Dr. Jeff Masters' WunderBlog

Lesser Antilles may see a tropical storm late this week
Posted by: Dr. Jeff Masters, 1:51 PM GMT on August 01, 2010 +3
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
Categories: Hurricane
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4051. AstroHurricane001 2:11 PM GMT on August 02, 2010    
Quoting ncforecaster:
Hey everyone...I only have just a minute to make a very quick post. In doing so, I just wanted to explain that the NHC doesn't typically designate a tropical disturbance as a TC until the convection consolidated around the COC has persisted for at least a period of 6 hours.

This has been the general rule of thumb since way back when I did my second student internship there in 1994. That is why it may appear to some that the NHC is too conservative in initiating TC advisories.

With the aforementioned in mind, the NHC may designate 91L as TD #4 by the 11 am EDT advisory.

Well, I gotta go-but want to wish each one of you a great rest of the day!:)


Darn it, by the it's going to have an eye.
Member Since: August 30, 2008 Posts: 8 Comments: 2811
4052. Chicklit 2:11 PM GMT on August 02, 2010    
Re: #4039 ...right. the P word.
Member Since: July 11, 2006 Posts: 14 Comments: 10253
4053. KEEPEROFTHEGATE (Mod) 2:11 PM GMT on August 02, 2010    
X
Member Since: July 15, 2006 Posts: 143 Comments: 40476
4054. oracle28 2:12 PM GMT on August 02, 2010    
Quoting WildWillyFL:


Are you going to believe the models or your lying eyes?


There ain't no way to hide your lying eyes.
Member Since: July 25, 2008 Posts: 0 Comments: 572
4055. GeoffreyWPB 2:12 PM GMT on August 02, 2010    
invest_RENUMBER_al912010_al042010.ren
Member Since: September 10, 2007 Posts: 0 Comments: 9117
4056. psuweathernewbie 2:12 PM GMT on August 02, 2010    
While most of the convection has dissipated with 91L, the convection it has left is over its coc, which means an organizing system is taking place. It would seem with the ASCAT pass Drakeon showed it is still rather disorganized which is a little strange. Last night the upper level winds CIMSS showed that the upper level anticyclone was strengthening over 91L. Also equatorward outflow channel is finding it hard to develop, but slowly and surely it is, as well as a potential poleward channel. Also it is far and above that ITCZ now and the convection is sustaining as well as creating its own convection which is great news for becoming a tropical cyclone soon.
4057. GetReal 2:12 PM GMT on August 02, 2010    




In difference to those early model runs, the steering looks more westward over the next 72 hours....
Member Since: July 4, 2005 Posts: 204 Comments: 8196
4058. scott39 2:13 PM GMT on August 02, 2010    
Quoting weatherguy03:
Latest Video Blog On Invest 91L
Thanks your video on invest 91L
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4059. cirrocumulus 2:17 PM GMT on August 02, 2010    
Member Since: September 30, 2005 Posts: 0 Comments: 1606
4060. truecajun 2:18 PM GMT on August 02, 2010    
so i was reading dr. m's update to a friend yesterday, and she wondered what the lesser antilles were. i'm sure most of us on here know the whole greater, lesser (leeward, windward), but I certainly didn't learn it in school. It wasn't until i was interested in storms that i figured it out. so in case any newbies are wondering....

The Greater and Lesser Antilles are south of the Bahamas. the Greater Antilles are made up of the "big islands" of Cuba, Jamaica, Hispaniola (Haiti and Dominican Republic), and Puerto Rico.

The "small islands" make up the Lesser Antilles. The Lesser Antilles are divided into the Leeward Islands (the northern half) and the Windward Islands (the southern half)

the prevailing winds are south, so that's how they got the name windward.
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4061. Chicklit 2:22 PM GMT on August 02, 2010    
Re #4059: Is the center moving SSE? Or are things just shifting around because there is no real center yet?
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4062. DaytonaBeachWatcher 2:24 PM GMT on August 02, 2010    
al four on the old navy site
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4063. StormChaser81 2:25 PM GMT on August 02, 2010    
Quoting Chicklit:
Re #4059: Is the center moving SSE?


Looks like the computer is having center location problems. Center is still west bound.
Member Since: August 11, 2008 Posts: 0 Comments: 2315
4065. DaytonaBeachWatcher 2:25 PM GMT on August 02, 2010    
FNMOC Satellite Data Tropical Cyclone Page

2010 Storms
All Active Year


Atlantic
04L.FOUR

East Pacific

Central Pacific
92C.INVEST

West Pacific
96W.INVEST

Indian Ocean

Southern Hemisphere

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4066. truecajun 2:25 PM GMT on August 02, 2010    
i think there is a new blog
Member Since: August 24, 2006 Posts: 0 Comments: 1848
4067. truecajun 2:26 PM GMT on August 02, 2010    
Quoting whs2012:
Can this storm make it into the gulf of mexico?


yes, it's definitely possible
Member Since: August 24, 2006 Posts: 0 Comments: 1848
4068. BadHurricane 2:27 PM GMT on August 02, 2010    
Quoting whs2012:
Can this storm make it into the gulf of mexico?


Of course it can!
Member Since: December 5, 2008 Posts: 0 Comments: 59
4069. Chicklit 2:28 PM GMT on August 02, 2010    
NEW BLOG
Member Since: July 11, 2006 Posts: 14 Comments: 10253
4070. StormChaser81 2:30 PM GMT on August 02, 2010    
Quoting whs2012:
Can this storm make it into the gulf of mexico?


Is water wet.

Anything is possible.

Wait 8 to 10 days and see what happens.
Member Since: August 11, 2008 Posts: 0 Comments: 2315
4071. cirrocumulus 2:34 PM GMT on August 02, 2010    
Holy smokes! Look at all the energy coming into 91L from the east. It looks like another batch of moisture reaching 32W.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
4072. Dakster 2:38 PM GMT on August 02, 2010    
I noticed that "TD4" is out... I also noticed that the NHC doesn't expect this to make it to Hurricane status... Everyone here agree?
Member Since: March 10, 2006 Posts: 0 Comments: 4932
4073. wunderkidcayman 3:14 PM GMT on August 02, 2010    
ok now we have TD4 now I just wait until its at 45-50W then see what the models say
Member Since: June 13, 2009 Posts: 2 Comments: 5406
4075. hydrus 5:29 PM GMT on August 02, 2010    
Quoting Dakster:
I noticed that "TD4" is out... I also noticed that the NHC doesn't expect this to make it to Hurricane status... Everyone here agree?
T.D. 4 will make hurricane status.jmo
Member Since: September 27, 2007 Posts: 1 Comments: 14252
4076. bammbamm2020 1:18 AM GMT on August 03, 2010    
Can someone tell me the origin of the word "Invest", as it relates to the name of an atmospheric disturbance?
4077. Laughingmomma 7:15 PM GMT on August 08, 2010    
I have a paid membership and got the same Norton warning about "unsafe site" related to (and I quote:) Threat Report
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About JeffMasters
Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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