Don battling dry air and wind shear

By: Dr. Jeff Masters , 7:37 PM GMT on July 28, 2011

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Tropical Storm Don continues to be an unimpressive low-end tropical storm as it continues northwest towards the Texas coast. Don formed yesterday afternoon from an African tropical wave that moved into the Gulf of Mexico under a region of low wind shear. Don's formation date of July 27 is nearly a month ahead of the usual August 23 date for the arrival of the season's fourth named storm of the year. There is currently no hurricane hunter airplane in Don, and a new airplane is not due in the storm until tonight. The last center fix at 1pm EDT found surface winds of 45 mph and a central pressure of 1005 mb, a 4 mb rise from earlier this morning. 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 5 - 15 knots of shear from strong upper level winds out of the north. This shear is creating problems for Don by injecting dry air into the system. Visible satellite imagery from early this afternoon showed the presence of surface arc-shaped clouds expanding outwards to the north from the center of Don. These type of clouds are a sign that the storm is struggling with dry air. When dry air at middle levels of the atmosphere gets injected into thunderstorms due to wind shear, the dry air tends to create strong downdrafts that rob the storm of moisture. These downdrafts spread out at the ocean surface and create arc-shaped surface cumulus clouds.


Figure 1. Visible satellite image of Don from pm EDT July 28, 2011, showing arc-shaped surface clouds--the tell-tale sign of dry air interfering with the storm's organization.


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 bring significant rains to Texas? According to the National Climatic Data Center, the six-month period ending in June 2011 was the driest on record. Average rain between January and June was more than eight inches (203 millimeters) below average in Texas, and the state experienced record heat between April and June. The heat and lack of rain have brought exceptional drought--the highest category of drought--to over 75% of the state. 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. We don't 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. Don's small size makes it prone to dry air and wind shear, though, and it is uncertain whether the storm can overcome these problems enough to become a significant rain maker. NHC gave Don a 12% chance of intensifying into a hurricane in the 11am advisory, which is a reasonable forecast, since Don is running out of time to get its act together in time to become a hurricane. None of the computer models is predicting Don will become a hurricane.

For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast. The 11 am version of this forecast shows that Port O'Connor, Texas has the highest chance of tropical storm-force winds (39+ mph): 45%.

New hurricane archive search feature
The autocomplete entities in the wunderground search box has been extended to include hurricanes, so you can now search for a storm by name, year, or basin. Here are some examples in case you feel like exploring your new options:

By name:

Hurricane David - Atlantic, 1979
David, Major Hurricane - Atlantic, 1979
Major Hurricane David - Atlantic, 1979

By year:

2005 Hurricanes Atlantic
2007 Hurricanes Eastern Pacific

By basin:

Hurricanes Western Pacific 2011
Hurricanes Atlantic 2008

By category:

Tropical Storms Atlantic 2005
Tropical Depressions Indian Ocean 2011
Subtropical Storms Eastern Pacific 2010
Extratropical Storms Western Pacific 1988

I'll have a new post Friday morning.

Jeff Masters

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Quoting Patrap:



Looking a lot better tonite......Pressure will be falling and for once we might get some convergence going in the lower levels so it can streghten some. The larger ball means a larger distance of rain for Texas....NICE
Member Since: September 2, 2007 Posts: 178 Comments: 20439
Recon nearing Don's circulation to measure pressure again...
Member Since: October 3, 2010 Posts: 40 Comments: 4129
1219. Patrap
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Quoting JrWeathermanFL:

Diminish?


Diminish, not dissipate...IR already shows the cloud tops are slowly shrinking.
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1217. jpsb
Quoting want2lrn:


Thank you Kori, trying to figure out if i need to get the boat off of the lift on the back side of the island or just tie it down. I appreciate your response and answers to any of my future questions. Thanks again.
You can expect hurricane force winds for a few hours. It looks to me like you will be on the dirty side of Don. I would definitely tie the boat down if it is high enough above water that waves will not hit it. If waves are a problem then move it. Cat 1 will bring 5-7' tide.
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Quoting muddertracker:


Yes...that's it..it is absolutely beautiful here! Not a tropical storm, hurricane, tornado or drought in sight!


Yeah...moved here from Florida about a year ago....
We did have some waterspouts a few months ago....VERY rare in these parts!
Enjoy Ko'Olina....BUT you have to explore. Get up to Haleiwa Neat little town with shops and restaurants!

Check out Windward...my home....Kailua - Great Beaches.
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Just spoke to CycloneOz.........he is in place to go live tomorrow from Corpus Cristi.....He has a HIGH DEF camera and ready to go. I will be carrying his live feed. Go to my blog if you need the link. I don't wanna get banned by posting it.
Member Since: September 2, 2007 Posts: 178 Comments: 20439
Quoting TropicalAnalystwx13:


Tropical cyclones naturally make their own convergence, don't they?
What level are you asking about?

A tropical cyclone, or even a large thunderstorms complex, causes lower level convergence and upper level divergence as it pulls in air at the ground and shoots it up and out at the upper level.
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Quoting TXStormWatcher2011:
Probably nothing, but over the years I've noticed that a great many storms once in the center of the Gulf start to turn towards the East, that is they don't continue on straight paths. Looking at these models I don't think it would be surprising if it turned towards Houston. There seems to be a critical point when crossing the Gulf and once passed the storm isn't going towards Mexico or Louisiana. I'm thinking these models are pointing roughly to Corpus Brownsville. If so I'd look West of that for a landfall.

Interesting you say that because, anecdotally, here in Galveston Lots of us have that opinion. People always say, "well they always come further up the coast [east] than predicted.". I wonder if it is actually true, or if it happened a few significant times and we all think it will always happen.
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1212. DFWjc
Quoting caneswatch:


Strange days have found us.....


yes, they have, and a good song indeed...
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Quoting MrstormX:
Convection should diminish soon, but with the anticyclone in place a fresh flare up can be expected in a few hours.

Diminish?
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1210. Patrap
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Quoting ElConando:


Strange days on planet earth indeed.


Strange days have found us.....
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Quoting floridaboy14:

this mean its weakining?


Convection might be, but everything is in place for more strengthening.
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1207. scott39
Don has dropped to 998mb with winds at 45knts.
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Quoting NoNamePub:

ummm You Mean Ko'Olina..... ????
Welcome to the islands!!!!


Yes...that's it..it is absolutely beautiful here! Not a tropical storm, hurricane, tornado or drought in sight!
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Quoting MrstormX:
Convection should diminish soon, but with the anticyclone in place a fresh flare up can be expected in a few hours.

this mean its weakining?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
1204. j2008
Quoting HurricaneDean07:
Time: 00:07:30Z
Coordinates: 24.5333N 91.75W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,520 meters (~ 4,987 feet)
Extrap. Sfc. Press: 1000.3 mb (~ 29.54 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 135° at 41 knots (From the SE at ~ 47.1 mph)
Air Temp: 22.9°C (~ 73.2°F)
Dew Pt: 15.8°C (~ 60.4°F)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 43 knots (~ 49.4 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
(*) Denotes suspect data

Low rain rate.... Confirmed sustained winds...


Satellite estimates state Don is a 65 mph TS, but HH havent found proof to back it up so at this current point...

Tropical Storm Don:
Latest found winds: 50 mph
Latest found pressure: 995 MB
Direction moving: WNW

The HH must be flying around the 65 MPH winds so that people dont freak out. HAHA wonder why there is such a big difference between satellite and HH wind speeds.
Member Since: December 19, 2008 Posts: 1 Comments: 224
Convection should diminish soon, but with the anticyclone in place a fresh flare up can be expected in a few hours.
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1202. DFWjc
Quoting ElConando:


Strange days on planet earth indeed.


nopers, just typical Texas weather...I've lived here all but 10 months of my life from Houston area, to DFW and even Tyler.... I'm used to it all, and I don't mind it one bit. I love my state :)
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1201. Patrap
00z 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 TXStormWatcher2011:
Probably nothing, but over the years I've noticed that a great many storms once in the center of the Gulf start to turn towards the East, that is they don't continue on straight paths. Looking at these models I don't think it would be surprising if it turned towards Houston. There seems to be a critical point when crossing the Gulf and once passed the storm isn't going towards Mexico or Louisiana. I'm thinking these models are pointing roughly to Corpus Brownsville. If so I'd look West of that for a landfall.


It's because many Gulf storms have to contend with approaching cold fronts, which steers them poleward (easterly).
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Ameister, Don actually hasnt improved much today, but now its beginning to crank up...
Member Since: October 3, 2010 Posts: 40 Comments: 4129
1197. scott39
Quoting KoritheMan:


I'd say generally WNW.
I was hoping for up the spine of Texas.
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1187.

SAB is at 3.0, which is 45kts
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Quoting KoritheMan:


There are far too many variables in the synoptic pattern to say for sure. But it WILL be a threat to the northern Caribbean.
Thanks buddy :)
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Looking more like a N MX landfall or extreme S TX.
Member Since: April 29, 2007 Posts: 0 Comments: 7862
Quoting DFWjc:


and to think we had 8+ inches of snow here in the D/FW area back in March, 4 hail storms between May/June...and now 27 days of 100+ weather....


Strange days on planet earth indeed.
Member Since: Posts: Comments:
Probably nothing, but over the years I've noticed that a great many storms once in the center of the Gulf start to turn towards the East, that is they don't continue on straight paths. Looking at these models I don't think it would be surprising if it turned towards Houston. There seems to be a critical point when crossing the Gulf and once passed the storm isn't going towards Mexico or Louisiana. I'm thinking these models are pointing roughly to Corpus Brownsville. If so I'd look East of that for a landfall.
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Quoting scott39:
Which way is Don forecasted to go once on land?


I'd say generally WNW.
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1189. GHOSTY1
Quoting jdjnola:


Yikes let's hope nothing gets in that sweet spot between LA and FL this year headed to LA. I am on the last-out crew at work and I don't want to even entertain that possibility. Send all the storms to TX!


Yeah, we'll take them, send them our way!
Member Since: July 25, 2011 Posts: 0 Comments: 453
Quoting muddertracker:
Aloha from O'olina, Oahu, HI! The weather is perfect! Don looks better.

ummm You Mean Ko'Olina..... ????
Welcome to the islands!!!!
Member Since: Posts: Comments:
Time: 00:07:30Z
Coordinates: 24.5333N 91.75W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,520 meters (~ 4,987 feet)
Extrap. Sfc. Press: 1000.3 mb (~ 29.54 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 135° at 41 knots (From the SE at ~ 47.1 mph)
Air Temp: 22.9°C (~ 73.2°F)
Dew Pt: 15.8°C (~ 60.4°F)
Peak (10s) Flt. Lvl. Wind: 41 knots (~ 47.1 mph)
SFMR Peak (10s) Sfc. Wind: 43 knots (~ 49.4 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
(*) Denotes suspect data

Low rain rate.... Confirmed sustained winds...


Satellite estimates state Don is a 65 mph TS, but HH havent found proof to back it up so at this current point...

Tropical Storm Don:
Latest found winds: 50 mph
Latest found pressure: 995 MB
Direction moving: WNW
Member Since: October 3, 2010 Posts: 40 Comments: 4129
1186. DFWjc
Quoting cctxshirl:
I spoke with a local farmer here in Rockport TX who had lost what little produce he had. I asked him when our last significant rain fell and he told me it was in January. I honestly cannot remember the last time we had rain here (not in 2011 anyway). We had a 5 minute shower one Saturday about a month or so ago and that was it. We would surely welcome Don's rain, as I know a lot of fellow Texans would.


and to think we had 8+ inches of snow here in the D/FW area back in March, 4 hail storms between May/June...and now 27 days of 100+ weather....
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1185. angiest
Quoting cctxshirl:
I spoke with a local farmer here in Rockport TX who had lost what little produce he had. I asked him when our last significant rain fell and he told me it was in January. I honestly cannot remember the last time we had rain here (not in 2011 anyway). We had a 5 minute shower one Saturday about a month or so ago and that was it. We would surely welcome Don's rain, as I know a lot of fellow Texans would.


Your Drought Information Statement: Link
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1184. jdjnola
Quoting twincomanche:
No. this is caused by limestone formations in Florida that don't for the most part exist in Texas.


Wait but TX is full of limestone, at least in the Austin/San Antonio area.
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Quoting TropicalAnalystwx13:


Great loop , says everything about Don
Member Since: April 29, 2007 Posts: 0 Comments: 7862
1182. drs2008
Quoting stormhank:
anyone feel the central atlantic system could pose a US threat down the road??
A monster is brewing in the central atlantic.
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1181. scott39
Quoting KoritheMan:


I think so, though I'm not sure how much. It really depends on how he consolidates and where he makes landfall.
Which way is Don forecasted to go once on land?
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Aloha from O'olina, Oahu, HI! The weather is perfect! Don looks better.
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How many hours to landfall 20-24 max! just guessing
Member Since: April 29, 2007 Posts: 0 Comments: 7862
Quoting stormhank:
anyone feel the central atlantic system could pose a US threat down the road??


There are far too many variables in the synoptic pattern to say for sure. But it WILL be a threat to the northern Caribbean.
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1177. angiest
Quoting bohonkweatherman:
Good Night Weather World, those who need rain I pray you get relief soon, those in flood areas I hope it dries out for you, if you have your health you have everything. Peace and God Bless Soldiers, my daughter is heading to Iraq in 1 day.


Good night and my thanks for your daughter's service to our country.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1176. GHOSTY1
From the satellite i would say that Don is moving NW and i don't think there forecasting a shift to the west so i dont expect rain for Brownsville, and the other thing is if Don hits CC then Brownsville is on the dry side of the storm, but with Don who knows what side of the storm will be wet or dry. Just continue to watch and see what happens.
Member Since: July 25, 2011 Posts: 0 Comments: 453
Quoting TropicalAnalystwx13:

It's amazing how much Don has improved throughout the day,
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Good Night Weather World, those who need rain I pray you get relief soon, those in flood areas I hope it dries out for you, if you have your health you have everything. Peace and God Bless Soldiers, my daughter is heading to Iraq in 1 day.
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I spoke with a local farmer here in Rockport TX who had lost what little produce he had. I asked him when our last significant rain fell and he told me it was in January. I honestly cannot remember the last time we had rain here (not in 2011 anyway). We had a 5 minute shower one Saturday about a month or so ago and that was it. We would surely welcome Don's rain, as I know a lot of fellow Texans would.
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Quoting dis1322:
Hey guys im from Brownsville Texas Tip of Texas are we getting any rain from Don?


I think so, though I'm not sure how much. It really depends on how he consolidates and where he makes landfall.
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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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