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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2421. HCW
Let's see how many images I can get in this radar loop that is being recorded over at FLhurricane

Link
Member Since: August 10, 2002 Posts: 0 Comments: 1408
Quoting hurricaneben:
Should I be concerned about 91L in South Florida or are there no chances of making it this way.



YOU NEED TO WATCH THIS VERY CLOSE.......read what i just posted......read back!
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Chart says TD#5 in a few days:

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Quoting TampaSpin:
The Timing of this is SCARY as heck.......this reminds me of the "A" storm with timing........the high breaks down just to pull it North then the HIgh builds back in.........I DON"T LIKE THE LOOKS OF THIS AT ALL!!!!!


Link?
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Should I be concerned about 91L in South Florida or are there no chances of making it this way.
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2415. scott39
Levi, Is it safe to say the weaker 91L stays...the futher W it will go? Do you see it ramping up as fast as some modules?
Member Since: June 13, 2009 Posts: 0 Comments: 6755
2414. barbamz
Quoting Levi32:
Lowest surface pressure was not found at the center of flight-level winds, but rather north of it. This means that Don's circulation is still vertically tilted southward.



So Levi, the EURO has been right?
From the NHC-Discussion:
HIGH PRESSURE RIDGING OVER THE SOUTHERN UNITED STATES WILL PROBABLY STEER THE
CYCLONE ALONG THAT GENERAL COURSE UNTIL LANDFALL...PERHAPS EVEN A
LITTLE MORE TO THE LEFT AS THE RIDGE BUILDS. THE GUIDANCE IS IN
GOOD AGREEMENT...ALTHOUGH THE USUALLY RELIABLE ECMWF MODEL IS
FARTHER TO THE NORTHEAST. AN EXAMINATION OF THE ECMWF FIELDS
INDICATES THAT THE MODEL DECOUPLES THE LOW- AND MID-LEVEL
CENTERS...ALLOWING DON TO MOVE MORE TO THE NORTHWEST. THIS DOES
NOT SEEM LIKELY
AS ENVIRONMENTAL CONDITIONS ARE NOT EXPECTED TO
CHANGE SIGNIFICANTLY FROM NOW UNTIL LANDFALL. THUS THE NHC
FORECAST WILL STAY CLOSE TO THE DYNAMICAL MODEL CONSENSUS AND THE
PREVIOUS FORECAST.

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Are HH flying into 91L?
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2411. Levi32
Movement will continue to jump around before landfall, as it has all night. The reason for this is that whenever a convective burst fires near the center, it is pulled nearly underneath and turns more westward. Once each burst weakens, the surface center is more susceptible to being pulled out to the NW by the low-level flow. These stepped increments of movement have been evident for the last 12 hours or so.
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Quoting usa777:


You don't happen to remember the street that was on do you? I was one of the idiots that didn't evac.


just ck'd internet...surge for hugo averaged 20 ft (there is a pole with a marker in awendaw with a line where the surge came to)...that would put it at the 3rd story of a home as well... and the school that was an evac shelter was 10 ft above sea level, the error was in a transcript that was sent out saying it had a 20ft elevation...
Member Since: September 16, 2007 Posts: 0 Comments: 3650
Quoting jasonweatherman2011:
look at my big high back up to the east coast!!

jason what model is that and is it recurving future emily?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
good chance 91 smashes into hispanola end of story bad enough
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Quoting Levi32:


It needs to show clear separation from the ITCZ and become more well-defined. The NHC doesn't like classifying broad circulations, closed or not.


I understand now. Thank you very much.
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2403. Levi32
Recon-based 2-hour motion is 320 degrees.
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2402. usa777
Quoting IFuSAYso:


I did home inspection in the Bay St Louis, MS area after Katrina. I saw a 30+ foot sailboat on top of a 2 story home and the high water mark in a home I inspected was in the 3rd floor of the 3 story residence.


You don't happen to remember the street that was on do you? I was one of the idiots that didn't evac.
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2401. Levi32
Quoting HimacaneBrees:

From Levi's tidbit
"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?


It needs to show clear separation from the ITCZ and become more well-defined. The NHC doesn't like classifying broad circulations, closed or not.
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2400. Levi32
Lowest surface pressure was not found at the center of flight-level winds, but rather north of it. This means that Don's circulation is still vertically tilted southward.
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2399. Gearsts
Quoting HimacaneBrees:

From Levi's tidbit
"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?
Needs to look better i think, and Pre Don had alot of shear close to the islands.
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Quoting Levi32:
Don has been pulsing since last night - 3-4mb swings up and down. Recon extrapolations in each leg will also differ from dropsondes almost every time. Wait for the new vortex message.

Levi i have to give you credit but i think you nailed don on intensity and track :)
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting CanesfanatUT:
When do I get to say Hebert's Box?? Too soon?
when it becomes a major storm.
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From Levi's tidbit
"The wave has a closed low, though broad, and in proximity to dry air to the north and west. The wave looks a lot better-defined than pre-Don did over in that area, and this system may have even better conditions ahead of it west of 55W than Don had."

If it has a closed low then should it be called a depression? Or is there more that needs to happen for it to be classed as a depression? Wind speed, pressure, etc.?
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Unless DON can go north, there will be hardly any rain for Texas. So is it going to hit Texas?
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2391. Levi32
Don has been pulsing since last night - 3-4mb swings up and down. Recon extrapolations in each leg will also differ from dropsondes almost every time. Wait for the new vortex message.
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Quoting MrstormX:
Pressure back down to 998-9

Texas coast is a good coastline for it to ramp up a little :) id say a 50-60mph tropical storm at landfall
Member Since: July 25, 2011 Posts: 0 Comments: 1102
NEW BLOG ALMOST COMING
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2386. scott39
Dry air ahead of 91L and a broad Low will keep it tame until the Caribbean. Some modules are bullish.
Member Since: June 13, 2009 Posts: 0 Comments: 6755
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2384. Jax82
It appears that the African Wave Train will be running at full speed soon. The season is going to ramp up quickly, strap in your seats and fasten your seat belts!
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Quoting FLWeatherFreak91:
Keep one thing in mind with systems forecasted to take a track over the antilles then get lifted north by a trof: If the system experiences significant weakening over the islands, many times the system will become weak enough to slip under the trof against model consensus. We saw this with a couple of storms in 2004.

Yes, they have the tendency to regenerated after the islands. Good point.
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Pressure back down to 998-9
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000
URNT15 KNHC 291353
AF303 0504A DON HDOB 25 20110729
134400 2609N 09438W 8436 01515 9998 +255 +092 066019 020 018 001 03
134430 2610N 09439W 8426 01527 0007 +241 +100 052017 018 008 001 00
134500 2611N 09441W 8429 01525 0002 +251 +104 045018 018 018 000 00
134530 2612N 09442W 8428 01528 0001 +254 +106 051019 020 020 000 00
134600 2614N 09443W 8431 01527 0008 +248 +107 048020 021 022 000 00
134630 2615N 09445W 8428 01533 0014 +245 +106 042021 022 024 000 00
134700 2616N 09446W 8428 01537 0023 +234 +103 035021 022 025 000 00
134730 2617N 09447W 8431 01535 0031 +225 +099 034022 022 025 002 00
134800 2618N 09449W 8425 01543 0036 +220 +094 034022 022 023 001 03
134830 2619N 09450W 8430 01539 0039 +217 +089 031021 021 021 001 03
134900 2620N 09451W 8432 01539 0043 +213 +084 029021 021 023 000 03
134930 2622N 09452W 8428 01546 0048 +207 +080 026020 020 022 001 00
135000 2623N 09454W 8432 01543 0052 +205 +077 030020 021 022 001 00
135030 2624N 09455W 8425 01553 0058 +201 +074 036019 020 024 000 00
135100 2625N 09456W 8431 01548 0056 +206 +071 042017 018 026 001 00
135130 2626N 09458W 8429 01549 0054 +210 +069 050018 018 024 001 00
135200 2627N 09459W 8430 01549 0057 +207 +068 055018 019 024 001 00
135230 2628N 09500W 8430 01552 0061 +205 +067 062019 019 024 000 00
135300 2630N 09502W 8430 01553 0064 +205 +066 065020 021 024 001 00
135330 2631N 09503W 8430 01553 0065 +200 +065 059021 022 024 001 00
$$
;
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Quoting tiggeriffic:


my 3rd cup was b4 7am est lol...work 6am-6pm today...yay me but agree...set up NOT GOOD! They usually don't fly into storms that far away...wonder if it will spin up enuf visually for them to classify it b4 Monday...5 named storms by end of July would be absolutely CRAZY


Yea I work from 12pm to 8:30pm so I get to see whats happening.... Although I hate being on call because I would be in TX right now waiting on a Storm :o)

Taco :o)
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2379. scott39
I wouldnt look for the trof to recurve 91L. It will flatten out first. This is going to be another GOM TC. IMO
Member Since: June 13, 2009 Posts: 0 Comments: 6755
Don's pressure sways back and forth every hour
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Quoting IFuSAYso:


I did home inspection in the Bay St Louis, MS area after Katrina. I saw a 30+ foot sailboat on top of a 2 story home and the high water mark in a home I inspected was in the 3rd floor of the 3 story residence.


the people that went to a shelter at a school...they were in the gymnasium for hugo...water came in...they had to crawl up into the ceiling of the gymnasium and this was an evac shelter...i saw things during/after Hugo i hope i NEVER see again...not home to get my book out with all the data from Hugo...will see if i can get it later
Member Since: September 16, 2007 Posts: 0 Comments: 3650
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2374. jpsb
Quoting Vincent4989:
Don looks like it's gaining an eye-like feature.
Wow, keep going North Don, you can do it!
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The Timing of this is SCARY as heck.......this reminds me of the "A" storm with timing........the high breaks down just to pull it North then the HIgh builds back in.........I DON"T LIKE THE LOOKS OF THIS AT ALL!!!!!


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2304. TampaSpin 1:19 PM GMT on July 29, 2011 +0
Quoting stillwaiting:
,more than likely our first hurricane of the yr,looks like Emmy could become a major imo


YES........looks like a MAJOR and i BIG MAJOR....this is a giant turning tons of atmosphere.......i mean a bunch too.

The "Emily" storm name has always been major. Perhaps this one will get retired this year.
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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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