Don battling dry air and wind shear
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
Reader Comments
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My take is that models are overdoing the intensity in the short term!
GFS brings it dangerously close to Shanghai, moving NW.
Euro much farther south bringing it very close to Taipei, Taiwan and then into the China coast.
Its pouring in Friendswood as well. Got alot of rain yesterday
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Arlen is a small town South of DFW probably down I-35 towards Austin. No rain for Arlen yet....
This fictional Town is right has severe drought conditions that is making Hank Crazy because he can't water his grass enough to due restrictions. Hank is furious because his lawn is starting to look like Bill's.
Dale is raking in the business as fire ants have been on the move as the drought continues.
Peggy as always is having issues with the low flow toilets that have been brought back due to the drought.
Boomhauer is mad because his chick magnet boat is high and dry in its boat slip and the lake is so low all the tree stumps are showing making any activity dangerous.
Propane sales are down as it is too hot to grill.
Drinking beer in the alley is only done in the early morning as Bill had heat stroke waving to cars on the highway. (Again)
The gang is riding out the drought by heading to the water park (Based on the very real water park Schlitterbahn and its Flowrider endless wave The Boogie Bahn)
0z ECM ensemble model
None of the other models show ex Don (when he becomes inland) touching my State, we had too much rain here !
So far our July: 3 storms (but if Emily forms on sunday then we could squeeze out 4.)
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 30/1100Z TO 31/1100Z JULY 2011
TCPOD NUMBER.....11-059 AMENDMENT
I. ATLANTIC REQUIREMENTS
1. NEGATVIE RECONNAISSANCE REQUIREMENTS.
2. OUTLOOK FOR SUCCEEDING DAY:
A. INVEST SUSPECT AREA AT 31/1800Z NEAR 14.0N 54.0W.
it also depends on the strength when it reaches those coordinates...i know you said most likely and those are key words but on sept 20-21 storm was at:
25.2 71.0 cat 2
26.3 72.2 cat 2
27.2 73.4 cat 3
28.0 74.9 cat 3
have noticed more so where it is when it rounds PR
Some very pretty clouds and stuff.
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