91L near tropical depression status

By: Dr. Jeff Masters , 2:08 PM GMT on August 02, 2010

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A tropical wave near 12N 41W, midway between Africa and the Lesser Antilles Islands, is very close to being a tropical depression. NHC labeled this system Invest 91L yesterday, and is giving it a 90% chance of developing into a tropical depression by 8am Wednesday. Wind shear is a low 5 - 10 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 negative for development appears to be the storm's small size, which makes it vulnerable to modest increases in wind shear or dry air entrainment. A Windsat pass from 5am EDT this morning did not show a closed circulation. Satellite imagery shows that the intensity and areal extent of 91L's heavy thunderstorms is very limited, but that a closed surface circulation may be close to forming. Low-level spiral bands and upper-level outflow are not apparent yet.


Figure 1. Morning satellite image of Invest 91L.

Forecast for 91L
There is modest model support for 91L developing into a tropical depression. Three out of six of our reliable models for forecasting tropical cyclone genesis predict 91L will develop into a tropical depression by Tuesday or Wednesday. A west-northwest motion at 10 -15 mph is predicted, which should carry 91L a few hundred miles northeast of the northernmost Lesser Antilles Islands. However, it is possible that 91L would track over the northernmost Lesser Antilles Islands, as predicted by the Canadian model. Squalls from the outer rainbands of 91L may affect islands such as Antigua and Barbuda as early as Wednesday afternoon. As 91L makes its closest approach to the Lesser Antilles on Wednesday, the storm will begin to encounter strong upper-level westerly winds associated with the counter-clockwise flow of air around an upper-level low pressure system centered between Bermuda and Puerto Rico. The latest SHIPS model forecast predicts that these winds will cause wind shear to rise to the moderate level, 10 - 20 knots, by Tuesday night, and to the high level, 20 - 30 knots, by Wednesday. There is also a great deal of dry air associated with the upper level low that may cause problems for 91L. The high wind shear and dry air should weaken and possibly destroy 91L late this week.

A trough of low pressure is expected to move off the U.S. East Coast on Friday, and this trough may be strong enough to recurve 91L far enough to the northwest that the storm will threaten Bermuda. The HWRF and ECMWF models predict 91L could pass very near to Bermuda on Saturday. It is uncertain at this time if the trough will be strong enough to recurve 91L all the way out to sea early next week, as predicted by the GFS model, or leave 91L behind to potentially move westward again into the U.S. East Coast, as predicted by the Canadian model. The amount of wind shear that might be present early next week is also highly uncertain.

Cyprus records its hottest temperature in history yesterday
The island of Cyprus recorded its hottest temperature in its history on August 1, 2010 when the mercury hit 46.6°C (115.9°F) at Lefconica. The old record for Cyprus was 44.4°C (111.9°F) at Lefkosia in August 1956. An older record of 46.6°C from July 1888 was reported from Nicosia, but is of questionable reliability.

The year 2010 is now tied with 2007 as the year with the most national extreme heat records--fifteen. There has been one country that has recorded its coldest temperature on record in 2010; see my post last week for a list of the 2010 records. My source for extreme weather records is the excellent book Extreme Weather by Chris Burt. His new updates (not yet published) remove a number of old disputed records. Keep in mind that the matter of determining extreme records is very difficult, and it is often a judgment call as to whether an old record is reliable or not. For example, one of 2007's fifteen extreme hottest temperature records is for the U.S.--the 129°F recorded at Death Valley that year. Most weather record books list 1913 as the year the hottest temperature in the U.S. occurred, when Greenland Ranch in Death Valley hit 134°F. However, as explained in a recent Weatherwise article, that record is questionable, since it occurred during a sandstorm when hot sand may have wedged against the thermometer, artificially inflating the temperature. Mr. Burt's list of 225 countries with extreme heat records includes islands that are not independent countries, such as Puerto Rico and Greenland. Seventy four extreme hottest temperature records have been set in the past ten years (33% of all countries.) For comparison, 14 countries set extreme coldest temperature records over the past ten years (6% of all countries). I thank Mr. Burt and weather record researchers Maximiliano Herrera and Howard Rainford for their assistance identifying this year's new extreme temperature records.

Next update
I'll have an update later today if 91L develops into a TD.

Jeff Masters

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Quoting ho77yw00d:
IMO not that mine matters but it's only August 2nd and we have a td4 I feel it is a big reminder of what is to come and people should get prepared and enjoy mother nature as she does her work...


Heart of season is well underway, it's already showing by what's coming off Africa
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ANYONE AND I DO MEAN ANYONE, that does not Currently have this as a Storm for the fishes is truly out in left field with nobody else to throw too....a lot of non-sense being posted now.
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32W is going to go through 30+C water.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Glad most of the Florida wish-casting crowd is gone. It would have to be one hell of a bridging effect of both areas of high pressure to have this disturbance (if it does not get sheared apart first)to make a bee-line to Florida or the Gulf.
Member Since: August 4, 2009 Posts: 0 Comments: 497
It's not as much as you think.
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IMO not that mine matters but it's only August 2nd and we have a td4 I feel it is a big reminder of what is to come and people should get prepared and enjoy mother nature as she does her work...
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Quoting PensacolaDoug:
oh yea!! Shear to the recue again... its kinda got me wondering if this season is even gonna be ANYTHING like what they are predicting...


Lets hope the early forecasts were wrong.
Won't bother me none! I live on da bayou in Pensacola.

Bayou Chico or Bayou Grande?
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398. IKE
12Z GFS @ 108 hours....

Member Since: June 9, 2005 Posts: 23 Comments: 37860
Quoting WeatherNerdPR:

Why do you post that so much?


Agreed. If we want to see a graphic we'll go pull it up. Posting graphics and photos here just slows down the reload time, which can be critical for someone with a narrow bandwidth connection sitting on a boat in the Virgin Islands. Let's think about someone else for a change.
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Quoting MiamiHurricanes09:
How do you know? Models are now beginning to lock on to the system since it is a tropical depression.
hi mh09- do you see any evidence that the high will build back in? the nhc has been doing pretty good on projection and i was just wondering what the steering pattern holds for the next 3-5 days? what is making the nhc think that it will go north or be a fish storm? thanks in advance:)
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Quoting louisianaweatherguy:
THE ENVIRONMENT APPEARS FAVORABLE FOR SOME STRENGTHENING OVER THE
NEXT COUPLE OF DAYS...BUT AFTER THAT TIME VERTICAL WIND SHEAR IS
EXPECTED TO INCREASE AS THE CYCLONE APPROACHES AN UPPER-LEVEL
TROUGH AROUND 65W.


oh yea!! Shear to the recue again... its kinda got me wondering if this season is even gonna be ANYTHING like what they are predicting...
Yea. Real bummer, isn't it? What a drag if we get no landfalling major cyclones. Phooie!
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Quoting Drakoen:


No but the models were split with a lot of them taking Ike out to sea and a few down south and the NHC split the difference.


I was trying to find nhc's track on Frances but I couldn't find it. Seems like the general expected motion in 3 days is close to what Frances did in '04 (not predicting another Frances like storm)
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NMFC Norfolk Tropical Feed
No Active Tropical Warnings in the Atlantic, Caribbean, or Gulf of Mexico
By Maritime.CDO@navy.mil (NMFC CDO) from Naval Maritime Forecast Center Norfolk Virginia. Published on .

As of Sun 01 Aug 2010 22:45:01Z

2010 Storms
All Active Year

Atlantic
04L.FOUR
East Pacific
NONE
Central Pacific
92C.INVEST(T.C.F.A.)
West Pacific
97W.INVEST
96W.INVEST
Indian Ocean
NONE
Southern Hemisphere
NONE
Member Since: July 15, 2006 Posts: 176 Comments: 55615
Is it my imagination or has the CMC model done the best job at tracking so far this season?
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Quoting AllStar17:

Why do you post that so much?
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Airmass EuMetSat
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oh yea!! Shear to the recue again... its kinda got me wondering if this season is even gonna be ANYTHING like what they are predicting...


Lets hope the early forecasts were wrong.
Won't bother me none! I live on da bayou in Pensacola.
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Quoting Hurricanes12:
MH09, what do you think about the future environment that TD-4 has to over-come?
Has to overcome dry air that is currently penetrating the NW quadrant. The TUTT axis will also keep it in check unless it splits up into upper lows and retreat towards the SW. However much remains uncertain.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting Floodman:


It is, but only for the rest of us typically
There that's what I was trying to say. If it hurt the carrier ahead of time they might try to avoid it. Guy in charge of screw up is walking around all hang dog now. Went from straw boss to chief horses pa-toot. Good fellow basically just doesn't think things trough. Too much "Git er Dun".

Most common last words of a Red Neck. "Hey y'all look at me."
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32West continues to strenthen as it is in the general vicinity of the original strongest wave axis of the season.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Quoting AussieStorm:

Thank you, My whole family saved my life that day,and till my last breathe on this earth, I will always remember that. Maybe that is why I am such a helpful person.


I have gone through my fair share as well I dont want to go into detail since this is a weather blog but I am the same way and good things happen to good people (so they say) but I commend you for that=)
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MH09, what do you think about the future environment that TD-4 has to over-come?
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Selective model tool, don't overlook the options under the graphic in the link for graphic overlays and options. Models selection is on the right side slide bar :)

Model Data for Tropical Depression (04L)


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Quoting AllStar17:
Wow. 1 hour after the NHC classifies TD 4, people are RIPping it and calling it a fish.

well they are looking at the 5 day forecast track, It has it going past Bermuda. If Colin hits Bermuda, it's not a fish, well to me it ain't.

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Quoting louisianaweatherguy:
THE ENVIRONMENT APPEARS FAVORABLE FOR SOME STRENGTHENING OVER THE
NEXT COUPLE OF DAYS...BUT AFTER THAT TIME VERTICAL WIND SHEAR IS
EXPECTED TO INCREASE AS THE CYCLONE APPROACHES AN UPPER-LEVEL
TROUGH AROUND 65W.


oh yea!! Shear to the recue again... its kinda got me wondering if this season is even gonna be ANYTHING like what they are predicting...


Looks like some dry air around 50-55W that may be starting to shear the leading edge of TD4 as we speak. Perhaps it's not strong enough to dig in and do any real damage, but it's there.
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Quoting oracle28:
"Wow. 1 hour after the NHC classifies TD 4, people are RIPping it and calling it a fish."

Not ripping it, but it's Bermuda bound.
How do you know? Models are now beginning to lock on to the system since it is a tropical depression.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting WeatherNerdPR:

People wanted it to be classified and now that it is they're RIPing it. People are weird!


I was saying what it looks like to me. I am allowed
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I have a new invest at 9N and 31W. I call it 32West.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
"Wow. 1 hour after the NHC classifies TD 4, people are RIPping it and calling it a fish."

Not ripping it, but it's Bermuda bound.
Member Since: July 25, 2008 Posts: 0 Comments: 597
Quoting WeatherMum:
119 down here in Navarre on the Beach. Not even the hot part of the day.

I think you mean the heat index...and yes thats hot...i'm in myrtle grove and the heat index from Naval Air Station 15 minutes ago was 111 degrees...went for a swim in the pool and pool temp was 92 degrees...not much relief
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THE ENVIRONMENT APPEARS FAVORABLE FOR SOME STRENGTHENING OVER THE
NEXT COUPLE OF DAYS...BUT AFTER THAT TIME VERTICAL WIND SHEAR IS
EXPECTED TO INCREASE AS THE CYCLONE APPROACHES AN UPPER-LEVEL
TROUGH AROUND 65W.


oh yea!! Shear to the recue again... its kinda got me wondering if this season is even gonna be ANYTHING like what they are predicting...
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Quoting AllStar17:
Wow. 1 hour after the NHC classifies TD 4, people are RIPping it and calling it a fish.

People wanted it to be classified and now that it is they're RIPing it. People are weird!
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368. unf97
Quoting AllStar17:
We do not have 92L.


For the time being that is. However, the impressive wave coming in off the African coast behind TD4 certainly appears soon to be a strong candidate in the short term to be our next invest. It looks very impressive on satellite imagery.
Member Since: September 25, 2007 Posts: 0 Comments: 1193
Quoting MiamiHurricanes09:
Where is 92L? I don't see it on the navy or ATCF site.


We don't have it. People are talking about the area just behind TD 4.
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Quoting texwarhawk:


Ike was never a fish

Link


No but the models were split with a lot of them taking Ike out to sea and a few down south and the NHC split the difference.
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Where is 92L? I don't see it on the navy or ATCF site.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting IKE:


I agree with what you said the other day...there's just something missing this season, so far, in the ATL.
i don't think its missing i think its too much of something kinda like adding more ingreients than requirer
Member Since: July 15, 2006 Posts: 176 Comments: 55615
Will this system be affected by shear or dry air?
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Quoting ho77yw00d:


hey... I listened to your story this morning I had to close my computer with all the thunder and lightening last night but that is an amazing survival story and thank goodness your mom knew CPR! Like I said last night you are very blessed to be here:)

Thank you, My whole family saved my life that day,and till my last breathe on this earth, I will always remember that. Maybe that is why I am such a helpful person.
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360. BDAwx
The Bermuda weather service has called the April-May-June period the driest 3 month period since 1963. Extreme drought conditions were erased by 3-6 inches of rain over the 4-day Cup Match Holiday weekend.
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Quoting Orcasystems:


It would be nice if we were getting some of this moisture heading towards the Islands. We have not had rain for over a month, They are finally calling for a bit of rain here this week :)


Is this typical of you guys in a La-Nina year? It seems here in the mid-west U.S. we become the deep south and they become something like Hades. C'mon December!
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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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