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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WTF?Only 20%?
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Morning Everybody:-

Well bearing in mind the day of the year and the pronunciations of the 'doubters,' plus the references to deceased parrots, it looks like,' Resurrection is the order of the day.

I wonder if this quote from some news Crowned whether type chappie might be a little premature?

Crownweather......

''This will likely be the last discussion concerning Invest 91-L.''
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488. MahFL
There seems to ne a couple of competeing coc's on 91L. Also the one likely to win seems to be drifting South.
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XX/INV/91L
MARK
25.55N/64.14W
Member Since: July 15, 2006 Posts: 178 Comments: 56141
Definitely a needle in a shear stack that's for sure.
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SPECIAL TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
850 AM EDT FRI APR 22 2011

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

SHOWER ACTIVITY ASSOCIATED WITH A NON-TROPICAL LOW CENTERED ABOUT
385 MILES SOUTH-SOUTHEAST OF BERMUDA IS A LITTLE BETTER ORGANIZED
THIS MORNING...ALTHOUGH WINDS NEAR THE SYSTEM APPEAR TO HAVE
DECREASED BELOW GALE FORCE. THERE IS A LOW CHANCE...20 PERCENT...OF
THIS SYSTEM DEVELOPING INTO A SUBTROPICAL OR TROPICAL CYCLONE
WITHIN THE NEXT 12 TO 24 HOURS BEFORE ENVIRONMENTAL CONDITIONS
BECOME HIGHLY UNFAVORABLE FOR ADDITIONAL DEVELOPMENT. THE LOW IS
MOVING NORTHWARD AT 10 MPH BUT SHOULD SLOW DOWN LATER TODAY.
ADDITIONAL INFORMATION ON THIS LOW CAN BE FOUND IN HIGH SEAS
FORECASTS ISSUED BY THE NHC/TROPICAL ANALYSIS AND FORECAST
BRANCH...UNDER AWIPS HEADER NFDHSFAT2 AND WMO HEADER FZNT02 KNHC.
ADDITIONAL SPECIAL TROPICAL WEATHER OUTLOOKS WILL BE ISSUED AS
NECESSARY.
Member Since: September 10, 2007 Posts: 0 Comments: 11557
000
ABNT20 KNHC 221247
TWOAT

SPECIAL TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
850 AM EDT FRI APR 22 2011

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

SHOWER ACTIVITY ASSOCIATED WITH A NON-TROPICAL LOW CENTERED ABOUT
385 MILES SOUTH-SOUTHEAST OF BERMUDA IS A LITTLE BETTER ORGANIZED
THIS MORNING...ALTHOUGH WINDS NEAR THE SYSTEM APPEAR TO HAVE
DECREASED BELOW GALE FORCE. THERE IS A LOW CHANCE...20 PERCENT...OF
THIS SYSTEM DEVELOPING INTO A SUBTROPICAL OR TROPICAL CYCLONE
WITHIN THE NEXT 12 TO 24 HOURS BEFORE ENVIRONMENTAL CONDITIONS
BECOME HIGHLY UNFAVORABLE FOR ADDITIONAL DEVELOPMENT. THE LOW IS
MOVING NORTHWARD AT 10 MPH BUT SHOULD SLOW DOWN LATER TODAY.
ADDITIONAL INFORMATION ON THIS LOW CAN BE FOUND IN HIGH SEAS
FORECASTS ISSUED BY THE NHC/TROPICAL ANALYSIS AND FORECAST
BRANCH...UNDER AWIPS HEADER NFDHSFAT2 AND WMO HEADER FZNT02 KNHC.
ADDITIONAL SPECIAL TROPICAL WEATHER OUTLOOKS WILL BE ISSUED AS
NECESSARY.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.
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Quoting RastaSteve:


Yeah, after I read Crown Wx's forecast for this invest I then ran the viz on it and was really surprised at how much organized it got overnight. Very solid circulation right now. Will be interesting to see if some of this onvection wraps around the COC now that is colocated under the ULL.



Definitely looking very spry this morning, best yet IMO. If it doesn't get an offical tag today, I'd be surprised.
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Quoting Levi32:


Keep in mind that most convection associated with subtropical storms is a direct result of instability/divergence from the ULL. You can't ask for fully tropically supported convection.

Correct if I am wrong, but is it not more conducive for a tropical system to be associated with an UL high not an ULL? An ULL I think would hinder convection.
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NVM
Member Since: November 8, 2009 Posts: 4 Comments: 13805
12Z update, pressure down, winds down.

AL 91 2011042200 BEST 0 253N 630W 35 1009 LO
AL 91 2011042206 BEST 0 261N 629W 35 1008 LO
AL 91 2011042212 BEST 0 269N 629W 30 1007 LO
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April and Invest should not be used in the same sentence Teddy :)

Member Since: November 1, 2004 Posts: 25 Comments: 8937
Well organized, especially for an April invest.

This should be 30-50% on the TWO IMO.

Member Since: July 8, 2005 Posts: 259 Comments: 24579
Good Morning everyone and Happy Earth Day
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List of Past and Present:-

Dead Parrot Casters.To be Copiled. To include point of launching and projected site of landing correct to no less than 3 decimal places in both lon. and lat. {No 'Squashed Spiders, please.}

This, Deceased parrot in question is currently located at:-

26.1N 63.1W
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469. IKE
Tallahassee...panhandle of Florida.....

LONG TERM (SUNDAY NIGHT-FRIDAY)...
DEEP LAYER RIDGE IN PLACE OVER THE SERN REGION WILL BEGIN TO WEAKEN
AND SHIFT EAST AS A BROAD UPPER TROUGH AND SURFACE COLD FRONT
APPROACH FROM THE WEST THROUGH THE MID-WEEK PERIOD. THE MODELS ARE
ALL IN DECENT AGREEMENT WITH THIS PATTERN TRANSITION AND SHOW THE
COLD FRONT CONTINUING EAST AND THROUGH THE LOCAL AREA WEDNESDAY
NIGHT INTO THURSDAY BRINGING ANOTHER ROUND OF SHOWERS/TSTMS ACROSS
THE REGION. OTHERWISE...CONDITIONS WILL REMAIN QUIET THROUGH THE
FIRST COUPLE OF DAYS OF THE UPCOMING WEEK AS HIGH PRESSURE REMAINS
IN CONTROL. THE LATEST GUIDANCE INDICATES TEMPERATURES REMAINING A
FEW DEGREES ABOVE AVERAGE THROUGH MUCH OF THE PERIOD WITH OVERNIGHT
LOWS ONLY DROPPING INTO THE LOW TO MID 60S AND HIGHS REACHING THE
UPPER 80S EACH AFTERNOON. SLIGHTLY COOLER TEMPERATURES WILL BE
POSSIBLE IN THE WAKE OF THE FRONTAL BOUNDARY THURSDAY NIGHT INTO
FRIDAY.
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Quoting MahFL:
"This parrot is dead".......next !


"It is an ex-Parrot!"
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Miami NWS Discussion

LOOKING INTO NEXT WEEK, AS THE INVERTED TROUGH
CROSSES S FL EARLY IN THE WEEK MOISTURE AND INSTABILITY WILL BE ON
THE INCREASE AND WILL ALSO MAINTAIN THE CHANCE OF SHRA/TSRA. BOTH
THE GFS AND ECMWF SHOW THE SAME TREND AND BRING THE TROUGH ACROSS
THE REGION MONDAY/TUESDAY TIME FRAME WITH ONLY MODEST AMOUNT OF
MOISTURE INCREASE. CHANCES OF RAINFALL MAY CHANGE IN TIME AS WE
GET CLOSER TO THE EVENT BUT FOR NOW IT DOES NOT LOOK TO BE A
SUBSTANTIAL AMOUNT OF RAINFALL BUT ANYTHING WILL HELP THE DRY
CONDITIONS.
Member Since: September 10, 2007 Posts: 0 Comments: 11557
465. MahFL
"This parrot is dead".......next !
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464. IKE
Crownweather......

Friday Morning Discussion On Invest 91-L Located To The North-Northeast of Puerto Rico

Rob Lightbown on April 22, 2011, 6:08 am


A low pressure system, designated
Invest 91-L, was located to the north-northeast of Puerto Rico near 25
North Latitude, 64.8 West Longitude this morning. Analysis of shear this
morning shows that this system is under 30 to 35 knots of shear and is
over ocean water temperatures of 75 Degrees Fahrenheit. The convection
that you are seeing on satellite imagery is a result of an upper level
low pressure system interacting with Invest 91-L. In fact, in my eyes
the %u201Cfeeder band%u201D you are seeing to the south of this system may
actually be a pseudo cold front developing and this system remains very
much cold-cored in nature.
Wind shear is expected to increase significantly over Invest 91-L by
tonight and especially this weekend and there should be little left of
Invest 91-L by later this weekend into early next week.
Given the fact that ocean temperatures are marginal at best, plus the
forecast of significant wind shear to affect this system within 18 to
24 hours, I do not expect Invest 91-L to be classified as a subtropical
storm.
Whatever is left of Invest 91-L will turn to the southwest tonight
into Saturday and is forecast to increase the likelihood of showers and
thunderstorms for the Bahamas on Sunday and maybe south and central
Florida for Monday and Tuesday.
This will likely be the last discussion concerning Invest 91-L.
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the COC of 91L seems to be now located at 26.1N 63.1W
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Just a reminder....

I've made a blog entry, If anyone is interested. Feel free to write your location on this blog. Just so as to keep track of where people are in the event of severe weather/hurricane.

Just go here
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What are some other good weather programs?

...besides SimuAWIPS
Member Since: August 24, 2010 Posts: 19 Comments: 4358
Quoting xcool:
SimuAWIPS very good weather program ;))))


I like it for Severe Weather but, how do you use it in the Atlantic. I can't see how to use it in the Atlantic!
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...actually, it can't be from that same front that brought the NC tornadoes...91L's parent frontal boundary was already in the mid-Atlantic April 17...tornadoes happened on the 16th...fronts don't travel that fast...
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Quoting PolishHurrMaster:

No,it wasn't,but Reedzone said that was Arlene.If yes,we've got Bret now(it's not named),and waiting for Cindy forecasted in mid-May.We could have three off-season storms in 2011!


LOL...This is nuts! Does anyone have images of 90L they saved or something? Man, I can't believe I missed that one....
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Quoting NCHurricane2009:
http://www.nhc.noaa.gov/tafb_latest/gehov2latest. gi f

Checked hovmoller diagram, looks like 91L frontal in origin, but I don't know what date this front came off the US east coast....

Does anyone know which front 91L is traced to? Is it the same front that brought the violent tornadoes in eastern NC (that would be so weird for me)....


oops...
Here is the correct link to the hovmoller....

http://www.nhc.noaa.gov/tafb_latest/gehov2latest. gif
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Quoting NCHurricane2009:


Wow...didn't even know about that one...I've been of my game for a while...but I'll be back once I start my job and have a much regular schedule...

Did that 90L ever get mentioned in the NHC TWO?

No,it wasn't,but Reedzone said that was Arlene.If yes,we've got Bret now(it's not named),and waiting for Cindy forecasted in mid-May.We could have three off-season storms in 2011!
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http://www.nhc.noaa.gov/tafb_latest/gehov2latest.gi f

Checked hovmoller diagram, looks like 91L frontal in origin, but I don't know what date this front came off the US east coast....

Does anyone know which front 91L is traced to? Is it the same front that brought the violent tornadoes in eastern NC (that would be so weird for me)....
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Quoting MiamiHurricanes09:
00z ECMWF does pretty much nothing with 91L.
Its window is very short.
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Quoting NCHurricane2009:

Did that 90L ever get mentioned in the NHC TWO?
Nein.
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00z ECMWF does pretty much nothing with 91L.
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Quoting KoritheMan:

90L was, as expected, of baroclinic origin, being declared on March 11 near the northwestern Canary Islands. It formed from a piece of energy that broke off a cold front.


Wow...didn't even know about that one...I've been of my game for a while...but I'll be back once I start my job and have a much regular schedule...

Did that 90L ever get mentioned in the NHC TWO?
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Quoting NCHurricane2009:
when did we have 90L?!
90L was, as expected, of baroclinic origin, being declared on March 11 near the northwestern Canary Islands. It formed from a piece of energy that broke off a cold front.
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Early morning all,

Been a long time since I posted, been super busy finishing my last semester here at school.

I can't believe we have Invest 91L right now...when did we have 90L?! (we start with 90L and count up, right?)
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448. xcool
SimuAWIPS very good weather program ;))))
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Quoting hurricane23:


I've been useing SimuAWIPS here at work.


SimuAWIPS is a pretty amazing app considering its free and all the upgrades it's receiving.
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Quoting Stormchaser2007:


GREarth.

Amazing program...


I've been useing SimuAWIPS here at work.
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Quoting Stormchaser2007:


Hey Adrian!

Congratulations on your daughter!


Hey thanks..
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Quoting hurricane23:


Agree... From my view there's been no warm convective process going just purely baroclinic as I expressed earlier. Just hoping for some precip here


So, the 36-hour tendency for convection to develop very near the center of circulation (albeit getting sheared off immediately) isn't a warm-core characteristic? It's quite clear that the main convective bombs going off are trying to do so right over the center, something you would not see with a purely baroclinic low.
Member Since: November 24, 2005 Posts: 635 Comments: 26701
Quoting hurricane23:


Agree... From my view there's been no warm convective process going just purely baroclinic as I expressed earlier. Just hoping for some precip here


Here in Central Florida, even with the severe weather outbreak a few weeks back, and the 13 inches of rain that came along with it, things are starting to get dry around here again. It gets dry fast as heck in Florida if its not pouring rain all the time.


I would agree, it appears most of the convection is from a baroclinic process, however its hard to say for sure, the eye can be deceiving. This much is known, its not a pure warm core, lol.

Subtropical storms do involve baroclinic forcing to as Levi said, keep that in mind.
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Quoting Levi32:


Wait that was on Google Earth? What weather program is that?


GREarth.

Amazing program...
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Recent AMSU passes:


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Quoting hurricane23:


Hey mike!! Loving that WV view on GR earth..Wish I could afford it.


Hey Adrian!

Congratulations on your daughter!
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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.

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