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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Looks like Mexico will get most of Don's heavy rain. I guess the blog has already moved on to 91L now.
Member Since: May 27, 2009 Posts: 0 Comments: 4355
Quoting jpsb:
Looks like northern Mexico is going to get some rain, well good for them, not sure they need it all that bad. Hopefully the Texas coast will get a little rain out of Don, but it looks like central Texas is going to get nothing. Well it's kinda too late already, maybe this winter will bring some rain then the cold front start making there way down to us.


There are some models hinting a possible return of La Nina in the fall/winter. Not good. It's matter of where in Texas will feel full brunt of its effect. If we don't get anything from tropical systems, hope for El Nino to come... that always bring us above average amount of rainfall.
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Don - pressure up several notches:

AL, 04, 2011072912, , BEST, 0, 258N, 943W, 45, 1004, TS, 34, NEQ, 90, 50, 20, 20, 1011, 100, 20, 0, 0, L, 0, , 0, 0, DON, M,

91L - chugging along, but strength unchanged:

AL, 91, 2011072912, , BEST, 0, 92N, 406W, 25, 1008, LO, 34, NEQ, 0, 0, 0, 0, 1011, 250, 80, 0, 0, L, 0, , 0, 0, INVEST, M,
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2217. Walshy
Quoting stormhank:
I know its way early in the game to ask this , but could 91L possibly be a US threat??


Possibly.
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Quoting floridaboy14:

:) haha well i think don looks better on satalite you agree or disagree?


All I do is post the graphics from Google earth and model runs. I very seldom make predictions or observations without available data to support it. So far... that data to make the call one way or the other is not available.

There are lots of self declared weather wizz kids on here who will make that call for you, based on a hunch, guess, or Ouji board.

When in doubt... ask the NHC or Levi.


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a weaker trough spells trouble for someone, if that's the case
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I know its way early in the game to ask this , but could 91L possibly be a US threat??
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Quoting jpsb:
Looks like northern Mexico is going to get some rain, well good for them, not sure they need it all that bad. Hopefully the Texas coast will get a little rain out of Don, but it looks like central Texas is going to get nothing. Well it's kinda too late already, maybe this winter will bring some rain then the cold front start making there way down to us.

well ive got some bad news... the CFS model trys to bring another moderate to strong la nina in the fall which would make the drought even worse..
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I am waiting on Levi's analysis on 91L
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Quoting Neapolitan:

I don't think so. Spencer is in favor of mandatory teaching of "Intelligent Design", which automatically removes from him the title of "credible". And he wrote in his own blog last year that his number one goal was to save the taxpayer money--not to pursue scientific truth.


So you're one of many that still believes that saturated fat and cholesterol consumption causes heart disease? That's the mainstream establishment's view yet they are completely wrong when you dig a little deeper in the literature...

Either way, both sides can be wrong.

Another great example - sun causing skin cancer... Dermatologists are screaming at us to avoid the sun and lather on massive amount of sunblock...

What does that do to us? Chronically low vitamin D level leading to significantly higher risk of getting one of 18 different cancers including skin cancer melanoma... multiple types of autoimmune disease, mental disorder, too many to list. Even autism...

So be careful of what you say... Even mainstream establishments can be wrong. Big time.
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2210. jpsb
Quoting Jax82:
Don on the Brownsville radar. Some rain is better than no rain at all, right?

Looks like northern Mexico is going to get some rain, well good for them, not sure they need it all that bad. Hopefully the Texas coast will get a little rain out of Don, but it looks like central Texas is going to get nothing. Well it's kinda too late already, maybe this winter will bring some rain then the cold front start making there way down to us.
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2209. HCW
Level 3 radar is on now with model tracks , Lightning , LSR's and more





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Quoting Orcasystems:


I have no idea if its in the centre.. its just the lowest I found... hehe I'm not flying the plane.. just along for the ride like the rest of us :)

:) haha well i think don looks better on satalite you agree or disagree?
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Quoting floridaboy14:

is that in the center and if it is does this mean don is weakining?


I have no idea if its in the centre.. its just the lowest I found... hehe I'm not flying the plane.. just along for the ride like the rest of us :)
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Quoting Orcasystems:
Time: 11:58:30Z
Coordinates: 25.8167N 94.3833W
Acft. Static Air Press: 842.1 mb (~ 24.87 inHg)
Acft. Geopotential Hgt: 1,533 meters (~ 5,030 feet)
Extrap. Sfc. Press: 1003.3 mb (~ 29.63 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 321%uFFFD at 20 knots (From the NW at ~ 23.0 mph)
Air Temp: 20.5%uFFFDC (~ 68.9%uFFFDF)
Dew Pt: 6.5%uFFFDC (~ 43.7%uFFFDF)


is that in the center and if it is does this mean don is weakining?
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Quoting floridaboy14:

hey teddy gfdl shows this as a cat 3 crossing the islands. HWRF as a cat 2 recurving east of the islands what are your thoughts on don? stronger or weaker?


IMO HWRF makes 91L too strong too early, in 12 hours its already a 55 knot system.
Member Since: July 8, 2005 Posts: 259 Comments: 23012
2203. Jax82
Don on the Brownsville radar. Some rain is better than no rain at all, right?

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Time: 11:58:30Z
Coordinates: 25.8167N 94.3833W
Acft. Static Air Press: 842.1 mb (~ 24.87 inHg)
Acft. Geopotential Hgt: 1,533 meters (~ 5,030 feet)
Extrap. Sfc. Press: 1003.3 mb (~ 29.63 inHg)
D-value: -


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Good ULAC with 91L.
Member Since: July 8, 2005 Posts: 259 Comments: 23012
Quoting CybrTeddy:
Good morning! 30% it is..

hey teddy gfdl shows this as a cat 3 crossing the islands. HWRF as a cat 2 recurving east of the islands what are your thoughts on don? stronger or weaker?
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Quoting CybrTeddy:
Good morning! 30% it is..



looks like 91L is starting too get some baning on the 1st hafe it still needs too work on the 2nd hafe
Member Since: May 21, 2006 Posts: 5089 Comments: 114053
Quoting jasonweatherman2011:
invest 91L WILL NOT BE A FISH STORM

I must agree.
Member Since: November 13, 2009 Posts: 2 Comments: 728
2197. JRRP
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at last a raal storm too track
Member Since: May 21, 2006 Posts: 5089 Comments: 114053
HWRF shows a cat 2 recurving north of the islands..
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Good morning! 30% it is..
Member Since: July 8, 2005 Posts: 259 Comments: 23012
2192. Walshy
Quoting jasonweatherman2011:
invest 91L WILL NOT BE A FISH STORM


You are most likely correct. Islands are in the path. It could then go out to sea but way to early to know...
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Quoting twincomanche:
Look at the researchers not the author of the piece. I agree that the tone of the article leaves some science to be desired, however the researchers are pretty credible.

I don't think so. Spencer is in favor of mandatory teaching of "Intelligent Design", which automatically removes from him the title of "credible scientist". And he wrote in his own blog last year that his number one goal was to save the taxpayer money--not to pursue scientific truth.
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Hurricane Hunters are curently not in the center of don correct? looks like the GFDL is going with a cat 3 in the northern islands...
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2189. barbamz
Quoting floridaboy14:

so basically as they get closer to the center they will find the stronger winds right? thanks


No, because there is no real eyewall. And Don don't seems to strenghten (Pressure now: 1004.9 mb.
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Quoting jasonweatherman2011:
invest 91L WILL NOT BE A FISH STORM

correct Jason it will hit the leeward islands and afterwards there are 3 scnearios. 1: into the Gulf
2: out to sea 3: towards the bahamas because now some models showing a WEAKER trough
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Quoting Dakster:


I think the windward islands may disagree with that statement...



I agree with you.. anyone who would call a CAT 3-4 into the windward islands is an obvious idiot.
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2186. Jax82
Looks like Don's sheared thunderstorms going south are going to benefit Mexico more than Texas.
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Quoting jasonweatherman2011:
invest 91L WILL NOT BE A FISH STORM


Thanks Nostradamous............ :)
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Quoting barbamz:


Little bit stronger winds:
Peak (10s) Flt. Lvl. Wind: 51 knots (~ 58.6 mph)

No center fix and vortex reading yet on Google Earth.


Lowest I have seen so far is around 1003
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2182. Dakster
Quoting aquak9:
AtHome is right. Sorry but 91L is gonna be a fish.


I think the windward islands may disagree with that statement...

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Quoting barbamz:


Little bit stronger winds:
Peak (10s) Flt. Lvl. Wind: 51 knots (~ 58.6 mph)

No center fix and vortex reading yet on Google Earth.

so basically as they get closer to the center they will find the stronger winds right? thanks
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Quoting Orcasystems:


Multiple models have it as a CAT 3-4


Where can i find this link?
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Quoting Gearsts:

Is that little blob the coc? If it is, it looks mighty innocuous at this point and far from the thunderstorms.
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2178. barbamz
Quoting floridaboy14:
Are the hurricane hunters finding don stronger?


Little bit stronger winds:
Peak (10s) Flt. Lvl. Wind: 51 knots (~ 58.6 mph)

No center fix and vortex reading yet on Google Earth.

Edit:
Extrap. Sfc. Press: 1004.9 mb (~ 29.67 inHg)

Minimal Pressure seems to be stable. No further dropping, as much as I can see.
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Multiple models have it as a CAT 3-4
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2176. NASA101
8AM TWO:

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 CONTINUES TO SHOW 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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91L certainly bears watching. if the models are correct and this turns out to be a big hurricane, Florida better hope that Bermuda High weakens and shifts eastward allowing for a recurve
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Quoting tatoprweather:
Did somebody mentioned that the CATL wave will have a yellow circle by tomorrow?


Told you!!!
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2172. Walshy
91L
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.