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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Fish storm, err Fish Depression.
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06z GFS starts developing another weak CV storm in a couple days near where 04L is.
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012300Z AUG 10
FM NAVMARFCSTCEN NORFOLK VA
TO WEATHER
BT
UNCLAS
MSGID/NLMC/OVLY2/0087/AUG
OVLY/TCFA/012300Z6/AUG/1OF1/TCFA AL9110/METOC
LINE/2//G/094800N1/0362400W5/103000N4/0422400W2
LINE/2//G/112400N8/0361200W2/080600N4/0364200W5
LINE/2//G/080600N4/0364200W5/084800N0/0424200W2
LINE/2//G/084800N0/0424200W2/120600N9/0421200W9
LINE/2//G/120600N9/0421200W9/112400N8/0361200W2
TEXT/20//G/070600N3/0392400W8/TCFA AL9110
TEXT/20//G/060600N2/0392400W8/VALID UNTIL 022300Z
TEXT/20//G/050600N1/0392400W8/WINDS: 20-25 KTS
TEXT/20//G/040600N0/0392400W8/MVG: WNW AT 15 KNOTS
ENDAT
BT
NNNN

Member Since: July 15, 2006 Posts: 165 Comments: 52265
Quoting KEEPEROFTHEGATE:
i think he mean aye aye


Oh, I took it as he spotted an eye because of his previous comment (see below). My bad. But there is definitely no eye yet, lol.

Quoting AstroHurricane001:
Does anybody see an eye? It could be the result of a hot tower collapse that set off its internal circulation leading an eye.

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X
Member Since: July 15, 2006 Posts: 165 Comments: 52265
Quoting KEEPEROFTHEGATE:
i think he mean aye aye


Between 11:45 and 13:15 UTC is what looked like an eye trying to form.
Member Since: August 30, 2008 Posts: 8 Comments: 2834
Heatwaves? In Europe?

Cannot be.

We wait in anticipation to break 70 at the end of the week.
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Quoting Drakoen:


TD 4

invest_RENUMBER_al912010_al042010.ren

And the Euro did not develop it? hmmm.
Member Since: July 7, 2005 Posts: 0 Comments: 5120
Interesting area of convection east of 04L thats persisted for a little while.
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Quoting AllStar17:


yes


i saved the nhc website to my favorites
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in 35 mins NHC will start issuing advisorys for TD 04L
Member Since: July 15, 2006 Posts: 165 Comments: 52265
Quoting fishcop:
Thanks for the update Doc. Another HOT day in Grand Cayman - walked outside 2 minutes ago and it was like walking into a steamer. Should be a great day on the water
Current temp is 89F.
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Quoting FloridaHeat:
now that this is a td does that mean the nhc will offer an official track


yes
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now that this is a td does that mean the nhc will offer an official track
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Quoting GetReal:




In difference to those early model runs, the steering looks more westward over the next 72 hours....


I've looked at the various models (CMC, GFS, NOGAPS, Euro) and none of them really seem to show the strong high over the southern US. The steering you just posted however, does show it.
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Quoting KEEPEROFTHEGATE:
04L/TD/L
MARK
13.10N/41.33W


Looks like some banding is taking place .... EH?
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Quoting HurricaneHunterGal:

There is no eye right now
i think he mean aye aye
Member Since: July 15, 2006 Posts: 165 Comments: 52265
Dr. Masters, you mentioned that one country set a coldest temperature record. Which country was that, and what was the temperature? I hear that Peru, Argentina and Australia are cold right now, but the entire Northern Hemisphere is under an intermittent heat wave.
Member Since: August 30, 2008 Posts: 8 Comments: 2834
Guys I just want you to know this is developing past 40W, which does NOT make this a Cape Verde Storm at all. So our first Cape Verde Storm has yet to form.
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USVI major hurricane threat...

imho
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AOI

AOI

AOI

AOI

AOI

TS BUSTED FORECAST ALIBI
Member Since: October 1, 2007 Posts: 81 Comments: 26493
Quoting Drakoen:
Don't understand why Dr. Jeff Masters says there is no upper level outflow when the cimss shear flow and water vapor imagery clearly depict that.


Here's the image again, lol. Coordinates are on previous page.

Member Since: August 30, 2008 Posts: 8 Comments: 2834
Ok we have TD 4. Now the $500,000 (hope thats enough) question is, where will it go next.
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Just like Bonnie, this storm probably won't amount to much past Puerto Rico..
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33. SLU
I'm a bit surprised at the renumber given the lack of concrete evidence of a well defined circulation.
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thanks Dr. Now all the it will hit So Fla wishcasters will bwgin to assemble lol
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91L looks like a TD to me, wait.. they already renumbered it, GAME ON!
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Quoting AstroHurricane001:


EYE! EYE!

There is no eye right now
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In difference to those early model runs, the steering looks more westward over the next 72 hours....
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Don't understand why Dr. Jeff Masters says there is no upper level outflow when the cimss shear flow and water vapor imagery clearly depict that.
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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 Mon 02 Aug 2010 14:00:01Z
2010 Storms
All Active Year

Atlantic
04L.FOUR(T.C.F.A.)
East Pacific
NONE
Central Pacific
92C.INVEST(T.C.F.A.)
West Pacific
96W.INVEST
Indian Ocean
NONE
Southern Hemisphere
NONE
Member Since: July 15, 2006 Posts: 165 Comments: 52265
Quoting Drakoen:


TD 4

invest_RENUMBER_al912010_al042010.ren

Wow Doc makes an update saying that it's near TD status and it turns into one a few minutes later! LOL
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Jeff: The high wind shear and dry air should weaken and possibly destroy 91L late this week.

Wow! That should be something to see! If this TD becomes Colin and then goes POOF, that would be far out!
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Quoting KEEPEROFTHEGATE:
04L/TD/L
MARK
13.10N/41.33W


EYE! EYE!
Member Since: August 30, 2008 Posts: 8 Comments: 2834
Thanks for the update Doc. Another HOT day in Grand Cayman - walked outside 2 minutes ago and it was like walking into a steamer. Should be a great day on the water
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Everyone will be completely wore out trying to predict 91L's fate by this time next week only to watch it have very little impact.
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Thanks Doc.

Wow, TD 4! It will be interesting to read the forecast discussion.
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Quoting KEEPEROFTHEGATE:
04L/TD/L
MARK
13.10N/41.33W


It looks very organized!!
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Quoting AllStar17:
Oooop! Dr. M. is going to have to update his blog again right after he just updated.

The long range forecast of now TD 4 remains very uncertain, to say the least.


The updates for an impending storm usually come just after 10 am, just to raise the excitement value.
Member Since: August 30, 2008 Posts: 8 Comments: 2834
15. IKE
The high wind shear and dry air should weaken and possibly destroy 91L late this week....

You will get hammered on here for that doc. Thanks for the info.

My temperature gauge in my car showed the outside temp yesterday at 105 degrees...inland Florida panhandle.

I've never seen it that hot here.
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04L/TD/L
MARK
13.10N/41.33W
Member Since: July 15, 2006 Posts: 165 Comments: 52265
Quoting weatherguy03:
Latest Forecast Video On Invest 91L(Ooops..TD #4..LOL)


LOL!
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Does anybody see an eye? It could be the result of a hot tower collapse that set off its internal circulation leading an eye.
Member Since: August 30, 2008 Posts: 8 Comments: 2834
Oooop! Dr. M. is going to have to update his blog again right after he just updated.

The long range forecast of now TD 4 remains very uncertain, to say the least.
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AL, 04, 2010080212, , BEST, 0, 123N, 403W, 30, 1006, TD, 0, , 0, 0, 0, 0, 1010, 180, 50, 40, 0, L, 0, , 0, 0, INVEST, M,
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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.