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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Flight-level winds have not exceeded 40kts for either plane so far in their respective trips into Don.
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Quoting Tazmanian:



and its trying too make a run for hurricane


He might well achieve that before landfall, but he's not nearly as well organized as some people are saying.
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818. MTWX
Just a little info on the aircraft for those of you who care...

WC-130J

On 12 October 1999, the U.S. Air Force took delivery of its first WC-130J aircraft. Nine others are scheduled for delivery by late-2000.

In September 1998, the C-130J Development System Office (DSO) at Wright-Patterson AFB, OH, signed a contract with Lockheed Martin Aeronautical Systems, Marietta, GA, to modify six C-130Js to the "W", or weather, configuration. This involved installing and integrating special avionics and weather sensors, as well as making structural modifications. The DSO later exercised contract options to modify an additional four C-130J aircraft.

The WC-130Js will replace the existing fleet of ten WC-130H-model aircraft. The "J-models" are based on the familiar C-130 platform that the Air Force has flown for more than 40 years, but with many improvements, including new engines and avionics, as well as the addition of two mission computers and two head-up displays.

Sensors mounted on the outside of WC-130Js provide real-time temperature, humidity, barometric pressure, radar-measured altitude, wind speed and direction. These are used to calculate a complete weather observation every 30 seconds. These aircraft also deploy dropsondes, instruments ejected out the aircraft and deployed by parachute through the storm to the sea. During descent, they gather real-time weather data and relay it back to the aircraft.

This information is transmitted by satellite directly to the National Hurricane Center for input into the national weather data networks. Forecasters use the data to better predict the path of a storm or hurricane.

Member Since: July 20, 2009 Posts: 23 Comments: 1391
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.....
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the old rule for vis clear eye =100mph
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806

P451 look at the actual HH data - the center is at roughly 24.5 and 92 - superimpose that on the latest IR and a much different picture shows.
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Quoting jpsb:
Would a weak (45 mph) TS have an eye? I thought you needed a strong TS-Cat 1 before you got an eye.


No, weak tropical storm's do not have an eye. What Don is doing now is building an eye-wall, right where that deepest convection can be seen on satellite. Once he is finished, which should be sometime tomorrow, he will likely begin to show an eye. They usually appear when the storm reaches 60 mph (from experience).
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Quoting P451:
SUMMARY OF 700 PM CDT...0000 UTC...INFORMATION
----------------------------------------------
LOCATION...24.7N 91.8W
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...WNW OR 285 DEGREES AT 16 MPH...26 KM/H
MINIMUM CENTRAL PRESSURE...1004 MB...29.65 INCHES

=============
Note: This image is 45 minutes OLDER than the center fix. The cloud mass is further west than in this image. But given that this will still suffice for analysis. So imagine if you will the red dot, the coc, about 10 miles further EAST in the image to account for forward motion over 45 minutes.






And at 5pm, again same applies, image is 45 minutes older than center fix, meaning the cloud mass is actually further west than depicted here. So imagine if you will the red dot, the coc, about 10 miles further EAST in the image to account for forward motion over 45 minutes.





Nice convection, remains disorganized and elongated - perhaps as much as 90% of the convective mass is south of the surface circulation.




LOL that info you this posted is way old DON is way better now
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JSL
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810. jpsb
Quoting Tazmanian:



yup looks like its starting too from a eye
Would a weak (45 mph) TS have an eye? I thought you needed a strong TS-Cat 1 before you got an eye.
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Quoting floridaboy14:
those who underestimated don look what we have.... a hurricane watch is needed for texas.. i am sticking with the path i said ever since don formed north of brownsville south of courpis christi


Not entirely convinced that Don is up near hurricane strength at the minute, will have better idea when the next recon gets to the centre in a few minutes. However, even if Don is at 50/55mph I'd say a hurricane watch is needed. I agree with landfall somewhere between Brownsville and Corpus Christi.
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Quoting KoritheMan:
Don certainly looks significantly better organized than earlier. The circulation no longer appears exposed on visible satellite images. High cloud motions also suggest that shear has lessened over the storm. Maybe Texas WILL get that beneficial rainmaker they've been beckoning for.



and its trying too make a run for hurricane
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Pressure down to 1006mb as they approach the circulation. Flying 100mb lower than Kermit.

Winds near 40mph.

000130 2448N 09134W 8429 01553 0063 +188 +178 131029 030 034 002 00
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Don is in the process of building an eye-wall. If this can continue through tomorrow morning, I suspect we will see an eye developing, if we do not have one now (unlikely).
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Don certainly looks significantly better organized than earlier. The circulation no longer appears exposed on visible satellite images. High cloud motions also suggest that shear has lessened over the storm. Maybe Texas WILL get that beneficial rainmaker they've been beckoning for.
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those who underestimated don look what we have.... a hurricane watch is needed for texas.. i am sticking with the path i said ever since don formed north of brownsville south of courpis christi
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Quoting belizeit:
Look at that green spot in the center thats what you look for if you are looking for the eye in the infrared pic



yup looks like its starting too from a eye
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Time: 23:41:30Z
Coordinates: 25.65N 90.6667W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,585 meters (~ 5,200 feet)
Extrap. Sfc. Press: 1011.4 mb (~ 29.87 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 144° at 36 knots (From the SE at ~ 41.4 mph)
Air Temp: 17.0°C (~ 62.6°F)
Dew Pt: 12.6°C (~ 54.7°F)
Peak (10s) Flt. Lvl. Wind: 37 knots (~ 42.5 mph)
SFMR Peak (10s) Sfc. Wind: 33 knots (~ 37.9 mph)
SFMR Rain Rate: 1 mm/hr (~ 0.04 in/hr)
(*) Denotes suspect data

Mission 4 will soon be there.
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Quoting Patrap:
23:45
Enhanced Infrared (IR) Imagery (4 km Mercator)

Look at that green spot in the center thats what you look for if you are looking for the eye in the infrared pic
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It is possible that the low pressure reading corresponds to a mid level reflection of the strong convection - winds are up from the last pass as well (at least at Fl)
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Quoting Patrap:


Don flexing and in a good schweet spot as he continues to improve and deepen.

don the zombie storm
Member Since: July 15, 2006 Posts: 163 Comments: 52198
Member Since: July 3, 2005 Posts: 414 Comments: 125497
Quoting Patrap:
23:45
Enhanced Infrared (IR) Imagery (4 km Mercator)




is that a eye starting too from looks like a pine hole eye
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twc really stepping up to the plate crystal e. said there are spots with water temps. up to 90f
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Kermit missed the circulation to the east.
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Link If you look at the last image you can see a eye starting to form where the outflow is coming from the center
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Quoting Levi32:


Every 58kt or similar SFMR reading I have seen is suspect.


Still, this system is running 50-60 mph right now, no doubt. Considering that it was very ill looking a couple of hours ago, its impressive.
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23:45
Enhanced Infrared (IR) Imagery (4 km Mercator)

Member Since: July 3, 2005 Posts: 414 Comments: 125497
Quoting Levi32:
Overshooting tops have been going off south of Don's center for quite some time now. We'll see if the 996mb extrapolated pressure is actually valid. It's from up at 750mb so a dropsonde may reveal a different pressure in reality. An 8mb drop in 2 hours would be impressive.
Have HH found the stormed stacked now?
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Quoting MiamiHurricanes09:
I don't think Kermit's data is too reliable.

I'd wait for the WC-130J to penetrate the circulation in a few minutes to know what the pressure really is.

A pressure of 996mb would not be too surprising though.


I just wrote basically the same thing, they'll fly lower!
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Quoting Patrap:
23:15 UTC Rainbow

its there lurking
Member Since: July 15, 2006 Posts: 163 Comments: 52198
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783. MTWX
Quoting MiamiHurricanes09:
I don't think Kermit's data is too reliable.

I'd wait for the WC-130J to penetrate the circulation in a few minutes to know what the pressure really is.

A pressure of 996mb would not be too surprising though.

Nothing like a good 'ol J model!!
Member Since: July 20, 2009 Posts: 23 Comments: 1391
I think Mission 4 number will fly lower than mission number 3.
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Quoting CybrTeddy:
Note the flight level winds though, more than likely a gust. The pressure though is pretty impressive, 996.1 mb.. that's a 8mb drop.



yup
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Note the flight level winds though, more than likely a gust. The pressure though is pretty impressive, 996.1 mb.. that's a 8mb drop.
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Quoting TropicalAnalystwx13:
If current trend continue, I expect a hurricane by tomorrow afternoon. 996 mb. and 58 knots???

Hurricane strength is 65 knots.


Every 58kt or similar SFMR reading I have seen is suspect.
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Overshooting tops have been going off south of Don's center for quite some time now. We'll see if the 996mb extrapolated pressure is actually valid. It's from up at 750mb so a dropsonde may reveal a different pressure in reality. An 8mb drop in 2 hours would be impressive.
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Quoting TropicalAnalystwx13:
Dare I say rapid intensification?



yup
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Quoting MrstormX:


Pat that is crazy, are those just overshooting clouds or is this trying to form an eyewall?


Don flexing and in a good schweet spot as he continues to improve and deepen.

Member Since: July 3, 2005 Posts: 414 Comments: 125497
Quoting MiamiHurricanes09:
I don't think Kermit's data is too reliable.

I'd wait for the WC-130J to penetrate the circulation in a few minutes to know what the pressure really is.


Thats the best part of having multiple missions at once, they can audit each-other.
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Don looks like it's developing a pin-hole eye. Oh, wait. That was a smudge on my monitor.

Ant report from NE Houston: Went and observed two ant mounds today. On one, I saw an ant yawning. On another, several were sprawled out on leaves enjoying the sun. I think that means that Don is not an ant-ticipated event for Houston.
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If current trend continue, I expect a hurricane by tomorrow afternoon. 996 mb. and 58 knots???

Hurricane strength is 65 knots.
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Wow, Don is certainly looking a lot more impressive. If that 996mb reading is right, that's a pretty large fall. Expect Don is at least 50mph at the minute.
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Time: 23:36:00Z
Coordinates: 24.4667N 91.7667W
Acft. Static Air Press: 753.0 mb (~ 22.24 inHg)
Acft. Geopotential Hgt: 2,507 meters (~ 8,225 feet)
Extrap. Sfc. Press: 997.1 mb (~ 29.44 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 121° at 27 knots (From the ESE at ~ 31.0 mph)
Air Temp: 21.8°C (~ 71.2°F)
Dew Pt: 8.9°C (~ 48.0°F)
Peak (10s) Flt. Lvl. Wind: 31 knots (~ 35.6 mph)
SFMR Peak (10s) Sfc. Wind: 31 knots (~ 35.6 mph)
SFMR Rain Rate: 0 mm/hr (~ 0 in/hr)
(*) Denotes suspect data
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About JeffMasters

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