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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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:

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Quoting Levi32:


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



No Question! He has struggled at the Lower Levels. He has always had a great Mid Level. But, as we have said that can change very quick with such a small spinner.
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Pretty dry even at 850mb. Not a T/Td spread one would expect to find in a tropical system.

And they are all like this.

Time: 13:52:30Z
Coordinates: 23.9N 89.8167W
Acft. Static Air Press: 842.6 mb (~ 24.88 inHg)

Air Temp: 17.8°C (~ 64.0°F)
Dew Pt: 5.8°C (~ 42.4°F)
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So do the HH data suggest intensification?
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Quoting TampaSpin:
To the surprise of many Don has never been a healthy system. It still lacks much in organization. But with small storms like this, that can change very quickly in just a matter of a few hours. BEWARE of WHAT he might become as a Hurricane is possible from him!


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

Member Since: November 24, 2005 Posts: 635 Comments: 26700
Thanks Levi for the blog post today.

One of the least talked about aspects of this storm that is good for us in South Texas is the relatively slow movement. Even if it doesn't get any bigger. a 10 mph storm that doesn't have high winds could do a lot of doggone good in the ranch country of South Texas and maybe get up to put some water in the aquifers N of San Antone.
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Quoting NotCircumventing:
Problem with NHC percentages: just a few days ago (or was it a couple?) Don had a near 0% chance of even developing over the next 48 hours.

So now we have to just watch real-time, because of course the NHC guidance can change suddenly -- that is the nature of the business.

And watch for an East Matagorda Bay area landfall, too.


Just curious, what makes you say East Matagorda Bay area?
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45kt surface wind reading from SFMR:

000
URNT15 KNHC 281417
AF303 0204A DON HDOB 26 20110728
140800 2320N 08912W 8429 01582 0117 +169 +043 188026 028 033 003 00
140830 2319N 08911W 8432 01577 0121 +161 +045 197029 030 033 001 00
140900 2318N 08910W 8430 01583 0125 +160 +046 196030 030 032 002 00
140930 2317N 08909W 8429 01583 0122 +164 +047 192029 030 031 001 00
141000 2316N 08908W 8436 01578 0124 +162 +048 186030 030 029 002 00
141030 2314N 08907W 8432 01582 0122 +165 +049 190030 030 028 001 03
141100 2313N 08906W 8433 01580 0121 +168 +051 191030 030 027 001 00
141130 2312N 08905W 8429 01585 0120 +170 +052 192030 031 030 001 03
141200 2311N 08905W 8433 01583 0121 +168 +054 192033 034 030 000 00
141230 2309N 08905W 8429 01585 0123 +165 +055 192036 036 029 002 00
141300 2308N 08904W 8432 01583 0125 +163 +056 190033 035 029 002 03
141330 2307N 08903W 8433 01584 0126 +165 +056 189031 033 030 003 00
141400 2306N 08902W 8423 01599 0126 +166 +057 193035 036 034 004 00
141430 2306N 08900W 8432 01583 0124 +165 +056 193033 034 046 004 00
141500 2305N 08859W 8431 01584 0123 +166 +055 197033 033 045 005 00
141530 2304N 08857W 8420 01602 0130 +160 +054 184032 038 044 011 00
141600 2303N 08856W 8443 01574 0135 +151 +050 185035 036 042 015 03
141630 2302N 08855W 8424 01591 0130 +160 +045 185035 037 036 003 00
141700 2302N 08853W 8416 01601 0127 +165 +041 181032 032 034 002 00
141730 2301N 08852W 8407 01611 0127 +167 +040 187032 032 035 001 00
$$
;
Member Since: November 24, 2005 Posts: 635 Comments: 26700




Stinks there is not a bouy in the area! We need one that area!
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Complete Update

TS BUSTED FORECAST ALIBI






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To the surprise of many Don has never been a healthy system. It still lacks much in organization. But with small storms like this, that can change very quickly in just a matter of a few hours. BEWARE of WHAT he might become as a Hurricane is possible from him!
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The high pressure that is supposed to steer Don WNW is slightly weaker than forecast, therefore a more pure NW track is likely. Intensity is difficult to peg, but with the water temperatures and very low shear over the next 24hours, a cat 1-cat 2 storm is possible, weakening due to dry air and upper level winds just before landfall to a minimal category 1 hurricane just about around Freeport, texas.

Visual shows the circulation under the north edge of the convection.
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46 knots
(~ 52.9 mph)
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I draw issue with this statement, and hopefully the Doc sees this and comments:

"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."

As of his June update (July not being over yet), the Texas State Climatologist says this is Texas' third worst drought:

"The current drought was officially named as the third-worst drought in Texas history with a Palmer Drought Severity Index value of -6.37 for June 2011."

Link

Is there some preliminary information that indicates things have changed this month?

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TS Don Update July 28th. 2011
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Recon finding surprisingly light winds at flight-level within the convective burst on the southeastern side of Don's circulation. SFMR is picking up some 40kt readings, but the health of this part of the system is in question if the flight-level winds are all below 30kts.

Member Since: November 24, 2005 Posts: 635 Comments: 26700
Gonzo flight today was cancelled, no upper air synoptic flight.
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Looks like we got 2 more invest coming soon!
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Tropical Tidbit for Thursday, July 28th, with Video
Member Since: November 24, 2005 Posts: 635 Comments: 26700
It's nice to see a detailed tropical discussion from you, Dr. Masters. Thanks.
Member Since: November 24, 2005 Posts: 635 Comments: 26700
Welcome back from your vacation Jeff, and please extend our collective thanks to Angela for her good work during your absence.
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43 knots
(~ 49.4 mph)
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Thanks for the update Doc, hope you enjoyed the vacation!
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The storm passing through the lesser antilles hit Dominica hard last night.

A natural dam formed a few years ago gave way, eliminating Miracle Lake and flooding the Layou River valley. Luckily, it appears no one was hurt, and the main bridge over the river linking the capital and the towns on the west road survived. However, the bridge further up on the road across the island is badly damaged, and the major road to the center of the island is reportedly washed out in places, so travel to the interior of the island will have to rely on other roads which in my personal experience are very susceptible to landslides.

No one is yet reported dead but the dam broke at midnight last night.

http://stormcarib.com/reports/current/dominica.sh tml
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Thanks Dr. Masters! Good to see you back!
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Thanks for the update

Taco :o)
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999.5 mb
(~ 29.52 inHg)

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thanks for the update
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Thank you, Dr. I reckon we'll see as the day goes on. Texas overall needs 15" of rain to erase the drought. I don't think Don will be the drencher everyone's hoping for..
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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.

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