Lesser Antilles may see a tropical storm late this week

By: Dr. Jeff Masters , 13:51:GMT den 01. august 2010

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A concentrated area of intense thunderstorms near 9N 36W, midway between Africa and the Lesser Antilles Islands, has become more organized this morning. NHC has labeled this system Invest 91L, and is giving it a 60% chance of developing into a tropical depression by 8am Tuesday. Wind shear is a moderate 10 - 20 knots, sea surface temperatures are at record highs, and the dust and dry air of the Saharan Air Layer (SAL) are far enough to the north of 91L to potentially allow further development. The main negatives for development are the current phase of the Madden-Julian Oscillation, which favors downward motion over the tropical Atlantic, and the fact that 91L is too close to the Equator to take much advantage of the Earth's spin to get spinning. Last night's ASCAT pass of the region did show a large region of westerly winds south of 91L, indicating that a surface circulation is trying to form. Satellite imagery shows that the intensity and areal extent of 91L's heavy thunderstorms is increasing. However, there are no signs of a surface circulation, and low-level spiral bands and upper-level outflow are not apparent yet.

Forecast for 91L
There is strong model support for 91L developing into a tropical depression. The GFS, ECMWF, NOGAPS, and UKMET models all develop 91L into a tropical depression by Tuesday or Wednesday. A west to west-northwest motion is predicted, and residents of the Lesser Antilles Islands should anticipate the possibility of a tropical storm arriving in the islands as early as Thursday. It is possible that 91L would pass northeast of the islands, as predicted by the UKMET model, and it is too early to speculate on which of the islands is at most risk. As the storm approaches the Lesser Antilles late this week, 91L will encounter a strong upper-level low pressure system centered north of Puerto Rico. This upper-level low will likely bring high levels of wind shear to the waters just north of Puerto Rico and Hispaniola. Thus, if 91L stays to the south, in the Caribbean, it is far more likely to attain hurricane status than if it pushes north of the Caribbean. As always, long range forecasts of this sort are speculative, and it is too early to reliably say what the long-term risks of 91L becoming a hurricane are. The latest intensity forecast from the SHIPS model shows 91L peaking in strength four days from now, then weakening as it encounters the high wind shear area north of Puerto Rico.


Figure 1. Morning satellite image of Invest 91L.

Next update
I'll have an update Monday morning.

Jeff Masters

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Quoting MiamiHurricanes09:
No renumber yet; no tropical depression at 5PM EDT. Now I'm out.


Interesting to note, there was never and still isn't a renumber from 97L (Bonnie)
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Quoting bappit:

I agree.
good news
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1524. Levi32
Quoting bappit:

Ummmm, I remember this discussion from a while back, and while I do not want to repeat that, I myself will not call a la nina a cold PDO.


No other way to explain why global temps cool when PDO is "cold" and warm when the PDO is "warm". Think Pinatubo....equator cooled....whole darn thing cooled. Same with La Nina.
Member Since: november 24, 2005 Posts: 635 Comments: 26686
1523. bappit
Quoting P451:


I don't know...Shear seems awfully high in it's path. Dry air could also be a problem as it gains latitude.

It looks to be in a healthy place right now but once it nears PR, unless something changes, it won't be by then.

I agree.
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Quoting bappit:

It is still disorganized. That's why it is not upgraded.


T-numbers do not indicate a disorganized system.
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1521. Levi32
Quoting bappit:
Interesting image, StormW.


I think the convection on the left is closer to the center of the upper level anticyclone. The convection on the right is closer to the lower level vorticity maximum. The system will get named for sure, I think, if the convection on the right moves under the anticyclone. Otherwise, the convection currently more nearly under the anticyclone center is going to compete with the convection to the east--which seems pretty significant with the shadows from the setting sun. I think those are the vortical hot towers I hear tell of.


Greatest 850mb vorticity is not under the convection on the right. CIMSS maps can be wrong when low-level clouds cannot be observed well by satellite. They fill in with numerical model data and it becomes unreliable. The CIMSS maps don't even agree with themselves sometimes.

Observe....the "Regular" 18z vort map:



And the rapid-scan one for 18z:



See the difference?

The "regular" map is more accurate in this case. The highest vorticity on that map is closer to the western area of convection and is near 11.5N, 35W on the CIMSS map, which may still be a little too far east, but it's better than the map you're looking at.
Member Since: november 24, 2005 Posts: 635 Comments: 26686
Good night, sleep tight.

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1517. bappit
Quoting Levi32:


Cooling the equator is a heck of a lot more significant than cooling the mid-latitudes. The negative PDO is colder than the positive PDO, hence the nicknames "cold PDO" and "warm PDO", respectively.

Ummmm, I remember this discussion from a while back, and while I do not want to repeat that, I myself will not call a la nina a cold PDO.
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1516. 7544
maybe td at dmax imo
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Member Since: september 30, 2005 Posts: 0 Comments: 1606
1513. bappit
Quoting MiamiHurricanes09:
It's about the ASCAT pass and how it only caught the SE semicricle of the circulation. Nothing major. But that might hint as to why they haven't upgraded it yet.

It is still disorganized. That's why it is not upgraded.
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If the area near 13n/80w had more real estate, it might have had a chance to develop.
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Quoting Hurricanes12:
Will 91L be initiated as a TD?


At 11 pm probably. Although.. imo we could see it at any time. My best guess is 35 mph pressure 1006 mb.
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1509. bappit
Interesting image, StormW.


I think the convection on the left is closer to the center of the upper level anticyclone. The convection on the right is closer to the lower level vorticity maximum. The system will get named for sure, I think, if the convection on the right moves under the anticyclone. Otherwise, the convection currently more nearly under the anticyclone center is going to compete with the convection to the east--which seems pretty significant with the shadows from the setting sun. I think those are the vortical hot towers I hear tell of.
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1507. 7544
Quoting xcool:
MiamiHurri .look very strong TS & MOVE WNW TO ME .



agree and larger in size by the hour
this will cover alot of land where ever it goes the way it looks now in size will cover the whole fla pinisuler lol but it does
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No renumber yet; no tropical depression at 5PM EDT. Now I'm out.
Member Since: september 2, 2009 Posts: 130 Comments: 21194
1505. xcool
MiamiHurri .look very strong TS & MOVE WNW TO ME .
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Will 91L be initiated as a TD?
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With the exception of Bonnie, it seems like our systems want to be big this year. We can most likely attribute that to the large build up of heat in the Atlantic.
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Quoting xcool:
Looks like a moderate TS there.
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1501. xcool
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1500. bappit
Quoting xcool:
Shear 36k 108 hr


18z SHIP

I think dry air would flood the circulation before then if a vortex develops and gains latitude.
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As far as I'm concerned 91L is already a TD approaching TS status, jmo. I think it will be our 1st major hurricane of the 2010 Season.
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2 ship readings suggest that 91L has a pressure of 1006mb in the S/SW quadrant. Be back later.

Member Since: september 2, 2009 Posts: 130 Comments: 21194
1497. Levi32
Quoting Snowlover123:


That's the 12Z GFS.


18z from yesterday, actually.
Member Since: november 24, 2005 Posts: 635 Comments: 26686


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Ever since I got on here and complained NO Rain...well, we finally got it...I don't undertstand everything on here, but appreciate the knowledge..I just wish I could get a pic on here..I have tried everything..
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Quoting BaltOCane:


That's the 12Z GFS.
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1489. Levi32
Quoting bappit:

Cold PDO seems like a misnomer. Looking at the plots here (http://jisao.washington.edu/pdo/)



it seems the only places that are cool in a cool phase are in the southern Pacific and up along the Alaskan coast south to maybe Vancouver. The "warm phase" seems a heck of a lot cooler overall.


Cooling the equator is a heck of a lot more significant than cooling the mid-latitudes. The negative PDO is colder than the positive PDO, hence the nicknames "cold PDO" and "warm PDO", respectively.
Member Since: november 24, 2005 Posts: 635 Comments: 26686
Tracks may more further east and loop.
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The wave axis energy is coming into the system from the northeast, and the southwest is firing convection for the first time in an outer band. The winds are probably beginning to increase at the center. Also, the size may be getting larger than expected earlier.
Member Since: september 30, 2005 Posts: 0 Comments: 1606
Quoting reedzone:
Funny thing is, JFV did not violate the terms of conditions, the admin is banning his posts for no apparent reasons because of his past.

Noticed that myself as well. Don't know anything much about JFV really as I am new. However, yesterday I got banned for a few hours until I wrote the real admin, and it was removed. I got banned for a picture of a flower with a raindrop on it. It appears that someone in here that considers themselves a little admin helper may have reported my picture because they got their feelings hurt from me laughing, and they thought that I was laughing at them, instead of just the situation in general. Quite frankly, I am happy when there aren't storms going on. I don't care for the destruction, at all. So perhaps this gives you an idea of what I am starting to find really goes on in here.
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1482. Dakster
Quoting rmbjoe1954:


It is too early to speculate on any CONUS threat.


If the future 'Colin' hits CONUS, I am sure that it will make landfall somewhere between Brownsville, Texas and Lubec, Maine.
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channel 10 said that we need to wait till 6:30 to find out....TS colin or TD...whats with that?
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Quoting Tazmanian:



wow this thing is geting strong


1006 mb isn't that strong, but some Depressions have had higher pressures.
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1479. bappit
Quoting StormW:


That and the Cold PDO.

Cold PDO seems like a misnomer. Looking at the plots here (http://jisao.washington.edu/pdo/)



it seems the only places that are cool in a cool phase are in the southern Pacific and up along the Alaskan coast south to maybe Vancouver. The "warm phase" seems a heck of a lot cooler overall.

Edit: the map on the left is labeled "warm phase" and the one on the right is the "cool phase".
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Quoting xcool:
Shear 36k 108 hr


18z SHIP
It goes down to 21 knots at 120 hours.
Member Since: september 2, 2009 Posts: 130 Comments: 21194
1477. xcool
Shear 36k 108 hr


18z SHIP
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.