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 Stormchaser2007:
GFS kills 91L over Hispaniola.

Grain of salt for now.

Not much to work with for now.



GFS Revives 91L north of Haiti at 180 hours.




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OK everyone! I got the Graphics and models all up on Invest 91L on the Website..........good NITE!
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Quoting xcool:
2011 Atlantic hurricane season wowww


For some Odd Reason it goes hand and hand with how our Tornado Season went....

Very Active Indeed....

Taco :o)
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1718. GHOSTY1
there's been alot of people saying they may have to eat crow, at this pace we might wipe out the entire crow population and rain won't bring them back...Dun Dun Dunnnnnnnnn
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Quoting CybrTeddy:
GFS shows 91L north of Hispaniola after being weakened due to land interaction and another system just off Africa getting its act together. The wave train has started.
If 91L were to miss the Greater Antilles entirely and go north of them, so very bad things are in store.

Luckily we have sufficient time to watch this.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
1716. sky1989
Quoting CybrTeddy:


Good model support, good anti-level conditions ahead. Other than the dry air, which I don't see being a huge problem due to the large moisture field associated with it, this one has no real barriers against development.


I have a bad feeling about Emily. It seems like a majority of the years that that name is used, that name seems to produce the worst storm of the year (1987, 1993, and 2005 although it was definately not the worst in that season).
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Quoting taco2me61:


Me either AtHome :o)

I will be having some BBQ "Crow" tomorrow for lunch because I thought it would be stronger today and be a High end Cat1 or Low end Cat2 by landfall....

"Boy Did I Blow This One"

But I was not looking at the "Hole Picture" thats for sure..... Next time I will....

Taco :o)


Lol. We've all eaten plenty of crow. Surprised there's not a cookbook. :)
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1714. ryang
The models are underestimating the high to the north of 91L. No way the system will move NW almost immediately. IMO of course
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GFS shows 91L north of Hispaniola after being weakened due to land interaction and another system just off Africa getting its act together. The wave train has started.
Member Since: July 8, 2005 Posts: 259 Comments: 24484
It will be interesting to see if the models shift south again, like they did with Don when he was a tropical wave. We have plenty of time to model watch.
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1711. pottery
Hi Ryang!
Long time...
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Looks like I may be eating my words already from earlier this afternoon regarding the Central Atlantic wave.
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1709. xcool
2011 Atlantic hurricane season wowww
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1708. JLPR2
Well I'm out.
Good night all!

This one has a chance at becoming our first hurricane.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
1707. MTWX
Quoting jasonweatherman2011:
modeles are out!!!

Wow the SHIPS ain't messin around with this one, now is it!!
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Quoting jasonweatherman2011:


hey jason i know u guys are worried about don and invest 91L but look at wats coming off Africa right now. Look at the color on your map. VERY INTERESTING!!!
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1705. GHOSTY1
the BAMS CLIP and LBAR will probably cause the greatest intensification, jmo.
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1704. sky1989
Quoting GHOSTY1:
Didn't Don's original models when he was an invest show him moving into Bahamas but look where he is now? Just food for thought.


They did indeed. When 90L (Don) was in the position 91L is in now, the models, off and on, predicted it to go through the Bahamas and up towards the Carolinas. Then they dropped it altogether as it struggled and traveled much further west towards the Gulf of Mexico.
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Quoting Stormchaser2007:
GFS develops 91L over the Lesser Antilles.

This is going to be a good one.


Good model support, good anti-level conditions ahead. Other than the dry air, which I don't see being a huge problem due to the large moisture field associated with it, this one has no real barriers against development.
Member Since: July 8, 2005 Posts: 259 Comments: 24484
GFS model is showing 91L (Emily) hitting Haiti in 150 hours from now. This will most likely be our first REAL threat of the Season.
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1701. MTWX
Quoting taco2me61:


Me either AtHome :o)

I will be having some BBQ "Crow" tomorrow for lunch because I thought it would be stronger today and be a High end Cat1 or Low end Cat2 by landfall....

"Boy Did I Blow This One"

But I was not looking at the "Hole Picture" thats for sure..... Next time I will....

Taco :o)

Save me some!! I made roughly the same call as you Taco...
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Quoting GHOSTY1:


Don't worry i've already angered some people on the blog right now unintentionally to the point where they just leave the blogs without any sort of conclusion, but i don't think you mean the same type of trouble as i mean.


Ah well. We're not going to make everyone happy. Don't sweat it though. Sometimes things don't come across the same on the blog as we mean them in our heads. But lesson learned it'll be ok. Looks like there's plenty to talk about now for sure. :)
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On happier note this may bring some much needed rain to Texas. My garden plants are all but goners in the heatwave we've been having. Even by Texas standards it's been a bad year for weather and gardeners. Usually my plants have gone that way in August, this year much earlier, they gave a meagre crop and died away.
Member Since: June 22, 2010 Posts: 0 Comments: 14
GFS kills 91L over Hispaniola.

Grain of salt for now.

Not much to work with for now.

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I did an update for 91L. Here it is.
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1696. JRRP
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1694. nigel20
Quoting GHOSTY1:
Didn't Don's original models when he was an invest show him moving into Bahamas but look where he is now? Just food for thought.
I agreee 100%
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1693. xcool
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1692. GHOSTY1
Yeah i don't know sure but im pretty sure i was wrong about something and should eat crow but all the crows are dead of dehydration in texas so i guess ill have to go for a buzzard
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1690. ryang
Very interesting with 91L. This weekend will be very interesting indeed.
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1689. pottery
Quoting GHOSTY1:
Didn't Don's original models when he was an invest show him moving into Bahamas but look where he is now? Just food for thought.

Exactly!
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1688. xcool
mybe 5 storm in July wow
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1687. JLPR2
Quoting sunlinepr:
Yeap, very close to PR and RD...




Seems the E name is our curse now. XD
Earl passed close last year and this one would be Emily.
Member Since: September 4, 2007 Posts: 7 Comments: 8747
1686. sky1989
Quoting MississippiWx:
Looks like 91L develops on the GFS, but could struggle with land interaction around the Greater Antilles.



I think this will end up traveling further south than that. Remember, the models kept shifting further and further south with what became Don. Of course, if this intensifies more quickly, and becomes named east of the islands, it may be more susceptible to a path further north.
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1685. GHOSTY1
Didn't Don's original models when he was an invest show him moving into Bahamas but look where he is now? Just food for thought.
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1684. KEEPEROFTHEGATE (Mod)
rains coming

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1683. Gearsts
Quoting Stormchaser2007:
GFS develops 91L over the Lesser Antilles.

This is going to be a good one.
link please?!
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1682. pottery
Quoting MississippiWx:
Looks like 91L develops on the GFS, but could struggle with land interaction around the Greater Antilles.


That model looks Bad for Haiti.
I hope it's wrong by a long way.
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Quoting AtHomeInTX:


Lol. Wouldn't be the first time my mouth gets me in trouble or fingers as it were. :)


Me either AtHome :o)

I will be having some BBQ "Crow" tomorrow for lunch because I thought it would be stronger today and be a High end Cat1 or Low end Cat2 by landfall....

"Boy Did I Blow This One"

But I was not looking at the "Hole Picture" thats for sure..... Next time I will....

Taco :o)
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Yeap, very close to PR and RD...


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Looks like 91L develops on the GFS, but could struggle with land interaction around the Greater Antilles.

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1677. GHOSTY1
Quoting AtHomeInTX:


Lol. Wouldn't be the first time my mouth gets me in trouble or fingers as it were. :)


Don't worry i've already angered some people on the blog right now unintentionally to the point where they just leave the blogs without any sort of conclusion, but i don't think you mean the same type of trouble as i mean.
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1676. pottery
Quoting bigwes6844:
I dont think it will fizzle because it has been really holding up really good lately. This mite be one to watch real soon.

I agree with that too.
I was just trying to make the point that noboddy can predict what will happen at this stage.
Looks pretty good though!
Nice conditions ahead too......
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1675. SLU
Quoting pottery:

For what it's worth at this early stage, that is showing a pretty consistent WNW movement.

Early days...


Very interesting weekend ahead .....
Member Since: July 13, 2006 Posts: 13 Comments: 5356
1674. j2008
Quoting GHOSTY1:
im just wonderin' whats goin to happen to Don when i wake tomorrow mornin' and what 91L will be seasons getting off to a fast start (and yes im kinda late to start saying this)? Does anyone still think a hurricane Don by tomorrow?

I would be surprised if we did but im sure not going to rule it out.
Member Since: December 19, 2008 Posts: 1 Comments: 224
1673. nigel20
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Quoting KoritheMan:
Ghost, you might want to watch the way you word things. People are almost going to universally be offended by statements along the lines of "I want a storm", because they feel that you are trivializing the suffering of others. A most understandable concern, if you ask me.

Over the years I've learned that much depends upon perception. Just remember that.
I'm glad you said it, because I would have probably said something in my less tactful way.
Member Since: May 26, 2011 Posts: 0 Comments: 1193
Quoting Tygor:
Wow I can't believe this isn't going to affect Central Texas. I thought for sure it would get us.


I was also holding onto to hope that we would get some further North. You just glare at the grass and it curls up into little yellow needles and combusts. Okay, that is a bit exaggerated but there are many ranches and farms here that could use a good rain. Oh well, will wait and watch.
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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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