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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1771. MTWX
Quoting GHOSTY1:
thanks, just a question is about the same area as katrina, ike, and rita formed or were there origins further north?


Looks strangly of Emily '05... weird
Member Since: July 20, 2009 Posts: 23 Comments: 1393
Quoting sky1989:


That seems to be the trend every year with the GFS on Cape Verde storms while they are still in their infancy stage. I remember it immediately recurving Bertha in 2008 while it was still just an invest. It was the same thing with Earl and Igor last year.
And all those you listed actually recurved ;)
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Quoting GHOSTY1:


Looks like if it does form it will be a large one, this may require some watching, but where do you think it may end up? This may be so large too that it may give Texas some rain. Things are starting to heat up.


It looks like it forms from another monsoonal circulation. We'll have to see if it focuses its energy in the Caribbean or East Pacific. The GFS believes it will form in the East Pacific.
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1768. sky1989
Quoting MississippiWx:
The NAM also has a 1004mb low in the Western Caribbean. Develops in the SW Caribbean and moves up to around the Yucatan. You can see 91L just coming into the islands:



Although I know that the NAM is actually not intended for forcasting cyclogenisis, it did very well with Don while nearly all of the reliable models ignored it.
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1767. JRRP
,
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1766. pottery
Quoting Floridian2011:
Yo Potter, ya gonna be keeping close tabs on this one because of your proximity to the ABC islands?

Indeed!
I am in Trinidad, 11n 61w.
Not too worried about wind here, as we are too far south for most systems to spin up.
But Tropical Storms can bring deadly rainfall, and we have steep mountains with towns in valleys.
A bad combination, sometimes....
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1765. KEEPEROFTHEGATE (Mod)
Quoting Floridian2011:


Good to see that, man, you've been missed. At least you came back. However, 456 appears to be gone forever, dang, I miss that blogger, WOW, =(. This gets me emotional and everything, NO JOKE.
teh good ones always disappear nothing stays the same everything changes
Member Since: July 15, 2006 Posts: 171 Comments: 53838
1764. GHOSTY1
Quoting MississippiWx:
The NAM also has a 1004mb low in the Western Caribbean. Develops in the SW Caribbean and moves up to around the Yucatan. You can see 91L just coming into the islands:



Looks like if it does form it will be a large one, this may require some watching, but where do you think it may end up? This may be so large too that it may give Texas some rain. Things are starting to heat up.
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This Invest is big, covers a big area...

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6z should be interesting.

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Sorry to get you all Messed up before Bed.........NITE NOW!
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1760. sky1989
Quoting reedzone:
GFS recurves 91L (Emily) after 192 hours... So typical of the GFS


That seems to be the trend every year with the GFS on Cape Verde storms while they are still in their infancy stage. I remember it immediately recurving Bertha in 2008 while it was still just an invest. It was the same thing with Earl and Igor last year.
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Quoting MississippiWx:
The NAM also has a 1004mb low in the Western Caribbean. Develops in the SW Caribbean and moves up to around the Yucatan. You can see 91L just coming into the islands:

Wait u gotta be kidding me are we or could be seeing two tropical systems in the carribean by next week! OMG SMH! This is our second season folks. here we go!
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1756. GHOSTY1
the LBAR looks the best for intensification and the worst for some one who may be in her path if she becomes too strong.
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Quoting OceanBlue78:


Its like that here in Pearland...one day I went outside, got in my gpas truck, looked at the house and was like huh? Got out, the brick around the garage leaned to the right, breaking the fascia board and pulling away from the wood around the garage door. 2 years ago the heat and dryness cracked our newly installed tile floor, and there are cracks in the sheetrock now. Thank God for the few crazy thunderstorms we have had, but we still need 15-20 inches of rain to catch up. Don was some hope, but now really no hope.


Soaker hoses around the slab. 15 mins a day. Necessity here in Corpus due to clay soil. Think about it, when you see the ground crack, it's not just shrinking horizontally.
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The NAM also has a 1004mb low in the Western Caribbean. Develops in the SW Caribbean and moves up to around the Yucatan. You can see 91L just coming into the islands:

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1750. GHOSTY1
thanks, just a question is about the same area as katrina, ike, and rita formed or were there origins further north?
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Evening Pottery,

I christened our current storm
Don Quixote
and am hoping that next will be
Emily Post
with good manners and the best etiquette
so as to bother no one!
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I'd watch the LBAR. Did very well with Don/90L.

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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.


And when he was a twave off Africa every was oooh, look at that.
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1746. ryang
Quoting pottery:

LOL, I thought they would NEVER get Certified.....
Big competition there for the rest.
About time, too.

91L looks like one to keep a close eye on for the next week, for us all down this side.


Yup, I've been so busy these days but I'll be keeping a very close on this one.
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Quoting Floridian2011:


EX, you're back, how I've missed ya, man.


Yup! Tryin to get back into the tropics.
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GFS recurves 91L (Emily) after 192 hours... So typical of the GFS
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Well, speculating on track after the Lesser Antilles is simply that...speculation. The trend so far, however, is for the trofs to be too strong on the models and the high too weak. We'll see if that's the case again with 91L. As for now, though, most of the models like a track near and north of the Greater Antilles.
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1741. MTWX
Quoting sky1989:


Hopefully not another Jeanne (2004).


Or Ivan...

Quoting GHOSTY1:
who is SF?


South Florida
Member Since: July 20, 2009 Posts: 23 Comments: 1393
1739. xcool
Floridian2011 .hehe
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1738. GHOSTY1
KEEPEROFTHEGATE, why do have weakening flag i see convection reintensifying?
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Quoting MTWX:

We should make one!! Hurricane Prediction: 1001 ways to prepare crow... LOL

I'm thinkin we been putting a decent dent in the crow population, we have to list them as an endangered species the way we are all going this year!


Lol. And its just begun. Poor crows. :)
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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.
yeah thats what i was telling jason just now. and watch that huge one behind the one thats just coming off africa.
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1734. pottery
Quoting ryang:


Hey Pottery....are u flying redjet yet? lol

LOL, I thought they would NEVER get Certified.....
Big competition there for the rest.
About time, too.

91L looks like one to keep a close eye on for the next week, for us all down this side.
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1732. GHOSTY1
who is SF?
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1731. sky1989
Quoting reedzone:


GFS Revives 91L north of Haiti at 180 hours.






Hopefully not another Jeanne (2004).
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Quoting taco2me61:


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

Very Active Indeed....

Taco :o)


might be 2004 again........Wow i hope not!
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1729. MTWX
Quoting AtHomeInTX:


Lol. We've all eaten plenty of crow. Surprised there's not a cookbook. :)

We should make one!! Hurricane Prediction: 1001 ways to prepare crow... LOL

I'm thinkin we been putting a decent dent in the crow population, we have to list them as an endangered species the way we are all going this year!
Member Since: July 20, 2009 Posts: 23 Comments: 1393
Quoting ryang:
The models are underestimating the high to the north of 91L. No way the system will move NW almost immediately. IMO of course


I'd have to agree with that.

This could stay in the Caribbean.
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1726. GHOSTY1
All aboard the wave train... Choo Choo... Chugga Chugga... Next stops the Carribean, Bahamas, and the Gulf of Mexico....
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1725. KEEPEROFTHEGATE (Mod)
04L/TS/D/Cx



weakening flag OFF
Member Since: July 15, 2006 Posts: 171 Comments: 53838
91L should be the first big player of the season.
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1723. KEEPEROFTHEGATE (Mod)
XX/INV/91L
MARK
10.45N/41.75W
Member Since: July 15, 2006 Posts: 171 Comments: 53838
1722. ryang
Quoting pottery:
Hi Ryang!
Long time...


Hey Pottery....are u flying redjet yet? lol
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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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About

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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