Invest 91L more organized, but has little time to develop

By: Dr. Jeff Masters , 2:31 PM GMT on April 22, 2011

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Hurricane season is more than a month away, but we have a tropical disturbance (91L) typical of what one might see in June or November. 91L is spinning over the waters a few hundred miles south of Bermuda, and has improved considerably in organization since yesterday, thanks to a drop in wind shear. The latest SHIPS model output is showing shear of 40 - 55 knots over 91L, but shear analyses from the University of Wisconsin CIMSS group is showing lower shear values of 20 - 30 knots over the main circulation center and to 91L's north, where the heaviest thunderstorms are. The system has a warm core at low levels, but a trough of low pressure lies over the storm at upper levels, and this trough is pumping cold, dry air into 91L, making it not completely tropical. One characteristic of subtropical systems like 91L is the presence of the main band of heavy thunderstorms removed several hundred miles from the circulation center, and 91L fits that description. 91L has two centers of circulation competing to be dominant, and this competition is slowing the storm's development. The storm was headed north at 5 - 10 mph early this morning, but that motion has halted, and 91L appears to be moving more south-southwesterly now, away from Bermuda. Sea surface temperatures are 23°C, which are very cold for a tropical storm to form in, but could support development of a subtropical storm.

As 91L moves south today, shear will steadily rise, and the storm likely has only until Friday night before shear grows too high to permit development. NHC is giving 91L a 20% chance of developing into a subtropical or tropical depression, which is a reasonable forecast. There has been only been one 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 a harbinger of a active season ahead. Had this been going on in the Caribbean, that would be a different story.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L. Note the two centers of circulation competing to be dominant. I expect the northern center will become dominant.

Jeff Masters

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Just got word that the CIMSS website has been fixed. Here is the first image. Notice the 100kt windshear.


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Damage reported in the Maryland Heights area
Member Since: July 3, 2005 Posts: 419 Comments: 127377
Five Fingers claw at the ball?

And who's To say ''is that all?''

The hand that wields the haze, is that, witch does prophecy the days!"!
Member Since: January 21, 2011 Posts: 0 Comments: 2058
SEVERE WEATHER STATEMENT
NATIONAL WEATHER SERVICE ST LOUIS MO
823 PM CDT FRI APR 22 2011

...A TORNADO WARNING REMAINS IN EFFECT FOR NORTHEASTERN ST. LOUIS...
NORTHERN ST. LOUIS CITY...SOUTHWESTERN MADISON AND NORTHWESTERN ST.
CLAIR COUNTIES UNTIL 845 PM CDT...

...DAMAGING TORNADO MOVING INTO THE NORTHERN PART OF THE CITY OF ST.
LOUIS...

AT 820 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING A
CONFIRMED TORNADO. THIS TORNADO WAS LOCATED NEAR RIVERVIEW...MOVING
EAST AT 45 MPH.
Member Since: July 3, 2005 Posts: 419 Comments: 127377
XX/INV/91L
MARK
24.95N/63.76W
Member Since: July 15, 2006 Posts: 168 Comments: 53285
Wow, the Raleigh area news is full of hard-to-believe-but-true tornado experience stories. Personal belongings being discovered 60 miles away, bathtub rides, crawling out from under rubble unharmed. Amazing, and scary.

Raleigh, N.C. — The National Weather Service said Friday that the number of confirmed tornadoes that tore across central and eastern North Carolina last Saturday is up to 28, which breaks the state record for the size of a tornado outbreak.
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I have a bad feeling about this tornado. It is over highly populated area. Went over St Ann and now Fergusion. The Debris ball has gotten bigger.
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And the 'Lance,'
That pieces the Trance,
Is but a stance of what is to be Advanced?
The trace, its form embraced!
Is yet to be Traced?
Dare not,those, Who repose
Impose on this!
Air in Viginity Exposed!
Member Since: January 21, 2011 Posts: 0 Comments: 2058
Quoting CaicosRetiredSailor:
The Dry Dragon:



has 91L in its paw


The Dry Dragon has been sleeping on the lower 1/8th of the country for some time now. 91L has a tenacious attitude, hoping it will provide a shred instability for an 1/8 of the 1/8 needing water. Parts south of Lake O can't catch a break this year.
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Nice debris ball with the tornado near St Louis. Went right over Lambert Field.
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Quoting Patrap:
there's a invest?

lol

ya but no name so its not a count maybe re check post season o well better luck next time we await 92L next
Member Since: July 15, 2006 Posts: 168 Comments: 53285


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Quoting cat5hurricane:

You sure?
why not can't sing but can dance
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Just tesing!!
Jefe
Member Since: January 21, 2011 Posts: 0 Comments: 2058
Quoting Patrap:

How did you know I had been in London??



Why,,were "the men who stare at sheep" of course...




Family blog. Any new images of the activity in the Midwest. My sites aren't too useful tonight.
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Member Since: July 15, 2006 Posts: 168 Comments: 53285
Quoting PlazaRed:
Nothing can be concealed?


Except hidden criteria.
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Quoting StormTop5000:
and so it has begun...
the dance the dance in the wind
Member Since: July 15, 2006 Posts: 168 Comments: 53285
Nothing can be concealed?
Member Since: January 21, 2011 Posts: 0 Comments: 2058
Quoting PlazaRed:
Toche!!
Or whatever they say in the tounge of the Franks?
Why cant we have the invest a name? just to satisfy thye ''lusters, screamers, wanters, casters? and the odd irrate's!!
This is going to be one hell of a season, Hefe!
Que Sera,Sera!!


That is Jefe to you, por favor.
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Quoting StormTop5000:
...and so it begins...
bring it on
Member Since: July 15, 2006 Posts: 168 Comments: 53285
Quoting GeoffreyWPB:


I have never seen so much excitement over an Invest. Going to be quite a season on the blog.


We had bigger thunderstorms. If the NHC named every swirl in the Atlantic, we would have to go through 5 alphabets per season.
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and so it has begun...
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Toche!!
Or whatever they say in the tounge of the Franks?
Why cant we have the invest a name? just to satisfy thye ''lusters, screamers, wanters, casters? and the odd irrate's!!
This is going to be one hell of a season, Hefe!
Que Sera,Sera!!
Quoting Grothar:


How did you know I had been in London??
Member Since: January 21, 2011 Posts: 0 Comments: 2058
Quoting GeoffreyWPB:


I have never seen so much excitement over an Invest. Going to be quite a season on the blog.
yes yes it will and we know what happens when things get out of hand well lets say empty space lots and lots of empty space
Member Since: July 15, 2006 Posts: 168 Comments: 53285
there's a invest?

lol

Member Since: July 3, 2005 Posts: 419 Comments: 127377

How did you know I had been in London??



Why,,were "the men who stare at sheep" of course...


Member Since: July 3, 2005 Posts: 419 Comments: 127377
Quoting Grothar:


Hey, Geoff. Anything new happen while I was gone. Heard there was a big swirl in the Atlantic. Heard those NHC guys weren't doing their job again.


I have never seen so much excitement over an Invest. Going to be quite a season on the blog.
Member Since: September 10, 2007 Posts: 0 Comments: 10932
Quoting cat5hurricane:

Little late over in England, no?


How did you know I had been in London??
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Quoting hydrus:
I have to go ...I just wanted to say Wuzup Gro..!


Stay cool, hydrus. Drop back later if you can. Still stormy by you I see.
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Not good.

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...and so it begins...
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I have to go ...I just wanted to say Wuzup Gro..!
Member Since: September 27, 2007 Posts: 1 Comments: 20345

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About

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