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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659. Lizpr
Quoting serialteg:


lizzzzzzzzzzzz! :D


Heya! I'm so gonna miss this hurricane season cus I not longer live in PR I move to Oregon! very different! But always come back to this blog never stop to amaze me lol.
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Hi all....question for Levi, Storm, anyone with sense...lol

2005..was it an "upward MJO" time with Katrina, Rita, Wilma, etc? or was it just perfect elements that helped them ramp up???
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Note that big dog is behind TD4.


Member Since: September 30, 2005 Posts: 0 Comments: 1606
652. Relix

There we have current steering layers.


And these were 3 hours ago.

The flow to the west has grown sightly and you can see the weakness growing a bit in there. It's now all about timing and system strength.
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I am seeing the models shift further south now and I think the models and more models will keep on moving more south and maybe with some of them getting stronger once in the eastern to central Caribbean then start pulling NW
maybe becoming a carib/GOM system
Member Since: June 13, 2009 Posts: 2 Comments: 11004
Quoting DestinJeff:
South Florida bloggers, please, form a single file line and have tickets ready at the door!



LMAO!
Member Since: August 4, 2009 Posts: 0 Comments: 497
Quoting Levi32:
TD 4's convection is weak and pathetic-looking because it's over cold SSTs of only 27C, 3C colder than what it's been over so far. It won't get into 28C waters again until it passes 50W. The stratocumulus field just northeast of the storm shows you that on the visible floater.



Actually it's in 28-29 degree water according to NOAA SST loops on the central Atlantic. Meanwhile, big dog, on it's tail, is approaching 30 degree Centigrade boiling water.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Quoting Floodman:


Jeanne, the classic "it's a fish, it's a fish, it's a...oh hell, evacuate!":

Now THAT'S recurvature.
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Well, TD4's convection is weaker than what it was before, but I've seen much worst! At least it's not dying off completely.
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Notice what I had said earlier about strengthening storm under a building ridge, then look at the CMC model.. Strengthens TD4 back to a well formed system before hitting Florida. It's not just intensifying it for no reason, the TUTT is away form it and it catches a ridge overhead, which ventilates it even more, gives it more favorable conditions.
Member Since: July 1, 2008 Posts: 13 Comments: 7340
Levi,

Superb job on the video, I really enjoyed it. Let's hope that the ridge doesn't build back in too strong after the shortwave trough lifts out.
Member Since: September 2, 2009 Posts: 130 Comments: 21091
643. 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.
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Quoting Lizpr:
Oh my how I miss this!


lizzzzzzzzzzzz! :D
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Quoting leelee75k:
Am I mistaken, I recall last week one Dr M's blogs saying that wave behind TD4 would have absorbed it and would be the bigger threat? I guess now that scenario isn't possible or did it happen and I missed it?
you missed it
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Quoting Floodman:


Jeanne, the classic "it's a fish, it's a fish, it's a...oh hell, evacuate!":



ROFL :D
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Quoting MiamiHurricanes09:
Very interesting 12z CMC...

Link


CMC-caster.
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637. xcool
cmc hit fL MIAM
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jason is ignored, man!
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Quoting DestinJeff:
South Florida bloggers, please, form a single file line and have tickets ready at the door!

Do not fear, the shower curtain is here!
Member Since: September 2, 2009 Posts: 130 Comments: 21091
634. Prgal
Quoting Relix:

Just saw it. Shows what I am saying there but a bit too much south. In that case I think a case like Jeanne is possible for us in PR

Yeap. The guy in Crown Weather told me a few minutes ago that he thinks there will be a shift to the west in the models. We need to keep an eye on this system...just in case.
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What's the basis for the intensity forecast on TD4? I know intensity is difficult to forecast, but unless there's a measurable amount of shear or dry air, what's to stop it from becoming a hurricane as soon as it hits the warmer water? I'm not trying to wishcast or anything like that, I'm just curious as to what the reasoning is for keeping it a TS through day 5.
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Very interesting 12z CMC...

Link
Member Since: September 2, 2009 Posts: 130 Comments: 21091
629. Relix
Quoting Prgal:


Did you see the new CMC?

Just saw it. Shows what I am saying there but a bit too much south. In that case I think a case like Jeanne is possible for us in PR
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Quoting Levi32:


Organization is good, but heat output is minimal...this will be limited until it gets over warmer water, which won't take too long given its rapid forward motion.


Early morning Wednesday if it stays on this heading should start encountering warmer waters.
Member Since: August 11, 2008 Posts: 0 Comments: 2315
Quoting reedzone:
12Z CMC shifted further south, also weakening TD4 into a wave, then redeveloping it near Florida.. This makes sense to me as well, if it weakens.
I Concur
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625. Prgal
Quoting Relix:
If it stays weak it's a problem. If you see the latest visible loop it's been heading W-WNW so I would say it's moving far more to the west. We will surely be seeing the tracks shift south and left for now, especially if it stays this weak. It wouldn't surprise me if it didn't feel much of the ride weakness at all. After 50W the system should start booming. Dry air is moving ahead of the system thanks to the TUTT north of PR. It will have a frame between 50-59W for strenghtening and that's when we could see a 55-60mph tropical storm passing about 100miles NW of the Greater Antilles. I don't expect much of a problem for the islands in that case. The wave behind shows more potential for the Caribbean at least as its moving slower and won't have to follow a weakness.


Did you see the new CMC?
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http://moe.met.fsu.edu/cgi-bin/cmctc2.cgi?time=2010080212&field=Sea+Level+Pressure&hour=Animation


i think this model run makes a lot of sense. if it verifies it would ruin my vacation :(
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CMC much south into the caribbean
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Disturbance behind 04L has been firing new convection throughout the afternoon.
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620. unf97
Quoting Floodman:


Jeanne, the classic "it's a fish, it's a fish, it's a...oh hell, evacuate!":



Good post and example Floodman. Jeanne is indeed the perfect model storm of showing how quickly conditions can change in the atmosphere in terms of monitoring tropical entities!
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Quoting reedzone:


LOL awesome evidence right there, though extreme is right, may fall apart. It's just too early to be convinced of anything right now.


Agreed.
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618. Relix
If it stays weak it's a problem. If you see the latest visible loop it's been heading W-WNW so I would say it's moving far more to the west. We will surely be seeing the tracks shift south and left for now, especially if it stays this weak. It wouldn't surprise me if it didn't feel much of the ride weakness at all. After 50W the system should start booming. Dry air is moving ahead of the system thanks to the TUTT north of PR. It will have a frame between 50-59W for strenghtening and that's when we could see a 55-60mph tropical storm passing about 100miles NW of the Greater Antilles. I don't expect much of a problem for the islands in that case. The wave behind shows more potential for the Caribbean at least as its moving slower and won't have to follow a weakness.
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Am I mistaken, I recall last week one Dr M's blogs saying that wave behind TD4 would have absorbed it and would be the bigger threat? I guess now that scenario isn't possible or did it happen and I missed it?
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Quoting IKE:


This system may be heading to a wave if the convection keeps dying off...then it would move more west....



Thats what I thought too, but right now the models say something else, even with the intenisity being factored in--- as a weak TS. Thats confustng!
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Quoting Floodman:


Jeanne, the classic "it's a fish, it's a fish, it's a...oh hell, evacuate!":



LOL awesome evidence right there, though extreme is right, may fall apart. It's just too early to be convinced of anything right now.
Member Since: July 1, 2008 Posts: 13 Comments: 7340
Quoting angiest:


GFDL and HWRF (among potentially others) moved back and forth between mid Texas and, as I recall, FL panhandle.


And the NHC wisely ignored them.
Member Since: July 25, 2008 Posts: 0 Comments: 596
full basin water vapour including aoi in east atlantic

Member Since: July 15, 2006 Posts: 169 Comments: 53296
Quoting KEEPEROFTHEGATE:
i know i took a chance com'on i'am not that bad at least you know theres only one of me


I have no problems with it, but from past experience admin does. Your ok in my book KOG.... well except for that fan thing. I will be rooting for bluer blues.


Member Since: September 23, 2005 Posts: 14 Comments: 10875
Quoting StormChaser81:


Regardless the convection is over the center.


Organization is good, but heat output is minimal...this will be limited until it gets over warmer water, which won't take too long given its rapid forward motion.
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12Z CMC shifted further south, also weakening TD4 into a wave, then redeveloping it near Florida.. This makes sense to me as well, if it weakens.
Member Since: July 1, 2008 Posts: 13 Comments: 7340

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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.