Lesser Antilles may see a tropical storm late this week

By: Dr. Jeff Masters , 1:51 PM GMT on August 01, 2010

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 cirrocumulus:
The position of 91L now looks like it is at 10.5N and 36.0W.


Yes I was about to mention that. It also appears to be moving more WNW.
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With the SST's as they are, if or when this breaks away, likely intensity may ramp up really fast.

That could be good though, as the weaker it satys, the lower it stays, the faster it strengthens, the higher it will go.

But, depends where it begins the ramp up.

Personally, with the ridiculous SST's, I doubt this will spend much time as a storm or CAT 1, be up to 2 or 3 really fast.

Remember though, I am a layperson, not a met.
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Quoting Hurricanes101:


I think it stays at 60%


I agree.
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AL, 91, 2010080112, , BEST, 0, 100N, 363W, 25, 1007, LO


will giveing that we have winds up too 25kt and mb down too 1007mbs and we have a T # up too 1.5/2.0

am going with %80 on the next two
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Quoting cloudy0day:
Levi

Thanks for the tidbit, I learned about effects of "kinks" in the ITCZ today.


You're welcome :)
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Hey yall, what's the deal with them changing it from 90L to 91L? Was 90L just the big spread out low in addition to the wave, and now that it's more compact and better organized they decided to rename it 91L?
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Quoting Levi32:


T8.0, 170 knots, or 195mph, the intensity of Hurricane Mitch, which I believe had the highest winds in the Atlantic.


I know Katrina got to 175 mph at some point
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Quoting Levi32:


Well as I said in the video, the behavior of the TUTT will be the determining factor on intensity, and if it's weak enough that the ridge can bust through and split the TUTT into cut-off lows, this may have a chance to maintain intensity or strengthen further after the islands, but it could just as easily weaken if the TUTT holds its own. At this point I'm not yet sure what the TUTT will do, and it's a long way off so things can change. We'll just have to monitor that as this approaches the islands to get a better feel for what may happen.
thanks
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Quoting MiamiHurricanes09:
I'm going with 70%-80%.
60%
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Again, this storm it too far out with far too many variables to predict where it may end up going. There is the ITCZ and several high pressure systems to deal with.

With that said there appears to be a developing LLC; I think we'll see a tropical depression within 24 hours if not later today and a tropical storm within 48-72 hours.
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be carefull on the TWO NHC screwed it up last nite. You had to click on the circle to see the update
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Im going with IKE .... 60%...No reason to go higher I dont think ...Not at this point ... I think by tonight we could see 80% though.
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Remember that the models will not properly initialise the system until it forms, until then the models will not have a good grasp of intensity nor track.

Better to watch carefully, pay attention to SST's wind shear etc.

Common sesne tells me that at 36 W already and so low, this has more of an Ivan track.
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Quoting scott39:
Thanks Levi, You said we could see a weakening or strengthing depending on certain factors once 91L heads towards SE Coast. What are your thoughts on re-strengthing after possibly weakening?


Well as I said in the video, the behavior of the TUTT will be the determining factor on intensity, and if it's weak enough that the ridge can bust through and split the TUTT into cut-off lows, this may have a chance to maintain intensity or strengthen further after the islands, but it could just as easily weaken if the TUTT holds its own. At this point I'm not yet sure what the TUTT will do, and it's a long way off so things can change. We'll just have to monitor that as this approaches the islands to get a better feel for what may happen.
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Quoting AstroHurricane001:
Some analog storms for future Colin:

Georges 1998...

Ike 2008...

Hanna 2008...

Frances 2004...

Fay 2008...

Jeanne 2004...

Rita 2005...

Karen 2007...

Kate 1985...

Fran 1996...

Betsy 1965...


You forgot Andrew 1992! LOL
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B
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folks this is what we've been waiting for, 91L is the real deal.
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Choice E 50%
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T-minus 4 minutes until deployment of F5 keys.
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Quoting weatherman12345:
POOL TIME!
2PM TWO 91L
A. 60%
B. 70%
C. 80%
D. 90%

C.
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Levi

Thanks for the tidbit, I learned about effects of "kinks" in the ITCZ today.
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Quoting Tazmanian:



B or C


I think it stays at 60%
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ok so what time did we see 91L last night
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Quoting Levi32:
Good morning all.

Blog Update:

Tropical Tidbit for Sunday, August 1st, with Video
Thanks Levi, You said we could see a weakening or strengthing depending on certain factors once 91L heads towards SE Coast. What are your thoughts on re-strengthing after possibly weakening?
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I'll be conservative and say 60%. Wouldn't be a terrible shocker to see 70%.
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Quoting weatherman12345:
POOL TIME!
2PM TWO 91L
A. 60%
B. 70%
C. 80%
D. 90%




B or C
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Quoting stormwatcherCI:
POSSIBLE NOAA G-IV
SURVEILLANCE MISSION OUT OF TISX AROUND CENTRAL
ATLANTIC SYSTEM FOR 04/0000Z IF IT DEVELOPS.




cool
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Quoting AussieStorm:

I'll go 70-80%, see ya's in the morning/evening



that seems about right
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Quoting IKE:
I'll go with 60% on the next TWO.....

I'm going with 70%-80%.
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642. IKE
Quoting AussieStorm:

I'll go 70-80%, see ya's in the morning/evening


Good night.
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Quoting MiamiHurricanes09:
Good morning Levi (actually afternoon, lol)! Great update.


Morning MH09 lol. Thanks.
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Quoting Tazmanian:
when the 1st HH fight for 91L
POSSIBLE NOAA G-IV
SURVEILLANCE MISSION OUT OF TISX AROUND CENTRAL
ATLANTIC SYSTEM FOR 04/0000Z IF IT DEVELOPS.

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Quoting will45:


has to reach 55W first



ok
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638. IKE
Quoting Tazmanian:



it is at 60%


I know.
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Quoting Levi32:
Good morning all.

Blog Update:

Tropical Tidbit for Sunday, August 1st, with Video
Good morning Levi (actually afternoon, lol)! Great update.
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636. HadesGodWyvern (Mod)
not until either Wednesday or Thursday, Taz if it develops.
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Quoting IKE:
I'll go with 60% on the next TWO.....


I'll go 70-80%, see ya's in the morning/evening
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Quoting Tazmanian:
when the 1st HH fight for 91L


has to reach 55W first
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Quoting IKE:
I'll go with 60% on the next TWO.....




it is at 60%
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The ascat data puts the invest at about 11 north.

ascending



descending


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Quoting will45:


posibally seeing a weakness to its north



ture but this is about a week out or so
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630. IKE
I'll go with 60% on the next TWO.....

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when the 1st HH fight for 91L
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Quoting weatherman12345:

why is it curving norhtward on the last clip


posibally seeing a weakness to its north
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627. IKE
Not much on the latest NOGAPS
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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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