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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My *guess at next advisory is 995-6mb 65mph TS
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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.



901 that would be cool if it did
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869. JLPR2
Quoting CybrTeddy:
Guys your acting like it went down to 901 mb lol, but if 995 is confirmed it is a substantial pressure decrease.


Hey! We are allowed to overreact if we want to. LOL! XD
Member Since: September 4, 2007 Posts: 7 Comments: 8732
Don is looking FAT on the last few RGB frames, wow!
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867. jpsb
Quoting P451:


The one thing is that the majority of the convection is south of the center so it might not give the central Texas coast what it needs. South Texas may instead see the heavy rains if this current state of organization continues.

That would really suck for central Texas. So close, but no cigar. Well here in the Galveston area we did get a little rain today, the garden lives for another couple of days :)
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Quoting JLPR2:


Then it seems Don is bombing out, but the winds haven't caught up with the pressure.
I agree. The highest winds they've found so far have only been in the 45-50mph range.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting JLPR2:


Then it seems Don is bombing out, but the winds haven't caught up with the pressure.



the way it works i think they do one pass for winds and one pass for MB
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 279 142 107010 013 024 002 03
Oh my....
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Guys your acting like it went down to 901 mb lol, but if 995 is confirmed it is a substantial pressure decrease.
Member Since: July 8, 2005 Posts: 259 Comments: 24151
I wonder if Don will look like this at 11:00.
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we may wake up too a 960mb storm in the AM
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Quoting TropicalAnalystwx13:
Q: What are the chances that DON becomes a hurricane before landfall Saturday morning?

A. Nonexistent
B. Low
C. Medium
D. High


C
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 279 142 107010 013 024 002 03


Is it clean?
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 +279 +142 107010 013 024 002 03


31 degree waters + 1 200W ceiling fan (spinning cyclonically) = Category 6 Hurricane in 30 seconds.

Seriously though that's 2 independent reports of sub 996mb pressure. Also, and more worrisome is the fast drop near the center which suggests a rapidly intensifying system and/or a forming eyewall.
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 279 142 107010 013 024 002 03

What?!
Oh Don...
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856. JLPR2
Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 +279 +142 107010 013 024 002 03


Then it seems Don is bombing out, but the winds haven't caught up with the pressure.
Member Since: September 4, 2007 Posts: 7 Comments: 8732
Quoting Tazmanian:



oh my RI RI


Indeed RI...wild stufz
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Quoting TropicalAnalystwx13:
Q: What are the chances that DON becomes a hurricane before landfall Saturday morning?

A. Nonexistent
B. Low
C. Medium
D. High
A. Don is only a boy.
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Don seems to be firing some nice convection as the sun sets, however it is not in a rather organized fashion. At this point my hope is it doesn't get over 60mph and becomes organized enough to give rain to the Eastern Half of Texas. At the moment as P451 pointed out, it would really only help S Texas at the current moment.
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 279 142 107010 013 024 002 03


Holly jesus!
Member Since: Posts: Comments:
Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 +279 +142 107010 013 024 002 03



oh my RI RI
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Hi, just getting into looking at this one now. Surely we've learned enough not to underestimate any storm.
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Wow Don...You continue to surprise us.

995.4 mb.
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Quoting MiamiHurricanes09:
995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 +279 +142 107010 013 024 002 03


wow...
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Where will the CATL wave/91L's track likely be? Will it follow a path like Don, further South, further North or something else?
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995mb! Uh-oh!

000930 2427N 09149W 8437 01489 9954 279 142 107010 013 024 002 03
Member Since: September 2, 2009 Posts: 130 Comments: 21194
843. MTWX
Quite a bit more info to show what these aircraft can handle/ improvements off of the old model series.

Link
Member Since: July 20, 2009 Posts: 23 Comments: 1393
Q: What are the chances that DON becomes a hurricane before landfall Saturday morning?

A. Nonexistent
B. Low
C. Medium
D. High
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Quoting Tazmanian:




LOL that info you this posted is way old DON is way better now


Thats as of 15 minutes ago Taz... know it all...
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Quoting Levi32:
Flight-level winds have not exceeded 40kts for either plane so far in their respective trips into Don.

their not at the center yet are they?
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Quoting Hurricanes101:


true, does seem to be getting its act together

hurricane status is not out of the question



yup oh DON now have a T # of 3.0
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837. skook
post 823

Please do not engage in personal attacks or bickering.
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Quoting WeatherNerdPR:
Hey guys. I see the wave in the CATL got a mention in the TWO.


Careful, you'll get harassed for mentioning something that isn't Don! Haha

That wave not looking bad though, certainly looks like it could steadily organise and we may see Emily some stage next week.
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We should see Don on radar before too long.
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Quoting Tazmanian:



DON would not be makeing a run for hurricane if it was


true, does seem to be getting its act together

hurricane status is not out of the question
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833. jpsb
Quoting TropicalAnalystwx13:


No, weak tropical storm's do not have an eye. What Don is doing now is building an eye-wall, right where that deepest convection can be seen on satellite. Once he is finished, which should be sometime tomorrow, he will likely begin to show an eye. They usually appear when the storm reaches 60 mph (from experience).
Ok, thanks, so what we are seeing and what the HH are finding is not inconsistent; a moderate TS gaining strength (maybe even rapidly).
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Quoting j2008:

Hurricane Hunters found that the center had relocated to about the center of the main convection. if what you are saying is true, it shouldnt be this organized and shouldnt be building an eyewall like it is.



thank you
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I'd take the plane observations over the NHC position - those often get revised as data comes in. Always take the actual data if at all possible - it is more reliable.
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Quoting Hurricanes101:


um he is right, the center is on the north side of the convection



DON would not be makeing a run for hurricane if it was
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829. j2008
Quoting P451:


The one thing is that the majority of the convection is south of the center so it might not give the central Texas coast what it needs. South Texas may instead see the heavy rains if this current state of organization continues.

Hurricane Hunters found that the center had relocated to about the center of the main convection. if what you are saying is true, it shouldnt be this organized and shouldnt be building an eyewall like it is.
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Quoting Tazmanian:



do you even no what you talking about i sure dont you need some classs dude plzs go by some and ware them


um he is right, the center is on the north side of the convection
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Quoting KoritheMan:


He might well achieve that before landfall, but he's not nearly as well organized as some people are saying.



+1
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Quoting P451:


The one thing is that the majority of the convection is south of the center so it might not give the central Texas coast what it needs. South Texas may instead see the heavy rains if this current state of organization continues.


True, but convection appears to be building over the center in the last couple of images (as per SSD). I don't really see any reason why that should change.
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Hey guys. I see the wave in the CATL got a mention in the TWO.
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04L/TS/D/CX
ON APPROACH
IR NIGHT VIEW


Member Since: July 15, 2006 Posts: 173 Comments: 54268
Flight-level winds have not exceeded 40kts for either plane so far in their respective trips into Don.
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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.