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 PcolaDan:
Off topic, but........ Levi?

I dont like there area were the earthquake is, might have been some large landslides to accompany it.
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Quoting NotCircumventing:


I suspect the steering influences are such that an ultimate landfall location 100 miles (or so) down the coast from Galveston Bay (or so) is likely.

East Matagorda Bay is (roughly) that area.

Just don't think the steering high will have quite as much influence, that is the only reason for my thought of EMB.


Sweet! That puts me in the NE quadrant. I need the rain!!
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She's gone....thanks Admin for the quick response.
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No lifer ... You can use a phonebook. Congratulations.
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I'm out...
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Quoting BrockBerlin:
Levi could you explain SFMR it can't be extrapolation as they are often greater than flight level, I am confused how they come up with it.


SFMR stands for Stepped-Frequency Microwave Radiometer. What it does is it senses radiation in the microwave band which is emitted from the sea surface. When strong winds blow over the ocean surface, it upsets the sea and creates waves and foaming, which emits higher levels of microwave radiation. The stronger the winds, the more foam there is, and the higher levels of energy that the SFMR detects. Calculations can then be done with computers to determine the wind speed from the microwave radiation readings, even through rainfall.
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We now have another troll -____- great... i came here to talk about the tropics and we get another one?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
My ignore list is expanding again today I see...
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OMG HE IS BACK!!1
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Off topic, but........ Levi?

Member Since: August 22, 2008 Posts: 12 Comments: 6010
Quoting TropicalAnalystwx13:
Good morning all. Just scrolling through the comments, it appears Don should be up to 45-50 mph at 11AM, despite its poor satellite appearance. It is having some problems with shear and dry air right now.

It's a small little system:

If it gets convection to its north ill give it a 70% of getting to hurricane satus.
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Come on! I just got up! Really? :|
Member Since: July 6, 2010 Posts: 112 Comments: 31337
04L/TS/D
MARK
23.11N/90.12W


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Quoting LoneStarWeather:


Just curious, what makes you say East Matagorda Bay area?


Well, applying the XTRP model to the two most recent VORTEX positions does support East Matagorda Bay. ;P
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting Levi32:


Many of his struggles were to be expected given the environment around him.


Don't forgot though other tropical storms developed in a much worse environment than don and looked a lot worse. It's nice to see this tropical storm take a path to texas. hope we get more waves that take a similar track but i dont want to see a major hurricane take this track
Member Since: July 25, 2011 Posts: 0 Comments: 1102
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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.