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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quakeman, tell Cantore he needs to move farther north, cuase Don isnt hitting there(the rain will) but landfall closer to corpus christi
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Heres a link for more information on Don and of Course 91L....

Link

Taco :o)
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Quoting ackee:
HOW strong will 91L get ?

A STRONG TS
B CAT 1
C CAT 2
D CAT 3




Cat 5!
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2518. angiest
Quoting NotCircumventing:
Are those tropical fish? Surely they are, so as to remain on-topic.

Don better do something dramatic, his stock is already declining in the wake of 91L's progress and potential.


Dramatic? Assuming the last two vortex messages record the true center, connect then with a straight line and then run XTRP. ;)
Member Since: August 26, 2006 Posts: 16 Comments: 4766
2517. HCW
Latest from the NHC and the expected times of impact

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Ackee, to early to tell, try that again in a couple of days...
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91L will become Emily...sooner rather than later...but it is absolutely ridiculous to over exaggerate the effects she will have on the CONUS. It's a wait and see game. No need to freak out, just yet.
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Quoting tatoprweather:


How about Cantore? Did he already showed up?
your guess is as good as mine. I am in Fort Worth crossing my fingers and toes for some of Don's moisture. Would not surprise me if Cantore showed up down there, since that is his game.
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Quoting tatoprweather:


How about Cantore? Did he already showed up?

Yep, they've already deployed the Cantore. They were showing him this morning...I think he's on South Padre Island.
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Emily (91L) might be like Bonnie in 1998 by track
Member Since: August 30, 2008 Posts: 1 Comments: 259
2511. ackee
HOW strong will 91L get ?

A STRONG TS
B CAT 1
C CAT 2
D CAT 3




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2510. ncstorm
If only Don was bigger, it could really cut into texas drought if so..

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Nice rain in downtown Houston. Looks like we are in for a wet day.
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2508. Jax82
Don, finally trying to become a Man.
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Here is some more info from Oz.

"Update: Staying put in Corpus Christi until the NHC 2 PM update. Broadcasting now from downtown marina at http://crazymother.tv/"
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Sounds like it is going to be all about the timing with this one. 91L is a long ways away, and we need to see how this system develops before a more confident track can be established.

NWS Melbourne

TUE-FRI...WEAK MID LEVEL TROUGH/SHEAR AXIS OVER FL AND THE ADJACENT
WRN ATLC WILL SPELL BEST CHANCE FOR RAIN TUE...WITH POPS A TAD LOWER
ON WED-THU AS TROUGH DAMPENS OUT. FROM LATE WED ONWARD...ECM AND GFS DIFFER W/R/T JUST HOW MUCH OF A WEAKNESS OR COL WILL BE LEFT BEHIND
OVER FL...WITH THE ECM OPTING FOR MORE RIDGING BUILDING WWD FROM THE
ATLC BY THU-FRI. GFS SOLN SHOWS H50 HEIGHTS ABOUT 20M LOWER...WHICH
IMPLIES LESS SUPPRESSION AND A LITTLE HIGHER POPS. OVERALL TREND OF
DECREASING POPS WAS FOLLOWED...50 AREAWIDE FOR TUE...30/40 WED/THU
AND 20/30 BY FRI.

EXTENT OF RIDGING LEFT OVER BY LATE NEXT WEEK WILL HAVE IMPLICATIONS
FOR TRACK OF MID ATLC TROPICAL DISTURBANCE ON DAYS 8-9 (NEXT WEEKEND)
AS THE SYSTEM (IN WHATEVER FORM IT TAKES) IS PROGGED TO APPROACH OR
REACH 75W BY THEN.
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Quoting metwombly:
Don is beginning to show up on Brownsville radar.


How about Cantore? Did he already showed up?
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Don has the convergence it needed to sustain itself, but now it is running against the clock. A hurricane is still possible, but very unlikely with it being so close to land now.

In other news, there was a big jump in vorticity with 91L. This is an indication that 91L is separating from the ITCZ. This will probably be TD #5/Emily Sunday or Monday.
Member Since: July 6, 2010 Posts: 113 Comments: 32286
I think it would be something if we get Emily and Franklin in the next week...
SW caribbean(pre 92L) Franklin...
91L, Emily
or the other way around... possible
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Quoting TampaSpin:
Don just might become a CANE Yet!


then again ........MAYBE NOT>.........LOL

TROPICAL STORM DON DISCUSSION NUMBER 8
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011
1000 AM CDT FRI JUL 29 2011

SATELLITE IMAGES SHOW THAT DON HAS BECOME A LITTLE BETTER ORGANIZED
THIS MORNING WITH INCREASED CONVECTIVE BANDING AROUND THE CENTRAL
CONVECTION...ESPECIALLY IN THE SOUTHERN SEMICIRCLE. HOWEVER...THE
STORM CONTINUES TO EXPERIENCE 10-15 KT OF NORTHERLY VERTICAL WIND
SHEAR...WITH AN ELONGATED OVERALL CLOUD PATTERN AND A STREAM OF ARC
CLOUDS IN THE EASTERN AND NORTHEASTERN QUADRANTS OF THE CYCLONE.
AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT HAS REPORTED 850 MB
FLIGHT-LEVEL WINDS OF 51 KT...AND THE CENTRAL PRESSURE ESTIMATED
FROM THE AIRCRAFT DATA IS NEAR 1002 MB. THE INITIAL INTENSITY
REMAINS 45 KT.

THE INITIAL MOTION IS NOW 300/12. DEEP-LAYER RIDGING OVER THE
SOUTHEASTERN UNITED STATES SHOULD STEER DON GENERALLY
WEST-NORTHWESTWARD UNTIL THE CYCLONE DISSIPATES OVER LAND IN
36-48 HR. THE NEW FORECAST TRACK HAS CHANGED LITTLE FROM THE
PREVIOUS TRACK AND IS NEAR THE CENTER OF THE TRACK GUIDANCE
ENVELOPE.

DON SHOULD CONTINUE TO EXPERIENCE MODERATE SHEAR UNTIL THE CENTER
MAKES LANDFALL IN ABOUT 18 HR. IN ADDITION...WATER VAPOR IMAGERY
AND MICROWAVE PRECIPITABLE WATER DATA SHOW DRY AIR CLOSE TO THE
WESTERN SIDE OF DON. THIS COMBINATION SHOULD CONTINUE TO IMPEDE
SIGNIFICANT STRENGTHENING BEFORE LANDFALL...AND THUS THE INTENSITY
FORECAST SHOWS ONLY SLIGHT STRENGTHENING. DON SHOULD WEAKEN
STEADILY AFTER LANDFALL...AND THE SYSTEM IS EXPECTED TO DISSIPATE
COMPLETELY BY 48 HR IN AGREEMENT WITH THE DYNAMICAL MODEL GUIDANCE.

INTERESTS IN EXTREME NORTHEAST MEXICO SHOULD CONTINUE TO CLOSELY
MONITOR THE PROGRESS OF DON. ANY SMALL DEVIATION TO THE SOUTH OF
THE FORECAST TRACK COULD NECESSITATE THE ISSUANCE OF WARNINGS FOR
THE EXTREME NORTHEASTERN COAST OF MEXICO.

FORECAST POSITIONS AND MAX WINDS

INIT 29/1500Z 26.2N 94.9W 45 KT 50 MPH
12H 30/0000Z 26.8N 96.7W 50 KT 60 MPH
24H 30/1200Z 27.7N 99.1W 35 KT 40 MPH...INLAND
36H 31/0000Z 28.3N 101.6W 25 KT 30 MPH...INLAND
48H 31/1200Z...DISSIPATED

$$
FORECASTER BEVEN
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Where is Arlen Texas located? Will Hank's grass get some loving from ol Don?
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Quoting metwombly:
Don is beginning to show up on Brownsville radar.


No doubt a sight for sore eyes...
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Don just might become a CANE Yet!
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Quoting LoneStarWeather:

Drive down to Corpus tomorrow. You'll get a rainy Saturday for sure!

I'm in CC right now-work here, live in Rockport.
Nothing on the horizon so far. And I'm on the bayfront.
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Don is beginning to show up on Brownsville radar.
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2492. beell
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SW caribbean could be 92L soon too, just saying
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Feeder bands from Don can be seen to the east of Harlingen, Tx. Got some rain coming.
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Don wants to come visit Texas, and im all for it, though it would be nice if his big sister(Emily) could come with him...
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Hey Jason, I have an even coolor avatar for you to use:

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Quoting jasblt:
Have to say, some of the comments and questions today about 91L are downright laughable. 2000 miles away, and people are asking where it's going, what the models say, strength?? really?? Hint... look back at the history of Don over the last week. It changes daily, get a grip folks. If you took this the wrong way, your probably asking the questions I'm talking about. Levi and the other weather wizards will let you know daily what is happening. Relax, read, and learn!


+100
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2485. ncstorm
Quoting TropicalAnalystwx13:


You and me both.


LOL!
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Quoting Levi32:
0z NAEFS says recurve near or east of Bermuda. The pink colors near Bermuda represent model variance, where some of the ensemble members show a storm, but others don't.


didnt they show don in the bahamas north of the islands? its a long way out but are there only 2 track scenarios levi?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2482. jasblt
Have to say, some of the comments and questions today about 91L are downright laughable. 2000 miles away, and people are asking where it's going, what the models say, strength? really?? *Head tilted to the side* Really?? Hint... look back at the history of Don over the last week. It changes daily, get a grip folks. If you took this the wrong way, your probably asking the questions I'm talking about. Levi and the other weather wizards will let you know daily what is happening. Relax, read, and learn!
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Think 91L is decoupling from ITCZ rigth now. IMHO.
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2480. Levi32
I'm off to work. Back later.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
Quoting HurricaneDean07:
Okay everyone, get this...
July 28th, 1995~ TS Dean formed, and July 30th hit texas
July 28th, 2011~ TS Don formed, and july 30th will hit TX

Climatology is creepy...
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2478. Levi32
0z NAEFS says recurve near or east of Bermuda. The pink colors near Bermuda represent model variance, where some of the ensemble members show a storm, but others don't.

Member Since: November 24, 2005 Posts: 635 Comments: 26661
48 hours until August, will 91L make it???
Member Since: July 6, 2010 Posts: 113 Comments: 32286
Okay everyone, get this...
July 28th, 1995~ TS Dean formed, and July 30th hit texas
July 28th, 2011~ TS Don formed, and july 30th will hit TX
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Quoting cctxshirl:
I'm dissapointed in Don. I was really looking forward to a rainy Saturday, but I just don't think it's gonna happen.

Drive down to Corpus tomorrow. You'll get a rainy Saturday for sure!
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Looking at the current motion of Don, looks to me like it might bring some badly needed rain to areas of central Texas after all.
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Quoting ncstorm:
Good Morning Everyone!

I am having flashbacks to Earl last year from the model runs for the new Invest..will it or wont it..watching in NC



You and me both.
Member Since: July 6, 2010 Posts: 113 Comments: 32286
2472. Walnut
Quoting cctxshirl:
I'm dissapointed in Don. I was really looking forward to a rainy Saturday, but I just don't think it's gonna happen.
Ummm... if you are in CCTX, it is headed right for you now.
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2471. ncstorm
Good Morning Everyone!

I am having flashbacks to Earl last year from the model runs for the new Invest..will it or wont it..watching in NC

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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.