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 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.
Impersonation?
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00z GFS


FULL IMAGE


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1917. JRRP
nada
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Quoting Floridian2011:


Well then, excuse us, dique, according to ''YOUR PAID'' site, ROFL, just kidding around, bro, laugh a little, it's 2am, man. LOL, but seriously, dude, why doesn't it develop it? -__________-.


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.
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1914. HCW
SPC has introduced a 2% Tornado risk for a large portion of Texas for Don

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Don appears to be on a WNW track. I wouldn't expect a landfall much farther north than Brownsville if it continues this track:

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1911. JRRP
so 30% ....
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Quoting Stormchaser2007:
According to my paid site, the 00z ECMWF does not develop 91L. Just has a strong TW passing through the southern islands.


Could mean that a weaker system goes south, while stronger goes north.
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1909. j2008
I'm out, Cheak in 8-9 Zona time. Keep an eye on our two systems for me.
Member Since: December 19, 2008 Posts: 1 Comments: 224
78 hours

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According to my paid site, the 00z ECMWF does not develop 91L. Just has a strong TW passing through the southern islands.
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Quoting sunlinepr:
A todos los que son bilingues. Favor de traducir en ambos idiomas SOLO si FZ traduce lo que dice tambien....

Buenas noches... Y GRACIAS ;)
Thank you, I think.
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Really not sure where the NHC is getting 1003mb from...

000
URNT12 KNHC 290607
VORTEX DATA MESSAGE AL042011
A. 29/05:35:40Z
B. 24 deg 59 min N
092 deg 56 min W
C. 850 mb 1455 m
D. 26 kt
E. 318 deg 17 nm
F. 081 deg 34 kt
G. 319 deg 13 nm
H. EXTRAP 1000 mb
I. 21 C / 1524 m
J. 27 C / 1527 m
K. 14 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 18
MAX OUTBOUND AND MAX FL WIND 46 KT NE QUAD 05:56:40Z
SLP EXTRAP FROM 850 MB
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Quoting nigel20:
I agreee 100%
I can remember 1 model picking up on Don and that was nogaps,which took him thru bahamas up to S/C N/C Area, that model has a NE Bias quite often. NOGAPS also had him as a closed circulation most of the time, which he didnt do til the very end. Of all the tropical models the BAMM SUITE has pretty much nailed Don JMI
Member Since: September 18, 2010 Posts: 0 Comments: 5100
1901. j2008
Quoting Hurricanejer95:
...DON CONTINUES MOVING WEST-NORTHWEST WITH LITTLE CHANGE IN STRENGTH...
1:00 AM CDT Fri Jul 29
Location: 25.0°N 93.0°W
Max sustained: 50 mph
Moving: WNW at 14 mph
Min pressure: 1003 mb

??? Upped pressure to 1003 should be 999 or 1000

Wonder if they made a mistake and will have to revise it in a bit.
Member Since: December 19, 2008 Posts: 1 Comments: 224
Gah, all of this tropical activity is killing my sleep. Lol. Now, I must stay up for the Euro run. My guess is that it will take 91L north of the islands too. Of course, since the GFS is showing it, it will probably show a track through the middle of the Caribbean. Those two are like siblings. If one likes one solution, the other likes something else.
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1899. HCW
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Quoting bigwes6844:
did yall see what the wunderground update on TS Don has the moving speed on this system. They got it moving N at 49mph! LMAO!!!!

rofl
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6z model plots should be interesting on 91L.
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Quoting bigwes6844:
did yall see what the wunderground update on TS Don has the moving speed on this system. They got it moving N at 49mph! LMAO!!!!


Someone mistook Movement speed as wind speed LOL
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did yall see what the wunderground update on TS Don has the moving speed on this system. They got it moving N at 49mph! LMAO!!!!
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Impressive

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1891. j2008
Back up to 1003. thats a five MB gain. Same wind speed, Don is pissing me off!! Wonder what kinda other tricks he has up his sleave. 30%on 91L wow we might have Emily before he month is out!
Member Since: December 19, 2008 Posts: 1 Comments: 224
Quoting Stormchaser2007:
60 hours:



Looks like most of the models are in agreement on a trof being present off the NE US coastline. CMC looks like it wants to take 91L north of the islands as well.
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Don up to 1003mb

1:00 AM CDT Fri Jul 29
Location: 25.0°N 93.0°W
Max sustained: 50 mph
Moving: WNW at 14 mph
Min pressure: 1003 mb
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...DON CONTINUES MOVING WEST-NORTHWEST WITH LITTLE CHANGE IN STRENGTH...
1:00 AM CDT Fri Jul 29
Location: 25.0°N 93.0°W
Max sustained: 50 mph
Moving: WNW at 14 mph
Min pressure: 1003 mb

??? Upped pressure to 1003 should be 999 or 1000
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Anyone think 91L could be a depression by tommorow night? Interesting to say the least
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60 hours:

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1884. KEEPEROFTHEGATE (Mod)
looks like a poss T.C.F.A. will be coming soon as first light in morning tells the tale with 91L see ya all around 730 8 am later all iam out
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Quoting FrankZapper:
To all bilingual bloggers. Please translate if what you say is important. If its just personal just say personal. Firefox has a Google translate button, but it doesn't work yet with FF 5.0. Thanks
A todos los que son bilingues. Favor de traducir en ambos idiomas SOLO si FZ tambien traduce lo que dice....

Buenas noches... Y GRACIAS ;)
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30% on 91L.
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To all bilingual bloggers. Please translate if what you say is important. If its just personal just say personal. Firefox has a Google translate button, but it doesn't work yet with FF 5.0. Thanks
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Strong wording!


A LARGE TROPICAL WAVE ACCOMPANIED BY A WELL-DEFINED LOW PRESSURE
SYSTEM IS LOCATED ABOUT 1200 MILES EAST-SOUTHEAST OF THE LESSER
ANTILLES. THIS DISTURBANCE IS SHOWING SIGNS OF ORGANIZATION...AND
ENVIRONMENTAL CONDITIONS ARE CONDUCIVE FOR GRADUAL DEVELOPMENT OVER
THE NEXT FEW DAYS
. THIS SYSTEM HAS A MEDIUM CHANCE...30 PERCENT...
OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES
WESTWARD OR WEST-NORTHWESTWARD AT 15 TO 20 MPH.
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Key words:

THIS DISTURBANCE IS SHOWING SIGNS OF ORGANIZATION...AND
ENVIRONMENTAL CONDITIONS ARE CONDUCIVE FOR GRADUAL DEVELOPMENT OVER
THE NEXT FEW DAYS.
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Removed...I think everyone got the point. LOL. 30%
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000
ABNT20 KNHC 290554
TWOAT

TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
200 AM EDT FRI JUL 29 2011

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM DON LOCATED ABOUT 285 MILES EAST-SOUTHEAST OF BROWNSVILLE
TEXAS.

A LARGE TROPICAL WAVE ACCOMPANIED BY A WELL-DEFINED LOW PRESSURE
SYSTEM IS LOCATED ABOUT 1200 MILES EAST-SOUTHEAST OF THE LESSER
ANTILLES. THIS DISTURBANCE IS SHOWING SIGNS OF ORGANIZATION...AND
ENVIRONMENTAL CONDITIONS ARE CONDUCIVE FOR GRADUAL DEVELOPMENT OVER
THE NEXT FEW DAYS. THIS SYSTEM HAS A MEDIUM CHANCE...30 PERCENT...
OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS AS IT MOVES
WESTWARD OR WEST-NORTHWESTWARD AT 15 TO 20 MPH.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$
FORECASTER STEWART/BLAKE
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1874. HCW
Quoting dsj11tsu:
Anyone think there is a possibility of TS Don taking a slight jog to the north and possibly making landfall near Corpus Christi as expected earlier this evening?


Don will make landfall in Mexico officially
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For those who missed it.

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I have a strong feeling 91L will end up in Central America or Mexico as with every other storm in that region since 2007.
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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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