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 TropicalAnalystwx13:
I think it is safe to say that Don is rapidly intensifying. But...how long will it last??
I don't see that happening. It still is a small blob of convection.
Member Since: May 26, 2011 Posts: 0 Comments: 1193
Suspected current steering for Don:

Port Lavaca still looks like a possibility if this holds. I continue not to understand where the models are getting the sharp bend west all the way through landfall.

Unless, CIMSS steering is as messed up as the sheer seems to have been.
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911:

The 999 surface reading indicated that the system is strengthening from the Mid-Levels and building it down to the lower levels. This explains why the winds haven't caught up to the pressure - a 15mb deep cyclone at sea level is only 12mb deep at 850 and 11mb deep at 750 which produces a shallower pressure gradient and lighter winds. This is compounded by the natural lag between pressure and winds.
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Good Evening...
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Are there any projected paths I can look at?
Member Since: June 22, 2010 Posts: 0 Comments: 14
AL, 04, 2011072900, , BEST, 0, 246N, 919W, 45, 998
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Quoting TropicalAnalystwx13:


Yes. 42 millibar drop in 24 hours is considered RI.
Or 2mb an hour.
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28/2345 UTC 24.5N 92.2W T3.0/3.0 DON -- Atlantic
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Quoting Max1023:
Based on superimposing the data from 750 and 850mb the center of Don is stacked to within 15nm between those two levels, indicating a system which is becoming better organized. The 850mb center is located at about 24.45N and 91.8W.



Don is still lopsided but it is MUCH better organized than before with convection now completely covering the 850mb center. I'm assuming that the surface center is not too severely displaced. At any rate the surface center would likely reform under the MLC based on the convective strength in that location.
is that a eye?
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VDM:

000
URNT12 KNHC 290026
VORTEX DATA MESSAGE AL042011
A. 29/00:09:50Z
B. 24 deg 26 min N
091 deg 50 min W
C. 850 mb 1424 m
D. 43 kt
E. 042 deg 6 nm
F. 138 deg 42 kt
G. 044 deg 8 nm
H. 999 mb
I. 22 C / 1524 m
J. 28 C / 1515 m
K. 14 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 1 nm
P. AF307 0404A DON OB 06
MAX FL WIND 42 KT NE QUAD 00:07:10Z
;
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Quoting NotCircumventing:
RI has specific criteria doesn't it?


Yes. 42 millibar drop in 24 hours is considered RI.
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it was 1001, 1000, 1004, 1005, 1004 and 995 no?
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Quoting SLU:
995mb?

No surprises given the overshooting tops in the last few hours.

what do you think about the to be invest? coming to a area near yours?
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Based on superimposing the data from 750 and 850mb the center of Don is stacked to within 15nm between those two levels, indicating a system which is becoming better organized. The 850mb center is located at about 24.45N and 91.8W.



Don is still lopsided but it is MUCH better organized than before with convection now completely covering the 850mb center. I'm assuming that the surface center is not too severely displaced. At any rate the surface center would likely reform under the MLC based on the convective strength in that location.
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Quoting Slamguitar:
Don is looking FAT on the last few RGB frames, wow!


Don breaks the record for the most persistent slacker in history to cross the Atlantic, thread the needle, then just go ***s up when entering the most enabling and protective GOM conditions in years.
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Quoting CybrTeddy:
Guys your acting like it went down to 901 mb lol, but if 995 is confirmed it is a substantial pressure decrease.
10mb drop in what? 5-6 hours?
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902. DFWjc
Quoting TropicalAnalystwx13:
I think it is safe to say that Don is rapidly intensifying. But...how long will it last??


True, especially with what happened last night...
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If Don gets stronger will this affect the path? Will it make it go further north or south?
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ATCF says Don is a little stronger:

AL, 04, 2011072900, , BEST, 0, 246N, 919W, 45, 998, TS, 34, NEQ, 85, 50, 0, 20, 1011, 90, 20, 0, 0, L, 0, , 0, 0, DON, M,
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899. SLU
995mb?

No surprises given the overshooting tops in the last few hours.

Member Since: July 13, 2006 Posts: 12 Comments: 4734
Quoting TropicalAnalystwx13:
I think it is safe to say that Don is rapidly intensifying. But...how long will it last??




all night
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I think it is safe to say that Don is rapidly intensifying. But...how long will it last??
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895. j2008
Quoting P451:


That will change if the convective burst maintains.


Well...off to lurk until the next official update in ~2.5hrs.


Have fun lurking, we will try to take care of Don and keep his situation monitored. Should be a fun night of HH data.
Member Since: December 19, 2008 Posts: 1 Comments: 224
Quoting duajones78413:
If Don is indeed gaining strength, would that mean a more southerly landfall?


yes, but probably still in Texas.
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Quoting ElConando:


Is it clean?
I believe that the 03 means it was contaminated, but it would make sense that the pressure really is 995mb because the reading before it was uncontaminated and had a pressure of 996mb.

Regardless, the system has intensified rapidly over the past few hours, and the pressure is somewhere around 995-996mb.
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00Z steering for a 995mb storm currently shows short-term wnw to w motion, with a turn to the nw later on, if the pattern holds.
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This is being very lightly reported down here, if at all. Is that the correct approach? I never seem to be surprised but I dismiss a lot of the stuff the weathermen tell us, apologies to some of you, but after the STUPID reporting of Ike we saw that anything is possible. The reporting of Ike and playing it down may actually have cost many lives, I posit that it did. People should have evacuated the Texas coast, and definitely got off the Island. The then Mayor of Galveston for instance telling people it was too late to leave, shows the ignorance and utter stupidity of our elected representatives and the local authroities under them. At times like that it needs correct coordination and the intervention of people who know what they are talking about. Any fool could see it was heading for Galveston, I'm not pleased or happy to say that I called it myself. If I could do it, then why couldn't they?
Member Since: June 22, 2010 Posts: 0 Comments: 14
this found a 995.4mb


995.4 mb
(~ 29.39 inHg)
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Pretty interesting pressure readings coming in from the Hurricane Hunters this evening. Definitely seem to support intensification with Tropical Storm Don as has been evident on satellite imagery throughout the afternoon and evening. Intense convection has built and continues to persist now over the circulation center of Don and outflow channels have become quite evident to the west, south, and east, but still struggling some to the north with some upper level northerly winds still impinging some of the system. Based upon the cloud pattern and significant pressure drop, it would suggest that Don is now building a solid core.
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A few hours ago everyone said 'How is this disorganized thing a TS?'
Now its 'Its got an eye! How is it a TS!?'
LOL
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If Don is indeed gaining strength, would that mean a more southerly landfall?
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We have such a long time to wait until the next advisory also....
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884. skook
.
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880. JLPR2
Meanwhile.



August is just around the corner...
Member Since: September 4, 2007 Posts: 7 Comments: 8459
If I were the HH I'd be making a center pass every hour only sampling 50 miles out in each quadrant in order to record the exact core strength of Don as it changes.
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Looking more symmetrical and expanding its CDO each frame. There comes a point, thermodynamically, where the feedback loops become all positive and I think we're there. Would'nt at all be surprised to see hurricane warnings/watches posted next update.
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if this keeps going a hurricane watch will be needed
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Quoting Max1023:
994mb corresponds to a 65mph system
991mb a 70mph system
987mb a 75mph hurricane

on average.

Considering Don's small size if the winds corresponded correctly to a 995mb pressure then it would likely be a 70mph system.


Alex kinda ripped those rules apart
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994mb corresponds to a 65mph system
991mb a 70mph system
987mb a 75mph hurricane

on average.

Considering Don's small size if the winds corresponded correctly to a 995mb pressure then it would likely be a 70mph system.
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My *guess at next advisory is 995-6mb 65mph TS
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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.