Don battling dry air and wind shear
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
Reader Comments
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Too bad we don't have a like button...
Is that a riddle?
Time: 21:53:00Z
Coordinates: 24.2333N 91.3833W
Acft. Static Air Press: 751.7 mb* (~ 22.20 inHg*)
Acft. Geopotential Hgt: 2,533 meters* (~ 8,310 feet*)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 211° at 4 knots (From the SSW at ~ 4.6 mph)
Air Temp: 14.1°C* (~ 57.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 6 knots (~ 6.9 mph)
SFMR Peak (10s) Sfc. Wind: 63 knots (~ 72.4 mph)
SFMR Rain Rate: 37 mm/hr (~ 1.46 in/hr)
(*) Denotes suspect data
what are they finding?
sheri
Coordinates: 24.2333N 91.3833W
Acft. Static Air Press: 751.7 mb* (~ 22.20 inHg*)
Acft. Geopotential Hgt: 2,533 meters* (~ 8,310 feet*)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 211° at 4 knots (From the SSW at ~ 4.6 mph)
Air Temp: 14.1°C* (~ 57.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 6 knots (~ 6.9 mph)
SFMR Peak (10s) Sfc. Wind: 63 knots (~ 72.4 mph)
SFMR Rain Rate: 37 mm/hr (~ 1.46 in/hr)
What's this?
I doubt that Don is anything more than a 50-55mph tropical storm (yes, I know they don't use 55mph anymore).
Zoom,Boxes,and Tropical Points active
no way??? almost a hurricane? hhaha
Strong winds, though possibly contaminated.
This what you are looking for?
http://www.nhc.noaa.gov/gis/
#1 of "Other NWS GIS Data"
give us a hint
Coordinates: 24.25N 91.35W
Acft. Static Air Press: 751.2 mb* (~ 22.18 inHg*)
Acft. Geopotential Hgt: 2,539 meters* (~ 8,330 feet*)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 154° at 11 knots (From the SSE at ~ 12.6 mph)
Air Temp: 14.3°C* (~ 57.7°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 12 knots (~ 13.8 mph)
SFMR Peak (10s) Sfc. Wind: 58 knots (~ 66.7 mph)
SFMR Rain Rate: 32 mm/hr (~ 1.26 in/hr)
Some very strong winds!
Contaminated reading. You don't have a flight level wind of 6kts and then have a surface report of 63kts.
yes and the answer is in your scriptholder
I agree, and this one isn't flagged either.
They dont use 55mph?
what if there not flagged they are false? or is it if there flagged the false? I hope i said that right.
sheri
The rest of the readings from that batch were flagged. The reading was definitely contaminated.
Troll: an annoying person on the internet which his actions may include spamming, posting mostly off topic, or in some cases, hacking.
Flagged means they are suspect, but this is weird...how can the flight level winds be so low. There might be unflagged contamination.
so is don weakining or stregnthining?
They used it once last year and the blog went crazy (I think it was with Igor), lol.
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