Don battling dry air and wind shear

By: Dr. Jeff Masters , 7:37 PM GMT en Julio 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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8-10 day 500mb Anomalies show mainly zonal flow across the US:

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Quoting Levi32:


It bares close watching, perhaps more so than pre-Don did. It has better vorticity than pre-Don had in that location, and as you mentioned, will have more of a positive MJO pulse on its side as opposed to the negative one that Don had/has.

i know you metioned in your video that a trough might come down and recurve that wave but if the trough missed it or never even came off the east coast, based on steering flow where would it be steered? like a don track maybe a little north?
Member Since: Julio 25, 2011 Posts: 0 Comments: 1102
Quoting TheDawnAwakening:
Convection is moving somewhat away from Don's coc which indicates wind shear is still hampering the system from the northeast, but over the next 12 hours this should begin to subside.
Havn't seen you in awhile :).
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G-4 NOAA Flight out in the GOM Aug 26 2005 as Katrina exited Fla. and into the GOM.


Note the WIDE pattern of dropsondes deployed from this flight as the pattern are displayed on the Screens onboard.

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TUTTs move and evolve a whole lot during the course of 10 days. Its current position and form is only that - current. Models have upper winds looking better later on once the wave nears the Caribbean, as the renewed upward motion over the central Atlantic will warm temperatures aloft a bit and cause the TUTT to shrink.
Member Since: Noviembre 24, 2005 Posts: 635 Comments: 26661
Quoting Cotillion:
As you may have noticed, a season with 4 or more storms prior to August 1st have also all had hurricanes.

So, this season is currently a first in that regard.


This season wants to be special, lol.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting cchsweatherman:
Here is exactly why I don't believe the tropical wave in the Central Atlantic will develop in the near future and may never develop:



1) Massive expanse of dry air and deep subsidence due to unfavorable MJO cycle (downward motion through atmosphere) and deep Saharan air exists all to the north and west of the tropical wave. This is some of the driest and most stable air to date over the Central Atlantic.

2) Well established TUTT over the Central Atlantic stretching into the Eastern Caribbean is creating hostile wind shear 10N to the North Atlantic essentially walling off the entire area. With no real signs of this easing up, upper level winds will become increasingly more unfavorable.


May never develop? Hold your horses, you know you can't predict future weather patterns that far out.

I agree with you about the near future, but never? It will change eventually and I'm sure it will get more favorable out there again eventually, that's how it works. Even if not, tropical cyclones will manage to pop when we head towards the busy cycle, they will just develop further west if that trend continues. And its not like it would matter at that point either. Because we all know tropical cyclones don't need to traverse the whole ocean to become dangerous. Some of the most violent ramp up right in our own backyard in a relatively short period of time.
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Quoting P451:
RGB ending 1915z - Lots and lots of shear. Newest burst already getting torn and displaced SW.




Catch you all late this evening. Going to let Don do it's thing through the rest of the afternoon while I do something else.
Is that a TS? Poor Texas.
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Quoting cchsweatherman:


Interesting because the upper level winds per CIMSS does not support a true upper level anitcyclone over the wave.


Nope. One of the products is wrong, but considering how the wave looks, I'm guessing that this image is wrong.

Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
As you may have noticed, a season with 4 or more storms prior to August 1st have also all had hurricanes.

So, this season is currently a first in that regard.
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Convection is moving somewhat away from Don's coc which indicates wind shear is still hampering the system from the northeast, but over the next 12 hours this should begin to subside.
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Quoting Levi32:


It bares close watching, perhaps more so than pre-Don did. It has better vorticity than pre-Don had in that location, and as you mentioned, will have more of a positive MJO pulse on its side as opposed to the negative one that Don had/has.


One more question: Should it take more of a southerly path like Don and enter the Caribbean, or skirt the islands and move into the Bahamas region like Don was supposed to originally.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting CybrTeddy:
The African wave has a ULAC over it.


Interesting because the upper level winds per CIMSS does not support a true upper level anitcyclone over the wave.
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Quoting CybrTeddy:
The African wave has a ULAC over it.
I see clouds.
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Quoting MagicSpork:


I can't believe you, Don! You make me wanna puke! You are the most pathetic excuse for a tropical system I have ever seen! I've seen ceiling fans create more wind than you! Why didn't you form into a Hurricane, Don!?!?

Sir, wind shear and dry air, sir!

That's right, Don, wind shear and dry air! Why don't you call your mommy next time you see a little dry air coming! I have no room in my squad for a weak storm like you that can't even handle a little wind shear!! Get out of my sight! I never want to see you again, Donna!!


haha, that made me laugh.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting JrWeathermanFL:
When does the first hurricane normally form?


Depends how many years you want to use as a range.
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Quoting TropicalAnalystwx13:


What is your opinion about the central Atlantic wave? It has a lot of model support, that while do not show much a tropical cyclone, shows it as a well-defined wave. The only factor going against it will be dry air, and the upward pulse of the MJO should be here soon.


It bares close watching, perhaps more so than pre-Don did. It has better vorticity than pre-Don had in that location, and as you mentioned, will have more of a positive MJO pulse on its side as opposed to the negative one that Don had/has.
Member Since: Noviembre 24, 2005 Posts: 635 Comments: 26661
The African wave has a ULAC over it.
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NOAA G-4 Liftoff and Climb out,Cockpit view.
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As said a few days ago, the following seasons have all had 4 storms or more prior to August 1st:

1886 (4 storms, 4 hurricanes); 12-10-4
1887 (5 storms, 1 hurricane); 19-11-2
1908 (4 storms, 3 hurricanes); 10-6-1
1909 (4 storms, 2 hurricanes, 1 major); 11-6-4
1916 (4 storms, 3 hurricanes, 2 majors); 15-10-5
1933 (5 storms, 2 hurricanes); 21-10-5
1936 (5 storms, 2 hurricanes); 16-7-1
1959 (5 storms, 3 hurricanes); 11-7-2
1966 (5 storms, 4 hurricanes, 1 major); 11-7-3
1995 (5 storms, 1 hurricane*); 19-11-5
1997 (5 storms, 2 hurricanes); 8-3-1
2003 (4 storms, 2 hurricanes); 16-7-3
2005 (7 storms, 3 hurricanes, 2 majors); 28-15-7
2008 (4 storms, 2 hurricanes, 1 major); 16-8-5

* Erin became a hurricane later.

There have been a couple of other close ones, but they weren't named in time.

All are above average, some way above particularly considering the timeframe. 1997 sticks out like a sore thumb.
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A year ago on this date, ATL was blank... No circles... even in 2009!
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Don is quietly getting his act together now, decent outflow seen on his southern side as cirrus are streaming away from the storm as divergence develops. I think if he lasts through the next 12 hours tonight with his convection even in its current state, he could have a pure 12 hours of eternal happiness in the western GOM of pure rapid strengthening given weakening wind shear and convection is able to develop and be maintained.
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Quoting cchsweatherman:
Here is exactly why I don't believe the tropical wave in the Central Atlantic will develop in the near future and may never develop:



1) Massive expanse of dry air and deep subsidence due to unfavorable MJO cycle (downward motion through atmosphere) and deep Saharan air exists all to the north and west of the tropical wave. This is some of the driest and most stable air to date over the Central Atlantic.

2) Well established TUTT over the Central Atlantic stretching into the Eastern Caribbean is creating hostile wind shear 10N to the North Atlantic essentially walling off the entire area. With no real signs of this easing up, upper level winds will become increasingly more unfavorable.



Good post. Pictures and backing up with explanations
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The center of Don has been pulled under the convection now, and yeah, its battling dry air, but its trying to moisten the air around it...

http://www.ssd.noaa.gov/goes/flt/t1/sloop-wv.html


Sure, its not a great looking storm, and the environmental conditions aren't exactly great, but I still find reason to believe that it will intensify a bit more before make landfall tomorrow night near Corpus Christi.
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Quoting cchsweatherman:
Here is exactly why I don't believe the tropical wave in the Central Atlantic will develop in the near future and may never develop:



1) Massive expanse of dry air and deep subsidence due to unfavorable MJO cycle (downward motion through atmosphere) and deep Saharan air exists all to the north and west of the tropical wave. This is some of the driest and most stable air to date over the Central Atlantic.

2) Well established TUTT over the Central Atlantic stretching into the Eastern Caribbean is creating hostile wind shear 10N to the North Atlantic essentially walling off the entire area. With no real signs of this easing up, upper level winds will become increasingly more unfavorable.


While it is dry right now, the MJO said come into our area of the world real soon. In addition to the second part, the wave has an ULAC over it, which should ease wind shear at least a little bit. But I do agree, that is a little of SAL/dry air to the north. This looks to be the MAJOR inhibiting factor.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting TropicalAnalystwx13:


The hype is that it is a well-defined wave that only has SAL to deal with.
It's also broad and disorganized. Upper level winds aren't the best right now.

As it nears the TUTT, convection should take off in response to upper level winds creating divergence aloft. However, these same upper level winds will increase shear.
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THE NATIONAL WEATHER SERVICE IN NEW ORLEANS HAS ISSUED A

* FLASH FLOOD WARNING FOR...
NORTHWESTERN JEFFERSON PARISH IN SOUTHEAST LOUISIANA...
THIS INCLUDES THE CITIES OF...METAIRIE...KENNER...

* UNTIL 500 PM CDT

* AT 158 PM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED
FLASH FLOODING FROM A THUNDERSTORM OVER THE WARNED AREA.

* THE STORM PRODUCING FLASH FLOODING WILL OTHERWISE REMAIN OVER
MAINLY RURAL AREAS OF THE INDICATED PARISH.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

RADAR ESTIMATES OF 2 TO 3 INCHES HAVE ALREADY FALLEN OVER SOME AREAS
AND ADDITIONAL RAINFALL AMOUNTS OF 1 TO 3 INCHES ARE POSSIBLE IN THE
WARNED AREA.
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Don being kept n check for the most part,with some hope to take advantage later if shear relaxes.

Its in that Phunny No Mans Land now..

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HH wheels up outta tampa
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Quoting Levi32:


Yup, when it's this far south. A massive ridge over the northern U.S. and southern Canada is very bad news though.


Do the 500mb anomalies still show the ridge retreating north into August?
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Quoting jasonweatherman2011:
watch out we got two blues on this map maybe iNvest 91L and maybe invest 92L TO WOW!!

Yellow circle placed on the blue in EPAC, NHC needs to put a circle on one of the blues in ATL
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Here is exactly why I don't believe the tropical wave in the Central Atlantic will develop in the near future and may never develop:



1) Massive expanse of dry air and deep subsidence due to unfavorable MJO cycle (downward motion through atmosphere) and deep Saharan air exists all to the north and west of the tropical wave. This is some of the driest and most stable air to date over the Central Atlantic.

2) Well established TUTT over the Central Atlantic stretching into the Eastern Caribbean is creating hostile wind shear 10N to the North Atlantic essentially walling off the entire area. With no real signs of this easing up, upper level winds will become increasingly more unfavorable.
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Quoting Patrap:


Don seems to have similar conditions to Bonnie last season. An upper low just to the west of it with the US ridge beating down on it from the north and east.
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Quoting Tazmanian:




a link would have been better


already said not good at them...and you quoted me to waste the same amount of space is even more rediculous
Member Since: Septiembre 16, 2007 Posts: 0 Comments: 3650
Don is trying to get a southern outflow channel developed. Convection is deepening within this band and could provide decent ventillation for Don over the next 24 hours before landfall. Still time for him to strengthen at a decent pace, probably talking around 65mph at landfall, down from the 75mph I was thinking yesterday.
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Quoting MississippiWx:


Says a lot for how the conditions around the US have been the past 2 to 3 years as well. The US ridge is a killer for tropical cyclones.


Yup, when it's this far south. A massive ridge over the northern U.S. and southern Canada is very bad news though.
Member Since: Noviembre 24, 2005 Posts: 635 Comments: 26661
Quoting P451:
WV - Don - 24hr - Ending 1915z



Almost looks like he recently exhaled a bunch of that dry air.
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Quoting MississippiWx:
Vorticity is impressive with the Central Atlantic wave:



Yeah, and it should be separating from the ITCZ before the end of the weekend.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting P451:
WV - Don - 24hr - Ending 1915z



Best it has looked since yesterday evening.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Vorticity is impressive with the Central Atlantic wave:

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Quoting Levi32:
Don looks a lot like Bret, which says quite a bit about the environmental conditions around the system. They are and have been much more unfavorable than the NHC and many others believed.


What is your opinion about the central Atlantic wave? It has a lot of model support, that while do not show much a tropical cyclone, shows it as a well-defined wave. The only factor going against it will be dry air, and the upward pulse of the MJO should be here soon.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Quoting Levi32:
Don looks a lot like Bret, which says quite a bit about the environmental conditions around the system. They are and have been much more unfavorable than the NHC and many others believed.


Says a lot for how the conditions around the US have been the past 2 to 3 years as well. The US ridge is a killer for tropical cyclones.
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Quoting floridaboy14:


teddy you think the trough is going to recurve the storm?


I have no idea.
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Quoting Patrap:
TS Don Storm Relative 1km Geostationary Visible Imagery Loop




Center not exposed anymore, but still has outflow boundaries indicative of dry air.
Member Since: Julio 6, 2010 Posts: 113 Comments: 32354
Before that wave in the Central Atlantic can do anything close to developing, it's going to have to separate itself from the ITCZ.
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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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