Don battling dry air and wind shear

By: Dr. Jeff Masters , 7:37 PM GMT on July 28, 2011

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Tropical Storm Don continues to be an unimpressive low-end tropical storm as it continues northwest towards the Texas coast. Don formed yesterday afternoon from an African tropical wave that moved into the Gulf of Mexico under a region of low wind shear. Don's formation date of July 27 is nearly a month ahead of the usual August 23 date for the arrival of the season's fourth named storm of the year. There is currently no hurricane hunter airplane in Don, and a new airplane is not due in the storm until tonight. The last center fix at 1pm EDT found surface winds of 45 mph and a central pressure of 1005 mb, a 4 mb rise from earlier this morning. Water vapor satellite images show a region of dry air to the northwest of Don, over the western Gulf of Mexico. Wind shear as diagnosed by the University of Wisconsin CIMSS group and the SHIPS model show a moderate 5 - 15 knots of shear from strong upper level winds out of the north. This shear is creating problems for Don by injecting dry air into the system. Visible satellite imagery from early this afternoon showed the presence of surface arc-shaped clouds expanding outwards to the north from the center of Don. These type of clouds are a sign that the storm is struggling with dry air. When dry air at middle levels of the atmosphere gets injected into thunderstorms due to wind shear, the dry air tends to create strong downdrafts that rob the storm of moisture. These downdrafts spread out at the ocean surface and create arc-shaped surface cumulus clouds.


Figure 1. Visible satellite image of Don from pm EDT July 28, 2011, showing arc-shaped surface clouds--the tell-tale sign of dry air interfering with the storm's organization.


Figure 2. The latest drought map for Texas shows that over 75% of the state is in exceptional drought--the highest category of drought. Image credit: U.S. Drought Monitor.

Forecast for Don
The big question for Don is, will it bring significant rains to Texas? According to the National Climatic Data Center, the six-month period ending in June 2011 was the driest on record. Average rain between January and June was more than eight inches (203 millimeters) below average in Texas, and the state experienced record heat between April and June. The heat and lack of rain have brought exceptional drought--the highest category of drought--to over 75% of the state. Don has the potential to bring some decent drought-busting rains to the state. If Don can expand in size and intensify to a 50 - 55 mph tropical storm, it has the capability to bring hundreds of millions of dollars worth of beneficial rains to the state. We don't want Don to stay in its current state, which is too small and weak to bring significant rains to Texas. If Don follows the current NHC forecast, which brings the storm up to a moderate-strength tropical storm, that would be just right. Don's small size makes it prone to dry air and wind shear, though, and it is uncertain whether the storm can overcome these problems enough to become a significant rain maker. NHC gave Don a 12% chance of intensifying into a hurricane in the 11am advisory, which is a reasonable forecast, since Don is running out of time to get its act together in time to become a hurricane. None of the computer models is predicting Don will become a hurricane.

For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast. The 11 am version of this forecast shows that Port O'Connor, Texas has the highest chance of tropical storm-force winds (39+ mph): 45%.

New hurricane archive search feature
The autocomplete entities in the wunderground search box has been extended to include hurricanes, so you can now search for a storm by name, year, or basin. Here are some examples in case you feel like exploring your new options:

By name:

Hurricane David - Atlantic, 1979
David, Major Hurricane - Atlantic, 1979
Major Hurricane David - Atlantic, 1979

By year:

2005 Hurricanes Atlantic
2007 Hurricanes Eastern Pacific

By basin:

Hurricanes Western Pacific 2011
Hurricanes Atlantic 2008

By category:

Tropical Storms Atlantic 2005
Tropical Depressions Indian Ocean 2011
Subtropical Storms Eastern Pacific 2010
Extratropical Storms Western Pacific 1988

I'll have a new post Friday morning.

Jeff Masters

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Quoting MyrtleCanes:
Euro looks to be heating up on the SE coast in the next week or so

http://raleighwx.americanwx.com/models/ecmwf.html

will be interesting to see the 12z


12Z shows the central Atlantic wave becoming a tropical cyclone and affecting the Bahamas before re-curving between the East Coast and Bermuda. Also shows possible activity off the SE Coast.
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I was out for lunch and when i come back i see Don has gotten control of his convection again GO DON you can do it.
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Euro looks to be heating up on the SE coast in the next week or so

http://raleighwx.americanwx.com/models/ecmwf.html

will be interesting to see the 12z
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Thx Doc for the update.
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Quoting cchsweatherman:
No reliable computer models show significant tropical development in the near future and I have to agree with the computer models here for a few reasons that I have noted here.

1) Developing TUTT over the Central Atlantic into the Eastern Caribbean: This will pretty much create a "wall" of hostile upper level winds and will inhibit tropical waves coming across the Atlantic from developing.

2) Deep subsidence across the Main Development Region: The combination of Saharan air coming across the entire Atlantic as well as downward motion and reduced instability in the region will limit convection.

3) Only isolated pockets of favorable conditions across the entire Atlantic Basin: The only area that is somewhat favorable for development at this time are the SW Atlantic as the rest of the basin deals with either deep layered dry air and high pressures or hostile upper levels or both.


Maybe not significant no, but every model besides the GFS develops this wave into a tropical cyclone, the one in the central Atlantic.



Even then, the GFS develops a well-defined low pressure area.
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Previous blog said this was the worst drought on record in Texas, current blog reads " According to the National Climatic Data Center, the six-month period ending in June 2011 was the driest on record." Thanks for the correction and citation. I think that does match the State Climatologist now.
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TS Don Floater - RGB Color Infrared Loop

Zoom,Boxes and TFP's availible
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Thanks! All that dry air coming off of Texas LOL
We can't win.
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Quoting j2008:

That plus the bermuda high being stronger thn normal doesnt stack up well for anybody in the Atantic.
It's not necessarily stronger, the negative NAO actually indicates that the high is weaker than normal. The high is just more spread out than normal, which is typical of negative NAOs since a negative NAO means a weaker pressure gradient between the Azores high and Iceland Low. This implies a weaker Icelandic low, which means less troughing over the W Atlantic. This allows the high to expand westward more and means less opportunities for troughs to pull out storms over the tropical Atlantic.
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Thanks for the update
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You're earnin your keep today Doc...thanks.
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No reliable computer models show significant tropical development in the near future and I have to agree with the computer models here for a few reasons that I have noted here.

1) Developing TUTT over the Central Atlantic into the Eastern Caribbean: This will pretty much create a "wall" of hostile upper level winds and will inhibit tropical waves coming across the Atlantic from developing.

2) Deep subsidence across the Main Development Region: The combination of Saharan air coming across the entire Atlantic as well as downward motion and reduced instability in the region will limit convection.

3) Only isolated pockets of favorable conditions across the entire Atlantic Basin: The only area that is somewhat favorable for development at this time are the SW Atlantic as the rest of the basin deals with either deep layered dry air and high pressures or hostile upper levels or both.
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I need a parker over north Central Texas...
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Thanks Doc. Lets get some rain to Texas!
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I like the idea of a better search function of the hurricane archive.

You could go further, but it's a nice start.
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Thanks Doc!
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Thank You
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Thank You!
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About

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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