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
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Extratropical Storms Western Pacific 1988

I'll have a new post Friday morning.

Jeff Masters

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2021. SLU
Deja Vu all over again according to stormfury?

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2020. ackee
EMILY IN 2005 FORM in almost similar postion of where 91L is now we could see EMILY 2011 do the same would be suprise if the track is similar too
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2019. SLU
Quoting stoormfury:
morning
91L is here and so is EMILY soon. all of the models develop this system. the track is very interesting. wonder what SLU might be thinking? is it devaju all over? the scenario looks like ALLEN 1980 all over again


Looks like a "potentially dangerous" situation for the Lesser Antilles.
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2018. ackee
I think that 91L is so far south that Unless it intensify quickly I cant see 91L not moving westard right throught the CARRB might be the frist hurricane of the seasons
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2017. tessa
One meterologist out of Dallas said that it would take a huge hurricane to get rid of the monster High Pressure that has been dominating us for so long.
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2016. tessa
Morning everyone. I had really hoped Don would go further north than what he is. Central & North & East Texas really needs the rain but I figured the High Pressure Ridge would push him south.
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morning
91L is here and so is EMILY soon. all of the models develop this system. the track is very interesting. wonder what SLU might be thinking? is it devaju all over? the scenario looks like ALLEN 1980 all over again
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CMC recurves 91l.

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Quoting reedzone:


Landfall is in Texas for now


O OK. Looked like Mexico. Wouldn't be surprised. About 91l, all of these models take him into the Caribbean Link
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Quoting AtHomeInTX:
Wow. Don really is going to Mexico. Of course.


Landfall is in Texas for now
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Wow. Don really is going to Mexico. Of course.
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2010. SLU
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Quoting sunlinepr:
Night shift is clear, Jaimito is gone...

shh.......... (lurking)
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000
WTNT44 KNHC 290839
TCDAT4

TROPICAL STORM DON DISCUSSION NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011
400 AM CDT FRI JUL 29 2011

DON CONTINUES TO HAVE A SHEARED APPEARANCE ON SATELLITE IMAGES WITH
THE LOW-LEVEL CENTER DISPLACED NEAR THE NORTHERN EDGE OF AN
ELONGATED AREA OF STRONG CONVECTION. THIS SHEAR CAN ALSO BE SEEN
IN RECONNAISSANCE DATA...WHICH SHOWED THAT THE SURFACE CENTER IS
DISPLACED SOMEWHAT TO THE NORTH OF THE 850 MB FLIGHT-LEVEL CENTER.
THE INITIAL INTENSITY REMAINS 45 KT...A BLEND OF THE ESTIMATES FROM
SATELLITE AND AIRCRAFT DATA.

MODERATE NORTHERLY SHEAR IS FORECAST UNTIL DON MAKES LANDFALL ALONG
WITH SOME DRY AIR ALOFT. THIS COMBINATION WILL PROBABLY LIMIT
SIGNIFICANT INTENSIFICATION EVEN THOUGH THE STORM IS MOVING OVER
VERY WARM WATERS. MOST OF THE GUIDANCE SHOWS JUST A LITTLE MORE
STRENGTHENING...AND THE OFFICIAL INTENSITY FORECAST IS BASICALLY AN
UPDATE OF THE PREVIOUS PACKAGE. AFTER MOVING THROUGH TEXAS...THIS
SMALL SYSTEM WILL NOT LAST VERY LONG AFTER INTERACTING WITH THE
HIGH TERRAIN OF MEXICO...AND DISSIPATION IS LIKELY BETWEEN 36 AND
48 HOURS.

AIRCRAFT FIXES SUGGEST DON IS MOVING ABOUT 295/12. HIGH PRESSURE
RIDGING OVER THE SOUTHERN UNITED STATES WILL PROBABLY STEER THE
CYCLONE ALONG THAT GENERAL COURSE UNTIL LANDFALL...PERHAPS EVEN A
LITTLE MORE TO THE LEFT AS THE RIDGE BUILDS. THE GUIDANCE IS IN
GOOD AGREEMENT...ALTHOUGH THE USUALLY RELIABLE ECMWF MODEL IS
FARTHER TO THE NORTHEAST. AN EXAMINATION OF THE ECMWF FIELDS
INDICATES THAT THE MODEL DECOUPLES THE LOW- AND MID-LEVEL
CENTERS...ALLOWING DON TO MOVE MORE TO THE NORTHWEST. THIS DOES
NOT SEEM LIKELY AS ENVIRONMENTAL CONDITIONS ARE NOT EXPECTED TO
CHANGE SIGNIFICANTLY FROM NOW UNTIL LANDFALL. THUS THE NHC
FORECAST WILL STAY CLOSE TO THE DYNAMICAL MODEL CONSENSUS AND THE
PREVIOUS FORECAST.

INTERESTS IN EXTREME NORTHEAST MEXICO SHOULD CLOSELY MONITOR THE
PROGRESS OF DON. AN UNEXPECTED 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 29/0900Z 25.3N 93.6W 45 KT 50 MPH
12H 29/1800Z 26.1N 95.5W 50 KT 60 MPH
24H 30/0600Z 27.0N 97.8W 45 KT 50 MPH...INLAND
36H 30/1800Z 27.8N 100.4W 30 KT 35 MPH...INLAND
48H 31/0600Z...DISSIPATED

$$
FORECASTER BLAKE/STEWART
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2007. SLU
OMG! Look at the latest intensity forecasts! THAT'S MADNESS. Will have to wait for the GFDL and HWRF for confirmation.
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Quoting sunlinepr:
Night shift is clear, Jaimito is gone...
The troll?
Member Since: May 26, 2011 Posts: 0 Comments: 1193
Night shift is clear, Jaimito is gone...
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Looking at water vapor imagery, there may still be some chance of a Texas landfall, though this would likely be confined to extreme south Texas, which would mean very little drought relief.
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Night Miami, night Mississippi, night Bigwes

Think I'm about to head out as well
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me too goodnight everyone!
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Quoting MississippiWx:


Lol...I put him/her/it on ignore just so I wouldn't say anything else to it.

Anyway, I'm out for the night. Don will probably remain a tropical storm, but I could see him becoming a strong tropical storm if this convection keeps firing.

I don't believe our next system will have such issues as Don, unfortunately. Night everyone.

I agree.

I'm out for the night too.

Have a good night everyone.
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Quoting TomTaylor:
no kidding.

I don't add him to ignore though. It's fun to try and test my Spanish skills with his posts.


Lol...I put him/her/it on ignore just so I wouldn't say anything else to it.

Anyway, I'm out for the night. Don will probably remain a tropical storm, but I could see him becoming a strong tropical storm if this convection keeps firing.

I don't believe our next system will have such issues as Don, unfortunately. Night everyone.

Member Since: July 15, 2006 Posts: 17 Comments: 10159
Quoting MississippiWx:


Since the blog has slowed down now, I'll say this...

Your JFV impersonation is really bad. It's also pretty pathetic that you'd want to impersonate a troll.
no kidding.

I don't add him to ignore though. It's fun to try and test my Spanish skills with his posts.
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Quoting FrankZapper:
Kori ,I now think of the 6 weeks from mid Aug to end of Sept as nail biting. Before Katrina my attitude was "it will curve away" like Ivan and Georges. Katrina chnged that for all in the Big Easy.We enjoy life, watch LSU and The Saints, but one eye is always on the tropics. And Jesus, my daughter's name is Emily.


I think most everyone was complacent prior to Katrina, myself included.
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The ultimate limiting factor is going to be the center location. If it remains on the northern edge of the convection, then Don will probably remain status quo. Hard to deepen when convection is offset from the center. But the outflow, to me, shows that shear has relaxed some. Also, Don is headed into a very low shear environment before landfall. If it remains that way, Don could strengthen into at least a strong tropical storm before landfall.
Member Since: July 15, 2006 Posts: 17 Comments: 10159
Quoting MississippiWx:
I think Don is headed for a landfall in Mexico, unfortunately. It doesn't appear that Texas is going to get the beneficial rainfall we all once thought. Looks like he is headed quickly to the west.



He's definitely south of the forecast track.
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Done is heading into a zone where there is hardly any shear. This could help him come close to that minimal hurricane status:

Member Since: July 15, 2006 Posts: 17 Comments: 10159
Got things to do later.


Have a great night/morning ya'll.
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Don looking better today, but wait, another invest?
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Quoting MississippiWx:
Don could really make a push for minimal hurricane status. This is the longest his intense convection has persisted. He'll probably stop shy though around 65mph or 70mph if this continues:




Don is actually located on the NE side of the outflow.

IR has been deceiving.
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00z CMC has 91L getting caught in a trough and recurving out to sea (moving NW/NNW in the image below).

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Quoting MississippiWx:
Don could really make a push for minimal hurricane status. This is the longest his intense convection has persisted. He'll probably stop shy though around 65mph or 70mph if this continues:



Agreed, looks best since start with spiral banding evident. But looks to be a Mexico landfall.

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Don could really make a push for minimal hurricane status. This is the longest his intense convection has persisted. He'll probably stop shy though around 65mph or 70mph if this continues:

Member Since: July 15, 2006 Posts: 17 Comments: 10159
Quoting MississippiWx:
I think Don is headed for a landfall in Mexico, unfortunately. It doesn't appear that Texas is going to get the beneficial rainfall we all once thought. Looks like he is headed quickly to the west.

I agree, extreme southern Texas might get some rain, but nothing more than an inch or two.
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Quoting MississippiWx:
I think Don is headed for a landfall in Mexico, unfortunately. It doesn't appear that Texas is going to get the beneficial rainfall we all once thought. Looks like he is headed quickly to the west.

sure hope you're wrong

I don't live in Texas, but I know how bad they need it, and how much everyone has been saying they will get this storm.

Should it miss Texas, I'd imagine the folks out in Texas would go nuts...if hey haven't already
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Nice TPW wave

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Quoting KoritheMan:


Climatologically, our state doesn't see the particularly destructive storms until late August into mid September.
Kori ,I now think of the 6 weeks from mid Aug to end of Sept as nail biting. Before Katrina my attitude was "it will curve away" like Ivan and Georges. Katrina chnged that for all in the Big Easy.We enjoy life, watch LSU and The Saints, but one eye is always on the tropics. And Jesus, my daughter's name is Emily.
Member Since: May 26, 2011 Posts: 0 Comments: 1193
Quoting Stormchaser2007:


Night shift is always the best.

Been that way since the blog started.
I haven't been around since the blog began, but yea, agreed.
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I think Don is headed for a landfall in Mexico, unfortunately. It doesn't appear that Texas is going to get the beneficial rainfall we all once thought. Looks like he is headed quickly to the west.

Member Since: July 15, 2006 Posts: 17 Comments: 10159
1981. Ryuujin
Quoting Stormchaser2007:


That one is MPH.


Oh well that's not QUITE as bad. Haha. Still. 134 mph.. weeeewww..


Anyway, night all!
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Quoting Ryuujin:

Wait, does that say 134kts or 134 mph? Cause if it's knots then...

Excuse MY french in saying... daaaaamn


That one is MPH.
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1978. Ryuujin
Quoting Stormchaser2007:

Wait, does that say 134kts or 134 mph? Cause if it's knots then...

Excuse MY french in saying... daaaaamn
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Wondering if the jump to the NNW in the 850 vort is related to it removing itself from the ITCZ:

300UTC



600UTC

Member Since: July 15, 2006 Posts: 17 Comments: 10159
Quoting FrankZapper:
What your saying is logical. I just hope it's a few weeks early for prime time


Climatologically, our state doesn't see the particularly destructive storms until late August into mid September.
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Quoting MississippiWx:
If 91L does develop and move through the Caribbean, it will probably try to move up to around the Yucatan. However, unless the pattern that has been in place across the United States for the whole summer finally changes, I think the US from Louisiana and east of there would be safe because of the giant death ridge. There might would be an opportunity for Central/South TX to receive the brunt. Obviously, that's just pure speculation at this point and something to keep in the back of your mind. If it tracks north of the islands, it will obviously be susceptible to any trof moving off the Eastern Seaboard. However, if there is no trof, it could be steered into FL. We'll see. Just some brainstorming that I'm doing out loud.
What your saying is logical. I just hope it's a few weeks early for prime time
Member Since: May 26, 2011 Posts: 0 Comments: 1193
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Impressive anticyclonic flow aloft associated with 91L:

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Quoting Stormchaser2007:
Red square is the center.



Excuse my French, but damn.
Member Since: July 15, 2006 Posts: 17 Comments: 10159

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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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