Tropical Storm Don unimpressive so far

By: Dr. Jeff Masters , 2:08 PM GMT on July 28, 2011

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Tropical Storm Don formed yesterday from an African tropical wave that crossed into the Gulf of Mexico, and the thus-far unimpressive storm appears poised to bring tropical storm conditions to the lower Texas coast by Friday night. An Air Force Hurricane Hunter plane arrived in the center of Don around 8am EDT this morning, and has found Don to be a small tropical storm with top winds near 40 mph. The 7:57am EDT center fix found a central pressure of 1002 mb, which is 2 mb higher than NHC was estimating in its 8am EDT advisory. However, a pass through the center at 9:49am EDT found the pressure had dropped 2mb, to 1000 mb. Top reliable surface winds seen by the Air Force plane with its SFMR instrument as of 9:45am EDT were 41 mph, at 8:10 am EDT. 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 10 - 15 knots of wind shear. The shear is from strong upper level winds out of the north. Since the atmosphere to the north of Don is relatively moist, the moderate shear will not be as damaging to the storm as if these winds had been blowing from the northwest, where the driest air lies. Thus the shear direction is often just as important as the strength of the shear, and in Don's case, the shear direction should not force significant amounts of storm-disrupting dry air into the core. Water temperatures in the Gulf of Mexico are near 29°C, which is 2.5°C above the 26.5°C threshold typically needed to maintain a tropical storm.


Figure 1. Morning satellite image of Don.

Satellite imagery shows that Don is a very small storm. Thus, the storm is vulnerable to pockets of dry air and modest-sized jets of wind shear that we can't see from the relatively coarse-resolution data collected by surface stations, hurricane hunter flights, and satellites. The moderate wind shear over Don is keeping Don's circulation tilted so that the surface center is displaced from the center at higher levels. This tilt is keeping the storm from intensifying. Latest visible satellite loops show a modest increase in the intensity of the thunderstorm's near Don's center began at 9am EDT, but this could be a transient burst and not a sign the storm is undergoing intensification.


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 be a boon or bane for Texas? The state is currently suffering through its worst drought in recorded history, and 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. However, we have a Goldilocks problem. We can't have Don intensifying into a hurricane, or its winds and flooding might bring hundreds of millions in damage. Neither do we 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. This forecast is low-confidence, though, since Don's small size makes it prone to sudden changes in strength, either upward or downward. NHC is giving Don just a 14% chance of intensifying into a hurricane in its 5am advisory, but this could easily change upward if Don manages to overcome its vertical tilt and start consolidating an eyewall. I put the odds of Don reaching hurricane strength at 30%. None of the computer models is predicting Don will become a hurricane.

A small system like Don is relatively difficult to resolve in some of the computer models we use to forecast tropical storm track, and the forecast tracks of Don from these models have a higher spread than usual. For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast, which is showing that Corpus Christi and Port O'Connor, Texas have the highest chance of 39+ mph winds: 40%.

I'll have a new post this afternoon.

Jeff Masters

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Quoting TropicalAnalystwx13:
10:00 AM CDT Thu Jul 28
Location: 24.1°N 90.1°W
Max sustained: 45 mph
Moving: NW at 14 mph
Min pressure: 1001 mb

Angiest, you beat us again!

the pressure rose yet the winds picked up.. interesting..
Member Since: July 25, 2011 Posts: 0 Comments: 1102
#16 in the ignore list........and growing!
Member Since: Posts: Comments:
Why doesn't the person go on ignore when I add 'em to my ignore list. Does this mean he has hacked Wunderground?
Member Since: July 6, 2010 Posts: 109 Comments: 30268
48 hour forecast NCEP...
Member Since: September 27, 2007 Posts: 1 Comments: 19564
136. Vincent4989

Quit quoting posts with personal info....remove it!
Member Since: September 23, 2005 Posts: 13 Comments: 10466
Quoting TropicalAnalystwx13:


I know, it doesn't work.


Troll came back with a slightly different name.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
DeborahNagy troll
Member Since: Posts: Comments:
10:00 AM CDT Thu Jul 28
Location: 24.1°N 90.1°W
Max sustained: 45 mph
Moving: NW at 14 mph
Min pressure: 1001 mb

Angiest, you beat us again!
Member Since: July 6, 2010 Posts: 109 Comments: 30268
Quoting TropicalAnalystwx13:
Looks like northerly shear...That isn't good for Don. Looks like it will always be troubling Don, all the way up to landfall.



And why can't I ignore that troll?!

about 10 to 15 knots.. once it moves more westerly it should reduce to 5 to 10 knots shear and dry air is the inhibator
Member Since: July 25, 2011 Posts: 0 Comments: 1102
90. Vincent4989

Edit to remove info
Member Since: September 23, 2005 Posts: 13 Comments: 10466
Quoting hurricanehunter27:
Ok im trying to ignore him and i updated my ignore list, yet he is still there.


I know, it doesn't work.
Member Since: July 6, 2010 Posts: 109 Comments: 30268
Tropical Storm Warning from Port Mansfield to San Luis Pass.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting Vincent4989:

TROLL ALERT! RING THE BELLS!


REMOVE THE QUOTE!!!!!!!!
Member Since: Posts: Comments:
I can't believe we have to deal this this spammer chum a second day in a row
Member Since: Posts: Comments:
Ok im trying to ignore him and i updated my ignore list, yet he is still there.
Member Since: Posts: Comments:
Finally, so whats the lowest pressure in don guys?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Looks like northerly shear...That isn't good for Don. Looks like it will always be troubling Don, all the way up to landfall.



And why can't I ignore that troll?!
Member Since: July 6, 2010 Posts: 109 Comments: 30268

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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.