Weather456's Tropical Weather Blog

Tropical Invest 90L

By: Weather456, 11:14 PM GMT on July 29, 2010

In this morning’s blog I mentioned an area of low pressure that was likely to become the next significant invest of interest, which has done so and been designated invest 90L. Invest 90L is associated with a broad area of low pressure in the Eastern Atlantic, along the ITCZ. RGB satellite imagery revealed clear mid-low level rotation centered near 7.5N-31W. This mid-level circulation is clearly defined with somewhat organized convection within 5 degrees west and south of the center. The latest surface analysis maps revealed a 1010 mb low at the surface, which I do not believe, is closed yet. Cloud tops have warmed over the past few hours which is an indication of diurnal minimum and that the system is becoming independent of the ITCZ. Wind shear is light above the system and SSTs are above 29C, with little dry air intrusion noted. I suspect under these conditions, that a depression will slowly form over the next couple of days, probably not before July 31. SHIPS keep conditions quite favourable ahead of 90L and this is reflected in the intensity guidances, which shows steady strengthening once the system is designated a depression.

Currently 90L is moving motionless based on the movement of the mid-level center and this is because the weak steering environment of the ITCZ. Ever heard of the doldrums? This motion should pick up off towards the west-northwest once the system manages to leave the ITCZ. A motion off towards the west-northwest should last up until 5 days. Afterwhich, a weakness develops in the subtropical ridge due to the passage of a mid-latitude trough over the NW Atlantic, which induces a more northwesterly motion through days 7. Beyond then, there are no guarantees that the storm will miss the extreme northern Leeward Islands or even recurve out to sea, thus interest in the islands should monitor the progress of 90L. The storm is about 7-8 days from Puerto Rico and almost a week from the Bahamas or USA.


Figure 1. RGB imagery of invest 90L this evening. If you were to loop the image, the center of the mid-level circulation can be clearly identified.

I am also watching another wave currently entering the Caribbean Sea, interacting vigorously with the TUTT and a ridge in the Caribbean Sea.

Watching an area of low pressure in the Eastern Atlantic

By: Weather456, 10:35 AM GMT on July 29, 2010

This morning infrared imagery revealed vigorous convection along an a surface trough in the ITCZ near 30W. Satellite images revealed some level turning along the feature which is rather organized. However the feature is embedded along the ITCZ so there is no indications that the area is self-sustaining. Conditions in the MDR south of 20N should be favorable over the next 5 days and models are developing a tropical storm from this area, particularly the ECMWF, CMC and NOGAPS. This maybe the next invest of interest and will be monitored closely as it heads west-northwest over the next few days.

A wave has begun to approach the Lesser Antilles and has increase in shower activity as expected.

I will have a detailed blog later this evening after the 12Z model runs.


Figure 1. Infrared image of the Central Tropical Atlantic showing the trough of low pressure along the ITCZ (lower right) and a wave approaching the islands (left).

Weather456

Last week of July 2010

By: Weather456, 1:33 PM GMT on July 25, 2010

Gulf of Mexico

Deep layer ridging dominates the area centered over the Eastern United States with anticyclonic flow from the southwest Atlantic into the Gulf of Mexico. Large upper level circulation moving over Eastern Texas helping to enhance diffluent shower activity over the Western Gulf along the remnants of Tropical Depression Bonnie just south of the Mississippi Delta and a broad area of low pressure over the Western Bay of Campeche. Due to the organization and proximity to land of both features, no development is expected. Expect this upper level circulation to continue slowly off towards the west over the next few days. Additional showers and thunderstorms can be expected over the Gulf region as a second upper low currently near Cuba, enters the area and with the aid of return flow and an accompanying surface trough, enhance moisture from Central America to the Gulf coast. At the surface, high pressure centered over the Southwest Atlantic will continue to maintain moderate southerly return flow over the next 3-5 days, but by week’s end, high pressure should shift west, giving way to more variable weather.

Southwest Atlantic

Deep layer ridging dominates the region; maintaining a dry and stable airmass, allowing for light winds and fair weather. This pattern is expected to continue for much of the week with little shower activity for the Bahaman Islands.

Caribbean Sea

Upper level circulation noted on water vapour imagery over Eastern Cuba is interacting with a surface trough of low pressure near 80W to produce widely scattered showers and thunderstorms over the island and Bahamas, with additional shower activity further south over the Northwest Caribbean in the diffluent flow around the low. These features are expected to continue off towards the west under the influence of deep layer ridging to the north and will monitored for development once in the Gulf of Mexico.

This upper level circulation is also helping to aid in the development of thundershowers along a broad area of low pressure in the Southwest Caribbean. A tropical wave along 70W should reach the area in 2 days enhancing additional shower activity. Much of this activity is expected to head west and across Central America.

Deep layer ridging over the Southwest Atlantic is helping to advect dry and stable air across the Eastern Caribbean, allowing for fair skies and dry weather. Trades are weak and variable due to the slack pressure gradient with showers mainly confined to patches of tradewind cumulus/stratocumulus clouds. This pattern should last until Thursday, afterwhich a tropical wave enters the region and begins interacting with the TUTT to produce showers and thunderstorms mainly over the Leeward Islands. This activity is expected to reach Puerto Rico and the Virgin Islands by weekend.


Figure 1. Water vapor image taken this morning at 8:45am EDT depicting several features discussed in today's tropical update.

Tropical Atlantic

The Azores High with a 1031 mb high centered 41N/23W dominates most of the Atlantic Ocean with mainly fair weather. Only patches of clouds are seen under the influence of this system from Africa to the Central Caribbean. Trades remain light to moderate and well below normal south of the ridge over the Tropical Atlantic and Caribbean. This pattern is expected to remain pretty much unchanged except for the Western Tropical Atlantic where a tropical wave near 40W will be interacting the TUTT circulation and over the Eastern Tropical Atlantic where a strong tropical wave is expected to emerge later this week.


Figure 2. Visible image of the tropical Atlantic and West coast of Africa showing several features discussed in today's tropical update. The tropical Atlantic is virtually convection-less but this is normal for late July. It appears that the most active years like 2005 and 2008 show a decrease in activity around this time of the year.

Downward motion has entered the Atlantic and none of the reliable computer models are hinting development over the last 7 days of July. Therefore, I suspect that the next tropical system of significant interest will occur in the month of August and it appears July is close off for business.

My August Outlook and July Verification is scheduled for next Sunday, 1 August.

Weather456

Disorganized Bonnie heading towards the Gulf Coast

By: Weather456, 3:33 PM GMT on July 24, 2010

From the latest advisory at 11am EDT, Bonnie was located 28.0N 86.7W or about 165 miles south-southeast of the mouth of the Mississippi River, moving off towards the west-northwest to northwest near 17 mph. Maximum sustain winds are described as barely 30 mph and minimum central pressure is above normal at 1014 mb. Satellite imagery showed an exposed low level closed circulation with much of the convection off towards the northwest due to southeasterly shear from adjacent upper low that has been moving in tandem with Bonnie since it was near the Bahamas. Shear has also been intensified by the fact that Bonnie is lodge between this deep layer upper low and a deep layer anticyclone over the Eastern United States. Earlier this morning, a hurricane hunter flight into the system managed to find some 30-knot flight level winds but not at the center of Bonnie, but rather in the thunderstorms to her northwest. Surface observations also support weak winds, with the highest marine station reporting 17-knot winds. I do not suspect Bonnie will improve significantly in organization due to the unfavourable upper environment in which she is embedded.


Figure 1. Visible image of the Western Atlantic, Caribbean and Gulf of Mexico showing Tropical Depression Bonnie, and two other areas of disturbed weather.

Bonnie steering is rather simple, as for most of her life. She is located between the deep layer flow of the upper low over the Western Gulf of Mexico and the Upper Ridge over the Eastern United States which is steering her off towards the west-northwest but increasingly northwest motions has been noted. This should bring Bonnie ashore over Eastern Louisiana over the next 12 hrs. Because the system remains asymmetrical, showers and isolated thunderstorms will occur several hours in advance of the system’s center. Bands of showers have already been noted on long-range Doppler radar. Wind...isolated gusts to tropical storm force winds in squalls are likely to spread over portions of the northern Gulf Coast...from southeast Louisiana eastward to the far western Florida Panhandle later today.
Rainfall...Bonnie is expected to produce total rainfall accumulations of 1 to 2 inches over portions of southern Louisiana...southern Alabama...southern Mississippi...and the far western Florida Panhandle...with possible isolated maximum amounts of 3 inches.


Figure 2. GOES-East water vapor imagery of the Western Atlantic, Caribbean and Gulf of Mexico depicting several features that are responsible for steering Bonnie. Water vapor imagery is one of the best tools for forecasting a storm's motion and track. It is more accurate and objective than other techniques such as numerical models. Water vapor imagery was mostly used to track Bonnie and came out with probably some of the best results.

Elsewhere, the leading southern edge of the upper low over the Northwestern Gulf of Mexico is enhancing showers and thunderstorms along a surface trough of low pressure over Western Bay of Campeche. The area will be monitored for development.

A second trough of low pressure over the Western Caribbean Sea is interacting with an upper low over Hispaniola. No development is expected in the near term, but the area will be monitored.

My August Outlook and July Verification is scheduled for Sunday 1 August.

Weather456

Tropical Storm Bonnie approaching Southern Florida

By: Weather456, 9:57 AM GMT on July 23, 2010

According to the latest advisory from 5am this morning, Tropical Storm Bonnie is located near 24.1N 78.6W or 250km southeast of Miami, Florida moving off towards the west-northwest near 18mph. Maximum sustain winds are near 40mph and minimum central pressure is down to 1008 millibars. Satellite images of Bonnie this morning showed a somewhat lop-sided tropical storm with the center of circulation on the southern periphery of a relatively small central dense overcast. Despite this, the storm appears to be located under 10-15 knots of shear and remains a tight concentrated tropical system. Intensity guidances have showed that there should only be slight strengthening of Bonnie as she heads into the Gulf of Mexico due to the proximity to an upper level circulation to the west, creating marginal upper air conditions. However, it appears the small area of low shear that Bonnie has consolidated under, is moving in tandem with this upper level circulation and combined with increasing sea surface temperatures, I suspect Bonnie to go slightly above the official guidance.


Figure 1. Infrared image of Tropical Storm Bonnie taken this morning at 4:45am EDT.

Bonnie is being steered by a combination of upper ridging to its north and a very large upper low to its west. Water vapour imagery clearly shows Bonnie being pulled briskly west-northwest around the counter-clockwise flow of the upper low, but at the same time, upper ridging is helping to steer her steady. Deep layer anticyclonic flow which should be centered where Bonnie is currently located, will steer her and the upper low off towards the west-northwest and then northwest over the next 2-3 days. The first landfall may occur today across Southern Florida, while a second landfall should occur by Sunday-Monday near Louisiana. After making landfall, the remnants may make an anticyclonic loop and re-emerge over the Atlantic in 5-6 days.


Figure 2. Water vapor imagery of Tropical Storm Bonnie this morning depicting the general flow in the atmosphere which is helping to steer the system.

Tropical storm forced winds only cover a tiny area and so the area affected by the maximum winds of Bonnie should be small. Currently, these winds are affecting Andros Island in the Bahamas and should approach the Southern Florida Peninsula and Keys later today. Higher gusts maybe reported under heavy thunderstorm cells, but minor to no damages are expected. Rainfall...Bonnie is expected to produce total rain accumulations of 1 to 3 inches over South Florida...with possible isolated maximum amounts of 5 inches. Additional rainfall amounts of 1 to 2 inches are possible over the northwestern Bahamas. Storm surge...storm surge will raise water levels by as much as 1 to 2 feet above ground level over portions of the northwestern Bahamas...the Florida Keys...and the southern Florida Peninsula. Tornadoes...isolated tornadoes are possible over extreme southern Florida later today.

Weather456

Tropical Invest 97L showing signs of organization

By: Weather456, 12:00 AM GMT on July 21, 2010

An area of disturbed weather dubbed 97L in associated with a surface trough of low pressure interacting with an upper low could become a tropical depression over the next few days. Satellite imagery, particularly, RGB imagery, no doubtedly, revealed a surface circulation forming near 20N-68W, just north of the eastern tip of Hispaniola as pointed out by the NHC. Surface observations support this by showing a weak west wind from the tip of Hispaniola and vorticity charts show an agreeable increase in circulation near the area. Water vapour imagery revealed even though the system is still interacting with an upper low to its northwest, anticyclonic flow is developing over the system likely due to the sustainable deep convection and as a result, vertical shear is down to 10 knots over the center, increasing to 30 knots in the periphery. Environmental conditions should continue to improve as the upper low dilutes and anticyclonic flow dominates aloft. Sea surface temperatures are very high, high enough to support a strong tropical storm or possibly a hurricane under weak vertical winds. Thus, with no major impediments but proximity to land, I suspect 97L to become Bonnie within a few days.

The system is being steered west to west-northwest under the influence of a sprawling ridge to the north, and this motion is expected over the next 3 days. I suspect a west-northwest motion over much of the system’s lifetime, taking it over the Turks and Caicos, Bahamas, Southern Florida or Keys and into the Gulf of Mexico in about 3-4 days. Afterwhich, a mid-latitude trough should swing across the Northeastern United States, inducing a weakness and more of northwesterly turn. The exacting timing of both features will dictate whether we have a repeat of a Rita track (statistical guidances) or a partial Dennis track (global models). Ridging is very pronounce so this turn might not be as abrupt. Interest in these areas should monitor the progress of 97L very closely.


Figure 1. Infrared image of Tropical Invest 97L at 7:15pm EDT.

Heavy showers and thunderstorms is an understatement to what occurred here in St. Kitts last night during the passage of 97L. Roads were washed away and we had flooding, making daily commute difficult today. What was most scary is that the lightning was so intense, it knocked out the power grid and we were without power for approximately 24 hrs, which is why I had no blog posted this morning. This system appears to be very wet so folks in the other Leeward Islands, Virgin Islands, Puerto Rico and Hispaniola are pretty much getting the same.

There are two African waves at 18W and 10E that I am watching for development.

My next blog will be posted in the morning.

Weather456

Tagging an Invest

By: Weather456, 10:08 AM GMT on July 19, 2010

An upper low over the Eastern Gulf of Mexico continues to interact with a tropical wave now along 88W and south of 27N to produce widely scattered showers and thunderstorms over the area from the Yucatan Peninsula, across Western Cuba and over the Florida Peninsula. This feature remains poorly organized and shows no signs of development as it head off towards the west-northwest. This is likely due to the marginal shear induced by the adjacent upper level circulation and because the upper low should move west in tandem with the lower level wave, I suspect little development from this feature in the near term. I continue to foresee a low chance of development. In the meantime, moisture will continue to spread across the aforementioned areas and the wave is expected to reach the Western Gulf of Mexico in about 2-3 days.


Figure 1. Water vapor imagery of the Gulf of Mexico, Mexico and Northern Central America depicting the first feature mentioned. The circular area of vapor over the central Gulf of Mexico is the upper low with the tropical wave on the right flank.

A tropical wave is located 79W, south of 22N, moving off towards the west. Shortwave infrared imagery revealed though thunderstorms have waned, they are more concentrated around a mid-level circulation at 16.8N-79.2W. This mid-level circulation can be clearly seen on close-up shortwave infrared image. Surface observations, however, indicate that there appears to be no surface circulation as yet. This feature continues to be located under a favourable upper ridge so I suspect that the mid-level circulation may slowly build towards the surface. I suspect this feature will move off towards the west, maybe a little north of due west and reach the Yucatan Peninsula by Tuesday or Wednesday. I will give this area a moderate chance of being tagged an invest and show some signs of development over the next few days. Regardless of development, showers and thunderstorms are going to spread across Jamaica, Eastern Cuba and the Caymans today.

A third tropical wave is along 65W, continue to interact with an elongated TUTT cell to produce widespread deep convection north of the Islands. Currently, the feature remains disorganized due to the southwesterly upper flow induced by the nearby upper level circulation. However, the wave should be located near the Southeastern Bahamas in about 3-4 days in an environment much more favourable for development under a ridge and adjacent to the TUTT cell, which is not expected to move much. I suspect this is where the feature will have to be watched the most. I will continue to give this area a low-moderate chance of development but a higher chance of being tagged an invest.


Figure 2. Shortwave infrared imagery of the Caribbean and southwest North Atlantic depicting the second and third features mentioned. On the left, we can clearly see a mid-level circulation evident from the cloud patterns and is more evident once looped. To the right is the forth wave interacting with the TUTT.

An Eastern Atlantic tropical wave, though possessing a well-define circulation, is entrenched in a layer of African dust and is void of any deep convection. No development expected in the meantime, but the wave will be watched as it heads off towards the west-northwest.

A very large African wave is near 17E and south of 22N. Infrared images show a large, organize, virgours mesoscale convection system associated with this wave, giving it the appearance of a tropical storm over land. The ECMWF thinks the feature will emerge with a surface circulation in about 1 week.


Figure 3. Visible image of the Eastern Atlantic showing the very dry stable environment that the wave is embedded in. There are a number of ways to assess the stability of the environment ahead such as the cloud-type, which in this case - stratocumulus. Stratocumulus are stable environment clouds and tropical disturbances moving into such a cloud deck will experience negative impacts on the development of deep convection.


Figure 4. Infrared image over Eastern Africa showing a strong tropical wave accompanied by a vigorous MCS.

Weather456

Several features to keep an eye on

By: Weather456, 10:24 PM GMT on July 18, 2010

There are several areas of interest across the Atlantic this afternoon that should be monitored for development over the upcoming week. First, we have a tropical wave along 85W and south of 21N moving off towards the west-northwest. Much of the convection along this wave is being skewed towards the north by an adjacent upper level low centered over the southeast Gulf of Mexico. Showers and thunderstorms are found over Western Cuba, the Florida Keys, Southern Florida Peninsula and the Bahamas. Lighter sporadic showers can be found further south over the Yucatan and Cayman Islands. The system is currently under marginal vertical shear from the upper low and because the upper low appears to be moving in tandem with the lower level wave, upper winds should only remain marginal for development. Current satellite images show a disorganized system and there have been no signs of organization. This feature should reach the Western Gulf of Mexico in about 3 days where it will be monitored for development. I give this feature a low chance of development.

The second feature is a tropical wave along 75W and south of 23N, which appears to have found itself on the favourable side of the TUTT as expected. Current satellite images show some signs of organization with possible weak rotation south-southeast of Jamaica. Further analysis of 925 mb vort, 850 mb vort and 700 mb GFS analysis confirmed this kink or turning. This feature is currently under a ridge that is adjacent to the TUTT, which makes the environment more favourable for some slow development. Vertical shear is still about 15-20 knots but this should gradually lower as the feature heads west into the Western Caribbean. Regardless of development, heavy showers are going to spread over Hispaniola, Jamaica and Western Cuba later this evening into Monday, reaching the Caymans by Monday evening. I give this feature a low-moderate chance of development.

The third feature is a vigorous tropical wave along 62W, south of 25N. This wave was once a naked rotation in the mid-low level cloud field over the tropical Atlantic but has dramatically increased in shower and thunderstorm activity over the past 24 hrs. Visible satellite imagery also reveals cyclonic turning centered near the northern-most Leeward Islands. Further analysis from surface observations showed the pressure down to 1010 millibars. This feature is currently under 20-25 knots of shear from the very same elongated TUTT cell so development will be hindered in the meantime. This TUTT is not expected to move much due to the blocking of the Continental Summer Ridge over the Southeast United States, so development, if any, will have to occur once the feature is on the western side of the TUTT in about 3 days. At this time, the feature maybe near the southeast Bahamas where some of the models have indicated development will take place. Regardless of development, heavy showers and thunderstorms are spreading over the Leeward Islands, Virgin Islands and Puerto Rico. I give this feature a low-moderate chance of development.


Figure 1. GOES visible image of the Gulf of Mexico, Caribbean Sea and Southwest Atlantic showing the three areas of interest. I suspect that the waves at 75W (middle) and 62W (right) have the highest chance of being tagged invests.

The fourth feature is now hundreds of miles further east near the African Coast. A vigorous tropical wave with an evident low level closed circulation near 14N-18W moving off towards the west-northwest. Satellite imagery showed clear cyclonic turning but little substantial deep convection and this is probably due to the dry, stable environment. The circulation, though closed and defined, remains rather broad and there are no signs of depression-forced winds. However, this feature is well organize, the most organized of the four areas of interest and stands a chance of becoming a depression since conditions ahead appear favourable to marginal. I give this feature a low chance of development.


Figure 2. METEOSAT-9 visible/infrared image loop of the vigorous tropical wave off the coast of Africa that almost fits the definition of a tropical depression.

Weather456

Caribbean favourable for development this week

By: Weather456, 3:54 PM GMT on July 17, 2010

A series of tropical waves across the Caribbean is interacting with TUTT cells north of the region to produce widely scattered showers and thunderstorms across the area. The first wave is near 78W, approaching the Western Caribbean and is interacting with an upper level circulation over the Florida Keys at 24N-83W. Widespread moderate to deep convection is ahead of the wave axis between 85W and 75W north of 15N. There have been no significant pressure falls over the past couple of days and there is not much model support, but the area will be monitored, as conditions will remain favourable for development. Regardless of development, heavy rains are going to spread across Jamaica, Cuba and the Caymans later today and over the next day or two, reaching the Yucatan Peninsula in about 2 days.

The second area of interest is a tropical wave near 65W in the Eastern Caribbean is interacting more with an elongated TUTT cell near 23N-65W. Vigorous convection is being generated along the axis and even in the diffluent flow around the TUTT cell as seen on visible and water vapour imagery. Similar to the western Caribbean, atmospheric pressures have shown no significant falls over the past day or two and there is not much model support for development. However, I suspect that an area of low pressure may develop in association with this feature as it heads off towards the west. The system is currently under 20-25 knots of shear from the adjacent TUTT cell but environmental conditions should become favourable for development so the area will be monitored. Regardless of development, heavy showers and thunderstorms are spreading over the Leeward Islands, Virgin Islands and occasionally Puerto Rico. Thundershowers should become more persistent over Puerto Rico and Hispaniola later today and tonight; spreading to Jamaica, Cuba, and the Caymans through Sunday and probably reaching the Gulf of Mexico in 3-4 days.

The tropical Atlantic remains quiet but there is one feature that’s worth mentioning. A highly amplified tropical wave is near 50W, south of 22N, moving off towards the west near 10-15 knots. Cyclonic turning is clearly evident along the axis centered near 12N, with rotation in the low-mid level cloud field. Several buoys in the MDR also depict the axis very well with the classic wind and pressure changes noted. This wave will be watched as it enters the Caribbean in a few days.

I give all these areas a low-moderate chance of development.


Figure 1. Visible image of the Caribbean and Western Atlantic showing three tropical waves of interest. Notice the cyclonic curvature in the third wave. Often times, interaction with upper level circulations leads to development that is why these areas will be monitored for development.

Weather456

Why has the Northwestern Pacific been so inactive?

By: Weather456, 1:45 PM GMT on July 11, 2010

The 2010 Northwestern Pacific Typhoon Season, which accounts for about a half of global tropical cyclone activity, has been noticeably inactive, even more inactive than the record low global activity we experienced in July 2009. Thus far, we have had one named tropical storm, one unnamed tropical storm and two other depressions, which yielded a total accumulated cyclone energy (ACE) of 2 as of July 7 2010, which pales in comparison to the normal year to date ACE of 42. By this time in 2009, we had numerous depressions, a few tropical storms and a number of typhoons and one super typhoon. There are number of atmospheric factors we can examine to determine the cause of such inactivity.


Figure 1. Number of named storms during the selective episodes of ENSO from 1975 to the present. There tend to be more storms overall during El Nino years but generally there is not much of a difference between individual years. In the Atlantic, the difference is far more noticeable with numbers reaching 19 in 1995 and as low as 9 in 2009.

During warm episodes of the ENSO (El Nino events), the Western Pacific Warm Pool (WPWP) shifts towards the east closer to the Date Line and the Central Pacific, causing cyclone genesis concentrations to shift towards the east. During cold episodes (La Nina events), the WPWP shifts towards the west, closer to its climatological position and thus cyclone genesis concentrations shift more towards the western regions of the Western Pacific. In both cases, the WPWP remains, largely remains in the Western Pacific Basin and though there are some differences in number of tropical storms in El Nino and La Nina years, the larger scale circulation should provide upward motion in both extremes. Pan (1981), Chan (1985), and Lander (1994) have described that ENSO events bring about in a change in location of storms in the Western Pacific Basin but not necessarily a change in frequency. The reason for this is that 1) as we commented before, the WPWP remains in the Western Pacific despite its shifts, and 2) the upward branch of the Walker Circulation remains in the Western Pacific. Therefore, there has to be an even more influential force for the inactivity in the Western Pacific Basin.


Figure 2. Storm activity to date during La Nina years from 1975 to the present. Most years appear to have 3 named storms by now so there has to be another factor with greater influence over the slow activity in the Northwest Pacific basin.

Another factor to explain the inactivity of the basin is the extensive areas of above normal sea level pressures over the Pacific. While above-normal pressures in the Eastern and Central Pacific can be tied to the weakening of El Nino and the uprising of La Nina, above normal sea level pressures in the Western Pacific is unusual for La Nina years; considering that Indonesia and Australia are wetter in La Nina years. Over the past 90 days, pressure anomalies across the global tropics are characterised by below normal pressures over the Atlantic and above normal pressures over the Pacific, a typical La Nina-like condition, but usually only extends to the date line. This pressure pattern has been synonymous with the sea surface temperature anomaly profile, where we have above normal SSTs in the Atlantic and below normal to near normal SSTs in the Pacific. The connecting factor now appears to be the atmospheric circulation. Significantly warmer sea surface temperatures in the Atlantic have intensified the upward motion over the basin than what we typically see during La Nina years and consequently, this upward motion reverses to downward motion over the Pacific.


Figure 3. 90-day reanalysis data of mean sea level pressure anomalies for the global tropics from April 10 to July 6 2010 showing lower than normal pressures (upward motion) over the Atlantic (purples) and higher than normal pressures (downward motion) over the Pacific (oranges and reds).


Figure 4. SST anomalies (C) for July 8 2010.

In addition, with upward motion favoured over the Atlantic, there has been a reluctance of the Madden Julian Oscillation to leave the basin. This, along with the ENSO and the SST profile, appears to have intensified a longitudinal circulation cell far beyond the Date Line causing a semi-global pattern of upward and downward motion across the Atlantic and Pacific. The overall atmospheric motion is known as the zonal overturning circulation.

The impacts on tropical cyclone formation is due to the suppression of convection due to the downward force and difficulty of deepening/intensification. Quite the opposite occurs on the opposite branch of this longitudinal circulation, here in the Atlantic.

Thus far, Alex alone has had a higher ACE than the entire Western Pacific Basin to date.

Weather456

Tropics are quiet following TD 2

By: Weather456, 1:35 PM GMT on July 10, 2010

Following the landfall of Tropical Depression 2 earlier this week, the tropics remain relatively quiet with few areas of interest. Weak surface pressure gradient is allowing for light easterly flow and fair weather over the Gulf of Mexico with maximum cloudiness in the extreme south-western Bay of Campeche due to speed convergence and upper low moving into Southeastern Mexico. Well-developed tropical wave moving into Central America and while it chances of developing on the Caribbean side are low, the system may develop once it emerges into the Eastern Pacific.

Further east, a tropical wave entering the Eastern Caribbean with little or no shower activity. Some of the wave is being amplified to the north by an upper low spinning near 25N/59W.


Figure 1. Visible imagery of the Western Atlantic.

Large area of Saharan Dust covers the tropical Atlantic from about 60W to 25W north of 12N. Water vapour and visible imagery revealed mid-level dry air subsidence resulting in numerous scattered to broken tradewind cumulus and stable air stratocumulus with little or no convective cloud cover, thus the perfect example of the tradewind inversion.

Another feature of interest lies on the eastern edge of the dust layer, where a nicely organized tropical wave is emerging off the coast of Africa near 9N. Numerous scattered showers and thunderstorms ahead of the wave axis where the monsoon southwesterlies converge and the AEJ is accentuating convection. For now, the feature will be watched as the 00Z GFS has been hinting on development, though conditions ahead appear only marginally favourable for sustained development.

There are two other waves over Africa that will be monitored for development, with each having noticeable mid-low level spins.


Figure 2. Visible imagery of the tropical Atlantic and West North Africa.

I will have a special blog tomorrow.

Weather456

Little change to our 4 areas of interest

By: Weather456, 9:39 AM GMT on July 05, 2010

A small area of low pressure dubbed 95L Invest continues to pulse itself over the Northern Gulf of Mexico. The system remains tied to a stationary frontal boundary and thus can be considered non-tropical, yet the generation of deep convection over the center of circulation yesterday and this morning resulted in tropical satellite estimates of 1.0. Based on satellite and surface observations, I believe the feature has secluded from the frontal boundary but winds remain below the required 25 knots to declare a tropical or subtropical depression. The system is moving off towards the northwest under the influence of return flow around a ridge parked over the mid-Atlantic states, and should be ashore over Louisiana later today or tonight, bringing scattered showers and maybe isolated thunderstorms. No development expected.

The second area of interest is an area of disturbed weather in the Western Caribbean Sea, which continues to generate cluster of deep convection across the area. Infrared satellite imagery revealed the system lost much of its deep convection during the last diurnal minimum, while shortwave imagery and surface observations revealed much of the circulation remains in the lower levels rather than the surface. Nevertheless, the system remains in a favourable environment for development and I suspect that after a few days of organization, a tropical depression may form. What we need with 96L is a surface circulation. 96L should track off towards the west-northwest to northwest over the next couple of days under the influence of surface ridging to its north and east. In the meantime, heavy showers and thunderstorms will continue across Jamaica and the Caymans for much of today.

A highly amplified tropical wave approaching the Lesser Antilles continues to interact with the TUTT to produce very vigorous convection, mostly along the northern section of the wave axis. Interacting with the TUTT will not allow development in the meantime, but the wave will be watched as it heads into the Central and Western Caribbean in 3 days, where environmental conditions are expected to be more than favourable. Regardless of development, the Leeward Islands can expect very heavy shower and thunderstorm activity over the next 24-36hrs. This may cause some flooding concerns around the mountainous regions of the islands.

The last area of interest is a broad area of showers near the Bahamas in association with a stationary frontal boundary. It will be monitored for development.


Figure 1. Infrared image of the Western Atlantic showing the four areas of interest being monitored this morning.

In case you missed it, I issued my July outlook yesterday, which can be accessed to right under “Recommended Links”.

Weather456

July Tropical Weather Outlook - Active July Expected

By: Weather456, 12:16 PM GMT on July 04, 2010

June Summary

If you ever read my outlooks during the 2009 Hurricane Season, you would release that each monthly outlook summarized and verified the forecast for the previous month. In my June Tropical Weather Outlook, I made a distinction between cyclogenesis occurring in the Northwestern Caribbean and the Gulf of Mexico and it followed:

“Climatology states that there is a 50% chance of seeing a named storm in June and based on the given conditions, there is a moderate chance of being above this mean. There is a moderate-high chance of cyclogenesis, if any, occurring in the Northwestern Caribbean, with a low-moderate chance of it occurring in the Gulf of Mexico.”

Therefore, I was pleased that the forecast for cyclogenesis areas verified when Alex formed in the Northwestern Caribbean and made landfall on the Yucatan Peninsula, one the high-risked areas pointed out.

However, the one storm that formed during the month, Alex, deviated from climatology and the tracks predicted during the month, and this was due to the unusual amount of ridging during the month of June and the July-like conditions we experienced during the last two weeks of June.

During the month of June, we saw four invests, one tropical depression, one tropical storm and one hurricane. Invest 91L formed from the remnant energy of Agatha that made landfall near Guatemala. It ventured into the Northwestern Caribbean where it was decoupled from the subtropical jet. This feature was given a low chance of developing.

Invest 92L was an unusual well developed Eastern Atlantic Tropical Wave that showed signs of development but was later decoupled by the Tropical Upper Tropospheric Trough (TUTT) as it approached the western Tropical Atlantic. This feature was given a moderate to high chance of developing. Despite the lack of formation, the invest brought squally weather to the Leeward Islands, Puerto Rico and Hispaniola with 3 days of heavy rains, thunder and lightning.

Invest 93L developed in the Southeast Caribbean and tracked west-northwest into the Western Caribbean where it developed into the first named system of the year, Alex. Alex made a first landfall in Belize and tracked across the Yucatan to emerge over the Gulf of Mexico about 18-24 hrs later. The system emerged with a central pressure of a strong tropical storm and only spent one advisory as a depression after being downgraded overland. For much of Alex’s life, the central pressure remained a category deeper than the winds and the lowest pressure of 947 millibars just before landfall in Northern Mexico, made Alex the 2nd most intense June cyclone on record.

Invest 94L developed from a non-tropical upper low and tropical wave east of the Islands. It never did developed.


Figure 1. Tropical weather activity during June 2010. Notice that all of this tropical activity occurred south of the Tropic of Cancer which often indicates an active season ahead.

Introduction to July Climatology


Figure 2. Track of all tropical cyclones occurring in July between 1851 and 2006 showing the majority of storms were clustered and tracked in the Western Caribbean and Gulf of Mexico but points of origin stretched further east than June.

Not much tropical activity occurs during the month of July, but the majority of hurricane seasons see the formation of one tropical cyclone during July. Using data from 1944 to 1996, on average, half of the hurricane seasons had their first tropical storm by July 11, with a second having formed by August 8.

Formation usually occurs in the eastern Caribbean Sea around the Lesser Antilles, in the northern and eastern parts of the Gulf of Mexico, in the vicinity of the northern Bahamas, and off the coast of The Carolinas and Virginia over the Gulf Stream. Storms travel westward through the Caribbean and then either move towards the north and curve near the eastern coast of the United States or stay on a northwestward track and enter the Gulf of Mexico.

Since 1851, a total of 105 tropical storms have formed during the month of July. Since 1870, ten of these storms became major hurricanes, the strongest of which was Hurricane Emily of 2005. This storm is the only known Category 5 hurricane during July, becoming the earliest forming cyclone of this intensity in the basin. The easternmost forming storm and longest lived during the month was Hurricane Bertha in 2008, forming at 22.9W and lasting for just over 17 days. The last storm to form in July was Dolly in 2008 so based on climatology; we are schedule for a named storm this July.

I am expecting a moderate to high chance of an above normal July.

Main Indicators


- El Nino Southern Oscillation and Vertical Shear Relationship
- Intraseasonal Rainfall Patterns
- Sea Surface Temperatures and Tropical Heat
- Steering Flow
- Mean Sea Level Pressure
- African Rainfall and the Saharan Air Layer

The El Nino Southern Oscillation (ENSO) and Vertical Shear Relationship

The Tropical Equatorial Pacific has continued to cool for June and the conditions for the development of La Nina are underway. I like to think that three consecutive months of below normal sea surface temperatures in the Equatorial Pacific is enough to signal a negative Oceanic Nino Index (ONI). Thus far we have had two consecutive months, May and June, of below normal sea surface temperatures as in figure 3. Therefore, continued cooling as projected by leading climate models should result in a La Nina as early as late July possibly August. La Nina conditions tend to favour cyclogenesis by providing the 200 mb anticyclonic flow needed to reduce vertical shear. Vertical shear was below normal over the entire Main Development Region (MDR) during the month of June. Further analysis of the 200 mb wind anomalies indicate we had more upper level easterlies than westerlies and more anticyclonic flow aloft, which is rather unusual for June. We saw the ramifications of this during Alex, which developed one of the most impressive upper anticyclones I have ever seen on a tropical cyclone.


Figure 3. Sea surface anomalies for the four NINO regions with NINO region 3.4 and 3.0 of greatest significance to ENSO events.

This pattern of continued below normal vertical shear should continue in tandem with increasing La Nina conditions and vertical shear as predicted by the GFS and CFS should be of little concern this month in relation to cyclogenesis.


Figure 4. GFS long-range wind shear forecast valid July 16 2010 showing the general upper pattern expected during the month. Large scale ridging in the deep layer should cause low shear for much of the month.

Intraseasonal Rainfall Patterns

Upward motion was more than usual during June thanks to warmer than normal sea surface temperatures, below normal sea level pressure and the Madden-Julian Oscillation. Most of the climate models foresee upward motion dominating the entire month of July with maybe a week of downward motion. We can use three factors as proxies to forecast upward motion for the month of July – below normal sea level pressure, warmer than normal sea surface temperatures and rainfall. All three factors indicate ample upward motion to get storms spinning during the month of July.

Sea Surface Temperatures and Tropical Heat

Sea surface temperatures continued their above average state without any significant cooling occurring except for the Western Gulf of Mexico in the wake of Alex. Sea surface temperatures are about 2-4 degrees above normal with the anomalies in most areas such as the Gulf of Mexico, higher than the anomalies on July 2 2005. This is largely due to weaker than normal surface trades we are experiencing during the negative phase of the North Atlantic Oscillation (NAO). These anomalies are expected to continue throughout the month of July. These anomalies are more significant than we think as they have contributed to the 2nd most intense June hurricane (no surprise as we are surpassing 2005 for warmest SSTs). I suspect that we may see another record this month along those lines.


Figure 5. Sea surface temperatures for July 2 2005 (above) and July 2 2010 (below). Both years appear to have record warmth with 2010 slightly cooler. We can see the cool sea surface temperatures in the wake of Hurricane Alex.

Steering Flow

Steering flow for June was unseasonable unfavourable and is expected to be the same case with July. The track of Alex was enough to dictate the presence of pronounce ridging across the subtropical central and western Atlantic rather than the eastern Atlantic and we can thank the negative NAO. For July, there are several indicators of the pattern we may see. We have already established from since May, that the negative NAO will dominate the season. The GFS long-range forecast shows ridging dominating the entire tropical and subtropical Atlantic with the polar jet stream and 500 mb storm tracks remaining as far north as 40-50N. This type of pattern is related to Neutral and La Nina years. I suspect we will have little recurvature this month and because I suspect our first Cape Verde type system this month, a Dennis and/or Emily type track seems to be in order.


Figure 6. Forecast of the location where the 500 mb pressure surface will be at a height of 582 decameters (5280 meters) above sea level based on the GFS. This is the accepted level for mid-latitude storm tracks.


Figure 7. GFS 200 mb analysis valid July 16 2010 showing the typical upper level pattern expected. With ridging so extensive and the polar jet stream far north, expect little re-curvatures this month. On a side note, notice the North American Monsoon High or Texas High which is a July type feature which brings unbearable heat and dry weather to Texas. This is one the best representation of July tropics.

Mean Sea Level Pressure

Mean sea level pressure for the month of June was below normal and again was displayed by the only storm to form during the month which was the 2nd most intense. Below normal sea level should continue this month and we can actually use it as a proxy for storm tracks as in figure 8, which is in agreement with the type of steering discussed earlier.


Figure 8. ECMWF Seasonal Forecast for July, August and September showing mean sea level pressure anomalies for the Western Hemisphere. Below normal pressures are expected across the Atlantic with above normal pressures in the Pacific (typical of La Nina years). We can indirectly infer storm tracks by these anomalies.

African Rainfall and the Saharan Air Layer

How was African rainfall and Saharan Dust like during June? Considering the peak of Saharan Dust outbreaks is June and July, dust was below average. How and Why? There are a number of factors that we can indirectly infer to obtain the level of dust experienced over the Atlantic during the month. For example, dust tends to increase the vertical stability over the Atlantic and for much of the June; the tropical Atlantic remained more unstable than usual. Second, dust tends to decrease the amount of convective cover over the tropical Atlantic which decreases the amount of water vapour pixels colder than -40C (cold cloud tops). During the month of June, water vapour pixels colder than -40C was above normal signifying there was more deep convection across the area.

The reason for the below average dust events was above normal rains over the Saharan Desert and the absent of eastern dominance of the Azores High. Both conditions are expected throughout the month and for atleast the first 9 days of July, we would have little dust events. Once of the reason why I suspect our first Cape Verde-type system may form this month.


Figure 9. MODIS composite loop of the Tropical Atlantic from June 01 to July 02, excluding several days in between that had no data. In this image there was about 2-3 major dust events, which none ever seemed to really affect the Eastern Caribbean. Also notice the combination of waves leaving the ITCZ.

Conclusion

In an average year, we see one tropical cyclone forming during the month of July and because the last storm to form in July was during the 2008 Hurricane Season, we are schedule for a cyclone this month. Given the continued favourable conditions following June, I suspect we will see more than one named storm this month. There is a 60-80% chance of seeing an above normal July, with a 50-60% chance of seeing our first Cape Verde type system. The Caribbean and Gulf of Mexico remains the two hotspots but the tropical Atlantic appears much more in play now.


Figure 10. Composite summary of the conditions expected for July 2010.

Tropical Update

Tropical Invest 95L remains void of any significant convection this morning and this is due to the insurgence of very dry air into the system. The system remains non-tropical as it has not secluded from the associated frontal boundary. Over the next couple of days, the system should seclude from the frontal boundary and head off towards the west-northwest then northwest into Louisiana under the influence of return flow from the ridge that accompanied the frontal boundary. Upper level winds, dry air and proximity to land causes me to believe that 95L should not of been designated in the first place and stands an almost 0 chance of developing into a subtropical or tropical cyclone. Regardless of development, some scattered showers will spread across the Central Northern Gulf Coast.

As predicted would happen, Invest 96L has developed in the Caribbean Sea and stands a better chance of developing than its predecessor. A tropical wave in the Western Caribbean continues to produce widely scattered clusters of deep convection, which is somewhat better, organized than yesterday at this time. The system has little shear to deal with an excellent upper level anticyclone above (all this discussed in the July outlook coming into play already), but it appears the convection remains east of the greatest surface vorticity which maybe due to some mid-level shear. Nevertheless, environmental conditions are conducive for further development of the system as it heads west-northwest towards the Yucatan Peninsula (Alex part 2). Both the reliable EURO and the not so reliable CMC develops 96L and takes it on a similar path to Alex, but they appear divided on landfall with the EURO in Northern Mexico and the CMC in Texas. The GFS does not develop the system but track the feature similar to the CMC. This appears to an Alex event all over again. Regardless of development, heavy shower and thunderstorm activity will spread across Jamaica and the Caymans today and on Monday.

We still have two other areas to discuss – talk about a full tropical update. A tropical wave approaching the Antilles will be watched for development, while development may occur near the Bahamas along the tail end of a frontal boundary and head northwest or north.


Figure 11. GOES-13 infrared image of the Western Atlantic this morning showing the potential areas of development.

Weather456

A few areas to watch now Alex is gone

By: Weather456, 1:49 PM GMT on July 03, 2010

RGB satellite imagery revealed a broad area of low pressure to be located near 27.5N-87.9N moving off towards the west-southwest. Surface observations indicate this low remains broad, ill-define and very weak with the lowest pressure reported near 1013 millibars. The feature is roughly vertically stacked with the upper level circulation just above the surface circulation but I suspect being tied to the tail end of a frontal boundary, it remains non-tropical. Scattered moderate convection is within a band to the south and east of the low bringing scattered showers to the Florida Peninsula and Western Bahamian Islands. Over the next 2-3 days, the low will seclude from the frontal boundary and head back towards the north and west under return flow. At this point, the system will be watched for development. I will give this a low chance of becoming subtropical and/or tropical but have to disagree with the NHC’s views on the wind shear environment, which appears a bit more favourable than they have indicated.

A broad area of convective activity has flared up in Caribbean in associated with a surface trough of low pressure and a tropical wave. This area will have to be watched definitely, since most of the reliable models develop this feature as it nears the Yucatan, taking the feature either to the Northwest Gulf Coast or into Mexico, a spread similar to Alex. Environmental conditions will be conducive for such a solution and it would be a double whammy for the Western Gulf Coast if this verifies.

A third, high amplitude wave is located along 49W south of 17N moving off towards the west near 15 knots. Visible imagery showed one of the best inverted-V patterns I have seen on a wave all season with cyclonic turning near 12N. Most of the models show this wave remaining strong as it reaches the Caribbean in 2-3 days and while development is not anticipated, showers and thunderstorms will spread across the Southern Antilles, Trinidad and Tobago, and Barbados.

Last, models are indicating development offshore the SE Coast in the SW Atlantic but from the same frontal boundary. I will also have to watch this area.


Figure 1. This morning's visible satellite imagery of the Tropical Western Atlantic showing the three features discussed.

I hope that my July outlook will be ready for tomorrow.

Weather456

Alex moving inland after making category 2 hurricane landfall

By: Weather456, 10:37 AM GMT on July 01, 2010

Hurricane Alex roared ashore last night over Northern Mexico, making landfall near 24.5N-97.7W at 10:25am EDT in the Mexican state of Tamaulipas with winds of 100 mph as predicted. Thankfully, the region that experienced the most intense winds is sparsely populated with the nearest town of San Fernando to the northwest of the point of impact, likely experienced strong tropical storm force winds. We should begin to see the extent of the damage inflicted as the day advances. Further north, maximum wind gusts were of tropical storm strength in areas like Brownsville, Texas and Port Isabel, Texas. The outer rainbands of Alex produce 4-6 inches of rain just in the 24 hr period yesterday causing flooding especially to South Padre Island.


Figure 1. Microwave image of Alex taken by the SSMI sensor onboard the DMSP satellites during landfall. The pass was taken 9:45pm EDT as Alex's eye crossed the coast (left). Brownsville radar image of the storm landfall around 10:25pm EDT (right). Landfall is defined as the point where the center of the eye crosses the coast. There are differences in these two sources of data but the radar image is more reliable since there discrepancies in microwave imagery that can cause the center to be off. These are referred to as paradoxes.

Alex should continue to move westward and inland under the influence of ridging to the north and gradually weaken throughout the day. I suspect he will be a minimum tropical storm or depression by day’s end. As Alex weakens, heavy rains will fall over the Sierra Madre Mountains producing life threatening floods and landslides.

Alex made landfall with a central pressure of 947 millibars which makes it the second most intense June hurricane on record, just 1 millibar higher than the previous record of 946 millibars by Hurricane Audrey in 1957.


Figure 2. 1-day observed precipitation from June 30-July 1 for the Texas region.


Figure 3. Dennis Barrett paddles his kayak down a flooded Padre Boulevard as the early effects of Hurricane Alex are felt along the Texas coast, Wednesday, June 30, 2010 in South Padre Island, Texas. (AP Photo/Eric Gay)

I will have an update on any other potential storms on Friday. My July outlook is scheduled for Sunday.

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.