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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1059. cirrocumulus
8:55 PM GMT on August 02, 2010
Member Since: September 30, 2005 Posts: 0 Comments: 1606
1058. MTWX
8:21 PM GMT on August 02, 2010
Quoting Thundercloud01221991:


that is a huge area under Excessive Heat Warning including the entire state of MS

here in Columbus, MS it is 102 with a 115 heat index
Member Since: July 20, 2009 Posts: 23 Comments: 1393
1057. Dodabear
8:12 PM GMT on August 02, 2010
Quoting angiest:
Wow, blog has ground to a halt this afternoon.


NEW BLOG!!!!! NEW BLOG!!!!! NEW BLOG!!!!!
Member Since: July 28, 2001 Posts: 0 Comments: 2279
1056. angiest
8:08 PM GMT on August 02, 2010
Quoting rmbjoe1954:


You have got to be kidding. On what basis?


The entire Gulf and Atlantic coasts should be evacuated at this time, until we know where it is going.
;)
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1055. angiest
8:07 PM GMT on August 02, 2010
Wow, blog has ground to a halt this afternoon.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1054. rmbjoe1954
8:04 PM GMT on August 02, 2010
Quoting NewYork4Life:
Should they begin evacuating the Outer Banks?


You have got to be kidding. On what basis?
Member Since: June 16, 2005 Posts: 0 Comments: 1380
1053. wunderkidcayman
7:59 PM GMT on August 02, 2010
convection is starting to pop with TD4 on the NW side
Member Since: June 13, 2009 Posts: 2 Comments: 12180
1052. Sfloridacat5
7:55 PM GMT on August 02, 2010
Quoting IpswichWeatherCenter:


By the looks of it the CMC keeps 04L weak however which means that it would go further south.


It will be interesting to watch. It shouldn't take too long to see if TD4 is going to follow the CMC path or continue on a more WNW path as the NHC forecasts.
Member Since: September 16, 2007 Posts: 0 Comments: 7541
1051. IpswichWeatherCenter
7:53 PM GMT on August 02, 2010
Quoting Sfloridacat5:
CMC looks to be too far South in Latitude. Following the CMC,it takes 94L until close to 60 West Longitude to get north of 15 degress North Latitude.
The NHC forecast has 94L reaching 15 degrees North at 50 West.


By the looks of it the CMC keeps 04L weak however which means that it would go further south.
Member Since: April 27, 2008 Posts: 29 Comments: 2097
1050. cirrocumulus
7:52 PM GMT on August 02, 2010
This is the year we may get a hurricane like Camille.
Member Since: September 30, 2005 Posts: 0 Comments: 1606
1049. Sfloridacat5
7:49 PM GMT on August 02, 2010
CMC looks to be too far South in Latitude. Following the CMC,it takes 94L until close to 60 West Longitude to get north of 15 degress North Latitude.
The NHC forecast has 94L reaching 15 degrees North at 50 West.
Member Since: September 16, 2007 Posts: 0 Comments: 7541
1048. TOMSEFLA
7:44 PM GMT on August 02, 2010
looks like cmc in that area
Member Since: October 19, 2005 Posts: 0 Comments: 234
1047. fmbill
7:43 PM GMT on August 02, 2010
Interesting GFS model

Cyclone Phase GFS 12Z
Member Since: May 27, 2008 Posts: 0 Comments: 461
1046. wunderkidcayman
7:42 PM GMT on August 02, 2010
I say wait for the G-IV to fly then we see what the models really say then
Member Since: June 13, 2009 Posts: 2 Comments: 12180
1045. TOMSEFLA
7:39 PM GMT on August 02, 2010
seems the cmc models is predicting td4 to weaken to an open wave by 68w then move wnw find environmental conditions fav for strenthing in se
bahamas and become a strong ts off se fla. this is a possibility??
Member Since: October 19, 2005 Posts: 0 Comments: 234
1044. wunderkidcayman
7:37 PM GMT on August 02, 2010
to answer your question TampaSpin with the CMC model and the cone is that the cone that you posted is from this morning and I would look for the new track at 5pm
Member Since: June 13, 2009 Posts: 2 Comments: 12180
1043. srada
7:34 PM GMT on August 02, 2010
I just dont see the CMC being the model that is right and the other models are wrong..the new and improved GFS looks like it has been consistent for the past two runs..
Member Since: August 17, 2006 Posts: 1 Comments: 774
1042. IpswichWeatherCenter
7:34 PM GMT on August 02, 2010
Quoting TampaSpin:



Is that giant Dry Air area moving at the same pace as 04L or faster/slower?
Member Since: April 27, 2008 Posts: 29 Comments: 2097
1041. KEEPEROFTHEGATE (Mod)
7:32 PM GMT on August 02, 2010
Quoting TOMSEFLA:
bill gray update for aug due out on the 4th. will see ifthey drop the # of storms. think they will drop numbers by 30%
we will know on wed. or thurs. i think there will be a downward turn in numbers if the euro verifies and nothing till after the 12th or 15th there could be a big drop
Member Since: July 15, 2006 Posts: 174 Comments: 54623
1040. XLR8
7:31 PM GMT on August 02, 2010
Quoting Orcasystems:


Last time KOG turned the fans on.. you guys screamed blue murder... heck you even had to wear socks with your flip flops.


No No not me I would never do that. Socks with flip flops that is....
Member Since: February 17, 2009 Posts: 0 Comments: 205
1038. LouisianaWoman
7:30 PM GMT on August 02, 2010
Quoting sailingallover:

One thing is the mid atlantic ridge has a weakness(area of lower pressure) along the cold front/trough that is sitting a few hundred miles off the east coast. TD4/Colin will go up into that weakness.
The lower pressure will suck it in plus it already has a natural trend to go north if it can due to the Coriolis effect.
While there is always a chance the track will radically change Louisiana doesn't have to worry about this one at the moment..


Thanks for the response! My question wasn't out of concern of it hitting Louisiana, just the curiosity of the curving that is scheduled to take place.
Member Since: August 10, 2009 Posts: 0 Comments: 64
1037. BrandiQ
7:30 PM GMT on August 02, 2010
I have a question, why is the CMC so different forecasting wise vs the GFS and the other forecasting models which call for TD4 to curve to the north much earlier and sharper then what the CMC calls for?
Member Since: May 25, 2009 Posts: 0 Comments: 164
1036. help4u
7:30 PM GMT on August 02, 2010
I think maybe 9 or 10 storms at most now with 3 hurricanes and no majors.Shear and dry hot air are the words for this season.
Member Since: September 18, 2006 Posts: 0 Comments: 1289
1035. pottery
7:29 PM GMT on August 02, 2010
Quoting Orcasystems:


Join the crowd


Take THAT !!
heheheh
Member Since: October 24, 2005 Posts: 0 Comments: 24452
1034. FirstCoastMan
7:28 PM GMT on August 02, 2010
New blog is out!!!
Member Since: August 7, 2009 Posts: 0 Comments: 251
1033. IpswichWeatherCenter
7:28 PM GMT on August 02, 2010
Quoting MiamiHurricanes09:
Unless TAFB goes with T3.0 it is likely that 04L will stay a tropical depression at 5PM. Reason I say this is because SAB is already at T2.0, if TAFB gives it a T3.0 I would assume that the NHC go down the middle with a "T2.5" which equals to a 35 knot cyclone.


I'd agree, or maybe decrease down to 25kt - its sat appearance isn't as good as it was earlier.
Member Since: April 27, 2008 Posts: 29 Comments: 2097
1032. Orcasystems
7:28 PM GMT on August 02, 2010
Quoting Thundercloud01221991:


that is a huge area under Excessive Heat Warning including the entire state of MS


Join the crowd

Member Since: October 1, 2007 Posts: 81 Comments: 26511
1031. pottery
7:27 PM GMT on August 02, 2010
Quoting Orcasystems:


Last time KOG turned the fans on.. you guys screamed blue murder... heck you even had to wear socks with your flip flops.

I remember that. Never content........
Member Since: October 24, 2005 Posts: 0 Comments: 24452
1030. sporteguy03
7:26 PM GMT on August 02, 2010
Quoting help4u:
So the # 1 EURO model shows nothing happening thru aug 12.Not an active season at all.


#1 model?
Member Since: July 7, 2005 Posts: 0 Comments: 5361
1029. TampaSpin
7:26 PM GMT on August 02, 2010



For sure the CMC is seeing something the others don't. Thata be for sure.....LOL

Member Since: September 2, 2007 Posts: 178 Comments: 20443
1028. Thundercloud01221991
7:26 PM GMT on August 02, 2010


that is a huge area under Excessive Heat Warning including the entire state of MS
Member Since: August 1, 2006 Posts: 28 Comments: 3716
1027. TexasHurricane
7:26 PM GMT on August 02, 2010
Are there any other areas of interest right now besides TD4?
Member Since: July 2, 2009 Posts: 0 Comments: 2811
1026. sailingallover
7:26 PM GMT on August 02, 2010
Quoting LouisianaWoman:
Ok so I have a question...

What are the models suggesting will be pulling TD4 toward the WNW-NW between 60-65w (if/when it gets there) ?

One thing is the mid atlantic ridge has a weakness(area of lower pressure) along the cold front/trough that is sitting a few hundred miles off the east coast. TD4/Colin will go up into that weakness.
The lower pressure will suck it in plus it already has a natural trend to go north if it can due to the Coriolis effect.
While there is always a chance the track will radically change Louisiana doesn't have to worry about this one at the moment..
Member Since: September 1, 2009 Posts: 23 Comments: 1007
1025. oakland
7:25 PM GMT on August 02, 2010
Quoting DestinJeff:
Don't forget now ... when it comes to models, shift happens.


LOL
Member Since: September 4, 2005 Posts: 41 Comments: 7526
1024. Orcasystems
7:25 PM GMT on August 02, 2010
Quoting XLR8:
103 and heat index 110, I cant take this. Orca turn the fans back on or else!


Last time KOG turned the fans on.. you guys screamed blue murder... heck you even had to wear socks with your flip flops.
Member Since: October 1, 2007 Posts: 81 Comments: 26511
1022. AstroHurricane001
7:25 PM GMT on August 02, 2010
Maybe this depression will dive underneath the dry air and redevelop farther west like pre-Alex (92L) did. If this happens TD4 will maintain a more southward track and may interact in a slight Fujiwara with the system behind it.
Member Since: August 30, 2008 Posts: 8 Comments: 2835
1021. pottery
7:25 PM GMT on August 02, 2010
Quoting DestinJeff:
Don't forget now ... when it comes to models, shift happens.

You meant 'muddles', surely?
But LOL to you!
Member Since: October 24, 2005 Posts: 0 Comments: 24452
1020. TOMSEFLA
7:24 PM GMT on August 02, 2010
bill gray update for aug due out on the 4th. will see ifthey drop the # of storms. think they will drop numbers by 30%
Member Since: October 19, 2005 Posts: 0 Comments: 234
1019. stormlvr
7:24 PM GMT on August 02, 2010
Quoting nrtiwlnvragn:


Ya and I can see their point. I think there is only 2-3 long range forecasters at HPC, at least from names I recognize in their discussions. There are about 10 NHC hurricane forecasters, so while HPC and NHC may agree for a few shifts, you get another forecaster come on board at NHC that may have a different forecast and now no longer coordinated between the two agencies.


Lordy--deviations from the NHC Tropical Forecast is allowed! LOL The 6 and 7 day point does give them a little wiggle room and a need for coordination. I have enjoyed the effort at HPC with the discussion focused on the Tropics this year. I hope they continue when things really heat up.
Member Since: August 12, 2007 Posts: 0 Comments: 323
1018. TampaSpin
7:24 PM GMT on August 02, 2010
Quoting Jeff9641:


That ULL will be a problem for Colin in the near down the road. Could be the cause for the models to trend left.


I would tend to agree with that thought as a weak system would go south is usually what does happen and stronger system would move North, but someone above just spoke of a stronger system moving South and a weaker moving North, which i am not sure of.
Member Since: September 2, 2007 Posts: 178 Comments: 20443
1017. Sfloridacat5
7:23 PM GMT on August 02, 2010
Quoting NewYork4Life:
Should they begin evacuating the Outer Banks?


Because there's a Tropical Depression in the middle of the Atlantic Ocean?
Member Since: September 16, 2007 Posts: 0 Comments: 7541
1015. help4u
7:23 PM GMT on August 02, 2010
So the # 1 EURO model shows nothing happening thru aug 12.Not an active season at all.
Member Since: September 18, 2006 Posts: 0 Comments: 1289
1014. XLR8
7:22 PM GMT on August 02, 2010
103 and heat index 110, I cant take this. Orca turn the fans back on or else!
Member Since: February 17, 2009 Posts: 0 Comments: 205
1013. cirrocumulus
7:22 PM GMT on August 02, 2010
Member Since: September 30, 2005 Posts: 0 Comments: 1606
1012. StormChaser81
7:21 PM GMT on August 02, 2010
Quoting cg2916:


Can't really tell.


Member Since: August 11, 2008 Posts: 0 Comments: 2315
1011. KEEPEROFTHEGATE (Mod)
7:21 PM GMT on August 02, 2010
Member Since: July 15, 2006 Posts: 174 Comments: 54623
1010. pottery
7:20 PM GMT on August 02, 2010
Quoting KEEPEROFTHEGATE:
we have civic holiday today too

Everyone is on Holiday??
I think I'll have a beer, for that!
Member Since: October 24, 2005 Posts: 0 Comments: 24452
1009. TampaSpin
7:20 PM GMT on August 02, 2010
So those that believe the CMC is so correct.....i wonder why the NHC has it so dismissed.....as it does not even put it in the Southern Cone.

Member Since: September 2, 2007 Posts: 178 Comments: 20443

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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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