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 neonlazer:
There is no vorticity with that area..so its just a bunch of storms


Seems to me that there is some organization to it but lets see what it's like tomorrow. That was just a quick observation on my part that something is developing there.
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not going to be much of a storm left when it nears the coast
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957. IKE
12Z ECMWF through August 12th...
Member Since: June 9, 2005 Posts: 23 Comments: 37858
Quoting pottery:

I'm...(gurgle, gasp)...under.. (paddle, grasp)... here!


You said you got 3/4" yesterday... how about today? We are in desperate need of a monsoon here.
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02/1745 UTC 12.8N 41.7W T2.0/2.0 04L
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IKE - EMCWF poofs it @ 144hrs?
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Quoting StormChaser81:


Let's not through gas on that fire.


It is too early to speculate on model resolutions beyond 3 days.
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Quoting hurricanejunky:


hurricane junky...not junkie...thanks for the posts though...they're great!


Sorry, was close though lol...
Member Since: July 1, 2008 Posts: 13 Comments: 7334
Quoting RobbieLSU:
Hey guys, I've got my August hurricane outlook up if you want to check it out : Link

Feel free to comment and let me know what I got wrong lol


Very nicely done! Thanks; bookmarked.
Member Since: November 8, 2009 Posts: 4 Comments: 13279
Quoting hurricanejunky:


hurricane junky...not junkie...thanks for the posts though...they're great!


R u a Rubio supporter?? I got his sticker on my car!! Go junkie... I mean Junky LOL

Afternoon everyone!!!
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If you look at the low level flow out in front of TD4, it's moving almost due west. Until TD4 strengthens any, don't expect it to gain too much latitude.
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This is why the CMC made sense this afternoon, and models starting to trend that way.
Member Since: July 1, 2008 Posts: 13 Comments: 7334
Quoting Hhunter:
interesting supports cmc scenario..


And where will TD4 be in relationship to PR this weekend?
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Quoting CaneBob:
Is it my imagination or is there a more organized system just to the east of TD4?
There is no vorticity with that area..so its just a bunch of storms
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Quoting Orcasystems:


Speaking of unobserved... where are you??

I'm...(gurgle, gasp)...under.. (paddle, grasp)... here!
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Quoting nrtiwlnvragn:
HPC Final Extended Forecast Discussion

Excerpt:


A SIGNIFICANT WRINKLE IN THE FORECAST IS TROPICAL DEPRESSION FOUR
IN THE CENTRAL TROPICAL ATLANTIC. THE DETERMINISTIC GUIDANCE
MOVES THIS SYSTEM NEAR OR OFFSHORE THE EAST COAST OF NORTH AMERICA
OVER THE NEXT WEEK. THE 12Z GFS/12Z CANADIAN ARE THE FARTHEST
WEST BRINGING IT TOWARDS FLORIDA IN 6-7 DAYS...WHILE THE 00Z ECMWF
IS WELL TO THE RIGHT TRYING TO RECURVE THE SYSTEM JUST EAST OF THE
70TH MERIDIAN. THE ENSEMBLE GUIDANCE WITH THIS FEATURE HAS ALSO
BEEN QUESTIONABLE WITHIN ANY OF ITS 00Z AND 12Z RUNS OVER THE PAST
SEVERAL DAYS...THOUGH ITS MULTI-DAY WAVERING FAVORS THE WESTERN
ROUTE MOVING THE LOW JUST NORTH OF THE GREATER ANTILLES UP THE
EAST SIDE OF THE BAHAMA BANK AND OFFSHORE THE FLORIDA COAST. NHC
TOOK THE EASTERN ROUTE AT 15Z WHICH WAS A SIGNIFICANT BREAK FROM
YESTERDAYS 16Z NHC/HPC AGREED UPON POINTS. SEE NHCS LATEST
DISCUSSIONS CONCERNING THEIR FORECAST REASONING. BEYOND FRIDAY
MORNING...THE WEAKENING TROUGH IN THE EAST IS NOT EXPECTED TO BE
STRONG ENOUGH TO CAUSE FULL RECURVATURE. THE COL IN THE FLOW
PATTERN SHOULD REMAIN TO ITS NORTH...SO THE AGREED UPON POINTS FOR
LATE THIS WEEKEND INTO EARLY NEXT WEEK TAKE THE SYSTEM NORTHWEST
IN THE GENERAL DIRECTION OF THE CAROLINAS SOUTH OF THE NORTHWARD
MIGRATING SUBTROPICAL RIDGE AXIS EXPECTED TO REACH THE 33RD
PARALLEL EARLY NEXT WEEK. SEE THE LATEST FORECASTS FROM NHC
CONCERNING THE FUTURE OF TROPICAL DEPRESSION FOUR.


I get the feeling they don't sound too happy about the NHC taking the eastern route.
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Is it my imagination or is there a more organized system just to the east of TD4?
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Quoting reedzone:


Hey buddy, just posted a new blog on HurricaneJunkies website, go check it out. I think you'll be interested.


hurricane junky...not junkie...thanks for the posts though...they're great!
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Quoting Jax82:


Jeff does this look like it could be a C. Fl monster?


Let's not through gas on that fire.
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Quoting pottery:

And now you claim Innocence, casually passing the Responsibility to my Dear Friend Keeper....
What a Wriggly Entity you are, assuming that your Dastardly Deeds will go un-observed while you lounge about in your DryZone sporting a dubious Orb of Bright Light that encircles your head like a neon sign advertising Goodness.
Goodness me, great globs gloriously gather, glowing, gleaming with gargantuan ghoulishness above.
And you smirk....


Speaking of unobserved... where are you??
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Quoting xcool:
SYNOPSIS...A TUTT LOW NORTHEAST OF PUERTO RICO WILL DRIFT
NORTHWEST AND WEAKEN DURING THE WEEK WHILE A RIDGE BUILDS OVER THE
AREA. THE RIDGE WILL CONTINUE THROUGH THE WEEKEND UNTIL ANOTHER
TUTT MOVES INTO PLACE NORTHEAST OF THE LOCAL AREA.


NWS San Juan this morning.
interesting supports cmc scenario..
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HPC Final Extended Forecast Discussion

Excerpt:


A SIGNIFICANT WRINKLE IN THE FORECAST IS TROPICAL DEPRESSION FOUR
IN THE CENTRAL TROPICAL ATLANTIC. THE DETERMINISTIC GUIDANCE
MOVES THIS SYSTEM NEAR OR OFFSHORE THE EAST COAST OF NORTH AMERICA
OVER THE NEXT WEEK. THE 12Z GFS/12Z CANADIAN ARE THE FARTHEST
WEST BRINGING IT TOWARDS FLORIDA IN 6-7 DAYS...WHILE THE 00Z ECMWF
IS WELL TO THE RIGHT TRYING TO RECURVE THE SYSTEM JUST EAST OF THE
70TH MERIDIAN. THE ENSEMBLE GUIDANCE WITH THIS FEATURE HAS ALSO
BEEN QUESTIONABLE WITHIN ANY OF ITS 00Z AND 12Z RUNS OVER THE PAST
SEVERAL DAYS...THOUGH ITS MULTI-DAY WAVERING FAVORS THE WESTERN
ROUTE MOVING THE LOW JUST NORTH OF THE GREATER ANTILLES UP THE
EAST SIDE OF THE BAHAMA BANK AND OFFSHORE THE FLORIDA COAST. NHC
TOOK THE EASTERN ROUTE AT 15Z WHICH WAS A SIGNIFICANT BREAK FROM
YESTERDAYS 16Z NHC/HPC AGREED UPON POINTS. SEE NHCS LATEST
DISCUSSIONS CONCERNING THEIR FORECAST REASONING. BEYOND FRIDAY
MORNING...THE WEAKENING TROUGH IN THE EAST IS NOT EXPECTED TO BE
STRONG ENOUGH TO CAUSE FULL RECURVATURE. THE COL IN THE FLOW
PATTERN SHOULD REMAIN TO ITS NORTH...SO THE AGREED UPON POINTS FOR
LATE THIS WEEKEND INTO EARLY NEXT WEEK TAKE THE SYSTEM NORTHWEST
IN THE GENERAL DIRECTION OF THE CAROLINAS SOUTH OF THE NORTHWARD
MIGRATING SUBTROPICAL RIDGE AXIS EXPECTED TO REACH THE 33RD
PARALLEL EARLY NEXT WEEK. SEE THE LATEST FORECASTS FROM NHC
CONCERNING THE FUTURE OF TROPICAL DEPRESSION FOUR.
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SAB still at T2.0 as of 17:45UTC.

AL, 04, 201008021745, 10, DVTS, CI, , 1280N, 4170W, SAB, LLCC, T, DT=2.0 BO CBND MET=2.5 PT=2.0 FTBO DT PA=50 NMI
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Quoting AllStar17:
The GFDL solution seems unrealistic. Even the HWRF is left of the GFDL.


I don't think the GFDL likes Bermuda very much, so no matter if we don't buy it or not, it'll send Tropical Cyclones there anyway. ;)
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Ok so I have a question...

What are the models suggesting will be pulling TD4 toward the WNW-NW between 60-65w (if/when it gets there) ?
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928. xcool
MiamiHurricane shear decreases 96hr
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Quoting AllStar17:
The GFDL solution seems unrealistic. Even the HWRF is left of the GFDL.


And NOGAPS is still losing the storm.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting DaytonaBeachWatcher:
Here comes 17 posts about how the xtrap isnt a model............ waiting.............


Let's see how many people can recognize a wink.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting Orcasystems:


Umm safe to assume that blob went over your house? ummmm it was KOG's idea.

And now you claim Innocence, casually passing the Responsibility to my Dear Friend Keeper....
What a Wriggly Entity you are, assuming that your Dastardly Deeds will go un-observed while you lounge about in your DryZone sporting a dubious Orb of Bright Light that encircles your head like a neon sign advertising Goodness.
Goodness me, great globs gloriously gather, glowing, gleaming with gargantuan ghoulishness above.
And you smirk....
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The GFDL solution seems unrealistic. Even the HWRF is left of the GFDL.
Member Since: June 29, 2009 Posts: 13 Comments: 5238
Big gap between the CMC and the GFDL
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The OFCI now lies on the eastern side of the guidance.
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920. Jax82
Quoting Jeff9641:
Models should shift left again on the next run too as the ULL will be shearing TD 4 until it reaches the Bahamas and the ULL away to the WSW.


Jeff does this look like it could be a C. Fl monster?
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Where is Patrap with the Early Statistical model and Dynamic model graphics? lol
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Expect the NHC to shift south of west ;)

This was exactly what I had predicted last night, too much ridging with the upward motion entering in. Sorry fishers, only 2-3 model runs recurve, the rest take it to the Eastern Seaboard.
Member Since: July 1, 2008 Posts: 13 Comments: 7334
901. xcool 2:44 PM EDT on August 02, 2010

Look at that, goes from 30+ knots to 12 knots in a day and a half.
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Here comes 17 posts about how the xtrap isnt a model............ waiting.............
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Quoting Hardcoreweather2010:
18Z Models are starting to roll in



The model hitting florida is noticeable.
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NMFC Norfolk Tropical Feed
Active Tropical Warnings in the Atlantic, Caribbean, or Gulf of Mexico
Atlantic Tropical Cyclone Formation Alert (TCFA) 91L
Tropical Depression 04L (FOUR) Warning #01

By Maritime.CDO@navy.mil (NMFC CDO) from Naval Maritime Forecast Center Norfolk Virginia. Published on .

As of MON 02 Aug 2010

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
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BAMM models point to North Carolina
24 HRS prior to landfall and it's still pointing to NC, that's when I'll get concerned.
Reminds me of Hugo, which had us in it's sights, until it veered west, crushed Charleston and gave us overhead, offshore pumping surf.
Member Since: August 16, 2007 Posts: 0 Comments: 73
Quoting Levi32:
See how the models flop.....now watch here come the calls for a US landfall from people who flop with each model cycle.



Whoa check out the XTRP model!!!

;D
Member Since: August 26, 2006 Posts: 16 Comments: 4766

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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.