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 hydrus:
Its a long way out, ( Aug-12 ), but this would be a monster trough for August...


This kind of looks like last year. I guess we might sneak one into TX here but we'll have to see how many make through the troughs again.

Last time I said something like that I got an irate WUmail. To that person there's no need to respond. I got your opinion on the matter loud and clear! Lol.
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Quoting Patrap:
Dons center Motion is really coming right to NW,,or slightly NNw
Can it recover if it starts going NNW?
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Quoting TampaSpin:



the intensity forecast has dropped like a rock!
,the highest forecast he had was around 65 I believe,not going to be our first hurricane,I'd be surprised if he makes it to shore as a TS,which would be better for tx
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Dons center Motion is really coming right to NW,,or slightly NNw
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Quoting Seawall:
When would the NHC generally classify a wave as an invest?
IMO should have a new nvest in each basin over the next 24hrs
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Right now Don looks like an egg that someone thru against a wall. Maybe he will pull himself together.
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its decoupling just like pat noted this may be an
un-recoverable dev.

04L/TS/D

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Quoting jasonweatherman2011:
the wind shear come down from the north is starting to go up!!


Good eye there Jason
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downdrafts on the wstrn flank?

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That NEast Shear is keeping Don in check
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Quoting Patrap:
Dons core is more than half exposed from the Convection


the wind shear come down from the north is starting to go up!!
Member Since: July 12, 2011 Posts: 0 Comments: 1532



the intensity forecast has dropped like a rock!
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Quoting TropicalAnalystwx13:


Yes, this may be our first serious storm of the season.
...this seems to be the one to watch next,its all about the environment ahead,if it goes north of PR,I'd guess a likely fish storm,if it goes south it could be a theater to the gom
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Quoting quakeman55:

The 06Z GFS takes that thing right into the central Caribbean...this'll get interesting for sure.

whooo (shudder) guess we should be hearing from Kmanislander once it gets around 50W (not funny)
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next invest coming soon!!
Member Since: July 12, 2011 Posts: 0 Comments: 1532
90.2W, 24N ... or so.
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Dons core is more than half exposed from the Convection


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When would the NHC generally classify a wave as an invest?
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Quoting hydrus:
Its a long way out, ( Aug-12 ), but this would be a monster trough for August...
u are going to see lots of storms going out to sea.
Member Since: July 12, 2011 Posts: 0 Comments: 1532
Quoting AtHomeInTX:
Our little Don not looking too strong today. I see a naked swirl racing more north than the convection. Here at the border I'm getting storms from the SW. Don't know what that says about the steering. But the rain is nice. :D

The weak La. disturbance will probably bring southeast texas more rain than I get from Don? Weathermen around here are not too optimistic with our 20 percent chance of rain. If Don does not come over your house then you will most likely not get much rain from him.
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Quoting Chicklit:
I believe we are approaching active part of hurricane season. Hoping Don is not too weak and not too strong. Welcome back Dr. Masters!!!

Link

That is one heck of a wave in the central atlantic.
Bet the guys at NHC are stocking up on coffee as we speak.

The 06Z GFS takes that thing right into the central Caribbean...this'll get interesting for sure.
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Central Atlantic wave has the head of a "dragon".
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Uh oh!!!,dons decoupling....its going to be a tough day for donny IMO,this won't be our first hurricane,and the LLC should make landfall close to galveston
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Quoting Patrap:
RGB loop shows some de-coupling and a Low Level Circ moving NW at the Surface,with the CDO being kicked Sw..

Floater - RGB Color Infrared Loop


That's what I see too. Hopefully even at that the convection should go over parts of the state that need it most. If he can hang on to it.
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I believe we are approaching active part of hurricane season. Hoping Don is not too weak and not too strong. Welcome back Dr. Masters!!!

Link

That is one heck of a wave in the central atlantic.
Bet the guys at NHC are stocking up on coffee as we speak.

Would someone please enlighten us about the MJO and its affect on that CATL wave?
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Quoting RitaEvac:


Be Texas's luck that'll be the one to come blast us away next week


Sure hope not! You need the water, but not the wind!
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318. 7544
Quoting aussiecold:
nice and very scary wave i central ATL


yeap and all the models are on to it could become big
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Quoting Patrap:
RGB loop shows some de-coupling and a Low Level Circ moving NW at the Surface,with the CDO being kicked Sw..

Floater - RGB Color Infrared Loop
That sounds like a knife to one of its "lungs"!
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nice and very scary wave i central ATL
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Don seems to be strengthening somewhat...Recon found 51-52 kt. flight level winds in their pass through the system, higher than last time IIRC.
Member Since: July 6, 2010 Posts: 113 Comments: 32524
RGB loop shows some de-coupling and a Low Level Circ moving NW at the Surface,with the CDO being kicked Sw..

Floater - RGB Color Infrared Loop
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Its a long way out, ( Aug-12 ), but this would be a monster trough for August...
Member Since: September 27, 2007 Posts: 1 Comments: 21751
312. jpsb
Quoting TropicalAnalystwx13:


Yes, this may be our first serious storm of the season.
Very impressive spin occupying a huge area, I sure hope nothing comes of it.
Member Since: June 30, 2009 Posts: 0 Comments: 1197
Quoting NotCircumventing:


Link
Thank you.
Member Since: September 27, 2007 Posts: 1 Comments: 21751
Quoting SouthALWX:
starting to look MUCH better on water vapor. Lots of outflow feathering. Look for some deepening soon.



DON is naked around the coc needs to get convection firing up there before he does anything...the dry air is doing a number on don..he seems to be slowing in speed also...
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Quoting TropicalAnalystwx13:


Yes, this may be our first serious storm of the season.


Saw an animation earlier which took it north of Hispaniola and towards Florida, but recurving back out to sea between Florida and the Bahamas.
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Member Since: July 12, 2011 Posts: 0 Comments: 1532
Quoting TropicalAnalystwx13:


Link
thank you
Member Since: September 27, 2007 Posts: 1 Comments: 21751
Don't be surprised to see 91L within the next 48-72 hours or sooner...This is one mean wave.

Member Since: July 6, 2010 Posts: 113 Comments: 32524
Quoting RitaEvac:


Just drove thru your area a few weeks ago on I-10 going and coming from La Berge casino resort in Lake Charles


Lol. Yeah, we might have gone there once or twice. Lots a Texas plates on the road on that stretch of I10. :)
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Last one for awhile... off to work :)



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Quoting OrchidGrower:
Re: #249, vapor image


Holy cow -- look at the spin in that Central-Atlantic wave!


Yes, this may be our first serious storm of the season.
Member Since: July 6, 2010 Posts: 113 Comments: 32524
starting to look MUCH better on water vapor. Lots of outflow feathering. Look for some deepening soon.
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I will take a wild guess and say Rockport Texas as a cat 1.
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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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Quoting scott39:
Crown Weather is being Bullish. He is saying a cat 1 WILL hit Texas or possibly....a cat 2!


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.
Member Since: July 6, 2010 Posts: 113 Comments: 32524
Quoting TampaSpin:



That might become a Monster for the Caribbean! They need to watch this close. CAPE VERDE is here now!


Be Texas's luck that'll be the one to come blast us away next week
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During the active season this blog totally stops me from being productive.
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Quoting reedzone:


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.


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.

img src="">
Quoting Ryuujin:

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?


No, the wind shear is 5-10kts. Not Don :)

Quoting jpsb:
Thanks!


Anytime.
Member Since: July 6, 2010 Posts: 113 Comments: 32524
Quoting NotCircumventing:
12Z GFS staying south with Don thru 24


Had a feeling the 12Z GFS would suggest that. Although the latest NAM (12Z) is still hinting just to the south of Galveston, TX.
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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.