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 Stormchaser2007:


Thats yesterdays run.

Tonights is only out to 45 hours.
Isn't this the latest 0z at 144 hours?


Member Since: September 2, 2009 Posts: 130 Comments: 21194
00z CMC 51 hours

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Anyone think there is a possibility of TS Don taking a slight jog to the north and possibly making landfall near Corpus Christi as expected earlier this evening?
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Looks like it will intensify before landfall....


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Quoting CybrTeddy:
00z CMC, near-Hurricane over the islands.


Thats yesterdays run.

Tonights is only out to 45 hours.
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00z CMC, near-Hurricane over the islands.
Member Since: July 8, 2005 Posts: 259 Comments: 24574
1863. Levi32
Don is still at 1000mb. Convection looks nice, but winds being found by the recon are still lackluster. My ideas for landfall and intensity remain the same.

Goodnight all.
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Quoting MississippiWx:


Yeah, that's the one that the NAM wants to bring into the NW Caribbean as a weak tropical cyclone at the end of the run. It will be interesting to see which side of Central America it chooses to develop, if it even does.
GFS and NOGAPS too. (18z run on the GFS...haven't looked at the 0z run).
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting Stormchaser2007:
Another AOI



NOGAPS, NAM, and GFS also wants to try to spin something up from this.
Member Since: July 8, 2005 Posts: 259 Comments: 24574
999.8mb in Don
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Quoting Stormchaser2007:
Another AOI



Yeah, that's the one that the NAM wants to bring into the NW Caribbean as a weak tropical cyclone at the end of the run. It will be interesting to see which side of Central America it chooses to develop, if it even does.
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Another AOI

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ASCAT showed a pretty well organized LLC with 91L.
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1854. wn1995
91l may try to spin up pretty quickly, been following it for a few days and its starting to organize nicely and may already have a closed surface circulation. Interesting for sure.
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1853. j2008
Quoting VAbeachhurricanes:


Nothing from the HH says that will be the case.

1001mb 45-50 mph winds

I dont think It will weaken that much if it weakens.
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91L looks pretty good for an invest. Could see this becoming a TD relatively quickly if trends continue.

Don looks fairly decent. Should make it upwards to 50-55kts at its peak, unless it undergoes another weakening trend.
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1851. scott39
91L looks impressive! Is it too early to know if the high is going to hold it more W, or is it going to turn out to sea?
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1850. 7544
what we seen so far this season and now 91 L is born anything can happen at anytime just stay on your toes . remember nothing is written in stone don already has proven that and the others will follow defying all the odds stay tuned
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Quoting Stormchaser2007:
CMC at 36 hours...

Still running.

Already a decent tropical storm it appears.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
CMC at 36 hours...

Still running.

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Quoting j2008:
Im gonna say Don will be at 55-60 MPH and 995-998MB.
Any thoughts?


At this advisory I don't see much change from the previous, maybe even an uptick in pressure. Curious to see if Don can start an actual strengthening trend or not.
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Quoting j2008:
Im gonna say Don will be at 55-60 MPH and 995-998MB.
Any thoughts?


Nothing from the HH says that will be the case.

1001mb 45-50 mph winds
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1845. j2008
Im gonna say Don will be at 55-60 MPH and 995-998MB at next update.
Any thoughts?
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Notable early August Atlantic Hurricanes:

Hurricane Connie 1955, landfall Aug 12
Connie struck as a Category 1, causing major flooding and inflicting extensive damage to the Outer Banks and inland to Raleigh.

Hurricane Celia 1970, landfall Aug 3
Celia moved towards the Texas coastline, it began to rapidly intensify again. The storm reached its peak as it made landfall near Corpus Christi, Texas, as a strong Category Three. Causing 15 fatalities.

Hurricane Charley 2004, landfall (2nd) Aug 13
Charley produced 16 tornadoes in Florida, North Carolina and Virginia. The total U. S. damage is estimated to be near $15 billion, making Charley the second costliest hurricane in U.S. history.




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1841. KEEPEROFTHEGATE (Mod)
XX/INV/91L
MARK
8.45N/41.75W
Member Since: July 15, 2006 Posts: 178 Comments: 56059
1840. KEEPEROFTHEGATE (Mod)
04L/TS/D/Cx



weakening flag OFF
Member Since: July 15, 2006 Posts: 178 Comments: 56059
Quoting extreme236:
Im definitely going with a 30% at the TWO. Id wanna say 40 but they tend to go on the lowside.


Yeah, I'd say 30% as well. 30% would actually be pretty high considering the NHC goes so slow. With 91L actually having model support, they might go ahead and say 30% though.
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It will be interesting to hear what Colorado State has to say about the remainder of the hurricane season.

I wonder if they will increase their numbers?

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1837. KEEPEROFTHEGATE (Mod)
passage is open

Member Since: July 15, 2006 Posts: 178 Comments: 56059
1836. Seawall
CV season looks like it's good to go... Just sayin'
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Im definitely going with a 30% at the TWO. Id wanna say 40 but they tend to go on the lowside.
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That strong spin can be noticed in Tot. Precipitable Water Anim.

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1833. Seawall
The crickets are out, and the ants in full steam.. LOL
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Recon about to make another center pass in Don:


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1831. KEEPEROFTHEGATE (Mod)
Member Since: July 15, 2006 Posts: 178 Comments: 56059
1830. xcool
Member Since: September 26, 2009 Posts: 2 Comments: 15684
You know, it would be interesting to know how many days this month we have been without an invest/tropical cyclone. I'd be willing to bet it's around 3 to 5.
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wow folks just wow.......
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1827. JRRP
Quoting MiamiHurricanes09:
One of the better S shapes I've seen in a while.

yea... at least for this season
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1826. KEEPEROFTHEGATE (Mod)
iam even considering updating numbers for the rest of the season in the season outlook on my blog page
Member Since: July 15, 2006 Posts: 178 Comments: 56059
1824. JRRP
Quoting Floridian2011:


Espero que me comiennzes a vigilar la situacion alla bajo muyd e cera, ahora que ya se formo, amigo, =(

puedes apostarlo... jejeje
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Quoting sunlinepr:


That S shape, points to organization...
One of the better S shapes I've seen in a while.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting sunlinepr:


That S shape, points to organization...


Indeed. It's a very well organized tropical wave. It also appears to be separated from the ITCZ now.
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Quoting jasonweatherman2011:
wow!! more tropical waves!!
Jason the one behind that one. Look at how HUGE that thing is.
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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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