season, Tropical Storm Agatha, formed on May 29th off of the coast of Guatemala from a broad area of low pressure within the Intertropical Convergence Zone (or ITCZ), a band of low pressure that circumnavigates the globe near the Equator where the trade winds converge. Although Agatha's maximum sustained winds were never estimated to be greater than 75 kph (45 mph) by the National Hurricane Center, it still turned out to be a very deadly storm as a result of flash floods and landslides brought about by Agatha's heavy rains.


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 estimates for the 1-week period 25 May to 1 June 2010 for Central America show that the heaviest rains fell just off shore and right along the Pacific coast side of Guatemala, El Salvador, Honduras and northwestern Nicaragua. Over 500 mm (~20 inches, shown in red) fell in two areas off the coasts of Guatemala and El Salvador. Over land, between 250 mm (~10 inches, shown in bright green) and 350 mm (~14 inches, shown in darker orange) of rain fell over the coastal areas of Guatemala and between 150 mm (~6 inches, shown in bright blue) and 250 mm fell over the coastal sections of El Salvador, Honduras and Nicaragua.

The counter-clockwise flow around the storm produce onshore flow along the coastal mountains which created orographic lift and enhanced rainfall. This heavy rainfall mixed with ash from the eruption of Pacaya the same weekend to produce a thick mud with the consistency of cement and clog drainage systems and led to more flooding especially in Guatemala City.

Death toll from Agatha is 179, making it the 6th deadliest Eastern Pacific storm.

Tropical Update

The tropics are quiet and none of the models is forecasting development over the next 7 days. I will have an update Sunday on the strong wave that is expected to emerge later next week.

Weather456

Watching East of the Bahamas; Tropical Waves A'Comin

By: Weather456, 10:33 AM GMT on June 03, 2010

The Atlantic tropics remain rather quiet this morning except for a few areas I’m monitoring. First, a broad area of disturbed weather has developed east of the Bahamas in associated with surface trough of low pressure from the tail end of a disintegrating frontal boundary. Shortwave infrared imagery along with falling pressures by several buoys and vorticity maps indicate there may be a low-pressure feature centered within this area with disorganized shower activity to the east due to strong upper winds. Over the next day or so, a low-pressure will likely form in the vicinity and head off towards the northeast with some chance of development as upper winds are expected to become a bit more favourable. Most of the models keep the low weak but a shallow warm-core system (subtropical) and it is eventually absorbed into an advancing frontal trough through day 5. It will likely affect Bermuda as it does so. I give this a low chance of becoming a subtropical depression over the next day or so. This is the area that I was watching north of Puerto Rico over the past 2 days.


Figure 1. Water vapor image of the Western Atlantic depicting the area of disturbed weather east of the Bahamian Islands.

There are three tropical waves in the Eastern Atlantic and over Africa that will need to be monitored over the next 2 weeks as they head west. While development of these waves may not take place in the Eastern Atlantic, they may develop in the long-term when they reach the Western Caribbean later this month. The first wave is located near 21W and south of 13N moving off towards the west near 10-15 knots. This wave is marked by pronounce cyclonic turning along the axis with convection in and around the wave axis. Last night’s ASCAT pass captured what appears to be the eastern semicircle of a pronounced low-level circulation. This ASCAT pass may not be meteorologically incorrect since models expect this wave to develop a low-pressure area along the axis as it heads west-northwest. Expect to reach the Caribbean in 5-7 days. While conditions are favourable for development, this is not a climatologically favoured area and its relatively embedded in the ITCZ; but the area will be monitored as it heads west.

The second African wave will be interesting when it emerges. The previous wave just discussed is the strongest of the season, but will take second place when a wave over Nigeria near 9E, emerges next Monday with August-like amplification. Both the GFS and ECMWF show mid-level turning with this wave and I cannot discount development at the time. Expected to reach the Caribbean by 11-14 June.

The last wave is far over Eastern Africa near 30E, expected to emerge late next week and reach the Caribbean beyond June 15. These are strong easterly waves and thus increase the chances of named storm further west later this month.


Figure 2. Infrared image of the Eastern Atlantic and West Africa showing the three tropical waves discussed in order from left to right.

Weather456

Tropical Update; Phet to become a category 5 storm

By: Weather456, 10:04 AM GMT on June 02, 2010

The Atlantic tropics remains relatively quiet this morning with few areas of interest to speak of. First, Invest 91L is located somewhere in the Northwest Caribbean. Satellite imagery along with surface observations revealed no signs of a low-level circulation but vorticity maps showed a region of low-level vorticity near the Yucatan/Belize coast, so this is where I assume the main system is. The combinations of dry air intrusion and vertical shear mainly in the mid-levels serve to weaken the thunderstorms associated with 91L. These conditions are expected to continue today and maybe even worse than yesterday and Monday. I highly doubt that this will become any classified system over the next day or so. It is difficult to tell where a system will go if you cannot even locate it, but the mean steering flow supports northerly motion. Regardless, it is becoming less likely that any moisture from this system will affect the Caymans, Cuba and Southern Florida so as someone asked yesterday from the islands, yes, put on those sprinklers.

The other area I’m watching is north of Puerto Rico where as expected a surface trough broke off from the original system draped across the Atlantic. Water vapour loops showed the system is under heavy upper westerly flow and CIMSS charts showed weak vorticity along this feature. However, conditions may become a bit more conducive for development over the next 5 days.

The third area I am watching is the southwest Caribbean where a broad area of low pressure continues to build instability with each passing tropical wave.


Figure 1. Water vapor imagery of the Western North Atlantic depicting the general pattern and their relationship with features of interest. Notice that dry air invading the Northwestern Caribbean as upper winds continue make their presence known. This should limit any development by 91L today.

The fourth and last feature is a tropical wave that just exited the coast of Africa. This wave is one of the largest of the season and actually has favourable conditions ahead of it. However, the wave axis extends to about 14N with the main vorticity and turning is at 8N, relatively too far south for development. It is impressive no doubt.

None of these areas have much model support but will still be monitored.


Figure 2. Infrared imagery of the East Atlantic and Western Africa showing a strong tropical wave emerging off the coast. While the axis is extends to 14N, notice the bulk of turning and convection is below 10N.

Tropical Cyclone Phet intensified into a powerful cyclone with winds of 126 mph, equivalent to a category three hurricane. Conventional satellite imagery and microwave imagery showed a somewhat well define eye around a relatively compact central dense overcast. The cyclone is moving off towards the northwest, expected to turn more north-northwest under the influence of an advancing upper trough and may brush close to the Arabian Peninsula. Phet is expected to become a category 5 equivalent hurricane by Friday.


Figure 3. NASA's MODIS TERRA captured this image of Tropical Cyclone Phet, a powerful category 3 equivalent storm in the Arabian Sea. It is so rare to see tropical cyclones right next to a desert.

Weather456

Tropical Update

By: Weather456, 9:47 AM GMT on June 01, 2010

Today marks the beginning of the 2010 Atlantic Hurricane Season, which most forecasters think, will be exceptionally active. Currently, things are quiet across the tropical Atlantic, as you would expect for June 1. However, this is the first year since 2006, where we did not had any classifications before June 1 – Andrea and Barry 2007, Arthur 2008 and Tropical Depression 1 in 2009.

A weak shortwave trough is established over the Gulf of Mexico with dry confluent flow at its base stretching from Mexico across the Southern Gulf to the Southern Florida Peninsula. A diffluent zone further north gives way to scattered moisture across the Southeastern United States and Eastern Florida Peninsula. Adjacent to this trough is large scale ridging which dominates the entire Caribbean Basin. Between these two features, lies the remnants of Agatha, which lies within a moisture, and shear gradient and expected to slowly dissipate as it heads north into the Gulf of Mexico with less moisture expected than previously forecasted.

Broad scale ridging dominates the entire Caribbean Basin with a moisture plume around the crest extending from Cuba to just north of the Antilles to join with a frontal trough draped across the entire subtropical Atlantic. Heavy shower activity lies along the diffluent zone of this feature over Puerto Rico and the Leeward Islands. Expect wet weather later today and tomorrow. Now, I will be monitoring this area very closely over the next day or two as global models are indicating that the tail end of this frontal trough will seclude (break off) and head back towards the northwest. The reason this occurs because deep layer ridging covers the tropical Atlantic creating a blocking pattern. Vertical shear is only marginally favourable at best for any significant development.

The other area I am monitoring is a broad area of low pressure over the SW Caribbean that will be energized later this week by a large tropical wave near 70W. The NOGAPS and GFS thinks the area will develop by mid-week but the former on the Caribbean side and the latter in the EPAC. If development occurred over the Caribbean, the system would be driven northwestward under deep ridging flow and thus interaction with central America may hinder or limit development.

I will continue to monitor these areas throughout the week.


Figure 1. Water vapor depiction of several features across the Tropical Atlantic this first day of the Atlantic Hurricane Season.

Tropical Cyclone 03A

An area of disturbed weather quickly organized over the Arabian Sea to become the North Indian Ocean’s third tropical cyclone according to the Joint Typhoon Warning Center. Satellite imagery showed impressive bands of heavy convection and microwave imagery revealed an eye-like feature and spiral bands as the system continues to organize. The system is moving over extremely warm waters of 31C and thus strengthening is likely. It is expected to recurve under the influence of advancing upper trough later this week. This should bring it ashore near the Indian/Pakistan as a category 2 equivalent storm.


Figure 2. Visible satellite imagery of Tropical Cyclone 03A over the Arabian Sea in the North Indian Ocean.

Guatemalan Sinkhole

A massive sinkhole swallowed an entire street intersection, several buildings and houses and a security guard. The street infrastructure was weakened by tropical storm Agatha,that being an unconfirmed news report. Everybody is astonished and people are trying to find an explanation for this bottomless crater.

Agatha is the Eastern Pacific's sixth deadliest tropical cyclone on record with a 148 deaths across Central America with 120 in Guatemala, the hardest hit region.


Figure 3. Large sink-hole opened up at an intersection in Guatemala after the passage of Agatha last weekend.

I am looking forward to working with you guys this season and may God protect you and your loved ones during what is expected to be "one hell of a season".

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.

Weather456's Tropical Weather Blog

About Weather456

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