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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Time: 00:09:30Z
Coordinates: 24.45N 91.8167W
Acft. Static Air Press: 843.7 mb (~ 24.91 inHg)
Acft. Geopotential Hgt: 1,489 meters (~ 4,885 feet)
Extrap. Sfc. Press: 995.4 mb (~ 29.39 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 107° at 10 knots (From the ESE at ~ 11.5 mph)
Air Temp: 27.9°C (~ 82.2°F)
Dew Pt: 14.2°C (~ 57.6°F)
Peak (10s) Flt. Lvl. Wind: 13 knots (~ 14.9 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots* (~ 27.6 mph*)
SFMR Rain Rate: 2 mm/hr* (~ 0.08 in/hr*)
(*) Denotes suspect data
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Member Since: August 2, 2010 Posts: 21 Comments: 9874
Dmin and Dmax are not that important to established tropical cyclones - anything below 1005mb usually produces enough convergence to generate its own convection provided the atmospheric dynamics are favorable. The effects of Dmin and Dmax are overwhelmed by the overall atmospheric conditions produced by the cyclone.
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30 frames of High Resolution Rapid Refresh IR Imagery over the Gulf of Mexico


Link
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Quoting jdjnola:
I think it's safe to say Don is now almost guaranteed to bring Texas some much needed rain...


I disagree to an extent. Since Don is gaining strength, he will likely move further west and south of the forecasted track and make landfall south of Corpus Christi. Only time will tell though.
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Time: 00:08:00Z
Coordinates: 24.5167N 91.7667W
Acft. Static Air Press: 843.1 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,514 meters (~ 4,967 feet)
Extrap. Sfc. Press: 999.2 mb (~ 29.51 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 139° at 34 knots (From the SE at ~ 39.1 mph)
Air Temp: 23.9°C (~ 75.0°F)
Dew Pt: 17.3°C (~ 63.1°F)
Peak (10s) Flt. Lvl. Wind: 39 knots (~ 44.8 mph)
SFMR Peak (10s) Sfc. Wind: 41 knots (~ 47.1 mph)
SFMR Rain Rate: 2 mm/hr (~ 0.08 in/hr)
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Quoting weatherman566:
Don is so confused. He really loves to blow up during Dmin huh? He's done that as a wave, depression, and now as a storm.

Silly Don.....
No kidding, just popped in, and convection is now firing up strong as ever...right in the middle of dmin lol
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Maybe later a center could clear out,,but Don has to Deepen a tad mo fo dat.
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Quoting jdjnola:
I think it's safe to say Don is now almost guaranteed to bring Texas some much needed rain...
+1........and that's a good thing.
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Quoting cajunkid:
wow


I'm a little cloudtop... not short nor stout.
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Quoting Patrap:
The Shadow from the Low Sun Angle behind the Central Tower is not a Eye forming
squashed my hopes
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Time: 00:09:00Z
Coordinates: 24.4667N 91.8W
Acft. Static Air Press: 842.4 mb (~ 24.88 inHg)
Acft. Geopotential Hgt: 1,511 meters (~ 4,957 feet)
Extrap. Sfc. Press: 996.7 mb (~ 29.43 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 139° at 18 knots (From the SE at ~ 20.7 mph)
Air Temp: 26.9°C (~ 80.4°F)
Dew Pt: 15.1°C (~ 59.2°F)
Peak (10s) Flt. Lvl. Wind: 19 knots (~ 21.8 mph)
SFMR Peak (10s) Sfc. Wind: 18 knots* (~ 20.7 mph*)
SFMR Rain Rate: 3 mm/hr* (~ 0.12 in/hr*)
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Product: Air Force Temp Drop (Dropsonde) Message (UZNT13 KNHC)
Transmitted: 29th day of the month at 00:31Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 307)
Storm Number: 04
Storm Name: Don (flight in the North Atlantic basin)
Mission Number: 4
Observation Number: 07

Part A...

Date: Near the closest hour of 0Z on the 29th day of the month
Highest Mandatory Level For Which Wind Was Reported: 850mb
Coordinates: 24.4N 91.8W
Location: 371 miles (597 km) to the ESE (107°) from Brownsville, TX, USA.
Marsden Square: 082 (About)

Level Geo. Height Air Temp. Dew Point Wind Direction Wind Speed
999mb (29.50 inHg) Sea Level (Surface) 28.8°C (83.8°F) 27.6°C (81.7°F) 250° (from the WSW) 29 knots (33 mph)
1000mb -5m (-16 ft) Other data not available.
925mb 690m (2,264 ft) 27.2°C (81.0°F) 23.2°C (73.8°F) 250° (from the WSW) 26 knots (30 mph)
850mb 1,440m (4,724 ft) 25.2°C (77.4°F) Approximately 18°C (64°F) 270° (from the W) 8 knots (9 mph)

Information About Radiosonde:
- Launch Time: 0:10Z
- About Sonde: A descending radiosonde tracked automatically by satellite navigation with no solar or infrared correction.

Remarks Section...

Dropsonde Location: Dropped in eye.

Splash Location: 24.43N 91.83W
Splash Time: 0:11Z

Release Location: 24.43N 91.84W
Release Time: 0:10:02Z

Splash Location: 24.43N 91.83W
Splash Time: 0:11:28Z

Mean Boundary Level Wind (mean wind in the lowest 500 geopotential meters of the sounding):
- Wind Direction: 255° (from the WSW)
- Wind Speed: 26 knots (30 mph)

Deep Layer Mean Wind (average wind over the depth of the sounding):
- Wind Direction: 250° (from the WSW)
- Wind Speed: 23 knots (26 mph)
- Depth of Sounding: From 843mb to 999mb

Average Wind Over Lowest Available 150 geopotential meters (gpm) of the sounding:
- Lowest 150m: 158 gpm - 8 gpm (518 geo. feet - 26 geo. feet)
- Wind Direction: 255° (from the WSW)
- Wind Speed: 25 knots (29 mph)

Sorry just got back to computer so if double post , sorry, note, Dropped in eye.
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The Shadow from the Low Sun Angle behind the Central Tower is not a Eye forming
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I think it's safe to say Don is now almost guaranteed to bring Texas some much needed rain...
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Ahhh, great. No steering for 17Z. That makes things more difficult.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
TS Don is currently attempting to get a way from the N/NE shear. So is relocating closer to the more intense convection.

If the trend continues the track would keep shifting south. As we currently stand... Don has relocated W to WSW of the earlier acquired fix.
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952. HCW
Quoting cajunkid:
That last vis. image, (prob. the last of today) lets the cat out the bag.


It let those overshooting tops out :)
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Guys that 999mb dropsonde had 29 knot surface winds - the actual central pressure is likely 997 or 996 based on the 1mb per 10 knot rule.
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00:15

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949. DFWjc
Quoting CybrTeddy:


100% guarantee that's an overshooting cloud top.


so what you're saying is he has a "outie" rather than an "innie"
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Quoting MTWX:

It that cumulative or hourly??


Specifically this is a 6 hour forecast.
Here's the main page for Don.

Link
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Quoting cajunkid:
wow
looks like trying to form an eye on that sat pic! wow
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Quoting Ameister12:
Do I dare say that Don has an eye?


100% guarantee that's an overshooting cloud top.
Member Since: July 8, 2005 Posts: 259 Comments: 24471
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944. SLU
Quoting islander101010:
what do you think about the to be invest? coming to a area near yours?


It's beginning to consolidate near 8n 39w. Looks like it may resemble the track of the previous version of Emily.



Member Since: July 13, 2006 Posts: 13 Comments: 5355
Here's another update from Oz in Corpus Christi, this time with art. The link below is a picture of him and that light he talks about.

"TS Don is projected to make a night time landfall in the Corpus Christi area. That is good news on two fronts! One, CC has 4G! I checked my webcam. It runs as fast as allowed by Ustream.tv. Two, I have this very special, very expensive light. It doesn't look like much with the sun out, but at night, it casts a beam 90 feet wide by 900 feet deep! We are a go in CC! :)"

Picture Link
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942. DFWjc
Quoting Ameister12:
Do I dare say that Don has an eye?


he's trying...
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Quoting MrstormX:


Recon would say otherwise...
None of the experts predict this to make it to hurricane strength. But who knows they are not very good with intensity forecasts.
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Don has a belly button!!
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Quoting NotCircumventing:
RI has specific criteria doesn't it?


NWS Glossary:
NHC terms

Rapid Deepening
An increase in the maximum sustained winds of a tropical cyclone of at least 30 kt in a 24-h period.

(Don't see Rapid Intensification listed)

http://www.nhc.noaa.gov/aboutgloss.shtml
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Do I dare say that Don has an eye?
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Time: 00:08:00Z
Coordinates: 24.5167N 91.7667W
Acft. Static Air Press: 843.1 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,514 meters (~ 4,967 feet)
Extrap. Sfc. Press: 999.2 mb (~ 29.51 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 139° at 34 knots (From the SE at ~ 39.1 mph)
Air Temp: 23.9°C (~ 75.0°F)
Dew Pt: 17.3°C (~ 63.1°F)
Peak (10s) Flt. Lvl. Wind: 39 knots (~ 44.8 mph)
SFMR Peak (10s) Sfc. Wind: 41 knots (~ 47.1 mph)
SFMR Rain Rate: 2 mm/hr (~ 0.08 in/hr
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936. MTWX
Quoting cycleranger:
Ensemble Tropical Rainfall Potential (eTRaP)

We'll be watching this...




It that cumulative or hourly??
Member Since: July 20, 2009 Posts: 23 Comments: 1393
Quoting KEEPEROFTHEGATE:
maybe its a cat 1 thunderstorm


IIRC, the entire mass of cloud that made up Marco was probably smaller than a good plains supercell.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
00z Early Cycle NHC model tracks
Don
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)




Dynamic Models (More sophisticated models)





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That last vis. image, (prob. the last of today) lets the cat out the bag.
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Quoting TXStormWatcher2011:
Are there any projected paths I can look at?


http://www.intellicast.com/Storm/Hurricane/Track. aspx
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maybe its a cat 1 thunderstorm
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Quoting HurricaneHunterJoe:
is that a eye?
That's what they said when they found Moe Green(Bugsy Siegel) in the barbershop. We've made a mistake in not giving The Don proper respect.
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04L/H/D/C1



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I see the NHC colored in the area in central atlantic..anyone have any thoughts on its potential track or strength down the road?? Thanks for any input
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Quoting cajunkid:
wow


Goodnight, Don.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Don is looking a lot better than it did this morning and pressure of 995, that's crazy. I'm thinking 65-70mph tropical storm before a Texas landfall. Can't rule out a chance of it becoming a hurricane.
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Quoting HurricaneHunterJoe:
hey tom, good job on the ins and outs of whats going on in there, no, what do ya say? lets hit the waves!
Beach sure does sound nice.

Haven't gone too much this summer, spending more time on the blogs lol

pretty lame really lol
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Ensemble Tropical Rainfall Potential (eTRaP)

We'll be watching this...



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Quoting FrankZapper:
I don't see that happening. It still is a small blob of convection.


Recon would say otherwise...
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wow
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Quoting TropicalAnalystwx13:
I think it is safe to say that Don is rapidly intensifying. But...how long will it last??
I don't see that happening. It still is a small blob of convection.
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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.