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 angiest:


Leaving home this morning there was a beautiful cloud well to the east, backlit. I never looked up the radar but it certainly resembled a CB. Too bad I wasn't in a position to get a picture of it.


Awwww, I missed that. I am anticipating some great cloud shows in the next few days.
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Quoting farhaonhebrew:
in the next two 40%?
percents again.
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Quoting caneswatch:


Well that's interesting.... Emily and Franklin maybe? It's starting to get to that time of year.
if Don gets organized better , he may go further south and likewise if he weakens,he go further north, i thought corpus was a good landfall site, where does corpus lie in your lines?
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692. BA
it's easy for ppl that don't live on the coast (that is to be impacted) to wish for storms to fire up, etc etc...

for those that live in the affected area, even a small tropical storm disrupts all sorts of businesses and organizations (people)...they are not a very friendly way to get rain

I, for one, hope Don doesn't strengthen at all...they are already evacuating Texas A&M campus...so kids with no place to go have to take a bus all the way to Laredo
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looks like some good news for don it certainly wont become a hurricane BUT its moving into a lower shear environment and its trying to pull some moisture from the north to cover up its center which is lacking convection to the north
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Don firing deep convection once again, and according to ATCF, no change in pressure since 12Z (1001 mb.). Could this be the start of Don's intensification? Possibly, but it is a little too early to see yet. Multiple outflow boundaries are seen coming from the system, which means that the main problem that Don is having is dry air.


Member Since: July 6, 2010 Posts: 113 Comments: 32056
Quoting hydrus:
Another system behind it on Aug-1..
in the next two 40%?
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according to University officials, the claim that the center of the storm will pass almost directly over our campus on Ward Island....i wonder if Impact Weather is pulling their leg.
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Member Since: September 23, 2005 Posts: 14 Comments: 11166
I still think Don will be Cat.1 by landfall or slight chance of Cat.2, and if i am wrong i would eat a crow but i think all the crows in texas have died of dehydration
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I've been looking at wind shear forecasts and have a question. I understand the concept of wind shear, but can someone explain to me what is shown on a "wind shear forecast" map versus what is shown on a "zonal wind shear forecast" map.

Thanks.
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Looks like he wants to pull something in from the north.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Member Since: September 27, 2007 Posts: 1 Comments: 21209
Quoting tiggeriffic:
hydrus or jason...popped on for just a few...don't have long...could either of you post a map of what the high is doing over the US please...thanks in advance


As of 12Z


Member Since: September 23, 2005 Posts: 14 Comments: 11166
That landfall location between Rockport and CC would be good for Houston too because it would bring more moisture towards us and it would be good for central texas, and not to be hatin' on southern texas but some of yall got rain from ts arlene and we need some relief up here in houston.
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Quoting hydrus:
Another system behind it on Aug-1..


Well that's interesting.... Emily and Franklin maybe? It's starting to get to that time of year.
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Quoting GHOSTY1:
I heard on our houston radio news 740 KTRH that they think landfall maybe between Corpus Christi and Rockport, TX


That is where the thin black line goes if you connect the forecast points.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting HurricaneHunterJoe:
he may be going into a lower shear environment both from a velocity and directional standpoint


I have a feeling thats what is happening and this will be what gets his act together for some development
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666. GHOSTY1 1:46 PM CDT on July 28, 2011

Hehe...
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Quoting KittyTwister:


I'm watching out my window at work in SugarLand. It is a beautiful sight to see the puffy white clouds with the darker grey ones looming just above them.
Would like to see those up this way(DFW). Looking at the regional radar though, we ain't gonna get squat until Don finally gets here (keeping fingers and toes crossed).
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Quoting TomTaylor:


Neutral, with a very slight warm bias




Models currently predict we will head back toward La Nina by the wintertime.


Thanks.
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I heard on our houston radio news 740 KTRH that they think landfall maybe between Corpus Christi and Rockport, TX
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Quoting GHOSTY1:
Looks like north of the Don's circulation that convection is starting to flare up hopefully the start of his new intensification
he may be going into a lower shear environment both from a velocity and directional standpoint
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Another system behind it on Aug-1..
Member Since: September 27, 2007 Posts: 1 Comments: 21209
Wow, the Atlantic has become very warm in the past week. It is now warmer than this time last year.

July 27, 2010:



July 27, 2011:

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Hopefully Don whatever it comes ashore as will add some instability to the atmosphere and with the increase moisture gives us texans some well needed rain.
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It would seem that Don still has fight, based on trop floater one wv imagery, last two frames. Go Don Go!!! We really really need that rain!!!!
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NotC

Heck, I'm happy when people spit at me anymore.
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hmmmm
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Looks like north of the Don's circulation that convection is starting to flare up hopefully the start of his new intensification
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Quoting KittyTwister:


I'm watching out my window at work in SugarLand. It is a beautiful sight to see the puffy white clouds with the darker grey ones looming just above them.


Leaving home this morning there was a beautiful cloud well to the east, backlit. I never looked up the radar but it certainly resembled a CB. Too bad I wasn't in a position to get a picture of it.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Vorticity with CATL wave moved a little south, now at 7N - 38W which clearly seen in CyberTeddy's post #638
Hence I don't see this system moving north of the Islands and due to its broad nature it will remain weak for a quite a while yet hence move generally Westwards!
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hydrus or jason...popped on for just a few...don't have long...could either of you post a map of what the high is doing over the US please...thanks in advance
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Member Since: September 27, 2007 Posts: 1 Comments: 21209
Quoting nrtiwlnvragn:


Yes new, no change in pressure from 12Z
it shows 1001mb that a drop from 1005
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I'm downtown Houston and it is just barely starting to rain.
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Member Since: September 27, 2007 Posts: 1 Comments: 21209
Quoting oreodogsghost:
Nice dark clouds forming up over Houston now.


I'm watching out my window at work in SugarLand. It is a beautiful sight to see the puffy white clouds with the darker grey ones looming just above them.
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Quoting nola70119:
El Nino, or La Nina conditions?


Neutral, with a very slight warm bias




Models currently predict we will head back toward La Nina by the wintertime.
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Quoting metwombly:
Waiting for the next image of Floater One. Looks like Don is the little engine that could, he's just a chugging along, best he can.
yup and he may still suprise as he has done on his long trip from africa many times
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Quoting floridaboy14:
is this new information? and has the pressure droped?


Yes new, no change in pressure from 12Z
Member Since: September 23, 2005 Posts: 14 Comments: 11166
Nobody should give up Don because so far, many people have given up on him, the NHC, TWC, and some wunderbloggers but he keeps fighting on and after reemerging multiple times you can't tell when he really is doomed
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soon to be 91L has a a large cyclonic circulation, which is still feeding off the ITCZ/MONSOON TROUGH. this will help the wave to moisten it's surroundings. convection is on the increase and we should soon see a yellow circle
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Air getting drier on TX coast

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Quoting nrtiwlnvragn:
AL 04 2011072818 BEST 0 245N 907W 40 1001 TS 34 NEQ 75 50 0 20 1011
is this new information? and has the pressure droped?
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AL 04 2011072818 BEST 0 245N 907W 40 1001 TS 34 NEQ 75 50 0 20 1011
Member Since: September 23, 2005 Posts: 14 Comments: 11166

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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.