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 MrstormX:


That was from the Vortex message, 2 planes are about equidistant from the center and should be there within the hour.
imagine if they find a strong tropical storm... wow im gettin excited :)
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Depth of 26C Isotherm

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Quoting MrstormX:


The QPF? That was interesting, another curiosity this evening...


Correct. It is strange. My assumption is that the convection is basically going to stream north and east until after landfall. Not too untypical, but just suprising given the distance from the center.
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Don looking good thus far! Let's see how it fares the rest of this evening and overnight!
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Quoting floridaboy14:

have they reached the center yet?


That was from the Vortex message, 2 planes are about equidistant from the center and should be there within the hour.
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665. TI882
The Highest winds I saw were ~72mph @ 21:53
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This could be a phase of RI for Don. Cloud symmetry looking more in line with that of a strong TS.
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Quoting MrstormX:


Pressure 1004, wind are all contaminated.

have they reached the center yet?
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time to go till all the kids on here are in bed...
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HHmight be sampling that convection blowup down in the southern part of storm, the lat and long are about right
Member Since: September 18, 2010 Posts: 0 Comments: 5115
Quoting hurricaster:


I was actually talking about the HPC Precipitation Map that was posted. The majority of forecasted precip is well north and east of Don which was suprising to me as it doesn't correlate to the projected track.


The QPF? That was interesting, another curiosity this evening...
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Quoting Levi32:
Back after work.


Hi Levi,Can you give me a heads up on TW approaching the Leeward & Windward Islands....I live in Barbados and see more and more the models developing this system..
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I would be shocked if Don stays at 45mph.
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Quoting HurricaneHunterJoe:
u talking about flight 3 or 4?


I was actually talking about the HPC Precipitation Map that was posted. The majority of forecasted precip is well north and east of Don which was suprising to me as it doesn't correlate to the projected track.
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Quoting floridaboy14:
whats the lowest pessure and strongest wind speed the hurricane hunters found in don?


Pressure 1004, wind are all contaminated.
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I thought I just saw the start of an eye?
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ok got it now,thought u were talking about 04,hes up north. Athe HH able to sample winshear?
Member Since: September 18, 2010 Posts: 0 Comments: 5115
whats the lowest pessure and strongest wind speed the hurricane hunters found in don?
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After 8 loops, the plane is now making a run at the center again. Looks like everything is ok.
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They just left a message so i think the'll head in again
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18Z gfs model develops CA Wave
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Quoting belizeit:
I remember recon doing the same last year after one of there reading meters failed i think they are waiting for word from head quarters if they should reenter the storm or just fly back home.


Why would they show a public Vortex message if the equipment was foul though?
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Looks to me like Don has entered an intensification, if not a rapid intensification, phase with outflow improving in all quads. Next update should be interesting.
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644. JLPR2


Don is doing the opposite it did yesterday, looks like it hit a sweet spot. With convection growing and stronger while we get closer to D-min Don should strengthen tonight.

And that has been said thousands of times but ehh... mind you I wanted to say something. xD
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Could they be checking the strong wind shear ?
Member Since: September 18, 2010 Posts: 0 Comments: 5115
Quoting HurricaneHunterJoe:
which flight are you referring to?


Flight 3, NOAA HH
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I remember recon doing the same last year after one of there reading meters failed i think they are waiting for word from head quarters if they should reenter the storm or just fly back home.
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Quoting MrstormX:


Wow, now Im really confused. What are they doing?
which flight are you referring to?
Member Since: September 18, 2010 Posts: 0 Comments: 5115
000
URNT12 KWBC 282225
VORTEX DATA MESSAGE
A. 28/2153Z
B. 24 DEG 13 MIN N
91 DEG 26 MIN W
C. NA
D. NA
E. NA
F. 136 DEG 36 KT
G. 046 DEG 80 NM
H. EXTRAP 1004 MB
I. 16 C/2566 M
J. 17 C/2549 M
K. 17 C/NA
L. NA
M. NA
N. 1345 / NA
O. 1/2 NM
P. NOAA2 0304A DON OB 07 AL042011
MAX FL WINDS 36KT NE QUAD 2130Z
SLP EXTRAP FROM 750MB

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I think the Orion is waiting so it and the Low plane can pass through the center at the same time and sample all 4 quadrants on the same pass. It will help and determine the true strength and organization of Don at all levels of the atmosphere.
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Quoting MrstormX:


Wow, now Im really confused. What are they doing?
How long until the AF plane gets there? Maybe the NOAA plane is checking their equipment due to those bizarre wind readings?
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Quoting tiggeriffic:


they showed a nice brand new one last year on the news...did a whole segment when they were tracking a storm...got one the year before that too...


No tiger, just looked it up. The plane in question known as "Kermit" was built in 1975.
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Quoting MrstormX:
Recon has now done 5 loops in the middle of nowhere, I hope their equipment isn't failing; those are pretty old planes they use.


Wondering how much fuel are they wasting on those loops O_O
Member Since: August 30, 2008 Posts: 1 Comments: 224
Quoting Walnut:
StormW also seems to have Poofed himself.

hes on HCW now.
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632. xcool
hmm look like eye try start ? don
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Quoting tiggeriffic:


they showed a nice brand new one last year on the news...did a whole segment when they were tracking a storm...got one the year before that too...


Wow, now Im really confused. What are they doing?
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Quoting MrstormX:
Recon has now done 5 loops in the middle of nowhere, I hope their equipment isn't failing; those are pretty old planes they use.
Curious, indeed.
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Quoting hurricaster:


Not certain how this jives with the forecast path. Confusing. Is this a case of the left hand not talking to the right?
u talking about flight 3 or 4?
Member Since: September 18, 2010 Posts: 0 Comments: 5115
Quoting txjac:


What would make them do that? Are they seeing something strange?

They are being controlled by aliens! LOL
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Quoting MrstormX:
Recon has now done 5 loops in the middle of nowhere, I hope their equipment isn't failing; those are pretty old planes they use.
so the storm isnt stregnthining anymore?
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Quoting MrstormX:
Recon has now done 5 loops in the middle of nowhere, I hope their equipment isn't failing; those are pretty old planes they use.


they showed a nice brand new one last year on the news...did a whole segment when they were tracking a storm...got one the year before that too...
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623. Mikla
Area at 38W,10N is looking stronger all the time. Looking at RGB, it appears to be developing good circulation except to the SW. Water temps are good, shear is good for now. Starting to get some decent, organized convection. It just has dry SAL to fight off.
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Recon has now done 5 loops in the middle of nowhere, I hope their equipment isn't failing; those are pretty old planes they use.
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621. txjac
Quoting red0:
Meanwhile in Fort Smith, Arkansas...

This is the 27th straight day over 100, but I could feel something different in the air today from the beginning. The clouds were looking rather ominous and, it was nearly 1:00 before the signs around town started registering triple digits. I was thinking maybe today's high would be only like 102 instead of a more typical for this heat wave 107.

As I walked out of the theater this afternoon the humidity hit me in the face and I spotted what looked suspiciously like the aftermath of a brief shower on the pavement. The little spots of quickly evaporating wetness were no match for the high pressure ridge of doom though, and were fading fast. I put my headphones on and headed towards my bike when it started raining again, and I took shelter under an awning. As the humidity went from ridiculous to hard-to-breath I stood watching this piece of pavement in front of me get rained on for 18 minutes (2.5 plays of Radiohead's Street Spirit heh) without it getting wet. It was as if the pavement didn't quite remember what to do when it got rained on. 30 minutes later though, the sky opened up and we got our first measurable rain of July and the temperature is now in the 80's. Quite nice!



Good for you, happy that you received some rain
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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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