Tropical Storm Don unimpressive so far

By: Dr. Jeff Masters , 2:08 PM GMT on July 28, 2011

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Tropical Storm Don formed yesterday from an African tropical wave that crossed into the Gulf of Mexico, and the thus-far unimpressive storm appears poised to bring tropical storm conditions to the lower Texas coast by Friday night. An Air Force Hurricane Hunter plane arrived in the center of Don around 8am EDT this morning, and has found Don to be a small tropical storm with top winds near 40 mph. The 7:57am EDT center fix found a central pressure of 1002 mb, which is 2 mb higher than NHC was estimating in its 8am EDT advisory. However, a pass through the center at 9:49am EDT found the pressure had dropped 2mb, to 1000 mb. Top reliable surface winds seen by the Air Force plane with its SFMR instrument as of 9:45am EDT were 41 mph, at 8:10 am EDT. 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 10 - 15 knots of wind shear. The shear is from strong upper level winds out of the north. Since the atmosphere to the north of Don is relatively moist, the moderate shear will not be as damaging to the storm as if these winds had been blowing from the northwest, where the driest air lies. Thus the shear direction is often just as important as the strength of the shear, and in Don's case, the shear direction should not force significant amounts of storm-disrupting dry air into the core. Water temperatures in the Gulf of Mexico are near 29°C, which is 2.5°C above the 26.5°C threshold typically needed to maintain a tropical storm.


Figure 1. Morning satellite image of Don.

Satellite imagery shows that Don is a very small storm. Thus, the storm is vulnerable to pockets of dry air and modest-sized jets of wind shear that we can't see from the relatively coarse-resolution data collected by surface stations, hurricane hunter flights, and satellites. The moderate wind shear over Don is keeping Don's circulation tilted so that the surface center is displaced from the center at higher levels. This tilt is keeping the storm from intensifying. Latest visible satellite loops show a modest increase in the intensity of the thunderstorm's near Don's center began at 9am EDT, but this could be a transient burst and not a sign the storm is undergoing intensification.


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 be a boon or bane for Texas? The state is currently suffering through its worst drought in recorded history, and 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. However, we have a Goldilocks problem. We can't have Don intensifying into a hurricane, or its winds and flooding might bring hundreds of millions in damage. Neither do we 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. This forecast is low-confidence, though, since Don's small size makes it prone to sudden changes in strength, either upward or downward. NHC is giving Don just a 14% chance of intensifying into a hurricane in its 5am advisory, but this could easily change upward if Don manages to overcome its vertical tilt and start consolidating an eyewall. I put the odds of Don reaching hurricane strength at 30%. None of the computer models is predicting Don will become a hurricane.

A small system like Don is relatively difficult to resolve in some of the computer models we use to forecast tropical storm track, and the forecast tracks of Don from these models have a higher spread than usual. For those of you wondering about your odds of experiencing tropical storm force winds, I recommend NHC's wind probability forecast, which is showing that Corpus Christi and Port O'Connor, Texas have the highest chance of 39+ mph winds: 40%.

I'll have a new post this afternoon.

Jeff Masters

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Quoting angiest:


Are you sure that's all outflow? At least some of it looks to be pulling inward, like it is attempting to build some feeders (not necessarily succeeding.)
Yeah, some inflow is apparent, but there are a lot of outflow boundaries.
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Quoting tkeith:
Taz if you slow cook that crow and put a little BBQ sauce on it, it aint that bad...

:)



lol
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.
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Quoting CorneliaMarie:
What is the name of the E storm?


Emily.
Member Since: July 6, 2010 Posts: 113 Comments: 32001
591. jpsb
Quoting floridaboy14:

it will peak out 50 60mph if it gets going
Don's Doomed! Dry air wanted for questioning. Rain over Texas now suspect.
Member Since: June 30, 2009 Posts: 0 Comments: 1191
There is good model support for Tropical Storm Emily next week. Looks like we will not be as quiet as originally thought.
Member Since: July 6, 2010 Posts: 113 Comments: 32001
Quoting Tazmanian:
wow DON has gone all most POOF
Taz if you slow cook that crow and put a little BBQ sauce on it, it aint that bad...

:)
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scoring system?
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Quoting tkeith:
Luca would never harm "the Don"...
Noted...lol
.
.
.
We shouldn't mention where Luca wound up. The blog would go to the mattresses.
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wow DON has gone all most POOF
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Don is pretty pathetic, at the rate it is going it may only be a wisp of clouds by the time it gets to TX. Pressure is rising according to HH, convection is again waning, circulation is variable and this storm remains tilted.
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0Z ECMWF @ 168 hours - Tropical cyclone affecting Puerto Rico:



0z ECMWF @ 192 hours - Weakening due to interation with Hispaniola.



0z ECMWF @ 216 hours - Affecting the Bahamas as a strong system.



0z ECMWF @ 240 hours - Trough split off GA/SC coast, system affecting Bahamas now curving out to sea (I think?).

Member Since: July 6, 2010 Posts: 113 Comments: 32001
hi
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Quoting CosmicEvents:
That dry air is choking Don.
I'm visualizing Luca Brasi with a piano wire around his neck.


That's how the Don takes care of business
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still watching(ick) TWC, they showing a visible sat view of Don, he looks to have a great/strong surface circ! too bad the upper levels arent cooperating to allow it to grow and enlarge to get the precip shield bigger for Texas.
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Don is pulling a Bonnie. Fook.
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Impact Weather is sticking to their guns, which is strange since they normally downplay everything.
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Like Don, pre-91L may have to deal with land interaction once in the Caribbean, but to a lesser extent. The GFS develops this into a low pressure area, and the less reliable NOGAPS and CMC develop this into a tropical cyclone. However, shear is low, waters are warm, and despite some dry air, the wave has a favorable environment ahead of it until it makes it into the Caribbean. The trade winds are rushing across the area, which is what killed 90L when it first entered the eastern Caribbean. Lots of heavy rain can be expected for the GA's once again though, and in one weak from now, we may be talking about TS Emily in the Gulf of Mexico.

Member Since: July 6, 2010 Posts: 113 Comments: 32001
Quoting MiamiHurricanes09:
Don isn't doing well right now. Multiple lower-level outflow boundaries can be noted moving towards the north.

As you probably know, this is the result of downdrafts crashing at the surface and moving outwards. Expect convection to gradually weaken, sort of what happened yesterday, but this time on a grandeur scale.



Are you sure that's all outflow? At least some of it looks to be pulling inward, like it is attempting to build some feeders (not necessarily succeeding.)
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Quoting CosmicEvents:
That dry air is choking Don.
I'm visualizing Luca Brasi with a piano wire around his neck.
Luca would never harm "the Don"...
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Extend/Depth of Dry Air currently affecting Don as previously discussed:



You can see from the WV SAT image above that the W GOM remains dry above 850MB, which also verifies with the drop by the HH on the last center fix.

Significant Temperature And Relative Humidity Levels...
Level Air Temperature Dew Point
1005mb (Surface) 28.2°C (82.8°F) 27.1°C (80.8°F)
896mb 22.6°C (72.7°F) 21.6°C (70.9°F)
874mb 24.0°C (75.2°F) Approximately 19°C (66°F)
850mb 23.2°C (73.8°F) Approximately 17°C (63°F)
843mb 22.4°C (72.3°F) Approximately 10°C (50°F)
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Day 7 of the North American Hemisphere..if I am reading this map right, it looks like the bermuda high isnt that strong..leaving the east coast door wide open..

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I am starting to think that in about 8-12 hrs Don will restrengthen...it has to be past the shear it is in...it has time to mositen up it's surroundings and if it can do that and get into the area of low shear that it is moving towards, I think that is a good chance for Don to become the 60mph storm that is being predicted or even a min cat 1 hurricane. The circulation seems to be pretty decent, so I think Don could really ramp up quick if the conditions become right.
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Quoting TropicalAnalystwx13:


Agreed. However, unless it develops before entering the Caribbean, it probably won't have a chance until it makes it into the western Caribbean.

Most storms that form outside of the Cape Verde will probably form in the western Caribbean, like we've seen with Don.


It appears that the models do develop it before entering the Caribbean, at least the CMC and NOGAPS do.
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Just the TWC trop update, guys said that shear was keeping convection confined to the southern side of Don. he said wind shear was"going to be variable and less off to the NW of Don. Anonr got a current wind shear'wind shear forecast handy?
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That dry air is choking Don.
I'm visualizing Luca Brasi with a piano wire around his neck.
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the GFS has not been on the vball this season. check out ECMWF, very agressive with the CATL wave
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Based on the final recon fixes is looks like Don's weakening has peaked for the day @ 1005. Surely wasn't expecting that here, I was personally expecting Don to maintain today.
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Quoting hydrus:
The CMC 144 hour vort map .
a more westerly track i see
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Don isn't doing well right now. Multiple lower-level outflow boundaries can be noted moving towards the north.

As you probably know, this is the result of downdrafts crashing at the surface and moving outwards. Expect convection to gradually weaken, sort of what happened yesterday, but this time on a grandeur scale.

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Product: Air Force Tropical RECCO Message (URNT11 KNHC)
Transmitted: 28th day of the month at 17:45Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 303)
Storm Number: 04
Storm Name: Don (flight in the North Atlantic basin)
Mission Number: 2
Observation Number: 19

Mandatory Data...

Observation Time: Thursday, 17:38Z
Radar Capability: Yes
Aircraft Altitude: Below 10,000 meters
Coordinates: 26.1N 91.0W
Location: 274 miles (441 km) to the SSW (192°) from New Orleans, LA, USA.
Turbulence: None
Conditions Along Flight Route: In the clear
Pressure Altitude: 7,920 meters
Flight Level Wind: From 120° at 27 knots (From the ESE at ~ 31.0 mph)
- The above is a spot wind.
- Winds were obtained using doppler radar or inertial systems.
Flight Level Temperature: -20°C
Flight Level Dew Point: Not available, probably because the dew point hygrometer was not working.
Weather (within 30 nautical miles): Broken clouds (5/8 to 7/8 cloud coverage)
D-value: 50 geopotential meters

Remarks Section...

Surface Wind Speed (likely by SFMR): 25 knots (~ 28.8mph)
Mission Status: Concluded (Last Report)
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Quoting CybrTeddy:
The wave out in the CATL has good model support, the GFS develops a low pressure area but doesn't strengthen it. The NOGAPS & CMC develop it into a tropical cyclone. It is probably one of the most defined waves yet.


Agreed. However, unless it develops before entering the Caribbean, it probably won't have a chance until it makes it into the western Caribbean.

Most storms that form outside of the Cape Verde will probably form in the western Caribbean, like we've seen with Don.
Member Since: July 6, 2010 Posts: 113 Comments: 32001
The CMC 144 hour vort map .
Member Since: September 27, 2007 Posts: 1 Comments: 21173
Quoting TropicalAnalystwx13:
New convection firing on the northern side of Don.


it will peak out 50 60mph if it gets going
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Quoting floridaboy14:

teddy your thoughts on don and do you have any new info from the hurricane hunters?


Weakening is possible today as Don attempts to mix out that dry air from its core but I expect strengthening tomorrow to a 50-60mph TS before landfall on Saturday. The new info indicates there is a lot of dry air in its circulation at this time. IMO, I think the tropical wave in the CATL could be a bigger threat.
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quite agree with you SLU the area of disturbed weather in the CATL needs some recognition. the ingestion of dry air to it;g north is limiting convection. most of the models are hinting of development near the antilles come sun/monday time frame. SLU watch it same position and date of hurricane ALLEN 31 yrs ago
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New convection firing on the northern side of Don.

Member Since: July 6, 2010 Posts: 113 Comments: 32001
Quoting floridaboy14:
does anyone have new data from the hurricane hunters?


HH is heading home now.

The latest update you saw from NHC is pretty much the latest the HH reported.
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FWIW, from the TROPDISC:


TROPICAL STORM DON IS CENTERED NEAR 24.6N 90.7W AT 28/1800 UTC
OR 410 NM ESE OF CORPUS CHRISTI TEXAS OR ABOUT 370 NM E OF
BROWNSVILLE TEXAS MOVING NW AT 13 KT. ESTIMATED MINIMUM CENTRAL
PRESSURE IS 1005 MB. MAXIMUM SUSTAINED WIND SPEED IS 40 KT WITH
GUSTS TO 50 KT. DON IS LOCATED ON THE SOUTHERN PERIPHERY OF A
MIDDLE TO UPPER LEVEL RIDGE WITH AXIS ALONG 90W ACROSS THE
NORTH-CENTRAL GULF OF MEXICO THAT CONTINUES TO PROVIDE AMPLE
UPPER LEVEL DIFFLUENCE AND ENHANCEMENT TO THE ONGOING SCATTERED
MODERATE TO STRONG CONVECTION WITHIN 120 NM OF THE SE QUADRANT
AND WITHIN 150 NM OF THE SW QUADRANT. SEE LATEST NHC
INTERMEDIATE PUBLIC ADVISORY UNDER AWIPS/WMO HEADERS MIATCPAT4/
WTNT34 KNHC AND THE FULL FORECAST/ADVISORY UNDER AWIPS/WMO
HEADERS MIATCMAT4/WTNT24 KNHC FOR MORE DETAILS.
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Quoting CybrTeddy:
The wave out in the CATL has good model support, the GFS develops a low pressure area but doesn't strengthen it. The NOGAPS & CMC develop it into a tropical cyclone. It is probably one of the most defined waves yet.

teddy your thoughts on don and do you have any new info from the hurricane hunters?
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Quoting angiest:
Funktop shows Don trying to throw up some new green. He had lost the green earlier. Wonder if it will persist.


ewwwww
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The wave out in the CATL has good model support, the GFS develops a low pressure area but doesn't strengthen it. The NOGAPS & CMC develop it into a tropical cyclone. It is probably one of the most defined waves yet.
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Quoting OceanBlue78:


Honestly thats what I have thought the whole time...I think strong tropical storm to cat 1 hurricane just south of Brazoria county, the county I live in.


Thats what i said last night too.
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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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