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 CanesfanatUT:


Looks like more of a threat in terms of winds than Don ever was.


depends on how you look at it...Don is approaching NOW, it is going to hit land, this we know...if the rains fall too hard then they wash out and cause flooding...what little is left for the farmers will be gone...91L is a 10-15 days away...don't know where it is going or if it will go...
Member Since: September 16, 2007 Posts: 0 Comments: 3650
Quoting NotCircumventing:
Consider blog focus officially shifted to 91L as of 1100z.


Looks like more of a threat in terms of winds than Don ever was.
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Quoting MahFL:
New NASA peer reviewed paper says global warming won't be as bad as the doomcasters are saying.

Link

Although I do think some warming is occuring.




Scientists aren't buying it. (In short: Spencer's wrong. Again.)
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Quoting NotCircumventing:
Is there a Matagorda in northern Mexico?


LOLZ!
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Quoting Cotillion:
One of these days, it'd be nice to see a proper average for the post '95 era for named storms and hurricanes to dates.

I'm not sure of the averages for 1995 through current, but the 1966-2009 average says the fourth named storm normally appears on August 23, the first hurricane on August 10, and the first major on September 4. Lots of info here.
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2066. MahFL
New NASA peer reviewed paper says global warming won't be as bad as the doomcasters are saying.

Link

Although I do think some warming is occuring.



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2065. Walshy
Quoting floridaboy14:
so guys do you guys think don is weakining or stregnthining?
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so guys do you guys think don is weakining or stregnthining?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
ASCAT 7:30 PM EDT last night






OSCAT 9:53 PM EDT last night





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2062. Gearsts
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Happy Friday to all, our rain chances are now less than 20 percent just south of Austin. Honestly I am not surprised because of this High Pressure which has not allowed anything that is going to give me rain close to me. There is a chance this drought does not end this year and that is what people should prepare for, we don't know when this state will start getting the rains it needs. Nothing we can do but keep praying, have a great day!
Member Since: July 5, 2009 Posts: 0 Comments: 1348
white model run on character jasons post reminds me of hugos track
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I just woke up. does don look better worse or the same to you guys?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2057. Gearsts
Quoting aquak9:
gearsts and tampa- let's review in about a week, ok? Getting "my troll" on...geeeesh....

laterz- -
Yea you are, what if this becomes a hurracane and hits the islands are you gona keep saying fish storm?
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Quoting aquak9:
gearsts and tampa- let's review in about a week, ok? Getting "my troll" on...geeeesh....

laterz- -

I didn't call ya a troll.. ;)
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Quoting WxLogic:
Good Morning...



Morning, WxLogic. What do you know about the MJO, Bermuda High and anything else that would influence steering 91L? Am sure you know, but NHC has it at 30% as of 4:35 a.m.
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Good Morning Guys, looks like we have invest 91L models take this to a cat 3?? wow.. looks like don is looking better right?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2052. aquak9
gearsts and tampa- let's review in about a week, ok? Getting "my troll" on...geeeesh....

laterz- -
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Quoting jasonweatherman2011:
i am not calling invest 91L A FISH STORM!! its still to far out to know anything yet!!

jason ur right
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2050. WxLogic
Good Morning...

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Morning, Cotillion. I thought I saw a little northward jog to Don. Also would like to see it become a bigger system before landfall in order to spread the joy around. It's likely not going to become hurricane strength anyway, so longer over the water the better imo.

LinkWVLoopDon
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Quoting Chicklit:
LinkIRLoop91L

Can't wait to hear discussion on this one.
Would someone please wake up Kman and Pottery?
All hands on deck!


roflmbo...add stormjunkie, presslord and patrap and we have a steel deck party (navy term for being topside)for 91L...
Member Since: September 16, 2007 Posts: 0 Comments: 3650
Oh, and salutations to all.

A happy, summer filled song to begin the day: Link

Don's looking the best he has so far, a good amorphous amount of convection, some of it deep. Luckily, he doesn't have forever until landfall. Hopefully, the drought stricken areas will get some rain, but it's not looking hopeful.
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2046. MahFL
I saw on TWC this am that 2 inches of rain for S TX would not even be beneficial, it would only be neutral, ie, stopping things from actually getting worse.
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LinkIRLoop91L

Can't wait to hear discussion on this one.
Would someone please wake up Kman and Pottery?
All hands on deck!
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really watchin 91L right now...if it goes south of PR chances are it is a GOM storm...hit PR or just north of it and it could be a east coaster...and just in time for school to start....
Member Since: September 16, 2007 Posts: 0 Comments: 3650
Quoting aquak9:
AtHome is right. Sorry but 91L is gonna be a fish.

She was not reading the chart right.
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2041. Gearsts
Quoting aquak9:
AtHome is right. Sorry but 91L is gonna be a fish.
Getting your troll on i see...
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Well 06Z NOGAPS keeps 91L south of Hispaniola, and doesn't really develop it. Weird, I thought that NOGAPS was the first to blow up a storm, its lagging the rest.
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One of these days, it'd be nice to see a proper average for the post '95 era for named storms and hurricanes to dates.
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Quoting 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.


Tessa, sorry about that but those of us in SW Texas are desperate for rain too. Don looks like it will pass directly over us here near Del Rio and bring much needed moisture to our area. Now watching 91L, it looms large out there!
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2034. Gearsts
Quoting jasonweatherman2011:
wow winds over 100 mph!! look likes the modeles are moving more south
More south? I see that they are moving more north towards PR, i guess they see a stronger system...
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2033. aquak9
AtHome is right. Sorry but 91L is gonna be a fish.
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AtHome - how do you know the CMC recurves it? The model on Huffman's page only goes out 144 hrs and it has it just above PR at that point as a significant system.
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If 91L became a major hurricane, it would also be several weeks ahead of the usual date for the season's first major.
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2028. ackee
ANYONE read crown weather DIsscution on 91L I do agree that this will be first hurricane of the seasons
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2027. SLU
Quoting yonzabam:


The CATL wave is more to the south of Emily's track.


Yes it is. It may also move more to the WNW too.
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2026. SLU
Quoting stoormfury:
SLU have you seen the intensity forecast. it looks frightening. hope is an error.


The SHIPS predicts 1kts of shear in 24 hrs and a 44% chance of rapid intensification. Once the inner core tightens up, there might be precious little anyone can do to stop this from making a steady march to major hurricane status.

SHEAR (KT) 13 10 8 6 1 3 1 1 2 3 N/A N/A N/A


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2025. 34chip
Invest could do anything at this point. Calm down Texas! Wait till its really something to care about!
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Quoting SLU:
Deja Vu all over again according to stormfury?



The CATL wave is more to the south of Emily's track.
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SLU have you seen the intensity forecast. it looks frightening. hope is an error.
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2022. SLU
Emily's track during her previous life.

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

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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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