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 TropicalAnalystwx13:


10-20 knots of northerly shear. It should weaken somewhat as it heads NW.

Weaken to....?
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Quoting JrWeathermanFL:

I just got back to my house. I didnt know shear was affecting it. How strong is it?


10-20 knots of northerly shear. It should weaken somewhat as it heads NW.
Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting NotCircumventing:
Will have to look at this same map in a few minutes for the 1500 update ... but I think the ridge is eroding just a hair in the NW gulf, enough to allow Don to steer more on a true NW heading of 315 (or so), with ultimate landfall near Matagorda, TX (or so)



Been looking at that this morning, and I just don't see the westward push. Step backward in time on those charts and you see that the trough over the AZ area was apparently weaker. Now it looks a little stronger, and the western edge of the ridge looks to be eroding slightly.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
240. jpsb
Quoting NotCircumventing:
Don already bearing NW -- will have to see to what degree -- but that steering high will need to bulk up some to push Don back to a wnw heading I think.

I just don't think Don will get much more west of true NW.
In the old blog Ritaevac post # 1595 posted a nice graphic of Don being pushed more north. Wish I knew how to get to the old blog so I could repost that.
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I am going to venture to say that there may be more Nward tweaks of the track as time goes on. Winds out ahead of Don are heading almost straight Nward.
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GHCC Loop 8 images per hour started an hour ago. There will be a gap from 1045-1115, GOES had to let the maid in (daily housekeeping).
Member Since: September 23, 2005 Posts: 14 Comments: 11053
237. Jax82
Don must have ate too much mexican food last night.
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Fingers crossed that Don will bring much needed rain for our neighbors in TX without the destruction that they don't need.
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Quoting TropicalAnalystwx13:


No, he is being sheared.

I just got back to my house. I didnt know shear was affecting it. How strong is it?
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233. SLU
While Don continues to steal the headlines, let's not forget to spare a thought for this little "playette" in the Central Atlantic.

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Quoting hydrus:
Link if you have time please..


Link
Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting NotCircumventing:
179 ..

Sure like that product, never have seen that.



I still have the link to the original SFWMD site. The new public site does not have this product available. I will be very sad if they ever pull my plug.
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12 hr forcast



24 hr forcast



48 hr forcast

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Quoting JrWeathermanFL:
Is Don wrapping convection around his COC? It looks like it.


No, he is being sheared.
Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting NotCircumventing:
6Z GFS +48

Link if you have time please..
Member Since: September 27, 2007 Posts: 1 Comments: 20935
Is Don wrapping convection around his COC? It looks like it.
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Quoting floodzonenc:
Dry County... we're swimmin' in the sand
Prayin for some holy water
To wash these sins from off our hands...

One of my favorite Bon Jovi songs...  hope TX gets a much-needed soaker from Don. 

What are the prospects (factors) of Don's speed decreasing around the time of landfall or afterward?  The speed could be an important factor in its precipitation potential.  TIA.


The Classic Rock station in Houston apparently has someone with an odd sense of humour. Over the last few months, they have been playing Stevie Ray Vaughn's Texas Flood quite a bit.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Not looking so good Don...

Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting aburttschell:


Had to go do an appraisal on the Bolivar after Ike and it was sad to see all of the destruction. We go to Galveston at least once a year and each time we go back it gets better and better. There are still buildings left to ruin but more and more are being demolished for new homes or commercial sites. I'll have to remember to takes some pictures next time I go. And I'll see if I still have those originals from 2008 too.


Thanks....I was part of the relief effort for that storm based out of Seguin, TX
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Quoting wayfaringstranger:


Id love to see some pictures of Galveston since 2008. Let me know if you have any...


Our favorite place to go to the beach on Galveston was Galveston Island State Park. Pictures of Ike's destruction are here: Link

Pictures of recovery are here: Link

There's more things there as well.

GISP is about 5 miles west of the end of the Seawall, and about 25-30 miles WEST of where Ike made landfall. In others words, you are looking at the "weak" side of the storm.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
THE INITIAL MOTION IS NOW 305/12. DON IS TO THE SOUTH AND SOUTHWEST
OF A DEEP-LAYER RIDGE OVER THE SOUTHEASTERN UNITED STATES...AND THE
LARGE-SCALE MODELS FORECAST THIS FEATURE TO PERSIST FOR THE NEXT
36-48 HR. THIS SHOULD STEER DON GENERALLY NORTHWESTWARD TO WEST-
NORTHWESTWARD UNTIL LANDFALL ON THE TEXAS COAST BETWEEN 36-48 HR.
WHILE THE TRACK GUIDANCE AGREES ON THIS SCENARIO...THERE HAS BEEN A
SIGNIFICANT NORTHWARD SHIFT IN THE GUIDANCE SINCE THE PREVIOUS
ADVISORY.
..WITH THE CENTER OF THE GUIDANCE ENVELOPE NOW NORTH OF
CORPUS CHRISTI. THE NEW TRACK FORECAST WILL ALSO SHIFT NORTHWARD
BUT STAYS TO THE SOUTH OF THE CENTER OF THE NEW GUIDANCE ENVELOPE.
IT SHOULD BE NOTED THAT THE ECMWF AND UKMET FORECAST TRACKS ARE ON
THE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE TO THE SOUTH OF THE
CURRENT FORECAST TRACK.

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Sheared again...
Member Since: September 27, 2007 Posts: 1 Comments: 20935
THE INITIAL MOTION IS NOW 305/12. DON IS TO THE SOUTH AND SOUTHWEST
OF A DEEP-LAYER RIDGE OVER THE SOUTHEASTERN UNITED STATES...AND THE
LARGE-SCALE MODELS FORECAST THIS FEATURE TO PERSIST FOR THE NEXT
36-48 HR. THIS SHOULD STEER DON GENERALLY NORTHWESTWARD TO WEST-
NORTHWESTWARD UNTIL LANDFALL ON THE TEXAS COAST BETWEEN 36-48 HR.
WHILE THE TRACK GUIDANCE AGREES ON THIS SCENARIO...THERE HAS BEEN A
SIGNIFICANT NORTHWARD SHIFT IN THE GUIDANCE SINCE THE PREVIOUS
ADVISORY...WITH THE CENTER OF THE GUIDANCE ENVELOPE NOW NORTH OF
CORPUS CHRISTI. THE NEW TRACK FORECAST WILL ALSO SHIFT NORTHWARD
BUT STAYS TO THE SOUTH OF THE CENTER OF THE NEW GUIDANCE ENVELOPE.
IT SHOULD BE NOTED THAT THE ECMWF AND UKMET FORECAST TRACKS ARE ON
THE SOUTHERN EDGE OF THE GUIDANCE ENVELOPE TO THE SOUTH OF THE
CURRENT FORECAST TRACK.

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Quoting MrstormX:
I'm going to make a bold statement, I doubt this will ever be a hurricane. The tilt of Don is preventing any true strengthening, and the convection is not at all constant.


Yeah, the chances of this becoming a hurricane are very low at this time. Of course...things change.
Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting lovemamatus:
Don will hit Houston as a Cat 1, almost a 2.


I dont see how it will almost be a Cat.2.
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Ok, I had to do a blog thing so I could do the ignore thing, and it changed my "member since" and comments, why?
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Quoting wayfaringstranger:


Id love to see some pictures of Galveston since 2008. Let me know if you have any...


Had to go do an appraisal on the Bolivar after Ike and it was sad to see all of the destruction. We go to Galveston at least once a year and each time we go back it gets better and better. There are still buildings left to ruin but more and more are being demolished for new homes or commercial sites. I'll have to remember to takes some pictures next time I go. And I'll see if I still have those originals from 2008 too.
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Quoting cctxshirl:

In Rockport gas went up 10 cents a gallon overnight-hmmm! But other than that, nobody's getting too excited.
Gas has not gone up here yet-but would not be surprised to see it go up. I am just a little north of you, and am hoping we get some rain here.
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I'm going to make a bold statement, I doubt this will ever be a hurricane. The tilt of Don is preventing any true strengthening, and the convection is not at all constant.
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Quoting BrockBerlin:


My friend at NWS Corpus said the NHC said it was moving at 305 degrees, which is kind of a gray area between NW and WNW so it is not quite at a true 315 degrees (NW)


You could have just looked at the NHC advisory :P

PRESENT MOVEMENT...NW OR 305 DEGREES AT 14 MPH...22 KM/H
Member Since: July 6, 2010 Posts: 113 Comments: 31873
Quoting MrstormX:


WRF, doesn't that stand for Wretchedly wRong Forecast?

Snark Flag: ON
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Member Since: September 27, 2007 Posts: 1 Comments: 20935
Quoting roxycc:
well, there are no preperation being made in Corpus, it is business as usual.

In Rockport gas went up 10 cents a gallon overnight-hmmm! But other than that, nobody's getting too excited.
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Not a Met and been lurking this last week. Is the high supposed to weaken and move off to let Don go more northward? Been hearing off and on that it would and then again yesterday got the impression it was stronger than thought. Now weaker again. I'm one of those SWLA peeps and would like a little more rain (wishcasting) if it came closer to Houston/Port Arthur area - NE quadrant wisher.
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197. BDAwx
"Did you know that...
On this date in 1609, a hurricane tracking near the Bahama Islands crippled The Sea Venture, forcing the people aboard to take shelter on the islands. The captain initially named the islands Somers Islands after himself. Most of the colonists moved to Virgina the next year, but the island still celebrates Somers Day on this date."

That's how Bermuda was founded!!! :D
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Member Since: September 27, 2007 Posts: 1 Comments: 20935

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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.