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
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Hurricanes Western Pacific 2011
Hurricanes Atlantic 2008
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Tropical Storms Atlantic 2005
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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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Floater - Rainbow Color Infrared Loop
Zoom,Boxes and Tropical Points active
on approach
And their right i believe it will be and i have never left that stance and wont till landfall most everyone on here see's a weakness in Don then they say he's dead, how many times can you keep calling him dead when he's still alive and kicking?
Conditions were somewhat similar.
I'm thinkin that's at least 50 mph!!!!!!
And i expect that number to steadily increase because some people just don't see the whole picture that he's moving into favorable environment and will strengthen and possibly weaken but its not over till its over.
If they were the only 2 shakes in 2 days, probable...
But we've been having an average of maybe 7 shakes per day, and that releases energy, bit by bit..
But, we never know... when it will happen..
Im going to stick by my prediction of 55 by 8pm
I agree it has survived everything so why would it die now
What would be causing them to do that, and it is likely that the NHC track will tend to stay more where it is unless the tracks stop that back and forth?
And she peaked at 65 mph. Very possible at this time.
Its their projection on the strength of the ridge, and if I had my guess based on sat imagery i would say a little to the south of the current forecast track.
I have the same question as you. After lurking in the blog all afternoon, I got the feeling that the models were going to shift north again. Then the 4:00 post shifted back south. I am so confused! :S
Whats upp Stormjunkie???? Good to see ya on here :o)
and want to say thank you for all you did for Joplin and in Alabama :o) My mom lives up there in Joplin and from what she has told me it is still very "Bad" if you know what I mean....
Anyway Thank You my Friend
Taco :o)
I'm trying to learn and get a handle on these maps... it looks like Don is moving into wind shear at 5-10 kts, so will he have a better shot at continued development as he moves out of the higher shear?
Thanks, and hope it brings some much-needed rain for TX!
Don shouldn't make landfall until Saturday morning.
other 65mph TS
Not sure what causes it, but my guess is that the NHC would want to keep an average of what the models are saying. That is how they make thier cone, so I think it is safe to say that the cone will stay the same if all the models stay the same and vice versa.
by alot of shear you mean 10 to 15 knots right?
He isn't being affect by a lot of shear, it is his small size that makes weaker shear that wouldn't normally affect larger systems shear him.
Either way, he isn't having as much of a problem now.
Guilty as charged and damn proud of it!!!!!
How are you SJ? Don't get to see you much these days.
Regardless of what happens, Don can only be seen as a good thing. (Minus the random tornadoes in the NE Quadrant)
my background is geophysics, earthquake seismology is one of my major interests (i don't practice in the field, however.)
A few days ago a vid on here said a 9.0 could happen off shore of PR. This would mean a tsunami for the U.S. The guy said that activity should be increasing.
Yeah I was gonna say its not that there is a lot of shear affecting Don, its more that it's a small system that is easily impacted by 15kt shear.
Thanks, good facts to take in consideration....
So that would mean that even if we had 10 / 3.3 in mag. shakes, that would be similar to having just one 5.0?
Okay cool, thanks for the heads up!
lol it looks like DON wants too go too MX
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