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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I know this has been posted a few times today, but there is something noteworthy here. Look how much moisture the low pressure area associated with this wave is wrapping into its circulation. It is tapping the monsoon trough plus a nice slug that is over the South Atlantic.This should aid in fighting off the dry air to its north..jmo..looks a little healthier the last few frames..
Member Since: September 27, 2007 Posts: 1 Comments: 21173
Let's not forget about our friends in the WPAC dealing with Tropical storm Nock-Ten, and Tropical Storm Muifa. Nock-Ten is poised to make a second and third landfall! Meanwhile Muifa is expected to threaten Japan as a formidable typhoon in the long range. Track is sure interesting on Muifa making a "S" like motion.

Both impressive on satellite imagery.






Member Since: October 17, 2010 Posts: 0 Comments: 1267
443. Jax82
Did Don not get the memo that Texas needs rain?
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Quoting yoboi:


by looking a the bermuda high and how it's shifting

its just showing the high weaken because of a trough and if the storm gets west before the trough comes, it will curve into the east caost. way to far out to call this a fish storm
Member Since: July 25, 2011 Posts: 0 Comments: 1102
441. yoboi
Quoting caneswatch:


How can you be so sure if it's way far out in the Atlantic?


by looking a the bermuda high and how it's shifting
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been sing'n that houston song fur 3 days now he come round'lol
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what is the latest recon finding for pressure of don?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting IceCoast:


Levi when do you think the NHC will mention the wave in the Central Atlantic? What is preventing this from developing over the next 48-72 hours? It has good cyclonic curvature, anticyclone building overhead, a 1010mb low embedded in the wave axis, and convection looks to be on an uptick.





Wrapped-up dry air and broadness of the wave. This is just like pre-Don, which had no chance east of the islands, but flared up farther west. This one may do the same kind of thing.

Later.
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Quoting floridaboy14:
pressure may be rising by are the winds falling down?


The winds can often lag the pressure, so if they're not falling yet with the pressure, they may fall off a bit later this afternoon.

I've got class now. Back later.
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Don needs to hurry to find weaker shear
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Quoting yoboi:


don is not hitting texas yet might fall apart


I'd listen to Floridaboy if I were you.
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Quoting Levi32:


A rising pressure, higher than yesterday, implies weakening, yes.
pressure may be rising by are the winds falling down?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
yall are so negative, it'll bring some rain to td no matter what
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Quoting Levi32:


Hopefully not...it won't be a very heavy rainmaker, but I'm sure we won't get away without some thunderstorm clusters making it onshore. Here's hoping lol.


Yeah still got my fingers crossed someone gets some rain out of this. :)
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Convection needs to get going over the center soon or Don will be toast. Some dry air and subsidence associated with the high to the north, and additionally, upper level convergence (as a result of upper level winds from the NE) on the north and east side, also a result of the high over the US, seem to be the main issues inhibiting convection. Shear is pretty light, but since convection is so poor (in both intensity and coverage, due to the reasons mentioned above) these modest shear values have given Don more trouble than most were probably expecting. As a result, fairly light shear values are actually becoming quite an issue for this weak tropical storm.

CIMSS vort analysis and satellite imagery reveals that the surface circulation is becoming increasingly exposed/decoupled from the mid level circulation. Although one could argue that the mid level and low level circulations were never stacked very well to begin with.
Member Since: August 24, 2010 Posts: 19 Comments: 4357
427. afj3
Quoting Patrap:


Become a Paid member,,and all the ads go away,,and you get 40 frames of radar as well.


Hey! I was thinking about doing that. It seems like it's worth it...
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Quoting yoboi:


it will be a fish storm


How can you be so sure if it's way far out in the Atlantic?
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Quoting afj3:
Not to change the subject but the ads on the wunderground tropical pages are getting bigger and better (big one from Wells Fargo). Is that because more people are creating accounts or that more people log on this time of year and afterwards, the ads diminish? (It's good for the site regardless)


Become a Paid member,,and all the ads go away,,and you get 40 frames of radar as well.

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Quoting Levi32:
Latest vortex message from recon shows Don's pressure up to 1005mb, rising after falling a bit earlier. Convection just south of the center is struggling due to dry air entrainment, subsidence to the north, and wind shear out of the northeast. These conditions have all been lined out to be inhibitors of Don's intensification, and should keep him in check as me moves across the gulf. He needs to stop weakening though, or his surface center may get pulled up farther north than the current forecast track.


Levi when do you think the NHC will mention the wave in the Central Atlantic? What is preventing this from developing over the next 48-72 hours? It has good cyclonic curvature, anticyclone building overhead, a 1010mb low embedded in the wave axis, and convection looks to be on an uptick.



Member Since: October 17, 2010 Posts: 0 Comments: 1267
The COC seems to be displaced north of most of the moisture. It would suit me fine down here on the Laguna Madre a bit north of Brownsville if the COC goes well north of us as long as we get some much needed rain and then that sweeps up into Central Texas.
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Quoting AtHomeInTX:


LOL! Exactly! Like I said scary thought! :D


Hopefully not...it won't be a very heavy rainmaker, but I'm sure we won't get away without some thunderstorm clusters making it onshore. Here's hoping lol.
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418. DFWjc
I'm jealous TampaSpin, it's looks great outside!!!!
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Quoting floridaboy14:

so basically don is weakining?


A rising pressure, higher than yesterday, implies weakening, yes.
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Quoting TropicalAnalystwx13:


No, recon confirms a system that, while its pressure is rising, it is also gaining in strength.

50 mph observed, not contaminated.
Well pressure determines winds...so if pressure is rising, Don is weakening. Also, cloud tops warming also indicate convection is not firing at the moment and the surface circulation is becoming more and more exposed with time, which are both indications of a weakening system.
Member Since: August 24, 2010 Posts: 19 Comments: 4357
Low Cloud Don Loop
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great don is weaking..........
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting TampaTom:
"Texas... let me make you an offer you can't refuse..."



Let's hope it's a rainmaker for Texas... the way your luck is going this year, this could fizzle out at the coast..


LOL! Exactly! Like I said scary thought! :D
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Im going with Levi with that Low Level center tracking more NW to NNW all the time
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Yikes 1005mb...

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Jason, ECMWF has a TC in the EPAC and a small system just north of DR/Haiti but that's in 192hrs.


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Quoting Levi32:
Latest vortex message from recon shows Don's pressure up to 1005mb, rising after falling a bit earlier. Convection just south of the center is struggling due to dry air entrainment, subsidence to the north, and wind shear out of the northeast. These conditions have all been lined out to be inhibitors of Don's intensification, and should keep him in check as me moves across the gulf. He needs to stop weakening though, or his surface center may get pulled up farther north than the current forecast track.

so basically don is weakining?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting TampaTom:
"Texas... let me make you an offer you can't refuse..."



Let's hope it's a rainmaker for Texas... the way your luck is going this year, this could fizzle out at the coast..
We are expecting it to fall apart before it hits Texas that is why we are in Worst Drought ever. High Pressure Lives over Texas
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Don is very unorganized aloft and may Lose the Convective Level to shear as that Neast Shear is whooping his behind bad like.

ESL by LSU GOES-13

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had a pigeon land on a owl like that
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Latest vortex message from recon shows Don's pressure up to 1005mb, rising after falling a bit earlier. Convection just south of the center is struggling due to dry air entrainment, subsidence to the north, and wind shear out of the northeast. These conditions have all been lined out to be inhibitors of Don's intensification, and should keep him in check as me moves across the gulf. He needs to stop weakening though, or his surface center may get pulled up farther north than the current forecast track.
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Quoting belizeit:
The pressure has not risen in Don yet this morning its just the dropsond found the pressure at 1005 the planes radar still has it at 1002


The instrument that was actually at the surface is more accurate then the radar that's 5,000 feet up.
Member Since: September 6, 2007 Posts: 0 Comments: 6481
401. yoboi
Quoting floridaboy14:
Fish Storm? Just because models show a trough doesnt mean its going to be a fish storm.. euro showed don it like 3 runs recurving and now its hitting texas. stop saying fish storm unless the storm actually forms... Now can someone tell me if don is weakining or stregnthininh


don is not hitting texas yet might fall apart
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persistant system for the past wk. it will come back quick its going to take land to get rid of this one
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Quoting Patrap:
System looking like that will not bring any rain to anyone IMO. Don may not make it anywhere the way he is looking.
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Member Since: September 2, 2007 Posts: 178 Comments: 20439
Quoting NotCircumventing:
Maybe a name change to TS Done?

Don ain't done yet, Nice loop here to check out.
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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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