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 angiest:


Ike covered a large area, and therefore dropped rain over a large area. I don't remember the amounts, but since Ike came in and left it probably wasn't too bad.

Tropical systems are the only reasonable chance of making a dent in the drought in the near to medium term. It's just kinda dicey. Need the rain, not the wind and surge. And we don't need too much rain falling on the very dry ground too quickly or it all runs off.


I agree about the moisture part. But it is bad form to cite a storm which left tens-to-hundreds of thousands without power for anywhere from 1-3 weeks and access to Galveston Island cut off for over a week afterwards. Many lost their homes and have yet to recover.
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Quoting VAbeachhurricanes:


Not what the latest vortex says...


The latest vortex wasn't dropped in the eye.
Member Since: July 6, 2010 Posts: 113 Comments: 32714
1469. GHOSTY1
Quoting ScoobyMoo:


Ike was devastating for many of us in the Houston-Galveston area. I'm hoping I understood your comment incorrectly.


Im sorry i said it incorrectly i didn't mean in windspeed and strength i mean i would want a large system that would be able to aleviate our drought, sorry for the misunderstanding. I live in Houston and remember Ike i was without power for 3 weeks and was lucky all the trees didn't fall on my house or the neighbors. I know many were horribly ruined by the hurricane such as Bolivar Peninsula, Galveston, etc. i know the consequences that Ike brought and i wouldn't welcome anything like that here. Sorry for miscommunication.
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Quoting ackee:
THE ECMWF has been performing well so far this seasons it has hint devlopment of both DON and BRET I even notice that the wave in the Cental atlantic the Ecmwf has been hinting devlopment as well , also show another system as well. The GFS on the other hand, has dissapoint me so far this seasons


The ECMWF isn't actually developing the CATL wave. When I looked earlier, it was only indicating one closed isobar, which isn't at all equivalent to a full-fledged tropical cyclone.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
Quoting TropicalAnalystwx13:


They can declare an invest at any percentage, it is up to the forecaster. Remember the other day when Don was at near 0%, they reactivated him. Even before that, I think he was first declared at 10%.

We will see 91L tomorrow, hopefully.


Yep , you're right, I was just speaking generally!
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Since the GFS model run at 8 pm shows Don coming pretty much right over my house, and I live right on the Laguna Madre,I'm glad y'all assure me it won't amount to much. I'm not going to bother with my storm shutters, that is such a pain.I hope everyone is right and it's just some rain for me.
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Mexico in the Cone of Doom
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Quoting j2008:

Thanks for the info. That makes a lot of sence, If I think about it I think when Don was an invest he was only at 10%. I still stick by my statement that we should have 91L by tommorow morning at latest.

They are both right, but I think 20% more than 12 hours it becomes an invest!
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Quoting TropicalAnalystwx13:


No...Don is at 998 mb.


Not what the latest vortex says...
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Quoting doabarrelroll:


Also keep in mind that very few people who say they have been in a hurricane have actually seen hurricane force winds.


Eh, it's kind of a mixed bag. I've been through 100 mph winds, yet I have refused to evacuate in the event of another one (unless it's exceptionally severe). My family says they are, but I will stay behind and capture the footage on film.

Then again, I am 70 miles inland, so the storms I do get typically weaken quite markedly by the time they make it this way.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
1459. 7544
gota just love this blog ;
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I think we'll get our fourth in July.

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1457. ackee
THE ECMWF has been performing well so far this seasons it has hint devlopment of both DON and BRET I even notice that the wave in the Cental atlantic the Ecmwf has been hinting devlopment as well , also show another system as well. The GFS on the other hand, has dissapoint me so far this seasons
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Quoting ElConando:


Yes?


Lol...That's funny, but messed up.
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Quoting VAbeachhurricanes:
So Don back up to 1002 mb... wonder why they didn't put that for the 11 pm advisory. Won't make up his mind i see.


No...Don is at 998 mb.
Member Since: July 6, 2010 Posts: 113 Comments: 32714
Quoting MississippiWx:


Oh God.


Yes?
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Quoting stormpetrol:

Usually when at 20% for more than 24 hours , I think, not sure though!


They can declare an invest at any percentage, it is up to the forecaster. Remember the other day when Don was at near 0%, they reactivated him. Even before that, I think he was first declared at 10%.

We will see 91L tomorrow, hopefully.
Member Since: July 6, 2010 Posts: 113 Comments: 32714
1451. MTWX
Quoting floodzonenc:
When do you know that you've been following invests too long?

Tonight, I noticed a "blob" of soap suds on the shower floor, and the word "vorticity" popped into my head.

Thanks, WU bloggers, for my continuing education :)

Back to you, Don!


LOL!! I'm the same way with swirling water down the drain! My mind instantly says "tornado"
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Its closed and tight, not even broad!
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So Don back up to 1002 mb... wonder why they didn't put that for the 11 pm advisory. Won't make up his mind i see.
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1447. JLPR2
Quoting TropicalAnalystwx13:


Probably not open, but elongated somewhat.





Yep, it needs some nice convection to tighten that circulation. It's lacking that.

Member Since: September 4, 2007 Posts: 7 Comments: 8747
TEX or MEX?
Member Since: July 19, 2011 Posts: 12 Comments: 2532
Quoting j2008:

Someone correct me if im wrong but when a system has a 30 % is that when it can become an Invest?

Usually when at 20% for more than 24 hours , I think, not sure though!
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For a second I thought there was a malicious poster on here. I was wrong though.
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Quoting Floridian2011:
Good evening, =).


Oh God.
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1442. GHOSTY1
Quoting doabarrelroll:

you have now asked for 2 storms to hit Texas in 10 minutes. Ignore listed.


I dont think you understand the seriousness of the situation here in Texas, since we haven't had the rain in texas our crops are destroyed and ranchers lives are being severely impacted, were losing millions, tens of millions, and im pretty sure more than that because of the drought. What is the problem anyway with asking for good luck and plentiful rain but no since you think im just weird for some reason just forget the Texans, Go Ahead Ignore me its not like im gonna change my mind.
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1440. ackee
Quoting AtHomeInTX:


Good evening again everyone. Just had a surprise visit from my Grandkids Emily and Levi. Lol.
agree think this will become much further south to greater threat to land will be shock if this is not 91L very soon
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1439. angiest
Quoting ScoobyMoo:


Ike was devastating for many of us in the Houston-Galveston area. I'm hoping I understood your comment incorrectly.


Ike covered a large area, and therefore dropped rain over a large area. I don't remember the amounts, but since Ike came in and left it probably wasn't too bad.

Tropical systems are the only reasonable chance of making a dent in the drought in the near to medium term. It's just kinda dicey. Need the rain, not the wind and surge. And we don't need too much rain falling on the very dry ground too quickly or it all runs off.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Looked like the CMC took the 40W wave into the Carib below Hispaniola at the end of the run.
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Quoting TampaSpin:


Looks open to the NE to me........not sure tho.


Probably not open, but elongated somewhat.



Member Since: July 6, 2010 Posts: 113 Comments: 32714
1434. j2008
Quoting KoritheMan:


No. I've seen invests when systems are at 10%. There is no set criteria for an invest, other than the discretion of a forecaster.
Quoting doabarrelroll:

it can have a 10 percent chance and still be an invest.

Thanks for the info. That makes a lot of sence, If I think about it I think when Don was an invest he was only at 10%. I still stick by my statement that we should have 91L by tommorow morning at latest.
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Ghost, you might want to watch the way you word things. People are almost going to universally be offended by statements along the lines of "I want a storm", because they feel that you are trivializing the suffering of others. A most understandable concern, if you ask me.

Over the years I've learned that much depends upon perception. Just remember that.
Member Since: March 7, 2007 Posts: 602 Comments: 21231
Quoting Stormchaser2007:
Should be 91L by tomorrow.

ASCAT is impressive. This might become Emily.



Good evening again everyone. Just had a surprise visit from my Grandkids Emily and Levi. Lol.
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1431. MTWX
Quoting RitaEvac:
I've seen MCS over the plains bigger than this thing

Like this one...

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1430. angiest
Quoting 7544:


so far this the gfs has been oh well u know


GFS was horrible in the early part of 2010 but did very well with the CV train starting with Danielle.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting TropicalAnalystwx13:


It has a closed circulation.


Looks open to the NE to me........not sure tho.
Member Since: September 2, 2007 Posts: 179 Comments: 20448
We MAY finish our July with Emily, but it will be a close call. We'll see...If we don't have Emily before August 1, we'll probably have her between August 1-4.
Member Since: July 6, 2010 Posts: 113 Comments: 32714
Quoting GHOSTY1:
im hopin' for an extra busy year for Texas, maybe that wave will head for texas and hopefully avoid land masses, shear, dry air for perfect strengthening conditions. If it does impact land i hope the Ike solution works out for it where the land causes an increase in size and therefore more rain. what am i doing, i guess wishful thinking.


Ike was devastating for many of us in the Houston-Galveston area. I'm hoping I understood your comment incorrectly.
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1426. 7544
Quoting angiest:


18Z GFS loses it in the Central Caribbean. 0Z run starts shortly.


so far this the gfs has been oh well u know
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Quoting Stormchaser2007:
Would be something if we saw 4 storms in July...

CATL wave looks good.





It has a closed circulation.
Member Since: July 6, 2010 Posts: 113 Comments: 32714
1424. angiest
Quoting GHOSTY1:
hopefully CATL will strengthen and for texas too. im asked for more than a tropical storm. Which way is it supposed to head anyway?


18Z GFS loses it in the Central Caribbean. 0Z run starts shortly.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1423. GHOSTY1
Quoting doabarrelroll:


and let me guess, you live near those places...


Actually....No...I live in Houston and if i could have it my way i would like it to hit maybe 10 15 miles west of Galveston. I never choose where a system will go because of my location because that is just being stupid it never happens that way. Sometimes i wish it did though :P
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Should be 91L by tomorrow.

ASCAT is impressive. This might become Emily.

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Don might and i say might and only might make a landfall just below Hurricane force. It is still not very organized at the Lower levels by the evidence of a lack of Convergence. Until you have Convergence at the Lower Levels you WILL NOT HAVE much of a storm. That is just the way it is!!!!!
Member Since: September 2, 2007 Posts: 179 Comments: 20448

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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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