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 HurricaneDean07:
Hey everyone!
Im back from my Galveston vacation, and actually heard buzz about Don while I was there...
Hope Don gets stronger, we need the drought buster NOW... 106 today, some small heat showers from Houston to Austin, but not very exceptional in terms of rainfall amounts...

So the latest ive seen is,
Tropical Storm Don:
50 mph
996 MB
moving WNW @ 10

Welcome back. This blog has gotten me very confusd about Don, so I know as much as you. LOL
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1020. Patrap
Only Atlantic GREEN BALL is Don on the Navy page
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1019. angiest
Quoting LPStormspotter:


I'm sorry.. what does that mean?
Thanks for your answer!


That's the steering flow for storms 970-989mb. Those are the winds that would guide it. Looks like very little to move it anywhere there.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting TomTaylor:
idk, you'll have to talk to Don if you want those kind of specifics lol

Storm is pretty small though, so yea rainfall will be pretty confined. Probably more rainfall falling to the south of the storm too. Which is unfortunate because central Texas needs the rain more than south Texas does.
This is true, Extreme South, extreme southeast east and parts of northeast and northcentral Texas are in better shape than the rest of the state as far as the drought goes.
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1017. Gearsts
Quoting TampaSpin:
Evening everyone! Looks like Don is trying hard to strengthen. We also have a new Invest also.....




New invest? WHERE!?
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CI# /Pressure/ Vmax
3.2 / 995.7mb/ 49.0kt


Final T# Adj T# Raw T#
2.9 3.3 3.4

Latest Satellite Estimates from ADT...

So ADT thinks 60 mph 996 MB
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CDO continues...
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1014. jpsb
Quoting bohonkweatherman:
Here in Austin they have said we may get a little rain and we may not hit 100 Saturday but due to movement in deep south Texas most rain will be along his path. I am waiting on the next rain system for Central Texas just not sure when it will come or if it will be this year and I am being serious. Waiting 3 or 4 months for every rain is getting old and it really sucks.
Yeah, not looking good, Don seems to be headed to south Texas the one place in Texas that is NOT in exception drought conditions. Thanks a lot Don and I have been rooting for you for three days now.
Member Since: June 30, 2009 Posts: 0 Comments: 1191
Quoting TomTaylor:
idk, you'll have to talk to Don if you want those kind of specifics lol

Storm is pretty small though, so yea rainfall will be pretty confined. Probably more rainfall falling to the south of the storm too. Which is unfortunate because central Texas needs the rain more than south Texas does.


Not true--we need the rain in S. Tx. Wildlife is dying. I think we all need rain equally as bad.
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Quoting angiest:
If Don gets to <990mb:



I'm sorry.. what does that mean?
Thanks for your answer!
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Quoting JLPR2:


No, it was circled, but we don't have 91L officially anywhere.
If it strengthens more than yes it will move more to the NW. Check out the chart angiest posted below.
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Quoting MTWX:
Coming from a agricultural aspect: If you are in front of Don, I would suggest tilling the soil around your plants/crops as deep as you can (without damaging the roots) tomorrow, before the storm comes. This way the water has a better chance of soaking in, instead of just running off on the sun hardened ground. On the subject of saving parched trees: dig a culvert around the drip line (tips of outermost branches). This is where the "feeder" roots are, which will allow a tree maximum absorbtion.
Very good and intelligent info, I am watering my small yard tonight just in case it pours at my house with Don.
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Quoting JLPR2:


No, it was circled, but we don't have 91L officially anywhere.
I humbly apologize for jumping the gun. Does anyone have any crow leftover?
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Quoting HurricaneHunterJoe:
Dropsonde Location: Dropped in eye


It always says that...
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Evening everyone! Looks like Don is trying hard to strengthen. We also have a new Invest also.....




Member Since: September 2, 2007 Posts: 178 Comments: 20439
Don is growing in size, more rain for a larger area.
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Hey everyone!
Im back from my Galveston vacation, and actually heard buzz about Don while I was there...
Hope Don gets stronger, we need the drought buster NOW... 106 today, some small heat showers from Houston to Austin, but not very exceptional in terms of rainfall amounts...

So the latest ive seen is,
Tropical Storm Don:
50 mph
996 MB
moving WNW @ 10
Member Since: Posts: Comments:
Quoting jpsb:
Not really, it needs to make land fall north of CC to help with the exceptional drought. South of CC does not need the rain nearly as bad as central Texas.


75% of the state is in exceptional drought. Hard to find a spot that doesn't need rain, eh?
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1001. angiest
Curious, I wonder if, as he moves further west away from the sheer, if Don will pull in all that convection to his NE. I think he's been wanting it all afternoon and evening and just can't get it.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting twincomanche:
Whatcha think? TS bringing rain 70-80 miles each side of center?
idk, you'll have to talk to Don if you want those kind of specifics lol

Storm is pretty small though, so yea rainfall will be pretty confined. Probably more rainfall falling to the south of the storm too. Which is unfortunate because central Texas needs the rain more than south Texas does.
Member Since: August 24, 2010 Posts: 19 Comments: 4357
Quoting JLPR2:
Hello, soon to be 91L.



Very nice pass by ASCAT this evening. Clearly shows a defined low level circulation in association with the Central Atlantic tropical wave.
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Quoting HurricaneHunterJoe:
can anyone figure out The Don's heading from all the HHinfo?

WNW at 285 degrees
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OBTW, I got about 5 minutes of light rain at my house this evening.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Dropsonde Location: Dropped in eye
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993. JLPR2
Quoting DisasterResponder:


It was made 91L about 20 min ago


No, it was circled, but we don't have 91L officially anywhere.
Member Since: September 4, 2007 Posts: 7 Comments: 8690
Quoting DFWjc:


I couldn't agree more...
Yep needs to start heading NW. TWC chick said something about the High steering it to the NW. Not sure I understand what she means by that.
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Quoting DisasterResponder:
NHC has that wave in the Atlantic as an invest now. 20% chance of formation in next 48 hours.


Its not an invest yet
Member Since: March 10, 2010 Posts: 1 Comments: 7683
Quoting weatherman566:


I disagree to an extent. Since Don is gaining strength, he will likely move further west and south of the forecasted track and make landfall south of Corpus Christi. Only time will tell though.
Here in Austin they have said we may get a little rain and we may not hit 100 Saturday but due to movement in deep south Texas most rain will be along his path. I am waiting on the next rain system for Central Texas just not sure when it will come or if it will be this year and I am being serious. Waiting 3 or 4 months for every rain is getting old and it really sucks.
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Quoting JLPR2:
Hello, soon to be 91L.



It was made 91L about 20 min ago
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Is there much of a chance of any moisture from Don comming up in the N.W. OK- S.W. MO area?
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Quoting NotCircumventing:
And by "matagorda" I mean something completely different.

Let us say corpus.


Sell-out.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
985. DFWjc
Quoting jpsb:
Not really, it needs to make land fall north of CC to help with the exceptional drought. South of CC does not need the rain nearly as bad as central Texas.


I couldn't agree more...
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can anyone figure out The Don's heading from all the HHinfo?
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983. jpsb
Quoting jdjnola:
I think it's safe to say Don is now almost guaranteed to bring Texas some much needed rain...
Not really, it needs to make land fall north of CC to help with the exceptional drought. South of CC does not need the rain nearly as bad as central Texas.
Member Since: June 30, 2009 Posts: 0 Comments: 1191
NHC has that wave in the Atlantic as an invest now. 20% chance of formation in next 48 hours.
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Quoting Max1023:
Dmin and Dmax are not that important to established tropical cyclones - anything below 1005mb usually produces enough convergence to generate its own convection provided the atmospheric dynamics are favorable. The effects of Dmin and Dmax are overwhelmed by the overall atmospheric conditions produced by the cyclone.
Right, but even when this was a wave it wasn't following ocean-based convective cycles. Proximity to land and divergent conditions aloft were probably to blame for this. But still it was interesting to watch.
Member Since: August 24, 2010 Posts: 19 Comments: 4357
What seems to be helping Tropical Storm Don more than anything right now is that the storm is now gradually moving away from the shear axis present across the Central Gulf of Mexico and into a more favorable upper level wind environment. This should allow for steady intensification to continue and I'm anticipating that Tropical Storm Don will come ashore as a strong tropical storm.
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977. MTWX
Coming from a agricultural aspect: If you are in front of Don, I would suggest tilling the soil around your plants/crops as deep as you can (without damaging the roots) tomorrow, before the storm comes. This way the water has a better chance of soaking in, instead of just running off on the sun hardened ground. On the subject of saving parched trees: dig a culvert around the drip line (tips of outermost branches). This is where the "feeder" roots are, which will allow a tree maximum absorbtion.
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If Don gets to <990mb:

Member Since: August 26, 2006 Posts: 16 Comments: 4766
975. JLPR2
Hello, soon to be 91L.

Member Since: September 4, 2007 Posts: 7 Comments: 8690
I'm sure Texas is gonna love Don.
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i think the lowest pressure thus far

Time: 00:09:30Z
Coordinates: 24.45N 91.8167W
Acft. Static Air Press: 843.7 mb (~ 24.91 inHg)
Acft. Geopotential Hgt: 1,489 meters (~ 4,885 feet)
Extrap. Sfc. Press: 995.4 mb (~ 29.39 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 107° at 10 knots (From the ESE at ~ 11.5 mph)
Air Temp: 27.9°C (~ 82.2°F)
Dew Pt: 14.2°C (~ 57.6°F)
Peak (10s) Flt. Lvl. Wind: 13 knots (~ 14.9 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots* (~ 27.6 mph*)
SFMR Rain Rate: 2 mm/hr* (~ 0.08 in/hr*)
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Member Since: August 2, 2010 Posts: 21 Comments: 9814
Time: 00:09:30Z
Coordinates: 24.45N 91.8167W
Acft. Static Air Press: 843.7 mb (~ 24.91 inHg)
Acft. Geopotential Hgt: 1,489 meters (~ 4,885 feet)
Extrap. Sfc. Press: 995.4 mb (~ 29.39 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 107° at 10 knots (From the ESE at ~ 11.5 mph)
Air Temp: 27.9°C (~ 82.2°F)
Dew Pt: 14.2°C (~ 57.6°F)
Peak (10s) Flt. Lvl. Wind: 13 knots (~ 14.9 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots* (~ 27.6 mph*)
SFMR Rain Rate: 2 mm/hr* (~ 0.08 in/hr*)
(*) Denotes suspect data
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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.