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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809. Skyepony (Mod)
12Z runs are interesting. Most have an ULL well to the NNW stall up on front coming off the US. TD04 is greatly hampered or shredded by ULL as it runs into it well off shore. 12ZCMC drops it south, runs over the islands skirting the ULL, slams a big storm into SEFL in 5 days. Why feature the CMC? well..

We now have more than 24hrs of models data on TD04...models overall didn't grasp this one well at the start..well except the CMC..31nm error, followed by MM5B..42, the BAMs, HWRF, LGEM, GFDL & LBAR were in the upper 80s, the rest over 100.
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Quoting KEEPEROFTHEGATE:


here is another for ya pott also interesting area in cen carb as well


I wonder if any of these areas (Area behind 04L, Central Carib. or the area near the Virgin Islands) will be mentioned in the next TWO?
Member Since: April 1, 2010 Posts: 9 Comments: 2699
18z model plots should be out in less than 30 minutes... I'm expecting a shift towards the left.
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Quoting Levi32:


Enhanced by the colder water. I'm not interested in arguing about this. It's over far colder water than the last few days. That never happens without some adverse effect on the cyclone.

Back later.


Must be something in the water you Alaskan folk are drinking but i'll play along...

Yes very very cold water. Uh Huh
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Quoting Neapolitan:


And anal-retentive is an adjective. :-)


Well, we are talking about potential Hurricane...uh...Colon, right?

CMC does split the energy and sends a a TD to Nicaragua on day 4...i think this is new.
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Quoting Bordonaro:

Wanna trade, my thermometer is stating its 104F here in Arlington, TX.

Temp 77F
Humid 100%
Pressure 1009 steady.
Rain................

Sorry for you LOL.
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here is another for ya pott also interesting area in cen carb as well
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Quoting pottery:

Drizzles now.
Rumbles in the distance.
Last heavy rains came in with a west breeze. Unusual.


Thunder comin'?

Thunder gone. :)
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Quoting Drakoen:


Wrong. The 26C isotherm extends up towards near 20N in the eastern Atlantic. The SHIPS text placed TD 4 under 28C SSTs. SAL naturally causes suppression and with that comes the evidence of stratocumulus clouds.


Enhanced by the colder water. I'm not interested in arguing about this. It's over far colder water than the last few days. That never happens without some adverse effect on the cyclone.

If it was barely sustaining deep convection bursts for any significant amount of time before collapsing yesterday, then what do you think is going to happen when you drop the SSTs 2-3C?

Back later.
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It's pulling in a lot of lower level moisture from the south. You can see it filling in on the RGB image.

RGB
Member Since: August 11, 2008 Posts: 0 Comments: 2315
inner nest zoom click then click to animate

Link
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Quoting scott39:
Hey folks, Dont be thrown by Levis use of the word "cold water", he doesnt mean its cold like in the winter time! Hes referring to that adjective, because TD4 has to move over warmer water to feed off it and develope more!


The cool water is 83 degrees. That's bath water. But the warmer the water, the more intensification of the Cyclone, so I guess you're right to a degree.
Member Since: April 1, 2010 Posts: 9 Comments: 2699
Do a NOAA loop and click on SST and note that TD4 is well within the warm waters of 28 to 29 degree Celsius temperatures. Behind it is "Big Dog" 5 heading to even warmer waters of 30 degrees Celsius.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
TD 4 must be freezing in those 28C SSTs.

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Quoting pottery:

Drizzles now.
Rumbles in the distance.
Last heavy rains came in with a west breeze. Unusual.

Wanna trade, my thermometer is stating its 104F here in Arlington, TX.
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794. Gorty
I have two questions:

Why does a TC need to have a closed low to be called a TC?

And how does a west wind closes if off?
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Quoting cirrocumulus:


It's in warm water of 28-29 degree Celsius according to NOAA.


NOAA verse AOML....? Lol.

Satellites show it on the edge of the 27C isotherm, and SSTs were 2-3C warmer during the last few days.

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Hey folks, Dont be thrown by Levis use of the word "cold water", he doesnt mean its cold like in the winter time! Hes referring to that adjective, because TD4 has to move over warmer water to feed off it and develope more!
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GFDL doesn't do much with 04L until later in the run...HWRF is the opposite as it strengthens 04L a bit in the near term, then weakens it some and doesn't do much else.
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Quoting KEEPEROFTHEGATE:
should clear out in a bit for ya seems to be slackening off

Drizzles now.
Rumbles in the distance.
Last heavy rains came in with a west breeze. Unusual.
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Quoting xcool:
12


The GFDL is already wrong. As Jasoniscoolman2010x said, it looks like as if the system is going to the west. The GFDL shows it going to the WNW. This shows the slight poleward bias associated with the GFDL, and in order to get an accurate representation of the track, shift it west and south, and that's trouble in the CONUS.
Member Since: April 1, 2010 Posts: 9 Comments: 2699
788. Prgal
This is a very nice site.

Link
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Quoting Levi32:


27C, in this case, is not enough, and the dry air, with nearly down to 26C just off to the northeast. The dry air is SAL yes but subsidence, drying, and a stable atmosphere are naturally enhanced by the colder water to the northeast. You can see it quite clearly on large visible.

One should remember that although SSTs are warmer than the threshold for tropical development, in some situations the storm will require more than 26C or 27C to really strengthen. It was over 2-3C warmer water during the last few days, and a 3C drop will obviously have an effect.


Wrong. The 26C isotherm extends up towards near 20N in the eastern Atlantic. The SHIPS text placed TD 4 under 28C SSTs. SAL naturally causes suppression and with that comes the evidence of stratocumulus clouds.
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Good day guys! Do you think TD4 will feel the trough strongly enough to start picking a more wnw-nw motion soon or when should we start to worry if it keeps with a more w-wnw track? :) Thanks in advance.
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Quoting pottery:
Post 741 Keeper...
I am under the eastern side of the green area.
HISSS CRACKLE BOOOMMM
should clear out in a bit for ya seems to be slackening off
Member Since: July 15, 2006 Posts: 169 Comments: 53296
779. xcool
12
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Quoting Levi32:


A look at the visible loop will tell you that it's over colder water and near even colder water.


It's in warm water of 28-29 degree Celsius according to NOAA.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Quoting Levi32:


Possible but I don't think it will be quite that far south, but we'll see. Land would certainly mess it up on such a track.
thanks
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00Z Euro seems to have a good handle on the ridging in the southern US. Will be interesting to see the 12Z when it finishes.
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Quoting Levi32:


For a tropical cyclone, it can be.


I thought that 78 Degrees F was the criteria needed for a TC?
Member Since: April 1, 2010 Posts: 9 Comments: 2699
Quoting scott39:
Levi, If 91L stays just S of the islands, it will avoid strong wind shear, but will it avoid land as it tracks? If it stays weak-- just S of the Islands make since to me.


Possible but I don't think it will be quite that far south, but we'll see. Land would certainly mess it up on such a track.
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Post 741 Keeper...
I am under the eastern side of the green area.
HISSS CRACKLE BOOOMMM
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Quoting Drakoen:


LOL! Give TD 4 a jacket


No, it needs a scarf, and a winter coat. :P
Member Since: April 1, 2010 Posts: 9 Comments: 2699
Quoting Jeff9641:
I almost died when Levi posted a comment saying there is cold water near TD 4. LOL! Unless you call cold being 83 degrees.


For a tropical cyclone, it can be.
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Quoting Drakoen:


There is no very cold water at all. The SSTs are not the issue, the dry air is And the dry air is not due to the water it is due to the SAL.



27C, in this case, is not enough, and the dry air with nearly down to 26C waters are just off to the northeast. The dry air is SAL yes but subsidence, drying, and a stable atmosphere are naturally enhanced by the colder water to the northeast. You can see it quite clearly on large visible.

One should remember that although SSTs are warmer than the threshold for tropical development, in some situations the storm will require more than 26C or 27C to really strengthen. It was over 2-3C warmer water during the last few days, and a 3C drop will obviously have an effect.
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Quoting Jeff9641:
I almost died when Levi posted a comment saying there is cold water near TD 4. LOL! Unless you call cold being 83 degrees.


LOL! Give TD 4 a jacket
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Quoting jasoniscoolman2010x:
tropical depression four is moving fast over 17 mph..its look like to me in the last three hours the storm is moving more west.


It has a slight element to the north, but yes, it is heading a little more west than before.
Member Since: April 1, 2010 Posts: 9 Comments: 2699
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Quoting Drakoen:
HWRF 12z has shifted to the left some.
As expected. Let's see what the GFDL shows...
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Quoting Levi32:


A look at the visible loop will tell you that it's over colder water and near even colder water.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
Levi, If 91L stays just S of the islands, it will avoid strong wind shear, but will it avoid land as it tracks? If it stays weak-- just S of the Islands make since to me.
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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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