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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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: 7334
Quoting reedzone:
I bet you all had Frances lined up for a fish storm before it turned westward... sheesh guys, it's too early!!


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

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607. Lizpr
Oh my how I miss this!
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Just looking out ahead at the future horizon...the GFS still wants to develop another CV storm, but doesn't strengthen it much and doesn't move it very fast. Also shows a system in the NW Caribbean at 120 hours and moves it into the BoC.
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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.



Regardless the convection is over the center.
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Quoting reedzone:
I bet you all had Frances lined up for a fish storm before it turned westward... sheesh guys, it's too early!!


It may very well get ripped up by shear in a few days so it may not even matter.
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I bet you all had Frances lined up for a fish storm before it turned westward... sheesh guys, it's too early!!
Member Since: July 1, 2008 Posts: 13 Comments: 7334
602. IKE
Quoting angiest:


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


That is true.
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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.

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Quoting K8eCane:


well if thats the case , i spose the term fish storm makes no sense at all....sea storm would make more sense to me
I really don't mind "fish storm." It kind of has a ring to it, and mostly enjoy the controversy it invokes here sometimes. It's often very entertaining.
Member Since: September 1, 2007 Posts: 0 Comments: 1113
599. IKE
Quoting DestinJeff:


Ike? You okay? We need to get medical for you? I think that post may have been ... dare I say ... a bit of a --- no, I can't say it.


LOL.


Quoting oracle28:


Alex 5-day cone never pointed at LA/MS.

http://www.nhc.noaa.gov/archive/2010/graphics/al01/loop_5W.shtml


Ouch.
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Recent past runs of the ECMWF showed no appreciable strengthening until it reached the islands.
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Quoting oracle28:


Alex 5-day cone never pointed at LA/MS.

http://www.nhc.noaa.gov/archive/2010/graphics/al01/loop_5W.shtml


GFDL and HWRF (among potentially others) moved back and forth between mid Texas and, as I recall, FL panhandle.
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Quoting Levi32:
Good morning all.

Blog Update.

Tropical Tidbit for Monday, August 2nd, with Video


Awesome Job; love the "busted TUTT" analogy.
Member Since: August 8, 2006 Posts: 0 Comments: 8278
Quoting LightningCharmer:
Unless the fish are being washed onshore, I really don't see how fish really care about tropical systems....LOL


well if thats the case , i spose the term fish storm makes no sense at all....sea storm would make more sense to me
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Quoting MiamiHurricanes09:
Last comment before I go.

You're putting wayyyyyyyyy to much stock into the models. As we get more detailed information about the environment it is in from the aircraft's the investigate the system models will change. Do you remember Alex? When it was a tropical depression the models forecasted for it to hit Louisiana/Mississippi. Guess where it hit?

Too crazy on here, later everyone!


Alex 5-day cone never pointed at LA/MS.

http://www.nhc.noaa.gov/archive/2010/graphics/al01/loop_5W.shtml
Member Since: July 25, 2008 Posts: 0 Comments: 596
Quoting FLdewey:
Reed look at your home made map.

Really?

REALLY?

Wow.


It an go out to sea, not saying it wont, but I'm not convinced and it's not set in stone. We need a few more days to see what happens. It's a reasonable map, with alot of uncertainty.
Member Since: July 1, 2008 Posts: 13 Comments: 7334
Quoting TampaSpin:
A strong storm will move to the North and a weak storm will move further South....it always happens....if TD4 stays a very weak system then further West would me favored. Intensity will be the key to where TD4 goes.


Except when it doesn't happen:

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Quoting pottery:
Meanwhile, it is coming down in buckets here for the past 1/2 hr, with rumblings and grumblings in the sky.
Where is Orca?


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Quoting StormChaser81:


I think around 2pm or so. not positive on this.


18Z's will be out around 2pm.
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Quoting sammywammybamy:


When do the next models come out?


I think around 2pm or so. not positive on this.
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Quoting MiamiHurricanes09:
Yeah, it is a developed system, but how long has it been a developed system? Models need time to lock on to the developed system. I reckon that tomorrow's model plots will be the ones to pay attention to. 04L may very well be a "fish" but I just don't see it happening yet.


In 24 hours I would have more confidence in the models.
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582. IKE
Quoting scott39:
I dont understand the north track out to sea. Especially with the intensity having TD4 staying a weak tropical storm for the next 3 to 4 days. I thought with TD4 being that weak of a system it would go more W?


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



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In the near term I think it will get closer to the islands than currently forecast.
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A strong storm will move to the North and a weak storm will move further South....it always happens....if TD4 stays a very weak system then further West would me favored. Intensity will be the key to where TD4 goes.
Member Since: September 2, 2007 Posts: 177 Comments: 20430
People like to use the term "fish storm...what about dolphins and whales? Marine mammals have feelings, too!
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Quoting K8eCane:
#539


All tropical cyclones affect fish . all are fish storms
Unless the fish are being washed onshore, I really don't see how fish really care about tropical systems....LOL
Member Since: September 1, 2007 Posts: 0 Comments: 1113
Quoting IKE:


Don't believe the models until we have a developed system. TD4 is a developed system.

Rebuttal..don't believe the models whether it's developed or not right?


Do we have a model run *since* they declared it was a TD? Meaning the models were initialized with a TD instead of an invest.
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Meanwhile, it is coming down in buckets here for the past 1/2 hr, with rumblings and grumblings in the sky.
Where is Orca?
Member Since: October 24, 2005 Posts: 0 Comments: 23080
Quoting nrtiwlnvragn:


He may have 24hrs to do that.... admin does not like images of fish.
i know i took a chance com'on i'am not that bad at least you know theres only one of me
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Read and comment on my blog, y'all.
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Quoting ClearH2Ostormchaser:
Reedzone Makes good since to me.


Lots of uncertainty on that run.. need to see what happens the next few days.
Member Since: July 1, 2008 Posts: 13 Comments: 7334
Good morning Levi. I think we have a developing situation at 34West.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Probably better to monitor the 500-850 steering if your looking down the line, it's forecast to be a 60kt tropical storm at that time, maybe even the 400-850 if it's approaching cane status.


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models continue to shift south and west
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Quoting IKE:


Don't believe the models until we have a developed system. TD4 is a developed system.

Rebuttal..don't believe the models whether it's developed or not right?
Yeah, it is a developed system, but how long has it been a developed system? Models need time to lock on to the developed system. I reckon that tomorrow's model plots will be the ones to pay attention to. 04L may very well be a "fish" but I just don't see it happening yet.
Member Since: September 2, 2009 Posts: 130 Comments: 21032
EVERY STORM IS A FISH STORM....THINK ABOUT IT
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91L is not a large system.


But it does have spin.
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423 DestinJeff "You know what this is? A fish cast. [picture] "

Must be one of them thar newfangled BPfish.

MiamiHurricanes09 "How do you know? Models are now beginning to lock on to the system since it is a tropical depression.
415 oracle28 "I don't "know". For that matter, does anyone here "know"?? I'm just guessing from the models."

You're right about guessing... which I believe was MiamiHurricane09's point. As an Invest, 91L didn't have a well-defined center; and therefore didn't have a well-defined position to derive a decent model from.
Now that 91L has become TropicalDepressionFour, the models finally have a good position fix to start from. So the models will become far more accurate with their predictions than they have been.
And the modelers had yet to release their path&intensity predictions derived from that good position fix when MiamiHurricane09 posted.
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Quoting Levi32:
Good morning all.

Blog Update.

Tropical Tidbit for Monday, August 2nd, with Video


amen just in time!!!
Member Since: July 3, 2010 Posts: 0 Comments: 427

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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.