Atlantic tropical disturbance 91L poses little threat

By: Dr. Jeff Masters , 2:15 PM GMT on April 21, 2011

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A tropical disturbance (91L) near 24N, 63W, midway between the Virgin Islands and Bermuda, is moving north-northwest at about 8 mph. The system's heavy thunderstorm activity has increased since yesterday, but 91L has an elongated and poorly-organized circulation, thanks to a hefty 80 knots of wind shear. The storm is over waters of 25°C, and these waters will cool to 24°C by Friday as the storm continues to the north-northwest. Before 91L reaches Bermuda, steering currents will reverse and force 91L to the south-southwest on Saturday, into a region of higher wind shear. The latest SHIPS model forecast predicts wind shear will drop to 50 knots over 91L by Friday, then increase again to 70 knots by Sunday. The high shear and relatively cool water temperatures will make it difficult for 91L to organize into a subtropical depression. I give 91L a 10% of becoming a subtropical depression. Climatology argues against 91L becoming the first named storm of the year; there has only been once named April storm in the Atlantic since 1851, Tropical Storm Ana of 2003. The formation of a tropical disturbance at this location this time of year is unusual, but is not necessarily a harbinger of a active season ahead.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L.

Jeff Masters

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Invest 91L

Based upon the latest RGB imagery, the center of this disturbance exists around 25.6 N and 63.0W. The disturbance appears to be undergoing gradual organization as thunderstorms have developed and becoming more co-located with the somewhat elongated circulation. Some decent low level inflow into the disturbance is noted to the southeast of the circulation center with a notable band with diffuse showers.

Recent enhanced IR imagery shows that convection has deepened within the past few hours in a tight cluster very near the circulation center. Also, you can clearly see that the circulation remains somewhat elongated in a near north-south orientation.

Water vapor imagery shows very well the struggles the disturbance continues to encounter in attempts to develop further. An upper level low can be clearly seen very near to the west. This upper level low has been imparting some hostile wind shear as noted by the immense cloud debris spreading northward. In addition, dry air continues to get pumped into the disturbance from the south by the clockwise flow around the upper low. The disturbance has been able to sustain some due to its connection with deeper moisture well to the south as explained earlier with a notable inflow band.

All in all, Invest 91L has been and continues to attempt slow organization and may have a chance, albeit not high, to become something more in the next 24 hours.
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Quoting EYEStoSEA:


Keeper....your thoughts on 91L ?
it should flare up over the next 24 then weaken as it drifts sw during flare up then meanders next week as a area of showers over the bahamas

nothing more nothing less we shall see
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Would be nice is South Floida would get some rain from it, at least.
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ugh


we get 91L and we get the downcaster and wishcaster that comes with it
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Quoting KrippleCreekFerry:
Hurricane seasons are cyclical. This year should be busy in the Pacific and relatively quiet in the Atlantic. I feel comfortable about that.



have you even look at the hurricane season forcast # its goign too be vary busy this year
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383. 7544
Link dont know if its new but here sammy

also looks fla bound if it holds on
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Quoting KEEPEROFTHEGATE:
XX/INV/91L
MARK
25.25N/64.24W


Keeper....your thoughts on 91L ?
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XX/INV/91L
MARK
25.25N/64.24W
Member Since: July 15, 2006 Posts: 169 Comments: 53296
It's too bad the blog can't be this civil during the actual season!
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Quoting 7544:
91l is looking better the last few hours what u think can it surprise us and still hold on ?
It's window of opportunity is tonight and the day tomorrow. After that, (even more) hostile conditions should spell it's demise. A pesky little disturbance in April, nonetheless!
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376. 7544
91l is looking better the last few hours what u think can it surprise us and still hold on ?
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Quoting AllStar17:

Indeed!


Working on some quick analysis on this disturbance.
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Quoting cchsweatherman:
Invest 91L definitely becoming an interesting disturbance to track.
Indeed!
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Invest 91L definitely becoming an interesting disturbance to track.
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372. JRRP
Member Since: August 16, 2007 Posts: 0 Comments: 5482
Quoting JRRP:


Ridiculous how high the SOI still is even after the tropical Pacific has warmed all the way to neutral levels.
Member Since: November 24, 2005 Posts: 635 Comments: 26564
370. JRRP
Member Since: August 16, 2007 Posts: 0 Comments: 5482
The ASCAT pass on 91L reveals the circulation is very much elongated meridionally which matches the satellite observations. Gonna need to tighten up a bit in addition to organizing convection.
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If I was the forecaster on duty 91L would be around 40-60 percent and if it continued to maintain or improve by tomorrow I'd probably name it. I've seen worse looking subtropical and even tropical storms. If this was during cane season I bet it would be named but it's up to the forecasters and I've noticed subtropical has been in recent years having to be more tropical in the broad range before getting a name than before.
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It looks like there are some above normal SSTs present around and in front of 91Ls potential forecast track. The most notable just south of the Florida peninsula.
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Quoting Drakoen:

I think it is important that all meteorologist have some understanding of computer programming at least as it pertains to meteorology. Fortran is good, even though it is old, at least from what I have heard from others.


It's the oldest and I hated it but looking back it's been so much help with learning things like matlab and GARP. Both very useful and GARP is actually fairly fun to use.
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Quoting HurricaneKing:


Heck Im at North Carolina State University and for our met program we had to take a computer programing class. The year I had to take it there was only fortran or Java being offered. I took fortran. This year they are considering adding some others. So it's required.
I think it is important that all meteorologist have some understanding of computer programming at least as it pertains to meteorology. Fortran is good, even though it is old, at least from what I have heard from others.
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Quoting Drakoen:
91L's structure is steadily improving. It needs to maintain this level of organization or even increase for a couple more hours to be considered a subtropical storm along with an ASCAT or WindSat pass confirming a closed low level circulation.


See #360 for the 2nd part.
Member Since: November 24, 2005 Posts: 635 Comments: 26564
91L just had a large blow of convection right over the center, can really tell that the Upper level low is nearly on top of it.
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91L's structure is steadily improving. It needs to maintain this level of organization or even increase for a couple more hours to be considered a subtropical storm along with an ASCAT or WindSat pass confirming a closed low level circulation.
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Latest ASCAT only got a partial pass, but shows that there is a field of weak westerlies south of the center. There is also still an elongated trough evident in the wind field.

Member Since: November 24, 2005 Posts: 635 Comments: 26564
Quoting Drakoen:

That is incorrect. You are going to Florida State University and i'm assuming you have seen the FSU Meteorology Handbook. You will be required to take Meterological Computations (MET3220C): "This course covers the solution of meteorological problems using
computer and statistical programs; distributions of meteorological
variables; meteorological programming." You will be introduced to Fortran.


Heck Im at North Carolina State University and for our met program we had to take a computer programing class. The year I had to take it there was only fortran or Java being offered. I took fortran. This year they are considering adding some others. So it's required.
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Quoting j2008:
91 is getting impressive, I think NHC needs to up the percentage for development.


Forget that, we should have Subtropical Storm Arlene.
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357. j2008
91 is getting impressive, I think NHC needs to up the percentage for development.
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Quoting Jedkins01:


But scientists are expected or required to be super computer savvy. Don't get me wrong, as someone going to school to be a scientist, much of what I do involves computers, but not all of of us are Linux lovers. I really could care less about computer programming and getting into the deeper computer skills. I'm very thankful for it, and all power to those who love computer science, but it bores me to death. I would do whatever I could to avoid using Linux.

Degrees in meteorology don't require any classes in computer programming or deeper computer skills. I mean, there are grad programs for guys who want to do computer model programming, but its not required as a need, thankfully! I'll take theoretical calculus and differential equations over computer programming any day!
That is incorrect. You are going to Florida State University and i'm assuming you have seen the FSU Meteorology Handbook. You will be required to take Meterological Computations (MET3220C): "This course covers the solution of meteorological problems using
computer and statistical programs; distributions of meteorological
variables; meteorological programming." You will be introduced to Fortran.
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A band of showers just passed over us here in Nederland about a half hour ago. I checked the radar to see how extensive it was and I see a cell marked as possibly tornadic just south of Greeley in Weld County:



No warnings (not even for severe thunderstorms) have been issued...
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354. JRRP
where is 91L ???
jejeje
Member Since: August 16, 2007 Posts: 0 Comments: 5482
39 days remain
Member Since: July 15, 2006 Posts: 169 Comments: 53296
XX/INV/91L
MARK
25.25N/64.24W
Member Since: July 15, 2006 Posts: 169 Comments: 53296
The current structure of 91L on satellite imagery reminds me somewhat of Otto's structure last year while it was a weak subtropical storm.
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Quoting Levi32:
Linux is great though because you can do so much more with it. I recently installed Ubuntu and now I have toys that can post the SSD flash loops as gifs here on the blog:

New t-storm cluster going off on the NE side of the center.



Looks like Subtropical Storm Olga, they should classify it now, banding becoming better established with some strong convection firing near the center.
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Quoting Stormchaser2007:


Did you get GEMPAK installed yet? Thats a must have for Linux users...

On a side note, 91L's circulation appears elongated.



Once upon a time I tried it and I remember the manual installation was a nightmare that never ended, but I may try it again.
Member Since: November 24, 2005 Posts: 635 Comments: 26564
Quoting sunlinepr:


I think the same.... There is a lot of energy and WV in the atmosphere, Africa waves look impressive... and conditions are moving to what seems an early start...


Although it is important to remember that the amount of energy and water vapor in the atmosphere is far less than in 2005, because we are in the 2nd year of a La Nina which has cooled and dried out the tropical atmosphere relative to a neutral or El Nino state, which we had in 2005.
Member Since: November 24, 2005 Posts: 635 Comments: 26564
Quoting Levi32:
Linux is great though because you can do so much more with it. I recently installed Ubuntu and now I have toys that can post the SSD flash loops as gifs here on the blog:

New t-storm cluster going off on the NE side of the center.




Did you get GEMPAK installed yet? Thats a must have for Linux users...

On a side note, 91L's circulation appears elongated.

Member Since: June 9, 2007 Posts: 4 Comments: 15790
Quoting AllStar17:

Seems to be consolidating a bit?  The loops help illustrate your great analysis very well!  Nice to see you again!


Good to see you as well :)
Member Since: November 24, 2005 Posts: 635 Comments: 26564
Quoting stormpetrol:
Personally I'm beginning to think this year might be close to 2005, i know an early start has no bearing on what a season might bring or vice versa , but 91L is impressive for April.
I think we might get a named system before June 1 anyway, 91L or not. Everyone Have a happy and safe Easter



I think the same.... There is a lot of energy and WV in the atmosphere, Africa waves look impressive... and conditions are moving to what seems an early start...
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Quoting Levi32:
Linux is great though because you can do so much more with it. I recently installed Ubuntu and now I have toys that can post the SSD flash loops as gifs here on the blog:

New t-storm cluster going off on the NE side of the center.

Seems to be consolidating a bit?  The loops help illustrate your great analysis very well!  Nice to see you again!
Member Since: Posts: Comments:
Linux is great though because you can do so much more with it. I recently installed Ubuntu and now I have toys that can post the SSD flash loops as gifs here on the blog:

New t-storm cluster going off on the NE side of the center.

Member Since: November 24, 2005 Posts: 635 Comments: 26564

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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.