Tropical Storm Don unimpressive so far
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
Reader Comments
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My thought remains Matagorda Bay to High Island, but I am prepared to trim the eastern edge of that off, maybe to Freeport. My western edge is about 50-80 miles up the coast from the approximate landfall forecast from the NHC (if you connect the tropical points with a straight line).
That could allow more modest strengthening and stacking as Don pulls away from the shear axis.
Are you really suggesting that TS Don will erode to an open low because of the Northern shear or are you simply saying that the N shear is 5-10kts?
Hmmm.. what shear?? Please point out 20 knots or more shear on this map.
All I see is 5-10 knots.. Dry air is the factor with the storms fall.
Link
Elsewhere, I am keeping a close eye on P010L in the central atlantic which could become Emily down the road. Has a 1008mb low embedded along the wave axis with some very well defined cyclonic curvature. Convection needs to increase(it appears to be doing so) and it will likely be labeled our next invest.
Ill be back on later, gone for most the day.
I can't really agree based on the steering charts. Unless it makes it to mid cat 1-cat 2 strength. Those charts would support a more westward motion, but that has always been above my high end. But even at that, what we might see is more northerly motion followed by a bend further west, much closer to the middle or upper Texas coast.
Holy cow -- look at the spin in that Central-Atlantic wave!
Choke Canyon (near Corpus Christi)
Water Level
211.39 FEET
Thursday, July 28, 2011
7:30:00 AM
Level is 9.11 feet
below full pool of 220.50
http://chokecanyon.uslakes.info/Level.asp
Wishful thinking. My hubby says, "If its stupid and it works, then its not so stupid." LOL
Yup, it is easily seen on water vapor, and even better on TPW above^^. As i just noted convection is increasing, and i wouldn't be surprised if the crayons came out soon.
You need a newer image, the landfall has moved a tad north.
That might become a Monster for the Caribbean! They need to watch this close. CAPE VERDE is here now!
Just drove thru your area a few weeks ago on I-10 going and coming from La Berge casino resort in Lake Charles
91L soon ? when u think they will tag this one tia
Well, Don is located between the 10 and 20 knot lines, so....I just said 10-20 knots. Its probably closer to 10 knots though.
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No, the wind shear is 5-10kts. Not Don :)
Anytime.
Be Texas's luck that'll be the one to come blast us away next week
He is thinking a weak Category 1, but said it could range from a strong TS to Category 2. Anything is possible, you never know.
12z Early Cycle NHC model tracks
Don
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)
Dynamic Models (More sophisticated models)
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