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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Enhanced Infrared (IR) Imagery (4 km Mercator)

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Quoting stormpetrol:
FRom what I can tell about the setup this year so far, anything that exits the African coast will take a similar track to Don, jmo.


I wouldn't say anything will, but it certainly looks like the pattern is going to favor find kind of journey much more than last year.
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719. xcool
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...DON JOGS TEMPORARILY WESTWARD...EXPECTED TO RESUME A
WEST-NORTHWEST OR NORTHWEST MOTION TONIGHT...


SUMMARY OF 700 PM CDT...0000 UTC...INFORMATION
----------------------------------------------
LOCATION...24.7N 91.8W
ABOUT 410 MI...655 KM ESE OF CORPUS CHRISTI TEXAS
ABOUT 365 MI...590 KM ESE OF BROWNSVILLE TEXAS
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...1004 MB...29.65 INCHES
Member Since: September 2, 2009 Posts: 130 Comments: 21109
Quoting JLPR2:
Everyone is so impressed by Don they missed the 20% the CATL got. XD

First time I'm first posting one of those.


close but mo cigar look at the guy above
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Storm Relative 1km Geostationary Visible Imagery


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Looks like Don starting to build an eyewall 24.7N/92W
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Tropical wave in the CATL upgraded to 20%. Expect 91L soon.
Member Since: September 2, 2009 Posts: 130 Comments: 21109
713. xcool
91l come soon
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still 45mph 1004mb nw 16mph
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711. JLPR2
Everyone is so impressed by Don they missed the 20% the CATL got. XD

First time I'm first posting one of those.
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A LARGE TROPICAL WAVE OVER THE TROPICAL ATLANTIC ABOUT MIDWAY
BETWEEN AFRICA AND THE LESSER ANTILLES IS MOVING WESTWARD NEAR 20
MPH. ENVIRONMENTAL CONDITIONS APPEAR TO BE FAVORABLE FOR SOME SLOW
DEVELOPMENT OVER THE NEXT FEW DAYS. THIS SYSTEM HAS A LOW
CHANCE...20 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS.
Member Since: July 8, 2005 Posts: 259 Comments: 23882
Quoting belizeit:
Mission # 4 is now on the sence


any data yet....
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708. MTWX
Quoting Patrap:


Was great work for a good cause and you made alot of it happens MTWX,so thanx for that my man.

Making a difference takes a team effort, and it surely was and is.

Lets just be ready for August-September!! Have a feeling our luck is goin to run out real soon!
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707. JLPR2
A LARGE TROPICAL WAVE OVER THE TROPICAL ATLANTIC ABOUT MIDWAY
BETWEEN AFRICA AND THE LESSER ANTILLES IS MOVING WESTWARD NEAR 20
MPH. ENVIRONMENTAL CONDITIONS APPEAR TO BE FAVORABLE FOR SOME SLOW
DEVELOPMENT OVER THE NEXT FEW DAYS. THIS SYSTEM HAS A LOW
CHANCE...20 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS.

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HH Plan of the Day

000
NOUS42 KNHC 281445
WEATHER RECONNAISSANCE FLIGHTS
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
1045 AM EDT THU 28 JULY 2011
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 29/1100Z TO 30/1100Z JULY 2011
TCPOD NUMBER.....11-058

I. ATLANTIC REQUIREMENTS
1. TROPICAL STORM DON
FLIGHT ONE - TEAL 73 FLIGHT TWO - NOAA 42
A. 29/1200,1800Z A. 30/0000Z
B. AFXXX 0604A DON B. NOAA2 0704A DON
C. 29/1015Z C. 29/2000Z
D. 26.1N 93.8W D. 27.3N 96.0W
E. 29/1130Z TO 29/1800Z E. 29/2230Z TO 30/0230Z
F. SFC TO 15,000 FT F. SFC TO 15,000 FT

FLIGHT THREE - TEAL 74
A. 30/0000,0600Z
B. AFXXX 0804A DON
C. 29/2200Z
D. 27.3N 96.0W
E. 29/2330Z TO 30/0600Z
F. SFC TO 15,000 FT

2. OUTLOOK FOR SUCCEEDING DAY: POSSIBLE 30/1200 FIX.

II. PACIFIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
2. SUCCEEDING DAY OUTLOOK.....NEGATIVE.
JWP

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Mission # 4 is now on the sence
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Quoting TropicalAnalystwx13:


Don's center is well in the convection now, and recon is finding winds anywhere from 50-70 mph. It has improved significantly from earlier.


they won't up the winds above 60 if they cant find a pressure lower than 1004 mb hpa
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FRom what I can tell about the setup this year so far, anything that exits the African coast will take a similar track to Don, jmo.
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Still having a bit of dry air entrainment, but with this deep convection firing, dry air shouldn't be a huge problem later on like it was earlier today.
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Quoting red0:
Meanwhile in Fort Smith, Arkansas...

This is the 27th straight day over 100, but I could feel something different in the air today from the beginning. The clouds were looking rather ominous and, it was nearly 1:00 before the signs around town started registering triple digits. I was thinking maybe today's high would be only like 102 instead of a more typical for this heat wave 107.

As I walked out of the theater this afternoon the humidity hit me in the face and I spotted what looked suspiciously like the aftermath of a brief shower on the pavement. The little spots of quickly evaporating wetness were no match for the high pressure ridge of doom though, and were fading fast. I put my headphones on and headed towards my bike when it started raining again, and I took shelter under an awning. As the humidity went from ridiculous to hard-to-breath I stood watching this piece of pavement in front of me get rained on for 18 minutes (2.5 plays of Radiohead's Street Spirit heh) without it getting wet. It was as if the pavement didn't quite remember what to do when it got rained on. 30 minutes later though, the sky opened up and we got our first measurable rain of July and the temperature is now in the 80's. Quite nice!
Good for you Ark, you are as hot as Texas, today was my 50th day of 100 or greater, 63 days for San Angelo. I think 2011 will go down as hottest year ever, records being broken all time for streaks and hottest temperatures ever.
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Quoting MrstormX:


That was from the Vortex message, 2 planes are about equidistant from the center and should be there within the hour.
Is it unusual to have 3-4 planes in a minimal tropical storm at the same time? Do they know something we do not?
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699. JLPR2
Quoting weatherh98:


ive always thought that thats how storms intensify


Others do it without looking crappy. XD
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Quoting HuracanKY:
I see Don hasn't changed a whole lot in organization since earlier today. Shear is clearly still affecting the center, which still lies on the northern edge of the convective mass and not in the center. Highly unlikely he'll reach hurricane strength before landfall, with the wind shear not forecast to abate much. I raised my eyebrow when I took at look out in the Central Atlantic though. Looks like trouble brewing there....and possibly serious trouble eventually. Those low latitude waves can spell trouble, especially with a more pronounced high like we've seen recently sitting over the Atlantic.


Don's center is well in the convection now, and recon is finding winds anywhere from 50-70 mph. It has improved significantly from earlier.
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Quoting MTWX:

While it has been brought up, I would also like to thank Patrap, Presslord, Northof Atlanta, auburn, plywoodstatenative, TStormSC, Noil, AllyBama, snjgambbleSC, Floodman,and tornadodude for the tornado relief effort this spring!!! If I forgot anyone let me know!!


Was great work for a good cause and you made alot of it happens MTWX,so thanx for that my man.

Making a difference takes a team effort, and it surely was and is.
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Quoting Patrap:


WOW! Don really wants to be somebody!
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695. xcool
don kaboom soon
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Quoting JLPR2:


Maybe Don had us fooled and was just strengthening its core and now that its finished with the remodeling and everything is in order, it started to fire convection. Sneaky storm.


ive always thought that thats how storms intensify
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I see Don hasn't changed a whole lot in organization since earlier today. Shear is clearly still affecting the center, which still lies on the northern edge of the convective mass and not in the center. Highly unlikely he'll reach hurricane strength before landfall, with the wind shear not forecast to abate much. I raised my eyebrow when I took at look out in the Central Atlantic though. Looks like trouble brewing there....and possibly serious trouble eventually. Those low latitude waves can spell trouble, especially with a more pronounced high like we've seen recently sitting over the Atlantic.
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Reading back some, I see recon is finding some very interesting readings. While I'm confident the 72 mph reading is contaminated, I wouldn't doubt that Don is now between 60-65 mph. At the VERY least, they'll up it to 50 mph.

Don has REALLY improved since I was last here.
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Seems Don Tejas' has found the sweet spot and his groove this evening

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690. MTWX
Quoting taco2me61:



Whats upp Stormjunkie???? Good to see ya on here :o)
and want to say thank you for all you did for Joplin and in Alabama :o) My mom lives up there in Joplin and from what she has told me it is still very "Bad" if you know what I mean....

Anyway Thank You my Friend

Taco :o)

While it has been brought up, I would also like to thank Patrap, Presslord, Northof Atlanta, auburn, plywoodstatenative, TStormSC, Noil, AllyBama, snjgambleSC, Floodman,and tornadodude for the tornado relief effort this spring!!! If I forgot anyone let me know!!
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689. JLPR2
Quoting 1900hurricane:
Couple hours old, but something is definitely going on down there, although the resolution makes it hard to tell exactly what.



Maybe Don had us fooled and was just strengthening its core and now that its finished with the remodeling and everything is in order, it started to fire convection. Sneaky storm.
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I still think Corpus Christi Bay...
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687. j2008
Getting wild out there!!! Storm structure looks like a very strong TS or even a weak Cat 1.
Member Since: December 19, 2008 Posts: 1 Comments: 224
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Thanks for the links Pat. Don is really putting on a show this evening. This could interesting.
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Quoting Patrap:
That is starting to look pretty damned good for a TS.
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683. JLPR2
Quoting stormwatcherCI:
I think Levi left for a while but some if not most models develop it before it enters the Caribbean around Monday.


So far the models send it to the central Lesser Antilles, (north of Barbados) or to the NE Caribbean. (US Virgin Islands.)
But none the less all the people form the islands chain should keep an eye on it.
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Pinhole eye?
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ESL by LSU GOES-13 Low Cloud Product Loop
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Quoting belizeit:
If you look at the data from the dropsonde you notice the wind speed increase from 35 at flieght level to 45 at sea level indicating that wind are higher at the surface then at flight level


I saw that too, I think Air Force planes will help us determine if that is really the case.
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GOM Large Image, Low Cloud Product

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678. DVG
Check out the LSU low cloud product loop. Looks like some real strengthening at the core.
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If you look at the data from the dropsonde you notice the wind speed increase from 35 at flieght level to 45 at sea level indicating that wind are higher at the surface then at flight level
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Quoting gwadaman:


Hi Levi,Can you give me a heads up on TW approaching the Leeward & Windward Islands....I live in Barbados and see more and more the models developing this system..
I think Levi left for a while but some if not most models develop it before it enters the Caribbean around Monday.
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675. txjac
Quoting Patrap:


Always love that image Pat
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This is the best looking don ive seen the only one that ooked betterwas bret
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Couple hours old, but something is definitely going on down there, although the resolution makes it hard to tell exactly what.

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Quoting MrstormX:


That was from the Vortex message, 2 planes are about equidistant from the center and should be there within the hour.
imagine if they find a strong tropical storm... wow im gettin excited :)
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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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