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 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.
Member Since: March 7, 2007 Posts: 575 Comments: 20571
1170. jdjnola
Quoting Patrap:


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!
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Quoting TampaSpin:



It won't be too fast......a lot of dry air to its NORTH but, Low Shear will be in its favor so slow development will be likely.....also its a very large system and will take a while to get going. But, when it does look out.




It's at 20 percent and the cutoff for orange is 30, so it doesn't have to make a Bob Beamon jump to get there.
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Quoting HurricaneDean07:
Thundercloud the question is if it is rain created...


not sure ... recon has been finding lower pressure every time it goes into the storm and so I think that the winds have started to come up now
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Thundercloud the question is if it is rain created...
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Quoting TropicalAnalystwx13:


Tropical cyclones naturally make their own convergence, don't they?



That has been the problem with Don NO CONVERGENCE......without the Convergence of Winds in the LOWER LEVEL no storm will ever be very strong.......IT JUST CANT Happen......that was the reason why i didn't think it was a storm yesterday since there was little convergence.
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1165. Walnut
Quoting dis1322:
Hey guys im from Brownsville Texas Tip of Texas are we getting any rain from Don?
100% chance I would say.
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Quoting TropicalAnalystwx13:


Tropical cyclones naturally make their own convergence, don't they?


Typically it shows up on the CIMSS map. Naturally you would think it's there.
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1163. j2008
Quoting ProgressivePulse:



That's a 10mb drop in just a few hours, holy moly :-O

RI burst, might even continue through the night.
Member Since: December 19, 2008 Posts: 1 Comments: 224
Quoting jpsb:
Here in San Leon, Galveston County we have had 3 rains in the last 8 or 9 months. THREE counting the half inch or so we got today. Ground is like concrete with big cracks in it. Rock hard, I am hoping we will start getting tStorms off the gulf I can see them across the bay (Galveston) but sadly they rarely make it this far west. I had high hopes for Don, seems a perfect storm for us, houston and areas north, oh well, hope this drought ends soon, it's been terrible for 2+ years now.
I am praying for you, I have seen East and Southeast Texas with showers lately which is a big improvement but honestly I am not even getting a cloud and my average temp last 8 days is 105. I have had 4 or 5 decent rains the past 13 months, 2 of them last tropical season which saved us and filled up the lakes but not much since. The Drought is bad enough put 105 temps in the shade on top of that and it test you mentally.
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anyone feel the central atlantic system could pose a US threat down the road??
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Member Since: July 6, 2010 Posts: 113 Comments: 32030
1159. dis1322
Hey guys im from Brownsville Texas Tip of Texas are we getting any rain from Don?
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1158. angiest
Reds and pink in Funktop are getting huge.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting Thundercloud01221991:
Significant Wind Levels...
Level Wind Direction Wind Speed
995mb (Surface) 215° (from the SW) 48 knots (55 mph)
994mb 215° (from the SW) 49 knots (56 mph)

911mb 235° (from the SW) 40 knots (46 mph)
828mb 225° (from the SW) 36 knots (41 mph)
788mb 250° (from the WSW) 38 knots (44 mph)
758mb 250° (from the WSW) 31 knots (36 mph)
754mb 250° (from the WSW) 39 knots (45 mph)
753mb 195° (from the SSW) 48 knots (55 mph)
752mb 200° (from the SSW) 43 knots (49 mph)
751mb 235° (from the SW) 31 knots (36 mph)



That's a 10mb drop in just a few hours, holy moly :-O
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1156. GHOSTY1
Is it just me or is Don lookin' bigger? Does anyone think its gonna bigger? When i watching 13abc in Houston they said that Houston chances of rain is 40% of rain on Friday and Saturday, i understand that Don is small but it seems as he wants to grow so i feel that prediction is off, and they were still predicting CC or Rockport landfall.
Member Since: July 25, 2011 Posts: 0 Comments: 453
Quoting ProgressivePulse:
Perfect up top however, the convergence needs to be existent for this to keep up.



Tropical cyclones naturally make their own convergence, don't they?
Member Since: July 6, 2010 Posts: 113 Comments: 32030
1153. angiest
Quoting CybrTeddy:


it causes venting and that in turn causes strengthening.


When it sets up right, positive feedback loop (or negative, depending on your perspective).
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting KoritheMan:


No, this definitely isn't diurnal. Don is starting to take advantage of a more favorable synoptic environment, as he is essentially outrunning the strong shear. However, there still seems to be some subsidence (dry air) to contend with in the western Gulf, but this is lessening. IMO, Don could flirt with hurricane intensity before landfall.


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.
Member Since: July 28, 2010 Posts: 0 Comments: 285
Quoting RukusBoondocks:
new yello cirlce will be orange fast



It won't be too fast......a lot of dry air to its NORTH but, Low Shear will be in its favor so slow development will be likely.....also its a very large system and will take a while to get going. But, when it does look out.


Member Since: September 2, 2007 Posts: 178 Comments: 20439
Significant Wind Levels...
Level Wind Direction Wind Speed
995mb (Surface) 215° (from the SW) 48 knots (55 mph)
994mb 215° (from the SW) 49 knots (56 mph)

911mb 235° (from the SW) 40 knots (46 mph)
828mb 225° (from the SW) 36 knots (41 mph)
788mb 250° (from the WSW) 38 knots (44 mph)
758mb 250° (from the WSW) 31 knots (36 mph)
754mb 250° (from the WSW) 39 knots (45 mph)
753mb 195° (from the SSW) 48 knots (55 mph)
752mb 200° (from the SSW) 43 knots (49 mph)
751mb 235° (from the SW) 31 knots (36 mph)
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1148. angiest
Quoting twincomanche:
No. this is caused by limestone formations in Florida that don't for the most part exist in Texas.


Uhh, we get sinkholes out of mud quite frequently in SE Texas (see the huge one in Daisetta a few years back. Also, near Boling there is a sinkhole from erosion of the top of a saltdome. Then there is the Edward's Plateau, a huge expanse of limestone that is full of caverns. So, yeah, we *can* get them.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1147. JLPR2
Quoting RukusBoondocks:
new yello cirlce will be orange fast


I'm thinking it might be orange tomorrow morning, if D-max is good to it.
Member Since: September 4, 2007 Posts: 7 Comments: 8691
1146. Patrap
TS Don Floater - RGB Color Infrared Loop
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Well, back to lurking till the next advisory. L8R.
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Quoting want2lrn:
Good Evening all, Corpus here. Mostly just read and try and learn. Would someone please tell me if i am on the right track with my thoughts. Don appears to me to be getting it together tonight. Is this a normal flux in the system or a sign that the storm is indeed taking advantage of the environment which i understand to be conducive for some development? Really interested in your educated opinions, our meteorologists just seem to regurgitate information that i have seen elsewhere. Thank you!


No, this definitely isn't diurnal. Don is starting to take advantage of a more favorable synoptic environment, as he is essentially outrunning the strong shear. However, there still seems to be some subsidence (dry air) to contend with in the western Gulf, but this is lessening. IMO, Don could flirt with hurricane intensity before landfall.
Member Since: March 7, 2007 Posts: 575 Comments: 20571
Quoting Chicklit:

Good news.
Anyway, goodnight.
Texas rain.
Come again.



Night Chicklit.
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Quoting TropicalAnalystwx13:


Where the heck did that come from?
Those strong updrafts that occurred this afternoon caused it.

The latent heat from the intense thunderstorm activity was released aloft in the upper-levels of the atmosphere and expended (divergence).
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After being part of this blog for 7 years.....I have to say I'm surprised that it's so slow, considering the territory, size, location, and power that The Don currently has. How often do we have an imminent landfalling TS/hurricane in the US?
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Don is expanding and the core is finally getting some insulation. Good news for Texas as the rain event will be better.
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Quoting MrstormX:


Strengthening


Thanks, i'm semi-new and learning the ropes. Still confused about the steering and hoping Don will glide more Northerly
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Quoting MrstormX:
Anticyclone associated with Don.

Good news.
Anyway, goodnight.
Texas rain.
Come again.
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Don causes possible injury in GOM
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Quoting SETexas74:


Anticyclone allows Don to vent right? Is this a sign of strengthening or weakening?


it causes venting and that in turn causes strengthening.
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Quoting twincomanche:
Chances are this pattern may repeat and Texas will get the relief that Florida got. Funny how Mother Nature does these things.................what do we know.

Let's hope so. This drought has to end.
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I have the feeling that the CATL wave will become the first Atlantic hurricane this season, if Don doesn't become the first.
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Perfect up top however, the convergence needs to be existent for this to keep up.

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Quoting SETexas74:


Anticyclone allows Don to vent right? Is this a sign of strengthening or weakening?


It is indicative of a more favorable environment ahead.
Member Since: July 6, 2010 Posts: 113 Comments: 32030
Quoting SETexas74:


Anticyclone allows Don to vent right? Is this a sign of strengthening or weakening?


Strengthening
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Quoting JrWeathermanFL:
What will Don peak at?
A.45-50 mph
B.60-65 mph
C.70 mph
D.Cat. 1
E.Cat. 2
F.Cat. 3
G.Cat. 4
H.Cat. 5


C/D.
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Quoting MrstormX:
Anticyclone associated with Don.


Anticyclone allows Don to vent right? Is this a sign of strengthening or weakening?
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Good Evening all, Corpus here. Mostly just read and try and learn. Would someone please tell me if i am on the right track with my thoughts. Don appears to me to be getting it together tonight. Is this a normal flux in the system or a sign that the storm is indeed taking advantage of the environment which i understand to be conducive for some development? Really interested in your educated opinions, our meteorologists just seem to regurgitate information that i have seen elsewhere. Thank you!
Member Since: July 28, 2010 Posts: 0 Comments: 285
the north side of don just blew up I mean exploded
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new yello cirlce will be orange fast
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1122. Patrap
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What will Don peak at?
A.45-50 mph
B.60-65 mph
C.70 mph
D.Cat. 1
E.Cat. 2
F.Cat. 3
G.Cat. 4
H.Cat. 5
Member Since: July 19, 2011 Posts: 12 Comments: 2401

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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.