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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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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Quoting Levi32:
Step 1 here is the eastern Caribbean islands and Puerto Rico, which may very well have to deal with a named storm. After we figure out how strong 91L is going to get by that time and how fast, and after a few days have passed, we will have more of an idea of whether more land areas will be threatened. For now, a recurve after the eastern Caribbean could be very possible. It could also not be. It's another fragile pattern like with Don. Obviously he went west, but there's no guarantee that 91L will follow suit. When these things are this far out, no matter how many demands are made of forecasters, there is a high degree of uncertainty about what it is going to do 2000 miles away.

agreed levi but if i may ask, if 91L becomes a weak hurricane in the NE carribean does that gurantee a recurvutrue or do we just have to wait and see each day as the pattern progresses?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
I'm dissapointed in Don. I was really looking forward to a rainy Saturday, but I just don't think it's gonna happen.
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SFL surely needs to pay attention to 91L. Ends up in one of those places ya really don't want to see a cyclone. No disrespect to others along the path.


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Quoting NotCircumventing:
2447 ...

Migth still make it to Matagorda. Or so.

Seriously, I know I have said it a lot, but it is still a player with over 12 hours til landfall remaining.

Especially with this right-bias we see.

I sure hope you're right. We need the rain up here.
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just to put into perspective where 91L is and how fast a storm can spin up...

9/10/89 later named hugo was a TD 13.2n 20.0w
9/11/89 became TS Hugo 12.5n 29.2w
9/13/89 became Hurrican Hugo 12.8n 43.5w

understand that it was 2 months in the season later, however, our higher than normal sst's could be a BIG contributing factor as far as the season goes
Member Since: September 16, 2007 Posts: 0 Comments: 3650
2465. Levi32
Step 1 here is the eastern Caribbean islands and Puerto Rico, which may very well have to deal with a named storm. After we figure out how strong 91L is going to get by that time and how fast, and after a few days have passed, we will have more of an idea of whether more land areas will be threatened. For now, a recurve after the eastern Caribbean could be very possible. It could also not be. It's another fragile pattern like with Don. Obviously he went west, but there's no guarantee that 91L will follow suit. When these things are this far out, no matter how many demands are made of forecasters, there is a high degree of uncertainty about what it is going to do 2000 miles down the road.
Member Since: November 24, 2005 Posts: 635 Comments: 26684
Alright everyone, discount a MX/TX landfall... it would have to go directly west now for 200 miles just to hit Brownsville...
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What does the LGEM bring 91L up to? That's the one the NHC follows, and is usually quite right.
Member Since: July 6, 2010 Posts: 113 Comments: 32523
Quoting jasonweatherman2011:
if invest 91L slow down that will be bad to!!


This is gonna be all about timing.......pure and simple!
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if invest 91L slow down that will be bad to!!
Member Since: July 12, 2011 Posts: 0 Comments: 1532
2447 ...

Migth still make it to Matagorda. Or so.

Seriously, I know I have said it a lot, but it is still a player with over 12 hours til landfall remaining.

Especially with this right-bias we see.
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Quoting weathermanwannabe:


Nobody knows at this point......................


F. Emily is destined be be a storm with a retired name. No chance of being a fish, large circulation, the sky is the limit. Very very concerned.
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2458. Levi32
Quoting TropicalAnalystwx13:


Should I be concerned?


Not while it's 2600 miles away.
Member Since: November 24, 2005 Posts: 635 Comments: 26684
OUTER RAIN BANDS OF DON APPROACHING THE TEXAS COAST...

LOCATION...26.2N 94.9W
ABOUT 190 MI...305 KM SE OF CORPUS CHRISTI TEXAS
ABOUT 165 MI...260 KM E OF BROWNSVILLE TEXAS
MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H
PRESENT MOVEMENT...WNW OR 300 DEGREES AT 14 MPH...22 KM/H
MINIMUM CENTRAL PRESSURE...1002 MB...29.59 INCHES
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting scott39:
Goodmorning, I think our luck is going to run out my friend.


Ok I posted this once but it did not show up....

I think our time is running out and our luck is on a "Very Thin Line "

Not liking this at all and Good Morning to you too :o)

Taco :o)
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Quoting tiggeriffic:


i say the high is gonna lift up then flatten out before it can make a recurve...



That's been the general pattern to date. Otherwise would be a pretty big pattern change.
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2454. 10Speed
Quoting hurricaneben:
Should I be concerned about 91L in South Florida or are there no chances of making it this way.


Well, I finished my projection late Wednesday night and came up with landfall between Delray Beach and Boca Raton. That's just my own long range projection though and is for course only.
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Quoting tiggeriffic:


i say the high is gonna lift up then flatten out before it can make a recurve...


Which would send it here! Thanks!

lol.
Member Since: July 6, 2010 Posts: 113 Comments: 32523
Worries here in the NE coast about future hurricane Emily.....
91L could really be the first strong hurricane of the season...
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Quoting ProgressivePulse:
Have to see if High Pressure still rules the roost with respect to 91L. Little concerned here.


i say the high is gonna lift up then flatten out before it can make a recurve...
Member Since: September 16, 2007 Posts: 0 Comments: 3650




Finally..........look at the Convergence....That is the way a good looking Storm is suppose to have. This would be a Cane very soon with that kind of Convergence......it still just might make a Cane!
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Quoting Levi32:


As with when Don was at 40W, it's a bit early to say. 6 days of travel to get to 70W is a long time. To put that in perspective, if the models are off on 91L's average forward speed during that time by only 1mph, the position error after 6 days is almost 150 miles.


Wow.
Member Since: July 6, 2010 Posts: 113 Comments: 32523
look at this...
Member Since: July 12, 2011 Posts: 0 Comments: 1532
Quoting TampaSpin:
Don looks to be pulling NOrth some with his latest burst of energy. Looks like a Corpus Cristi land fall to me.


Surely is.



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Quoting TampaSpin:



Probably will be the first MAJOR and i Don't like the looks at where it might move toward!


Should I be concerned?
Member Since: July 6, 2010 Posts: 113 Comments: 32523
If Don were to keep his current NW motion it would head right into Matagorda Bay, which would be great(MORE RAIN FOR ME!!!)
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Don looks to be pulling NOrth some with his latest burst of energy. Looks like a Corpus Cristi land fall to me.
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Have to see if High Pressure still rules the roost with respect to 91L. Little concerned here.
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2442. Levi32
Quoting scott39:
Do you think the timing of the trof with a potiential recurve is overdone? TIA


As with when Don was at 40W, it's a bit early to say. 6 days of travel to get to 70W is a long time. To put that in perspective, if the models are off on 91L's average forward speed during that time by only 1mph, the position error after 6 days is almost 150 miles.
Member Since: November 24, 2005 Posts: 635 Comments: 26684
2441. HCW
Don & 91L model runs from the NHC. Have a great day :)


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Conduct becoming of a tropical cyclone, thar be the convergence it needs to sustain.

Morning All.

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2439. barbamz
Quoting Levi32:


ECMWF had landfall near Corpus on last night's run. That looks correct to me.


Thank you and as well for you tidbits.
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Quoting TropicalAnalystwx13:
Good morning guys, someone update me on 91L. Just TRYING to read through some pages of comments, models forecast this to become a major hurricane??



Probably will be the first MAJOR and i Don't like the looks at where it might move toward!
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2437. Levi32
Extrapolated 1003mb:

000
URNT12 KNHC 291414
VORTEX DATA MESSAGE AL042011
A. 29/13:43:10Z
B. 26 deg 06 min N
094 deg 38 min W
C. 850 mb 1452 m
D. 40 kt
E. 143 deg 30 nm
F. 188 deg 36 kt
G. 138 deg 56 nm
H. EXTRAP 1003 mb
I. 14 C / 1529 m
J. 23 C / 1524 m
K. 8 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 3 nm
P. AF303 0504A DON OB 08
MAX FL WIND 52 KT E QUAD 11:43:20Z
SLP EXTRAP FROM 850 MB
MAX FL TEMP 26 C 328 / 7 NM FROM FL CNTR
CONVECTIVE BAND TO S AND E OF CENTER
Member Since: November 24, 2005 Posts: 635 Comments: 26684
Well the GFDL likes a cat 3 for 91L... scraping the southern edges of the caribbean islands...

Canadian shows a hurricane attempting to get pulled out to sea by the trough...

GFS likes it to form south of PR, and go over or just west of it and cross into the turks and caicos, but no real sign of it getting pulled out to sea(less of a pull northward compared to CMC), though its a possibility.

HWRF likes a category 2, hitting 60 W and 20N at the same time, with the trough moving out of the picture, and possibly(likely) pushing it back westwards.(this solution would not be good, due to the fact there is a category 2 heading for the bahamas, and there is 29 to 31 C waters there...

Nogaps doesnt develop it much, but agrees to more monsoonal development, and the formation of an EPAC system(of coarse).
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Good morning guys, someone update me on 91L. Just TRYING to read through some pages of comments, models forecast this to become a major hurricane??
Member Since: July 6, 2010 Posts: 113 Comments: 32523
Quoting Levi32:


It will be slow to spin up, as pre-Don was while he was out in the central Atlantic, but not as slow as that. You're right that if it stays weaker it will probably track farther south and west. The GFDL is probably overdone on the short-term intensity, but I wouldn't be surprised to see this named before it reaches the islands.

Levi there a possibility the trough trys to pick it up and it gets out of the carribean but the ridge builds back in, would that be more of an east coast problem or a US gulf coast problem?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting jasonweatherman2011:
maybe invest 92L down the road!!


Maybe but should move into the Pacific!
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2432. P451
Quoting Levi32:
Don has been pulsing since last night - 3-4mb swings up and down. Recon extrapolations in each leg will also differ from dropsondes almost every time. Wait for the new vortex message.


Thanks for the recap.

See ya all on the new blog when it comes out.
Member Since: December 16, 2007 Posts: 7 Comments: 10202
2431. scott39
Quoting Levi32:


It will be slow to spin up, as pre-Don was while he was out in the central Atlantic, but not as slow as that. You're right that if it stays weaker it will probably track farther south and west. The GFDL is probably overdone on the short-term intensity, but I wouldn't be surprised to see this named before it reaches the islands.
Do you think the timing of the trof with a potiential recurve is overdone? TIA
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maybe invest 92L down the road!!
Member Since: July 12, 2011 Posts: 0 Comments: 1532
Quoting weathermanwannabe:


Nobody knows at this point......................

It was ment for your opinion.
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2428. Levi32
Quoting barbamz:



So Levi, the EURO has been right?
From the NHC-Discussion:
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.



ECMWF had landfall near Corpus on last night's run. That looks correct to me.
Member Since: November 24, 2005 Posts: 635 Comments: 26684
Quoting JrWeathermanFL:
91L will peak at_______.
A.Invest
B.TD
C.TS
D.Hurricane
E.Major Hurricane


Nobody knows at this point......................
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2426. Levi32
Quoting scott39:
Levi, Is it safe to say the weaker 91L stays...the futher W it will go? Do you see it ramping up as fast as some modules?


It will be slow to spin up, as pre-Don was while he was out in the central Atlantic, but not as slow as that. You're right that if it stays weaker it will probably track farther south and west. The GFDL is probably overdone on the short-term intensity, but I wouldn't be surprised to see this named before it reaches the islands.
Member Since: November 24, 2005 Posts: 635 Comments: 26684
91L will peak at_______.
A.Invest
B.TD
C.TS
D.Hurricane
E.Major Hurricane
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2424. scott39
Quoting taco2me61:


Way to far out to fly into

Taco :o)
Goodmorning, I think our luck is going to run out my friend.
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Quoting Waltanater:
Link?


Go to my WU blog and follow the links to my site......if i post it i will get banned. Maybe someone else can post it for you all. Sorry!
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Quoting JrWeathermanFL:
Are HH flying into 91L?


Way to far out to fly into

Taco :o)
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2421. HCW
Let's see how many images I can get in this radar loop that is being recorded over at FLhurricane

Link
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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.