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 DaytonaBeachWatcher:


I dont know why but i feel certain Drak was being facetous.


I wasn't
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For a season that was suppose to be active,91l going to die according to Dr Masters,and nothing else for awhile to watch,this season has scored a big fat O!!
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Quoting BadHurricane:


It seems that the biggest problem are dry air. What do you think?
looks like TD4 will need a camel!
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854. xcool
;
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AOI

AOI

AOI

AOI

AOI

TS BUSTED FORECAST ALIBI
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AL, 04, 2010080218, , BEST, 0, 128N, 418W, 30, 1006, TD
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TD4 is moving west at this time. I expect this motion to continue for the next 12 to 24 hours. This west movement is due to the small size of the system and the rapid forward speed. I am a little concern with the future track since it is very possible that the north turn will began rather later in the forecast period. I am praying that this storm does not make it close to Hispanola. Land interaction would destroy such a weak system but it would also cause disaster to those poor people in Haiti.
Member Since: August 10, 2009 Posts: 0 Comments: 164
Quoting MiamiHurricanes09:
Same here. Very interesting I've gotta say with the CMC as the king of the hill.


I dont know why but i feel certain Drak was being facetous.
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Quoting BadHurricane:


It seems that the biggest problem are dry air. What do you think?
Is that the desert?
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809 Skyepony "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..31, followed by MM5B..42, the BAMs, HWRF, LGEM, GFDL & LBAR were in the upper 80s, the rest over 100."

Thanks for keeping an eye on their track records. Handy to know which models deserve the most attention.
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Quoting IKE:
TD4 is in need of...badly. Not sure it's going to survive....



You're not sure if anything is going to survive these days :P .. Just messing with you, no hard feelings..
Member Since: July 1, 2008 Posts: 13 Comments: 7340
846. xcool
12z Euro is fishy
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Never turn the royal hiney on the tropics

OUCH

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It seems that the biggest problem are dry air. What do you think?
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Quoting Twinkster:
all you people take what levi said out of context. All he is stating is that the northern periphery of TD 4 is in much cooler waters than it has been the last couple of days. This acclamation taking place has most likely caused a slight decrease in convection and that plus the dry air to its north is limiting the outflow.

The acclamation period is important for any weather system. this can be compared to when the tropical waves over africa exit the coast and lose convection due to the acclamation to the ocean


Acclimation: physiological adjustment by an organism to environmental change

Acclamation: a loud eager expression of approval, praise, or assent
Member Since: August 2, 2006 Posts: 10 Comments: 9922
It will be interesting to see how this changes.

Click to enlarge





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Quoting IKE:
TD4 is in need of...badly. Not sure it's going to survive....

CLEAR!!
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Quoting Snowlover123:


What's the Link?


I'm on Google Chrome so you gotta copy and paste it.. sorry.

http://weather.smartgreenhelp.com/allan/tropical-depression-4-forms-colin-coming-soon
Member Since: July 1, 2008 Posts: 13 Comments: 7340
Quoting Drakoen:


Thanks for that forecast track skill update
Same here. Very interesting I've gotta say with the CMC as the king of the hill.
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Quoting Drakoen:


Green line = CMC


I though maybe that was what it was. Not sure why the legend isn't on the graphic yet.
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Quoting GBguy88:



Heat index here just broke 130. I feel like I should go outside and do something, just for the fact that we don't very often have a summer quite this hot.

You are welcome to THAT....
I would likely shrivvel-up and die in that heat.
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Quoting Levi32:


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.



Relatively warm waters however you wish to slice it.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
835. Skyepony (Mod)
Quoting Drakoen:
The CMC had the best track skill level beyond 84 hours in 2009


It's slowly improved this year. Both it & the MM5s started way off but showed well the last one too. They tweeked the CMC in i think beginning of 2009.
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834. IKE
TD4 is in need of...badly. Not sure it's going to survive....

Member Since: June 9, 2005 Posts: 23 Comments: 37858
833. xcool
12z Euro fish again open wave
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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.
Good blog! I pick scenario 2 as the most likely too.
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Quoting angiest:
12Z late dynamic models. Not sure who that Green line is.



Green line = CMC
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Quoting xcool:
LMAO
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Quoting Progster:


You are right, its a new (potential) development..but any feature upstream of TD 4 will likely affect its behavior.


It's too close to South America too. Not unheard of but that is one that I will believe when I see.
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Quoting Skyepony:
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.


Thanks for that forecast track skill update
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12Z late dynamic models. Not sure who that Green line is.

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Quoting angiest:


That didn't look to be related to TD04.


You are right, its a new (potential) development..but any feature upstream of TD 4 will likely affect its behavior.
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Quoting pottery:

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

Sorry for you LOL.



Heat index here just broke 130. I feel like I should go outside and do something, just for the fact that we don't very often have a summer quite this hot.
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824. xcool
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Quoting Snowlover123:


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.
Thats some "cold'" bath water! I like mine between 110--120degrees.LOL Im just messin with you,but seriously though if you took a bath in 83 degrees it would feel cold!
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Td4 looks quite ragged it needs to bump its thunderstorm activity a bit more , and slow down
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Quoting Drakoen:
The CMC had the best track skill level beyond 84 hours in 2009


But as observed with Alex, it did poorly on that storm.
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819. IKE
Member Since: June 9, 2005 Posts: 23 Comments: 37858
all you people take what levi said out of context. All he is stating is that the northern periphery of TD 4 is in much cooler waters than it has been the last couple of days. This acclamation taking place has most likely caused a slight decrease in convection and that plus the dry air to its north is limiting the outflow.

The acclamation period is important for any weather system. this can be compared to when the tropical waves over africa exit the coast and lose convection due to the acclamation to the ocean
Member Since: Posts: Comments:
Quoting reedzone:


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


What's the Link?
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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.
I'm on my way...! :)
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The CMC had the best track skill level beyond 84 hours in 2009
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Quoting Progster:


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.


That didn't look to be related to TD04.
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Quoting MiamiHurricanes09:
18z model plots should be out in less than 30 minutes... I'm expecting a shift towards the left.


Hey buddy, just posted a new blog on HurricaneJunkies website, go check it out. I think you'll be interested.
Member Since: July 1, 2008 Posts: 13 Comments: 7340
Quoting KEEPEROFTHEGATE:


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

Seeing that.......

Hi Aqua..........
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Quoting MiamiHurricanes09:
18z model plots should be out in less than 30 minutes... I'm expecting a shift towards the left.


Same here. Just like what happpened with Alex, when they shifted gradually to the south.
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810. xcool
ha
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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.
Member Since: Posts: Comments:

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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.