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Tropical Weather Update 8/15/09

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Geez, after two and a half months of no activity in the Atlantic, the past week and especially the past couple of days has really seen the basin’s behavior shift dramatically.  I had previously ignored an area of disturbed weather off the west coast of Florida.  It was a tropical wave that made its way across the west Atlantic, interacted with an upper-level low for a couple of days and made its way across the Florida peninsula.  As of tonight, it has been designated an area of interest to the NHC: Invest-91.

It is relatively small in extent and has only limited organization and no central surface circulation yet, so the NHC is only giving it a <30% chance of tropical formation in the next 48 hours.  Surface pressures are falling in the general area of the disturbance.  Still, it is an area of interest as it is expected to move northwest across the Gulf and move ashore, according to recent projections, near New Orleans, LA.  I want to be clear: it is not expected to be a Tropical Depression, Storm or Hurricane at that time due to the latest information.  It is an area of thunderstorms that is expected to move ashore in a couple days’ time.  To fill out the picture, this disturbance is currently northeast of a Gulf Loop eddy in the Gulf of Mexico northeast of the Yucutan Peninsula.  The heat content of the Gulf under the expected path of this disturbance isn’t as high as in the eddy, but it is at least as high as the water near T.S. Bill.

Elsewhere, T.S.s Ana and Bill continue to move across the mid-Atlantic.  No significant changes in their track or intensity forecasts have been made.

A tropical wave has moved over the Atlantic from the west coast of Africa.  It looked fairly vigorous just prior to moving offshore.  It is relatively close to Tropical Storm Bill.  It isn’t expected to develop; I just wanted to make note of it.  The next wave over Africa is a few days away from moving offshore.


This has been quite the day for the Atlantic basin.  This morning, Tropical Storm Ana was announced.  This afternoon, Tropical Storm Bill was announced.  They are a couple of days’ distance apart in the middle of the Atlantic tonight.  I was thinking Tropical Depression Three was likely a Tropical Storm this morning, but the NHC prudently waited for additional evidence to declare it as such.

Tropical Storm Ana’s vitals are:  Center located at 14.4N, 50.4W; moving W @ 17mph; maximum sustained winds of 40mph; minimum central pressure of 1004mb.

Nothing to get terribly excited about, Ana is a healthy storm that is likely to get stronger as the days go by.  Ana’s official track keeps getting pushed to the south, resulting in major differences in the 120-hr forecast location.  As of tonight, the models place her anywhere from Southern Florida to south of Cuba – a significant difference to be sure.  Her movement over the next 12-48 hours will have a major impact on where she eventually moves.  Her official track forecast puts her over Hispanola Tuesday afternoon, across northern Cuba Wednesday and into the Gulf of Mexico Thursday afternoon.  That official track forecast is to the northern edge of the model solution envelope.  Moving over Hispanola and in close proximity to Cuba would likely keep the storm much weaker than if she were to remain over open waters, of course.  Every model solution does put Ana in the Gulf of Mexico in the extended forecast (>5 days).  With a great deal of heat content available, that would allow whatever was left of Ana to likely restrengthen and threaten Gulf Coast communities.  Stay tuned.

Tropical Storm Bill’s vitals are:  Center located at 11.3N, 35.2W; moving WSW @ 16mph; maximum sustained winds of 40mph; minimum central pressure of 1004mb.

Bill didn’t move to the southwest quite as much as the official center designation suggests.  Rather, the dominant central low skipped around a bit over the past day.  This low pressure center is well established, giving the operational folks and the modelers a much better idea of his current state and anticipated future behavior.

T.S. Bill is forecasted to keep moving around the subtropical ridge to his north, which would mean a more westerly, then northwesterly course in a couple of days’ time.  There will be plenty of ocean heat content ahead of him on the official track forecast and minimal shear, so Bill is forecasted to be a Category 2 storm in 5 days’ time.

T.S. Bill’s official track forecast still keeps him to the northeast/north of the northern Windward Islands 5 days from now.  Any alteration in the ridge’s strength and position will of course alter that track.  All of the models’ track forecasts indicate this behavior except for the UKMET, which keeps Bill further south and moves him over the Windward Islands by Day 4.  If the remainder of the models are more correct, it is liklier that Bill will remain out to sea, curving around the west periphery of the ridge away from the U.S. coast.


As I did last season, I will place updated information at the top of posts.  My earlier write-up can be found below.

New model guidance is out for Tropical Storm Ana.  In the official 3-5 day period, she is now forecasted to skirt the northern coast of Hispanola before continuing to the northwest and potentially affecting southeastern Florida.  That’s right: there is the possibility that T.S. Ana could make landfall over Florida Thursday.

The GFS and UKMET are two models that contribute to this official forecast.  They keep Ana to the north of the Caribbean Islands.  The GFDL models diverge from this solution and keep Ana to the south of Hispanola, over Jamaica and to the south of Cuba.  The GFS and UKMET have perfomed well so far in this very young season.

And Tropical Depression Three has officially formed.  It was interesting when I wrote the initial post a few hours ago that the NHC hadn’t upgraded the system yet –  probably because they needed additional satellite confirmation of wind speed and organization.  They got it.  T.D. Three’s vitals are as follows:

11.5N, 34.0W; moving WSW @ 17mph; maximum sustained winds of 35mph; central pressure ~1006mb.

T.D. Three is forecasted to move in a similar path that T.D.2/T.S. Ana has taken: generally westerly for the next day or so, then a gradual turn toward the northwest thereafter as a mid-/upper-level trough passes by to their north.  Thursday could find T.D. Three north of the Windward Islands.  It has a lot of ocean to cover before potentially affecting any land.  Between now and then, T.D. Three also has to contend with any cold water that T.S. Ana upwells as she makes her way across the basin.  Lots of action to come, I think.


Well, the 2009 Atlantic Hurricane season has taken an interesting turn.  Last week, a tropical wave exited Africa near the Cape Verdes Islands.  It developed in Tropical Depression Two, but was held back from further development by dry, stable Saharan air to its north and west; high wind shear and relatively low water temperatures.  It decayed to a remnant low 36 hours ago and looked to slide into history.  Then, come convection popped up near the low’s center and the storm slowly matured yesterday.  It was forecasted to develop back into a Tropical Depression today if conditions held favorable.  Well, they’ve held favorable.  The system redeveloped into a Tropical Depression, then strengthened further into the Atlantic’s first named storm of 2009: Tropical Storm Ana.  Here are her vitals:

Center located near 14.6N, 46.8W; moving W @ 16mph; maximum sustained winds of 40mph; minimum central pressure 1005mb.

Ana is an extremely small storm, a result of the lack of development as she fought off the dry air, wind shear and low water temperatures.  Her environment should result in additional size development over the next couple of days, though she isn’t likely to grow to be a very large storm at her peak.

Ana’s official track forecast takes her over the northern-most Windward Islands Monday morning, and passing north of Puerto Rico by Tuesday morning.  I think it is likely she will be a Hurricane by Monday/Tuesday, but the official forecast keeps her at Tropical Storm strength through that period for now.

Invest-90 remains off to Ana’s east and the NHC has kept a >50% chance of tropical formation through the next 48 hours.  Like Ana, Invest-90 is going to pass over waters that are warmer in the next couple of days.  Combined with relativey weak wind shear, development of this system is highly likely.


One thought on “Tropical Weather Update 8/15/09

  1. Pingback: Atlantic Tropical Weather Update 8/16/09 #1 « Weatherdem’s Weblog

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