Tropical Tidbits from the Tundra

Ex-Agatha causing trouble in the Caribbean

By: Levi32, 3:11 PM GMT on May 31, 2010

The remnants of tragic Tropical Storm Agatha, which has been responsible for the deaths of at least 97 people in central America, does not appear to be done with us yet. After crossing central America and leaving a path of destruction behind, ex-Agatha may cause trouble in the western Caribbean.

There are several things which caused this unexpected possibility to come about. Agatha's sudden increase in forward movement over the Pacific caused her to make landfall nearly 24 hours ahead of the expected schedule, and this allowed steering currents to take her across central America much faster and farther to the east than anticipated. Yesterday she reached Belize and stalled with her low-mid level center onshore as steering currents weakened. This new timing of her movements changed the forecast, and with now more time to sit near the western Caribbean, we have a chance for some re-development.

Analysis of radar and visible satellite imagery shows that the center of ex-Agatha has jumped offshore this morning close to a new blow-up of convection, and is now located near 17.6N, 87.2W. The circulation does not yet appear to be closed at the surface, but is still fairly well-defined above 850mb. A circular cluster of new thunderstorms has been going off east of Belize for the last 6 hours, and if this can be sustained, we could see the circulation try to reach the surface, and at that point the NHC would likely label this Invest 91L. Ex-Agatha's previously fast forward movement was expected to carry her into the Gulf of Mexico rather quickly, allowing her little time to organize, as well as keeping her under high wind shear. The fact that she has stalled has allowed the upper ridge to expand northward over the western Caribbean, and wind shear is now down to 5-20 knots over the system.

Based on these factors, I give ex-Agatha a moderate chance of development into a tropical depression.

Ex-Agatha will eventually get drawn northward into the Gulf of Mexico, and a weak upper shortwave currently moving across the gulf south of Texas may help her start doing that tonight and tomorrow. If this fails and misses the system, it may take even longer for ex-Agatha to make it to the Gulf of Mexico. Once she does make it to the gulf, it will likely mean the final end of her life, as the subtropical jet racing across the gulf will inflict strong wind shear on the system, and only remnants of tropical moisture are expected to impact Florida as the system gets pulled out to the northeast. other significant threats of tropical development in the Atlantic are expected for the next 48 hours. A tropical wave in the eastern Caribbean will be interacting with the re-forming monsoon trough near Panama in 3-5 days, and may try to cause tropical mischief either north or south of that country. Development is more likely to occur in the eastern Pacific, but the area will be monitored. A well-defined tropical wave along 22E over central-eastern Africa continues to be tracked, and will emerge over the Atlantic in 6-8 days. Although this cannot be a concern for development at this time, this particular wave, if it remains intact, could become interesting down the road.

We shall see what happens!

Ex-Agatha Visible Satellite (click image for loop):

Caribbean/East Pacific Visible Satellite (click image for loop):

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Invest 90E and a dirty Caribbean

By: Levi32, 3:54 PM GMT on May 28, 2010

The tropics of the eastern Pacific and Caribbean have been becoming increasingly active over the last couple days, mainly due to an impressively strong upward motion pulse of the MJO, which is centered right over central America. This is resulting in a large amount of convection over the entire region.

The main feature is Invest 90E in the Pacific, located near 13N, 96W. 90E is not as organized as it was yesterday morning, but convection continues to fire fairly symmetrically around the center, and I expect this to eventually develop and become Agatha.

Another very interesting thing to notice this morning is that the monsoon trough has made its first invasion of the year into the Caribbean. The monsoon trough is a portion of the ITCZ where the northeasterly trade winds of the Caribbean converge with the monsoonal southwesterlies of the eastern Pacific. This boundary normally lies near or over Panama, but a WindSat pass from yesterday evening shows that it has migrated north into the Caribbean, now extending from Jamaica southwestward over Central America and out into the eastern Pacific.

A deep-layer trough north of the Caribbean has helped draw the monsoon trough northward. These northward migrations can happen periodically during the hurricane season, and when the monsoon trough is lying through the Caribbean it can act as a catalyst for tropical development.

Indeed, last night, convection associated with the monsoon trough started creeping across central America and emerging off the coast of Honduras into the western Caribbean. This convection was associated with two weak areas of low pressure, one over Honduras, and another emerging in the Caribbean near the Honduras/Nicaragua border, both lying along the monsoon trough. The former has now moved near the northern coast of Honduras, and the latter is now west of Jamaica this morning. There is no longer any significant convection over Central America due to nighttime cooling, and scattered moderate convection is associated with the Jamaica low. Wind shear is running 20-30 knots over the Jamaica low, as it is moving northeastward along the southern fringe of the subtropical jet, which is inflicting moderate upper-level westerlies over it. The jet, however, has been lifting north over the last 24 hours as an upper anticyclone starts to build into the western Caribbean, and wind shear is down to as low as 5 knots near the northern coast of Honduras.

The low near Jamaica would be a concern for possible tropical development, but it is being carried off to the northeast very fast by the monsoonal jet, which is directing it right into the arms of a deep-layer trough draped north of the Caribbean, of which ex-invest 90L is a part of. This trough will take the Jamaica low rapidly northeastward along the subtropical jet, likely near eastern Cuba tonight, and into the SW Atlantic tomorrow. Due to its rapid movement northward into hostile upper-level conditions, tropical development of this low is not expected. Jamaica, eastern Cuba, and Haiti, however, are looking to get some pretty nasty weather from this system.

Looking further ahead, some of the models continue to forecast 90E in the Pacific to make a crossing over central America and possibly re-develop in the Caribbean in a few days. I do not buy this solution, as the models have been initializing 90E too far east based on satellite imagery, and are not picking up well on the split nature of this system. I am still expecting more of a split system to continue, with one system over central America or the western Caribbean, and the real 90E out over the Pacific. There is really nowhere for 90E to go for at least the next couple days, as I don't believe the deep-layer trough to the north is yet strong enough to pull it into central America. It is, however, strong enough to put 90E in the middle of very weak steering currents, and thus I expect little movement of the system over the next couple days, and it should continue to meander around just south of the Gulf of Tehuantepec. Eventually, 90E may move inland, but I do not expect it to get into the Caribbean.

The western Caribbean will have to be watched though for the next couple weeks, as the MJO is just now strongly entering our area of the world, and upward motion will probably stick around through at least June 10th. This is a pattern we will be seeing all season long, as the MJO tends to jump to our area of the world and then sit there for as long as it can. It remains to be seen if the other circulation near Honduras will cause any mischief in the Caribbean once the Jamaican low leaves, but with an upper high building over the area, it is a very conducive situation for trouble.

Elsewhere....upper divergence over the eastern Caribbean will continue to spark showers and thunderstorms over Hispaniola, Puerto Rico, and the Antilles Islands for the next couple days, bringing localized rainfall amounts of up to several inches to those areas.

We shall see what happens!

Caribbean/East Pacific Visible Satellite (click image for loop):

Invest 90E Track Models:

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

90L sits off the coast; 90E organizes in the Pacific

By: Levi32, 4:25 PM GMT on May 26, 2010

Invest 90L continues to sit off the southeast U.S. coast this morning. Satellite imagery indicates the system has been gradually becoming better organized since yesterday. The surface low is now vertically stacked under the upper low, and wind shear has dropped to 5-10 knots. There is still some dry air being entrained into the system from the southeast, but not as dry as it once was, and the popcorn convection that has been going off since last night has slowly moistened the atmosphere enough to support more scattered convection, which is now starting to form into bands arcing into the center.

Model and AMSU microwave data indicate that 90L has indeed become a fully subtropical, warm-core system. However, the system as it is right now is not classifiable due to lack of organization. The forecast here was for 90L to not remain extratropical but to become a truly subtropical low, named or not, and that looks to have verified decently. 90L still has a window of about 24 hours to organize further, and we may see convection try to increase in coverage today, but most indications are that this system will not reach classifiable status.

90L will be gradually pulled eastward by a longwave trough diving southward over the Canadian maritimes. Interestingly enough, a lot of the models now have the trough not fully succeeding in picking up 90L, and leave a dissipating surface system behind south or southwest of Bermuda by the end of this week. Due to cooler SSTs in this area and the lack of upper air support, it is unlikely that 90L will be able to strengthen further if it is left behind, and will likely dissipate eventually. Meanwhile, rain and scattered thunderstorms will continue to rake coastal SE US today, and scattered showers are possible in Bermuda as well. Elevated surf and rip currents are still a concern along the SE US coast as 25-30 knot winds are still coming out of the east and northeast on the north side of 90L.

In other news, Invest 90E has formed in the eastern Pacific, and represents a large area of building heat west of central America. There are a couple possible centers, and the models all initialized the center in different locations this morning. There is not much around to move this disturbance at the moment, and it will likely sit there south of El Salvador and Guatemala for the next 24-48 hours. Thereafter, the deep-layer trough associated with 90L will be digging into the eastern Gulf of Mexico, and will likely draw at least a piece of the energy associated with 90E across central America and into the NW Caribbean. Depending on how the situation evolves, we may end up with a named storm in the east Pacific and another disturbance in the Caribbean, or the entire main disturbance may migrate into the Caribbean. There is really nothing to take this westward, so the mischief is going to stay near central America and any short-term movement will be to the northeast towards the Caribbean. Regardless of how the situation evolves, this could become a bad flooding event for central America as heavy rains associated with the disturbance sit over the area for potentially several days.

Potential for tropical development in the western Caribbean will be limited by the subtropical jet, which will be inflicting strong wind shear just north of the Gulf of Honduras, where any tropical disturbance is likely to be sitting in a couple days. The GFS takes the disturbance right off to the northeast with the deep-layer steering flow, but this is not cut and dried, as both track and timing could change depending on how the system organizes on the Pacific side, and how strong it is in the Gulf of Honduras if a piece comes across. The entire area will be monitored.

We shall see what happens!

Invest 90L Visible Satellite (click image for loop):

Invest 90L Track Models:

Southwest Atlantic/Caribbean Visible Satellite (click image for loop):

Invest 90E Track Models:

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

90L sitting offshore with potential for development

By: Levi32, 4:39 PM GMT on May 25, 2010

Invest 90L has not changed much during the last couple days. Continuous dry air entrainment from the south and westerly wind shear has caused the system to spit out multiple surface centers, which eventually die as new ones form to the north. However, the long-talked about trough-split has occurred over the eastern seaboard, and the upper piece that was left behind is currently over the Georgia coast and in the process of phasing with the large upper low to the east, which is nearly vertically stacked over 90L's surface center. This process is gradually lowering wind shear over 90L's center, allowing it to move more in tandem with the upper feature, and thus there is no longer a strong need to reform. This is resulting in a more permanent surface center, which is well-defined on satellite imagery, though still removed from the convection, which is mostly off to the NW now. There is long surface trough extending east of 90L's center, which is draped over Bermuda, bringing decent rains to them.

At first glance, 90L looks just as disorganized as it always has been. There are, however, some signs of change to be noticed. Firstly, the system is more symmetrical, and with no more need for radical replacing of low centers, the surface circulation has become more circular and permanent, with a good westerly component to the winds on the south side. Also, convection is starting to wrap around the system, and is now concentrated in the NW quad, instead of the northeast like it has been this whole time. This convection is firing over warmer SSTs to the west, close to the Gulf Stream, and this warm water should continue to support building convection on the west side of the system. This area of convection is firing directly beneath the upper low, which has not yet become perfectly stacked over the surface low, and is located to the NW. There is actually a brand new low trying to form underneath the upper low, near the convection to the northwest.

What all this means is that with 90L becoming vertically stacked, convection will now be able to start forming closer to the center and mix out the dry air. 90L will be drifting westward and eventually stalling off the SE US coast during the next 48 hours. With this movement taking the system over increasingly warmer SSTs, tropical processes will start taking over partial responsibility for maintaining the low pressure area as convection starts to fire around the center.

90L still has a chance to become fully subtropical, as it has about 2 days to sit off the SE US coast before getting picked up by a longwave trough to the north, which will kick 90L out to the northeast as it dissipates and gets absorbed. I am holding fast to the idea that there is still decent potential to get a name-worthy subtropical storm out of this. Whether the NHC will be inclined to name it if it does become one, we will have to see. The biggest problem for 90L will be its sheer size. The constant spitting out of old low centers has caused the circulation to become quite broad, and it will take a lot of effort to form a consolidated center that is needed for significant tropical development. There is time though, and the potential for trouble is still there. At any rate, strong winds on 90L's north side due to a tight pressure gradient will be increasing rough surf along the US east coast and Bermuda, with moderate rains continuing for Bermuda as well. Some scattered showers will likely also make it onshore into the Carolinas over the next couple days.

Elsewhere....there is still concern for tropical mischief in the western Caribbean sometime this week, as an area of building heat in the eastern Pacific may try to send a piece of energy across central America, which is developed by some models. Overall this situation is handled differently on each successive model run, and with most of the heat consolidating in the eastern Pacific, my confidence in Caribbean mischief is low, but the area will have to be watched as the upward motion pulse of the MJO remains over the area through the end of this week.

We shall see what happens!

Invest 90L Visible Satellite (click image for loop):

Invest 90L Track Models:

Southwest Atlantic/Caribbean Visible Satellite (click image for loop):

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Tropical concern for southeast coast next week

By: Levi32, 4:18 PM GMT on May 21, 2010

Rather quick update this morning.

Update: My tired eyes this morning didn't see that this low has already been designated Invest 90L, meaning that this is now a system under investigation for possible tropical/subtropical development by the NHC and Navy. This is the first invest of the year.

Not much has changed since yesterday. The concern continues to build for a hybrid low that is currently developing east of the Bahamas to cause quite a stir for people along the southeast U.S. coast next week. The current satellite view of the system shows a well-defined developing low northeast of the Bahamas. There are multiple low-level vortices rotating around the mean center, but this low is still being driven by baroclinic processes, and we cannot yet analyze it like a subtropical system.

This low will gradually strengthen as it drifts ENE, and will eventually get pulled northward later this weekend. This will be due to a trough-split off the east coast, which will be from the upper trough currently moving over the eastern US, which has been sparking severe weather over the south the last few days. This trough is being driven northeastward by an amplifying ridge behind it moving into the plains, and once it reaches the eastern seaboard it will hit the Atlantic ridge and get stuck. What will happen is the trough will dig southeastward, a piece will lift out into the jet to the north, and a large piece will get be left behind to move south off of Cape Hatteras on Monday and Tuesday as a cut-off low.

The hybrid low will be sitting off to the southeast moving slowly northward, and the upper trough-split piece will be right there to "adopt" the low, which will bring it westward underneath and make the system vertically stacked. This is when the low could really take off and go warm-core, becoming a full-fledged subtropical storm. With the blocking ridge building into New England, this system will retrograde west underneath the block and likely impact the southeast coast somewhere between Cape Hatteras and Charleston, South Carolina. The low will be moving slowly and eventually turning to the southwest, so it will be a close call on whether this commits to moving inland or tries to hug the coast and move back over water farther south. What is interesting is the Euro and Canadian models weaken the system upon landfall, and baroclinic systems don't do that. Warm-core lows weaken over land, so we know from the look on the model and the setup in the atmosphere that this will likely have a warm-core look to it and has a good shot at deserving a name. Hopefully this rogue storm won't take people on the beaches by surprise with high winds and heavy rain next week. The Euro is showing nearly hurricane-force winds at 5000 feet with this on the north side as it comes ashore, so this is going to be a beast of a storm with likely 50-60mph sustained winds. It's going to be nasty weather for a good stretch of the coastline.

The other area to keep an eye on into next week is an area of building heat over Central America and the western Caribbean, as tropical waves pile up air in the area. The upward motion pulse of the MJO which will be gradually intensifying over the SW Atlantic during the next 10 days will support increased convection and lowering pressures during the course of this week, and at some point a broad low is expected to form and move slowly off to the northeast as our hybrid low moves by to the north. This is the real deal in terms of true tropical development, and although likely to be a weak system if it does develop, it will have to be watched for spreading heavy rains to Hispaniola sometime next week.

We shall see what happens!

Invest 90L Visible Satellite (click image for loop):

Invest 90L Track Models:

Southwest Atlantic/Caribbean Visible Satellite (click image for loop):

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Could Alex be threatening the southeast U.S. next week?

By: Levi32, 4:27 PM GMT on May 20, 2010

Things are starting to get interesting in the tropics, as we will almost certainly have one if not two areas of interest to watch as we go through the weekend and into next week.

The first area to be watched is an area of disturbed weather over the Bahamas. An upper shortwave is currently moving eastward just east of Florida, creating a divergent flow aloft which is causing widespread showers and thunderstorms to break out from Cuba, across the Bahamas, and stretching out into the SW Atlantic. A weak surface low is trying to develop in this area northeast of Nassau, to the east of the shortwave trough axis. The shortwave will continue to move slowly eastward over the next few days, but the longwave trough currently over New England will be lifting out of the area, leaving the shortwave behind to get stuck east or northeast of the Bahamas on Saturday. As this happens, the developing surface low will slowly strengthen under the shortwave and drift eastward with it.

Meanwhile, the upper trough currently over the central US, which was responsible for the severe weather outbreak yesterday, will be rounding the ridge in the east and weakening as it moves northeast towards New England. This trough will become a shortwave within the ridge and once it passes the Ohio Valley it will dig southeastward towards the coast at 72 hours and split, lifting the trough out to the northeast and leaving a piece behind off the SE US coast.

This is the infamous trough-split I have been talking about for the past week. What we'll have is an already developing low east of the Bahamas, the split will come down off the SE coast, and then "adopt" the surface circulation by bringing it north and then west underneath. This situation is very lucrative for a nasty hybrid/subtropical cyclone to develop off the SE coast, because with the 2 upper troughs to the northeast lifting out, the upper convergence behind them causes surface pressures to rise rapidly around 40N southeast of New England. This sets up a strong convergence zone to the south near the developing low, and the big high to the north supports lowering pressures between the Bahamas and Bermuda as the surface low starts feeding back under the cold pocket aloft. Such a setup could easily wind this up into a beastly creature with gales gusting over 50 knots on the north side of the system.

This is the 0z European model's interpretation of the situation by Sunday night:

I continue to be more in line with the Euro than the other models on this system, as the trough-split should get caught under the blocking ridge which will be over New England, and after moving north this system should start backing west or southwestward towards the SE US coast. Some of the other models are fiddling around with this and making a mess. The latest GFS runs have been coming more in line with the Euro, but it continues to split off pieces of energy and throw them out to the east, which is typical of the GFS. But overall, the GFS is slowly coming to realize its error and is trending westward. Exactly how this all turns out remains unclear, but if the surface low can warm its core enough from the bottom up over sub-26C waters, it isn't much of a stretch to believe we could be looking at Subtropical Storm Alex off the southeast U.S. by early next week. We'll see what the NHC decides to do with it. It is even possible that the system could transition to fully-warm core, and thus tropical instead of subtropical, if it moves over the waters of the Gulf Stream, which are barely at 26C, the threshold for tropical development.

Although this feature will be capturing attention over the weekend, there is one more area to look out for as we head into next week. If you noticed on the model image above there is a broad low developing on the Euro in the SW Caribbean on Sunday night. This has been a consistent feature for several days now on most of the models, and it makes sense that with all these tropical waves piling into the southern Caribbean, the building heat will eventually have to be released. The release mechanism will be the MJO, which will be gradually increasing large-scale upward motion over the SW Atlantic and Caribbean over the next 10 days.

There are already areas of convection firing around Panama and over the NW Caribbean. This latter area is being caused by divergence aloft associated with a weak upper shortwave just east of the Yucatan. This has created a weak surface trough, which is enhancing thunderstorms over the NW Caribbean. This piece of energy will get stuck in this area over the next few days, and although development of this particular feature is not expected due to its position under the subtropical jet, it will be bringing rains to the Cayman Islands and portions of Cuba and central America, and will be adding to the overall energy buildup in the western Caribbean.

I am fairly confident that the energy buildup will eventually result in the development of a broad area of low pressure in the southern Caribbean sometime next week. The models have become much less aggressive with this feature on recent runs, and frankly I believe this is a step in the right direction, as the subtropical jet will likely be inflicting strong westerly shear over most of the Caribbean for a while yet, in part due to the trough-split to the north. Though a small upper ridge may start developing over the SW Caribbean next week, any tropical system that develops would likely stay fairly weak, and would drift slowly northeastward as the Bahamas system passes by it to the north. This northeastward movement may end up bringing some heavy rains to Haiti sometime next week, which is something they probably don't need.

Overall, next week will be very interesting, and I believe we have a good shot at seeing a subtropical storm get named off the southeast U.S. coast, depending on how the NHC decides to play it. I am beginning to become more confident in development of the Bahamas system than I am of anything in the Caribbean, but both areas need to be watched as we head into next week.

We shall see what happens!

Atlantic IR Satellite:

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Two tropical mischief-makers next week

By: Levi32, 4:14 PM GMT on May 19, 2010

Things remain fairly quiet in the Atlantic, for now, but there are signs that trouble may be brewing in two different areas next week.

The first area to watch is the southwest Caribbean, where a broad area of low pressure will be developing in 4-8 days, in response to the tropical wave train piling up air in that area. This air will be given the means to rise when the MJO moves over our area of the world during the next 5-10 days, which will generate large-scale upward motion, increasing convection in the Caribbean and lowering pressures. There is already evidence of more heat building up as convection is going off south of Panama and in the western Caribbean. There is good model agreement that at least a broad low will form in the SW Caribbean in several days, but whether it develops will depend largely on the orientation of the subtropical jet, which is currently inflicting strong westerly wind shear over the entire area. This jet should gradually start to lift northward as an equatorial ridge begins to expand over the area, and this should provide at least a small area of favorable conditions for tropical development over the southern Caribbean next week. Any low that develops will lift slowly northward due to weak steering currents and another feature to the north, which is our 2nd area to watch.

The 2nd area of concern is for possible subtropical development associated with a trough-split early next week, and this will likely begin to happen before the Caribbean really gets going. An upper longwave trough currently moving through the eastern US will be lifting out to the northeast over the next 2-3 days. As it does so, a piece from its tail will be left behind just north of the Bahamas, while the jetstream lifts northward out of the area. This piece of upper energy will be the catalyst for some sort of surface low to develop near the Bahamas. Such a low will likely be formed by baroclinic(nontropical) processes at first, but as it becomes vertically stacked under the upper cutoff low, it will likely become subtropical in nature. We may see signs of a surface trough start to develop east of the Bahamas as soon as tomorrow, as it appears the process will be initiated by a weak shortwave moving eastward out of the Gulf of Mexico.

The challenge here is that the models have been up and down and all around with this situation. There are 2 main possibilities. One is that a 2nd longwave trough, which will be digging into the north Atlantic in 5-6 days, will amplify into the piece that split off from the first trough, and any system down near the Bahamas will be pulled northeastward and exit the area as a mostly baroclinic entity. This solution is generally favored by the GFS. The other possibility is that the cut-off low misses the next longwave trough, and gets stuck to the south of the blocking ridge that will be building into New England next week. This would result in a subtropical low drifting westward towards the southeast US coast. This solution is more favored by the ECMWF. I am more inclined to lean towards the Euro in this situation, as in my experience it handles trough-splits better than the GFS. I think that even if a piece of energy gets taken out to the northeast into the longwave trough, another piece will still be left behind near the Bahamas that could cause mischief. The image below shows the European's depiction of the situation on Monday.

Overall, this is a very complicated synoptic situation, and we will likely have two different areas to watch for possible tropical trouble next week.

We shall see what happens!

Atlantic IR Satellite:

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Tropical Tidbit for Tuesday, May 18th

By: Levi32, 4:31 PM GMT on May 18, 2010

Things are fairly quiet in the Atlantic this morning. A broad area of showers and thunderstorms remains over the western Caribbean and Central America associated with a divergent flow aloft, bringing scattered areas of rain to those areas. Several tropical waves lie across South America and the central Atlantic embedded in the ITCZ.

The forecast challenge this week lies with two areas of interest that will have to be watched later this week and into next week. The first is a broad area of low pressure that will be developing by this weekend in the SW Caribbean, in response to the air being piled up in the area by all these tropical waves coming across South America. This air will be provided with a way to rise upward by an MJO upward motion pulse, which is expected to come across the western Atlantic during the next 5-15 days. This should result in increased convection over the Caribbean, and with tropical waves interacting with the broad area of low pressure sitting in the southwest Caribbean, there could be tropical trouble later on into next week.

The GFS ensembles are illustrating this setup well with an area of well-below normal heights centered over the NW Caribbean next week. This shows the build-up of heat that will be occurring in the Caribbean as tropical waves pile in and large-scale upward motion is initiated by the MJO. Enhancing this pattern is the blocking to the north over the eastern US, which tells us to watch underneath for fun and games in the tropics. You always watch underneath ridges. They always mean trouble. This is where we got the old saying: "High over low, look out below!", referring to the pattern of high heights to the north over low heights to the south in the tropics, which is a favorable pattern for tropical mischief.

The 2nd area that will be taking shape around the same time is a trough-split east of the Bahamas. A longwave trough currently moving through the eastern U.S. will be lifting northeast and leaving a tail piece behind in 3-4 days. This piece of upper energy, in conjunction with the subtropical jet, will likely spawn an area of surface low pressure in the SW Atlantic. Although this low will be created initially by baroclinic (nontropical) means, there is potential for this system to become subtropical in nature, as trough-splits commonly "adopt" their surface lows by bringing them underneath and allowing them to feedback in a tropical fashion by using the cold pocket aloft.

The difficulty here is that although there is strong model support for some sort of a low to develop in this area, the model solutions are all over the place, and the overall pattern is poorly handled by even the best of them. Two things could happen here. The trough-split could occur farther to the north, and the surface low could be pulled north by the next passing longwave trough, which would give less time and less favorable conditions for subtropical development. This is a solution the GFS showed us yesterday. The other possibility is that the low develops far enough south that it misses the longwave trough and gets blocked below a blocking ridge over New England. This was shown by yesterday's 12z ECMWF, and would be the best situation for getting subtropical development, as the system would drift westward towards the east coast underneath the block.

Another complication is the area in the Caribbean mentioned earlier. This large area of low pressure may end up competing with the trough-split system somehow, and this may prevent the subtropical system from winding up. The latest ECMWF 0z run from last night kind of shows this:

By next Tuesday you can see the broad low southwest of Jamaica and the trough of low pressure east of the Bahamas underneath the upper trough-split. Notice how they look connected by a surface trough, and this kind of a situation could lead to an elongated trough of low pressure extending up into the Bahamas that doesn't really have a chance to get going. On this run the Euro tries to get everything together in the Caribbean and then bring a developing low northeast across the islands and into the SW Atlantic.

Overall, we will have two areas of interest to watch later this week and next week. With the MJO coming across into our area of the world, the pattern is favorable for early-season tropical mischief.

We shall see what happens!

Atlantic IR Satellite:

Atlantic Tropical Surface Analysis:

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

2010 Atlantic Hurricane Season Outlook

Tropical Tidbit for Thursday, May 13th

By: Levi32, 4:48 PM GMT on May 13, 2010

There is not a whole lot going on in the Atlantic, as conditions remain mostly unfavorable across the basin, which is quite normal for May. Our area of the world is also being oppressed by large-scale downward motion associated with the dry phase of the MJO.

The one feature of interest today is a fairly well-defined tropical wave along 41W, which has been pretty impressive for May. This is the strongest wave of the month so far. Although the inverted-V signature is less distinct than yesterday, broad low-level turning about the wave axis is observed on visible satellite imagery, confirmed by elevated values of 850mb vorticity. The wave remains ill-defined above 700mb. The wave axis passed over a buoy in the central Atlantic near 4N, 38W earlier this morning, causing a wind shift from ENE to SE at about 9z.

Convective activity associated with the wave has greatly diminished since yesterday, likely due to the presence of strong westerly winds in the upper levels over the northern part of the wave causing moderate wind shear, evidenced by upper clouds racing off to the east and northeast on satellite imagery. Wind shear becomes even stronger as you go north of 10N. The wave is also moving into an area of dry air to its northwest with 700mb humidities below 50%, which is likely hampering convection as well. This is due to a confluent flow in the upper atmosphere forcing air to sink, which warms and dries it. The dry air shows up well on Total Precipitable Water imagery, and is noticeable on satellite imagery by the rapid drop-off in convection along the entire ITCZ just west of the tropical wave in question. The wave is also having to fight against the large-scale downward motion in the surrounding environment thanks to the MJO which was mentioned above.

There is not much development-wise to expect with this wave. As we have seen, conditions are less than ideal, and tropical waves such as this one are generally too far south at this time of year to develop at all. Due to even more hostile conditions just to the north, even a northward move would not help this wave. What this wave should be thought of as is a sign of potential things to come later this season, as tropical waves will likely tend to strengthen out over the Atlantic under favorable conditions.

Elsewhere in the Atlantic....there are no areas of concern for tropical development, and it should remain that way for a while yet. By the end of the month, around the 25th, the upward motion phase of the MJO will be moving into our area of the world, and with all these tropical waves piling into the southern Caribbean and central America, we will have to watch for heat build-up and possible mischief in late May and early June.

We shall see what happens!

200mb Vertical Velocity (green areas represent upward motion associated with the MJO):

Atlantic IR Satellite:

2010 Atlantic Hurricane Season Outlook

2010 Atlantic Hurricane Season Outlook

By: Levi32, 12:51 PM GMT on May 11, 2010

This will be my final outlook for the 2010 Atlantic hurricane season. I issued a preliminary outlook back in February, which can be viewed here and will be referenced at times during this post.

In overview, I am expecting a well above-average year with 18 named storms, 10 of which will become hurricanes, and 6 of which will become major hurricanes. This is up from my preliminary numbers in February, and would make the 2010 season the most active since the legendary year of 2005, as well as surpassing every other year since 1995. This season is also expected to have a high impact on the United States with at least 6 named storms making landfall, and a few of those could be major hurricanes. The various factors and methods behind the madness are explained in this outlook.

Let's take a look at some of the things that have been evolving during the past few months.

North Atlantic Oscillation (NAO):

The NAO is an atmospheric index which indicates the status of the surface pressure pattern across the North Atlantic Ocean. A positive phase of the NAO is associated with a stronger than normal Azores High and Icelandic Low. A negative phase of the NAO is associated with the opposite situation of a weaker than normal Azores High and Icelandic Low. The NAO has been strongly negative this winter and spring, which can be seen in Figure 1, with lower than normal pressures over the eastern Atlantic representing a weak Azores High, and higher than normal pressures over Greenland and Iceland representing a weak Icelandic Low.

Figure 1. Sea-level pressure anomalies for April, 2010.

This setup has a warming effect on tropical Atlantic SSTs, as a weaker Azores High results in a more relaxed pressure gradient over the tropics on its southern flank, resulting in slower northeasterly trade winds. This decreases evaporation of the ocean surface and upwelling of colder subsurface water, thereby allowing SSTs to warm. Indeed, the negative NAO this winter has played a big part in why SSTs have warmed to record levels across the tropical Atlantic during the past few months.

Figure 2. Surface wind speed anomalies for April, 2010.

The winter phase of the NAO tends to foreshadow the same phase during the following hurricane season, and this would support higher tropical activity. In addition to allowing SSTs to warm, slower trade winds make it easier for tropical waves and surface troughs to amplify, which increases surface convergence, and thus, lift. A weaker trade wind flow also pulls less Saharan dust and dry air off of the African continent. Steering currents are also affected, as a negative NAO is associated with a weaker Bermuda High that is displaced farther south and west of normal, which tends to steer more storms towards the Caribbean and United States. Also, a weaker Icelandic Low results in fewer upper longwave troughs trying to break down the Bermuda High and recurve tropical cyclones before they have a chance to hit land.

Sea Surface Temperatures (SSTs):

The big highlight of the last 4 months has been watching SSTs in the Atlantic MDR (Main Development Region) skyrocket to record levels, even surpassing 2005, the previous record holder. Although this is a scary development this year, there are some other things to notice about the current state of the ocean as well. Figure 3 points some of them out.

Figure 3. Current sea surface temperature anomalies as of May 10th, 2010.

One thing that has stood out all winter is how cold the Gulf of Mexico has been, and in fact smashed the record for the coldest SSTs ever observed since satellite measurements began. This can be attributed to the brutally cold winter over the southeast United States due to the extremely negative NAO and AO (Arctic Oscillation), which caused dramatic cooling of coastal waters. What has stuck out like a sore thumb though is the Loop Current, which has been quite well-defined all winter and spring, and is a reminder that the AMO (Atlantic Decadal Oscillation) is still warm, meaning that deep ocean currents like the Loop and the Gulf Stream will be supplying warmer-than-normal water into the gulf and off the SE US this summer, warming those areas up above normal. This has already started to show up in a profound way over the last 2 weeks, with the GOM and SE US coastal waters warming all the way back up to normal under a ridging pattern which has allowed warm water pouring in from the Caribbean, coupled with above-average air temperatures, to warm SSTs in those areas. The gulf will likely be well-above normal during the heart of the hurricane season.

The same negative NAO/AO pattern responsible for the cold GOM has also had effects across the entire north Atlantic. With arctic air being shoved down into the mid-latitudes all winter, this greatly enhanced the positive Atlantic tripole. The Atlantic tripole is a pattern that likes to show itself during warm AMOs where you have warm water in the tropics, cold water in the mid-latitudes, and warm water again in the arctic areas of the Atlantic north of 50N. The cold winter has served to enhance and expand the cold water belt north of 30N, which can be seen in the image above as an area of blue anomalies in the north Atlantic, smaller now due to the recent warming off the SE US.

Seasonal variations now start to play a key role, as during the winter and springtime, this belt of cold water sets up a strong temperature gradient with the warmer water to the south, resulting in a strong baroclinic zone along the gradient which pulls the subtropical jetstream to the south, creating a positive feedback with the negative NAO. This has favored lower-than-normal pressures in the northeast Atlantic during the winter and spring months, which slows the easterly trade winds over the tropics. In the summer, when the jet retreats to the north, the baroclinic zone dissapears, and the pattern switches so that the lowest pressures are found in the southwest Atlantic over warm tropical waters.

This is the kind of thing that will happen this summer as we have a classic setup of cold over warm in the Atlantic, where the cold water to the north promotes net subsidence (sinking air), and the warm water to the south promotes net upward motion. This really focuses heat and convergence over the deep tropical breeding grounds, lowering surface pressures and favoring lots of storm activity.

Another thing I am keeping an eye on is the SSTs in the south Atlantic and Gulf of Guinea (armpit of Africa). This area has been warmer than normal overall during the winter, but recent weeks have seen some cooling, and it will be interesting to see if that continues. A cooler south Atlantic helps focus heat and convergence in the tropical north Atlantic, and a cooler Gulf of Guinea increases the temperature gradient over western Africa, strengthening the AEJ (African Easterly Jet), and pulling the ITCZ farther north, which increases rainfall over the Sahel and promotes healthier tropical waves. There is some disagreement amongst the seasonal forecast models on whether SSTs in these areas will cool or remain warmer than normal during the hurricane season, but the gradient between the south and north tropical Atlantic is already strong, and the overall signal supports enhanced ITCZ activity.

The other big story this winter and spring has been the El Nino in the Pacific. This event ended up to be pretty impressive, peaking in the "strong" category. The El Nino peaked in December of last year, but decline off this peak was stalled by a series of strong Kelvin Waves in February and March. However, during late March the fall of Nino 3.4 SSTs resumed, and has not looked back since. The cooling in the central Pacific has really become quite rapid in the last couple weeks, with upwelling already bringing cold (blue) anomalies to the surface between 160W and 130W. This rate of decay is very similar to 1998, which was the 2nd-fastest reversal from a strong El Nino to a La Nina since 1950. Some signs of this reversal are already appearing in the tropical Atlantic, with wind shear trending below normal as the upper-level westerlies associated with El Nino diminish, and favorable anomalous upper-level easterlies have prevailed for the past month. After many procrastination predictions by the more reactive models like the CFS during the winter, which had El Nino lasting through the summer, most of the models have finally come in line with reality and are predominantly forecasting at least weak La Nina conditions during the heart of the hurricane season.

Figure 4. Various current model ENSO prediction plumes. The solid dark red line in each graph represents the SST threshold for El Nino (+0.5C), and the solid dark blue line represents the SST threshold for La Nina (-0.5C).

I believe we will be in a weak to moderate La Nina during the height of the hurricane season, and this is favorable for an active year. The worry here is that the El Nino has put a great deal of heat and moisture into the world-wide tropical atmosphere over the winter, and the Atlantic looks to be the main focus point for that heat this summer, making it the basin to go to for a bursting in tropical activity. Just look at the sheer magnitude of the heat forecasted to be over the Atlantic by the ECMWF (2m temperature). But this rapid crash of the El Nino makes it worse, because a more gradual transition before the hurricane season normally starts taking at least some of the heat out of the tropics by the peak of the year in August-September-October. Instead, we have all this heat built up and a rapid reversal to at least a weak La Nina right as the hurricane season starts. All that heat gets left over the Atlantic and something is going to have to be done with it. With the floor coming out from under the El Nino the atmosphere now has a way to effectively deal with that heat, and that will be in the form of tropical cyclones.

Some of the forecasts coming out of the seasonal models are flat-out scary, especially from the ECMWF. The sea-level pressure anomalies are absolutely textbook for a major hurricane aimed at the big land areas of the southwest Atlantic, and the Euro has been consistent since February while getting more bullish over time.

Figure 5. ECMWF Sea-level pressure anomaly forecast for the July-August-September period.

The situation depicted here is very favorable, with high pressures over the eastern Pacific due to La Nina forcing convergence over the Caribbean and SW Atlantic, and an area of much lower-than-normal pressures aligned across the Caribbean islands, Bahamas, Florida, and the Gulf of Mexico. High pressures from La Nina also extend into northwestern South America, weakening the Columbian Heat Low and slowing the easterlies that usually come racing through the Caribbean. This acts to allow SSTs to warm and results in more surface convergence as air piles up in the western Caribbean. These things are all reflected well in the ECMWF precipitation forecast, which shows higher than normal rainfall across the entire tropical Atlantic, Caribbean, and portions of the GOM and SW Atlantic.

Figure 6. ECMWF precipitation anomaly forecast for the July-August-September period.

Africa: Dust, Rainfall, and Tropical Waves

Knowing the expected conditions over western Africa is always very important in determining whether we will have an active Cape Verde year. Africa is the source of what we know as the Saharan Air Layer (SAL), which is a very dry, dust-laden airmass which, if expelled into the Atlantic, can greatly hamper tropical waves' efforts to develop, and often proves to be the death of many tropical storms that do develop and then suck in the dry air, which accelerates downdrafts and collapses convection within the storm.

The amount of dust, and thus, dry air, generated over the area of Africa that we call the Sahel, which lies along the southern border of the Sahara Desert, is mostly determined by rainfall. The rainy season in the Sahel only runs from about June-September, as the ITCZ is too far south the rest of the year to provide much rain. There is some disagreement among the models as to whether the Sahel will see above or below normal rainfall this summer. Some of the disagreement can be attributed to their similar dispute over the SSTs in the Gulf of Guinea, which have a significant effect on west African rainfall.

As mentioned earlier, a cooler Gulf of Guinea increases the meridional temperature gradient between southwest coastal Africa and the Sahara Desert. The hotter air to the north lowers surface pressures and tends to "attract" the ITCZ farther north (this is exactly what pulls the ITCZ north in the summertime in the first place) and the stronger temperature gradient increases convergence, and thus convection and rainfall, along it over the Sahel. This not only dampens dust output, but enhances the health of tropical waves exiting western Africa as well. The AEJ (African Easterly Jet), which is driven by this gradient, is also strengthened and pulled farther north, which increases cyclonic vorticity on its south side over both western Africa and the tropical eastern Atlantic. This results in stronger tropical waves that have a better chance to develop over the Atlantic.

Despite model disagreement, given the pattern shaping up I don't expect a very dry year for the Sahel, but more along the lines of normal to slightly above normal rainfall. Even with normal rainfall, dry air and dust will likely be lower than normal this year due to the last 3 rainy seasons which have been wetter than normal for most of the western Sahel. This has served to moisten the ground overall, setting up a less dusty pattern for this year.

Figure 7. Precipitation anomalies over the western Sahel for the 2007, 2008, and 2009 rainy seasons (left) and soil moisture anomalies for January-March 2010 (right).

Another thing that has an equally important effect on the SAL is the trade winds over the Atlantic. As I mentioned in my explanation of the NAO, the weaker than normal easterly trade winds over the eastern Atlantic associated with the negative NAO pull less dust and dry air off of the African continent, and this is becoming increasingly important here in May, the last month before the rainy season begins in western Africa. After all, the Sahara can produce as much dust as it wants, but if there are no winds to push it out over the Atlantic, then it won't have an effect on developing storms in our basin. With the NAO forecasted to remain predominantly negative during the summer, trade winds should remain slower than normal, and therefore duststorm outbreaks over the Atlantic should be less frequent.

Analogs and landfall impact idea:

In my February outlook, I compiled a package of 9 analog years exhibiting similar conditions to this year, which I have since narrowed down to 7 years, eliminating 1970 and 1978. The analogs I'm looking at are now 1958, 1964, 1966, 1995, 1998, 2005, and 2007 (a more detailed analysis on the similarities between the analogs and this year can be found in my February outlook). Yes I do consider the record year of 2005 to be an analog, and the reality is that it's one of the best analogs we currently have, along with 1998, in terms of overall SST profile in the Atlantic coupled with reversing El Ninos from the previous winter. Although I do not expect this year to see the kind of storm numbers we had in 2005, I do think we will rival it in terms of total impact on the United States and Caribbean.

The concern is that with cold water in the eastern Pacific due to La Nina and cold water in the north Atlantic due to the Atlantic tripole, we will end up seeing the record warm water in the MDR get sandwiched in between those two areas of subsidence, setting up a corridor of warm water and upward motion aimed through the Caribbean and right at the United States. As a rule, hurricanes like to follow the heat (that is what they thrive on, after all), and when you outline a clear, easy path for them, they are likely to take it. This is being reflected in a lot of the model forecasts now as they catch on to the La Nina.

Figure 8.

This is a very similar pattern to what was happening in 1998 and 2005 at this time with cool water over warm in the Atlantic, focusing heat and convergence in the tropical breeding grounds, resulting in big landfall years for the United States. If we look at the storm tracks in 1964, 1998, and 2005, the three biggest analogs, we see a clear pattern of tracks focused to the west, with high impact on the U.S. coastline from Texas to Cape Hatteras, as well as the Caribbean islands.

Figure 9. Named storm tracks during the Atlantic hurricane seasons of 1964, 1998, and 2005.

A combination of these 3 should define this season pretty well, with the main congregation of tracks focused south and west, unfortunately threatening most land areas of the SW Atlantic. There will likely be a few floaters that recurve in the central-eastern Atlantic as well, similar to 1998. I expect a very busy year for the Caribbean, more like 2005 and 2007. The entire U.S. coast from Brownsville to Hatteras looks to be under higher than normal risk, though I think tracks will be slightly more weighted west of Florida than north of Florida, based on current trends. I also think this may be the year where we get a long-track major hurricane landfall on the U.S. coast. We have not had a true long-track Cape Verde major strike on the U.S. since Ivan and Jeanne in 2004.

The warmer than normal Gulf Stream may be trouble for New England as well, as there could be a couple storms that break from the pack and come straight north up the eastern seaboard. This scenario will be especially likely if upper blocking sets up over the Canadian Maritimes. 2005 almost had this setup, but the blocking was a bit too far west, over the SE shore of Hudson Bay. With a predominantly negative NAO likely throughout the summer, the blocking pattern will be something to watch closely as far as the threat to New England goes.

The main challenge with overall tracks will lie in just how fast and how strong the La Nina comes on during the season, as stronger and faster would increase ridging over the SE US and tend to force tracks farther south, jamming more storms into Central America. 2004, although not technically an analog year, is a good representation of where I think tracks will tend to go this season.

The other curveball being thrown into the forecasting technique this year is the overall setup of things in the Pacific. The PDO, which although peaked positive with the El Nino, did not peak very high, and in reality was going into a cold phase in the north Pacific, apart from the El Nino signature. What resulted was a strong central Pacific-based El Nino framed by a cold PDO, and this is something we haven't really seen before. If it has occurred, it would have happened back in the mid-late 1940s, when the decadal PDO was last going negative. 1998 did reverse, but the PDO was warm before-hand, creating a different setup. 2005 was embedded within a 5-year warm PDO period. This simply throws something at us that is rarely seen, and it will be interesting to see how this plays out. At any rate, this will be crashing along with the ENSO indices during the summer, and plays right in along with everything else regarding favorability for active tropics in the Atlantic.


In summary, the Atlantic hurricane season of 2010 is expected to be very active, with a dangerous track congregation threatening most land areas of the western Atlantic, with a similar impact to 2008, in terms of years in recent memory. This has the potential to be considered an "extreme" season. I expect at least a couple major hurricane landfalls along the U.S. coastline, and the Caribbean is likely to be affected by one or more major hurricanes as well. The atmosphere has been primed this winter for a major season in the Atlantic, and the worldwide pattern favors a bursting of activity in our area of the world. This is indeed a scary situation, and I can only hope that everyone who lives in hurricane-prone areas has taken appropriate precautions this year. Every year is life-threatening to someone, and this season may go down as a top-10 impact year.

We shall see what happens!

PDF copy of this outlook for those who have trouble viewing it on the blog.

First Tropical Waves of 2010

By: Levi32, 5:53 PM GMT on May 07, 2010

Well yesterday we found out from the NHC that the first tropical wave of 2010 had already rolled off of Africa days days ago, and was already out in the central Atlantic. We did not pick up on this wave here on the blogs because it really wasn't that impressive, and is still fairly ill-defined this morning. The wave is now located approximately along 45W south of 10N, but the wave axis is very positively tilted, putting the northern end near 42W and the southern end near 47W, as of 12z. WindSat and ASCAT passes mostly missed the wave this morning, but visible satellite imagery shows the rather ill-defined structure of the wave, with no signs of the classic "V" signature in the low-level cloud streets that we look for when analyzing tropical waves. There is, however, a nice northward bulge in the ITCZ along the wave axis, and this shows up on the CIMSS TPW (Total Precipitable Water) product, shown below. The wave has been moving westward at a good clip of 22 knots over the past 24 hours, and this general motion will continue for the next 18-24 hours, with a slight turn to the WNW after the wave passes 50W. This trajectory will take most of the wave into South America in 48-72 hours, but the northern end of the axis may spark some showers and thunderstorms around Trinidad, which would provide some much-needed rain to that area.

While we just got our first tropical wave, the 2nd one is already rolling off Africa. Yesterday I analyzed an unofficial tropical wave over western Africa near 9W, and this analysis has been confirmed this morning by the NHC, which has now also analyzed the wave, now over the Atlantic along 15W, in their morning tropical discussion. They also added a 3rd wave along 36W behind the first one at 45W, but I can find no conclusive evidence to support the existance a tropical wave in this area, with no apparent wind shift or potential vorticity maximum in the region. The wording of the NHC discussion suggests that this analysis was mostly based on the TPW product, but I am of the opinion that the moisture bulge they referred to is more associated with the first wave near 45W. Hopefully things will become clearer in the next couple days. Meanwhile, the new wave which has emerged off of Africa will continue to move westward across the eastern Atlantic over the next few days, and will be monitored.

Elsewhere in the area of light showers and thunderstorms is persisting north of Panama due to a TUTT over the western Caribbean and a cut-off upper low over Costa Rica at its tail. The TUTT will be lifting out of the area over the next couple days, and scattered showers will continue to affect the coastal areas of Panama, Costa Rica, and Nicaragua over the next 24 hours.

We shall see what happens!

Current tropical waves analyzed over METEOSAT visible:

Current tropical waves analyzed over TPW:

Current West Atlantic Visible Satellite (click image for loop):

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

Tropical Tidbits from the Tundra

About Levi32

Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.

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