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

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2005 Hurricanes Atlantic
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Hurricanes Western Pacific 2011
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Tropical Storms Atlantic 2005
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Extratropical Storms Western Pacific 1988

I'll have a new post Friday morning.

Jeff Masters

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2115
Oh my cow!
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Unfortunately very little of Texas will get any rain from Don. AS FOR TROPICAL STORM DAN...THE FORECAST TRACK CONTINUES TO SHIFT
SOUTH AS IT IS BECOMING MORE APPARENT THAT THE UPPER LEVEL RIDGE
IS WINNING OUT. THE NHC FORECAST BRINGS THE STORM ASHORE NEAR
BAFFIN BAY TOMORROW EVENING WITH THE REMNANT CIRCULATION MOVING
WNW ACROSS SOUTH TX AND NORTHERN MEXICO. THIS TRACK KEEPS THE
REMNANT LOW A BIT TOO FAR SOUTH AND THE BEST RAIN CHANCES WILL BE
SHUNTED SOUTH AS WELL. NO CHANGES WERE MADE BEYOND THE FIRST
PERIOD.
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2119. WxLogic
Quoting SLU:


SHEAR (KT) 13 10 8 6 1 3 1 1 2 3 N/A N/A N/A


The SHIPS's shear forecast is the most worrying.


Is unfortunate... but I have a feeling which I sure didn't got from Don that this could be quite troublesome.
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Quoting Cotillion:


Neither really match, to be honest. Felix was lower and didn't properly begin to get going until he'd entered the lower Caribbean. As his windfield was small, the main impact was the Mosquito coast.

Dean was more problematic, but it was a Cape Verde storm, which this isn't.

Analog tracks and comparisons should be left until there's a better handle of the thing.

SHIPs forecasts aren't generally that great straight away. I doubt this invest will jump to a moderate TS in 24 hours.
I was only speaking in terms of strength and track.
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Quoting pottery:

I'm awake.
But I'm not awake.
Coffee.
I need coffee.....


Good Morning Pottery,hope ya had that cup of coffee, im on my second cup.......... looks like 91L could spell "TROUBLE" for the Islands...... Jus watchin from Barbados.....
Member Since: July 8, 2007 Posts: 0 Comments: 147
Quoting Vincent4989:

But it could become anything. Maybe a minimal hurricane.


the questions now are where will it go and how soon will it intensify, of course.
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Complete Update

HH enroute to Don

91L models have a CAT 4 splitting Matinique & St Lucia


TS BUSTED FORECAST ALIBI





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sorry i know everyone one is talking about that wave (invest 91L) but does anyone have the latest info from the hurricane hunters in terms of pressure and wind speed? thanks
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Hey "AT HOME",I know you remember when Rita was about 250 miles out from Houston and the evact disaster on I-45 was going on,she tack's a right hand turn and pile -drives Camoron,better known now as the Rita-right.When ever these systems are within 12hr's out from a Texas landfall,I cant help but relate back to that day.The track on Donnie boy I know is wright,as everyone has said,this is the tropic's and anything can happen.Just a haunting thought
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Quoting Chicklit:
Good point about Felix Cot.
That one went through RI after the wave passed the
Antilles and entered Caribbean.
But I think he was talking about intensity-wise.
Not saying anything else at this point.
It's just a big wave and NHC says conditions will be more favorable ahead.

But it could become anything. Maybe a minimal hurricane.
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Quoting MahFL:
When Galilieo suggested the Earth was not the center of the Universe, a lot of people at first refused to believe him.......scientists can have a hard time convincing people of reality.


Yep. Flat earthers, creationists and global warming deniers are proof of that
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Good point about Felix Cot.
That one went through RI after the wave passed the
Antilles and entered Caribbean.
But I think he was talking about intensity-wise.
Not saying anything else at this point.
It's just a big wave and NHC says conditions are favorable ahead of it.
This wave did come from Cape Verde originally.
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2109. SLU
Quoting WxLogic:


I agree...


SHEAR (KT) 13 10 8 6 1 3 1 1 2 3 N/A N/A N/A


The SHIPS's shear forecast is the most worrying.
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I've following your comments about Don for over a week now, but living in Puerto Rico, my attention now turns to 91L. Thanks to your analyses, I can keep updated about the latest forecasts with you more than with the local tv forecasters who keep repeating the same things over and over (mainly that its too far away to say anything definite about anything. So keep up the good work which 99.9% of you are doing.
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2107. SLU
Quoting WeatherNerdPR:

Aww man. But at what time frame? Monday-Tuesday? Or later?
Quoting Mixed:
I Hope The Word Gets Out Early This Time. Preparations For The Hurricane Season Should Have Been Done.
Quoting stormwatcherCI:
Got a "feeling" this might be another Dean or Felix.
Quoting Tropicsweatherpr:


Agreed. And that is why from today,I am preparing to avoid the last minute rush.


It should cross 55W by Monday and be in the Eastern Caribbean sea by Tuesday and near PR by late Tuesday through Wednesday.
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always look who funds the surveys polls or even science projects murdocks or wannabes are hiding around the corner with a agendas
Member Since: September 11, 2010 Posts: 1 Comments: 4358
Quoting stormwatcherCI:
Got a "feeling" this might be another Dean or Felix.


Neither really match, to be honest. Felix was lower and didn't properly begin to get going until he'd entered the lower Caribbean. As his windfield was small, the main impact was the Mosquito coast.

Dean was more problematic, but it was a Cape Verde storm, which this isn't.

Analog tracks and comparisons should be left until there's a better handle of the thing.

SHIPs forecasts aren't generally that great straight away. I doubt this invest will jump to a moderate TS in 24 hours.
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2104. WxLogic
Quoting SLU:


This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.


I agree...
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Quoting pottery:

I'm awake.
But I'm not awake.
Coffee.
I need coffee.....

I know...open your eyes and look east...have you seen that big honking wave?!
shiver me timbers.
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Quoting SLU:


This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.


Agreed. And that is why from today,I am preparing to avoid the last minute rush.
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2100. WxLogic
Quoting Gearsts:
And what about the SAL?


- 91L is doing a pretty good job to say the least at fending off the SAL. As it approaches 50W the moisture content in the environment has been and will be more conductive to dilute any further SAL in the Caribbean region and around 91L.



In the image above you'll see that SAL has being decreasing substantially over the past 6 to 9hrs and expect to to continue and making the environment more conductive for development.
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2099. Gearsts
Quoting Chicklit:

SAL doesn't affect steering.
Here's a SAL Map:
LinkSouthAfricanDust
Looking spotty ahead of 91L.
I dont think SAL is that bad:(
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Poll: What will 91L become of?
A.FFFIIIIIIISSSHHH!!!!!!!
B.Not Fish
For me, i pick B.
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Quoting SLU:


This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.
Got a "feeling" this might be another Dean or Felix.
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2096. pottery
Quoting Chicklit:
LinkIRLoop91L

Can't wait to hear discussion on this one.
Would someone please wake up Kman and Pottery?
All hands on deck!

I'm awake.
But I'm not awake.
Coffee.
I need coffee.....
Member Since: Posts: Comments:
2095. Mixed
I Hope The Word Gets Out Early This Time. Preparations For The Hurricane Season Should Have Been Done.
Member Since: Posts: Comments:
Quoting SLU:


This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.

Aww man. But at what time frame? Monday-Tuesday? Or later?
Member Since: July 7, 2010 Posts: 0 Comments: 5630
Quoting Gearsts:
And what about the SAL?

SAL doesn't affect steering.
Here's a SAL Map:
LinkSouthAfricanDust
Looking spotty ahead of 91L.
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This isn't the first time that models have wanted to put systems on a sight seeing tour along the Greater Antilles.

Think they did that with Don originally, though it ended up a bit more south than that.

Wonder if it's a pattern.

It's one to watch, especially as you head into August.



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2091. MahFL
When Galilieo suggested the Earth was not the center of the Universe, a lot of people at first refused to believe him.......scientists can have a hard time convincing people of reality.
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2090. WxLogic
In regards Don... I don't disagree with NHCs track. The illusion of the N jog is just due to the expanding convection and not the circulation.

The main WX associated with Don (Rain mainly) should be confined to the center and S of it as the upper level WNDS are not that optimal for equal expansion (or at least as equal as possible) on all sides of the TS.
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this time of yr it would be unusual to get development down in the sw carib. p541 dot map
Member Since: September 11, 2010 Posts: 1 Comments: 4358
2088. SLU
Quoting Mixed:


After Tomas Last Year I Dont Think We Can Take Another One For Now, Hoping That Its Gonna Be A Fish, SLU Whats Up.


This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.
Member Since: Posts: Comments:
2087. Gearsts
Quoting WxLogic:


:) you have quite a bit on your mind there... here're the answers to best of my abilities:

MJO:

- Currently we're in a "neutral" MJO phase as we're exiting a downward period and entering and active MJO in this upcoming week. Whatever develops will have a better chance of surviving that what the conditions Don encountered when it first "became" last week when the downward MJO was around.



Steering:

- Currently the steering will be mainly W to WNW:



- The Bermuda High is currently getting itself better establish as we speak and should be E of CONUS within the next couple days and bridging with the Azores High. This should allow 91L to proceed on a W to WNW for the time being until the time it enters the Caribbean. By then... it is expected that a TROF develops across the CONUS E coast. The depth, strength, and timing of this TROF will dictate if 91L moves into the GOM, E/SE CONUS, or re-curves out to see.

Forecast:

- I'm expecting 91L to be in the 40% mark by the 8AM TWO. It currently has a well established low level circulation which correlates to the moderate to high convergence values:



Divergence is good too as well as a nice ULAC which validates the good outflow aloft:



- Hope this high level overview helps.
And what about the SAL?
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Post 2078: - Hope this high level overview helps. WxLogic
Yeah, that is super.
Folks like you are one very good reason why this blog is so interesting and valuable.
Thanks for your time and expertise!

Of course the humor and the sorts of discussions that go on here regarding different opinions are also gratifying when having to work at the computer much of the day!
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Mission 5 well on the way to DON alreadyTime: 11:02:30Z
Coordinates: 27.5N 91.95W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,621 meters (~ 5,318 feet)
Extrap. Sfc. Press: 1016.0 mb (~ 30.00 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 148° at 25 knots (From the SSE at ~ 28.7 mph)
Air Temp: 17.5°C (~ 63.5°F)
Dew Pt: 7.2°C (~ 45.0°F)
Peak (10s) Flt. Lvl. Wind: 25 knots (~ 28.7 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots (~ 27.6 mph)
SFMR Rain Rate: 0 mm/hr (~ 0 in/hr)!
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91L has that look.... While Don is such a puffball. Seriously. He looks like a puffball.
Member Since: July 7, 2010 Posts: 0 Comments: 5630
Quoting stormpetrol:
Good morning, big blow up N of Panama this am as well!

hey is don weakining currently ?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
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Good morning, big blow up N of Panama this am as well!
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Quoting MahFL:
New NASA peer reviewed paper says global warming won't be as bad as the doomcasters are saying.

Link

Although I do think some warming is occuring.





It sounds like good news.

However, when a piece of 'groundbreaking' science is first broken by Forbes, and not by something like NewScientist, as well as some of the suggestive semantics used, the alarm bells go off.
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One estimate of Don rainfall :(


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2078. WxLogic
Quoting Chicklit:


Morning, WxLogic. What do you know about the MJO, Bermuda High and anything else that would influence steering 91L? Am sure you know, but NHC has it at 30% as of 2 a.m.


:) you have quite a bit on your mind there... here're the answers to best of my abilities:

MJO:

- Currently we're in a "neutral" MJO phase as we're exiting a downward period and entering and active MJO in this upcoming week. Whatever develops will have a better chance of surviving that what the conditions Don encountered when it first "became" last week when the downward MJO was around.



Steering:

- Currently the steering will be mainly W to WNW:



- The Bermuda High is currently getting itself better establish as we speak and should be E of CONUS within the next couple days and bridging with the Azores High. This should allow 91L to proceed on a W to WNW for the time being until the time it enters the Caribbean. By then... it is expected that a TROF develops across the CONUS E coast. The depth, strength, and timing of this TROF will dictate if 91L moves into the GOM, E/SE CONUS, or re-curves out to see.

Forecast:

- I'm expecting 91L to be in the 40% mark by the 8AM TWO. It currently has a well established low level circulation which correlates to the moderate to high convergence values:



Divergence is good too as well as a nice ULAC which validates the good outflow aloft:



- Hope this high level overview helps.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
2076. Mixed
Quoting SLU:
OMG! Look at the latest intensity forecasts! THAT'S MADNESS. Will have to wait for the GFDL and HWRF for confirmation.


After Tomas Last Year I Dont Think We Can Take Another One For Now, Hoping That Its Gonna Be A Fish, SLU Whats Up.
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Wow, look at these SSTs...
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We still have hope here on the S. TX Gulf Coast! (for rain that is). I know it probably won't be much, but it'll be something!
And then on to 91L.
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Quoting Neapolitan:

I'm not sure of the averages for 1995 through current, but the 1966-2009 average says the fourth named storm normally appears on August 23, the first hurricane on August 10, and the first major on September 4. Lots of info here.


Thanks, Nea. Of course, that works to 11-6-2 with the current AMO positive era at approximately 15-8-4, so it would be interesting to see how it fits in.

Especially with 4 storms prior to August 1 happening for the fifth time (97, 03, 05, 08 and now 11) in under fifteen years.
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back to work for me...
Member Since: September 16, 2007 Posts: 0 Comments: 3650
Quoting CanesfanatUT:


Looks like more of a threat in terms of winds than Don ever was.


depends on how you look at it...Don is approaching NOW, it is going to hit land, this we know...if the rains fall too hard then they wash out and cause flooding...what little is left for the farmers will be gone...91L is a 10-15 days away...don't know where it is going or if it will go...
Member Since: September 16, 2007 Posts: 0 Comments: 3650

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About

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