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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621. txjac
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, happy that you received some rain
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619. 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!
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Recon dropped a dropsonde in the center wich reads 1007 mb of pressure while at 213 feet it reads 1000mb pressure i think somthing is wrong with thedrosonde
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Quoting trollkiller2011:

Why, so the grass could turn green again and bugs and other critters start moving back into the area and my hair curl up and I'd be forced to go out and mow again? I loathed last year's rainy season.


would rather see you mow your lawn than the prices of meat and crops go up so much that the rest of the USA cannot afford to buy groceries...if you dont want to mow...dig up your grass and pave your yard with green concrete...
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Quoting MrstormX:


Not certain how this jives with the forecast path. Confusing. Is this a case of the left hand not talking to the right?
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Quoting txjac:


What would make them do that? Are they seeing something strange?


Your guess is as good as mine, maybe they are looping higher up or training for flying in an eye. It is crazy weird.
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Quoting trollkiller2011:

Cool, do you know where Stevens High School is at? I'm going to be an 11th grader there, this next year.
Yes I know where that is. Stay safe if it heads this way - Turn around, Don't drown. They are getting barricades ready just in case.
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610. JLPR2
Quoting MrstormX:
Recon just did a third loop, at the same spot...basically in the middle of nowhere.


Weird. Someone in that flight should take away the tequila from the pilot. XD
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Quoting SevereWeatherAddict:
As you can see I very rarely post. But what is the changes of Don becoming a cat 1 ? And thanks in advanced.
I Would say Yes 20% No 80%
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thank you for the links...will ck em later...always did trust anyone who was a met for the Navy...
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607. txjac
Quoting MrstormX:
Recon just did a third loop, at the same spot...basically in the middle of nowhere.


What would make them do that? Are they seeing something strange?
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Quoting duajones78413:


Your area as well as others need the rain badly
The ranchers need it the most. Buying feed for cattle is driving beef prices beyond insane.
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Recon just did a third loop, at the same spot...basically in the middle of nowhere.
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Quoting FrankZapper:
I think they ought to bury the hatchet. The more experts the merrier.
I recall him going to another site sometime last year. Looked forward to his synopsis. Most of us on here are non-pros and some just guess. As for me - I just like to learn.
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Quoting trollkiller2011:

Well I live in San Antonio too, West side. I definitely do NOT want any rain because I do not want my lawn to get any greener or taller because then I'll have to go out and mow!!! I'm loving this drought.


Your area as well as others need the rain badly
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Trollkiller, not having to mow the grass is great, but those cracks in the foundation are the pits!
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Quoting Patrap:
..Don is venting Houston,, Copy?




Not certain I understand. Can you elaborate Pat. Thanks!
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Lol Igor didn't go out to sea - it made a direct hit on Newfoundland and cause 250 million+ in damage.
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Quoting Walnut:
His analysis was decent. Don't know him personally.
I think they ought to bury the hatchet. The more experts the merrier.
Member Since: May 26, 2011 Posts: 0 Comments: 1193
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Quoting trollkiller2011:
Walnut you live in SA too?
Northwest Side.
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Enhanced Infrared (IR) Imagery (4 km Mercator)


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Quoting tiggeriffic:


me too please...wondered what happened...know you cant say but a link by email would be nice


You have it in your inbox.
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Recon seemed to turned in a circle at the Southern edge of the convection
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Quoting stormpetrol:


80-85mph Cat 1 IMO.
im going with strong ts weak cat 1 if it keeps getting organized
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586. JLPR2
Quoting WeatherfanPR:
WOW, the Tropical Wave in the Atlantic is getting better organized and very quick !!! I think 91L is coming soon !!!


Has convection today and if the models are right it should get going soon.

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Quoting JeffM:
I wish Don would expand in size. We in San Antonio, like the rest of the state, would LOVE some rain.
WOAI's weatherguy didn't sound all that optimistic - but I am holding out hope.
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582. DFWjc
Quoting tiggeriffic:


me too please...wondered what happened...know you cant say but a link by email would be nice


Wmail
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Quoting cat5hurricane:

60mph, maybe 65 at landfall. I guess it'll depend on whether or not it is consistent enough without any windfalls or shear issues like earlier. I give him 50mph perhaps by tonight.


80-85mph Cat 1 IMO.
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Quoting MississippiWx:


Lol...I'd be careful mentioning that name around this blog. While I like StormW personally, some on here do not like him. He has his own site. If you want it, I'll give you the link through e-mail. Can't give it to you over the public blog.
His analysis was decent. Don't know him personally.
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I do think the blog should show this cyclone the respect it deserves. Think of how it fought for days over the Caribbean. It's a fighter with a heart, and muscle if needed. Refer to it as The Don.
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Quoting WeatherfanPR:
WOW, the Tropical Wave in the Atlantic is getting better organized and very quick !!! I think 91L is coming soon !!!


That's the Orange Blossom Special heading for Florida.
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Yellow circle at 8/7 pm for CATL wave.
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I agree I would like that link via email as well.
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574. JeffM
I wish Don would expand in size. We in San Antonio, like the rest of the state, would LOVE some rain.
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Quoting MississippiWx:


Lol...I'd be careful mentioning that name around this blog. While I like StormW personally, some on here do not like him. He has his own site. If you want it, I'll give you the link through e-mail. Can't give it to you over the public blog.


me too please...wondered what happened...know you cant say but a link by email would be nice
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Quoting Walnut:
StormW also seems to have Poofed himself.
I heard he is Hurrycane
Member Since: May 26, 2011 Posts: 0 Comments: 1193
Outflow to the NW from Don has improved significantly in the last hour. Watch for convection to build over northern semicircle soon.
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About JeffMasters

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