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

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2005 Hurricanes Atlantic
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Hurricanes Western Pacific 2011
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Tropical Storms Atlantic 2005
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Extratropical Storms Western Pacific 1988

I'll have a new post Friday morning.

Jeff Masters

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Quoting cchsweatherman:


Notice all the stratocumulus clouds surrounding the entire wave to the north. Very indicative of a stable, dry environment.


That remains the only inhibiting factor of development imo, which is why I do not believe it will develop until just before it reaches the islands.
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Im still for a Rockport landfall just north of CC
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same stregnth lookin better though faster forward motion.
Final stregnth at landfall is:
a: 40-45mph
b:50-55mph
c:60-65mph
d:70mph- or a cat 1 hurricane
im going with C
Member Since: July 25, 2011 Posts: 0 Comments: 1102
167. HCW
Member Since: August 10, 2002 Posts: 0 Comments: 1346
Quoting StormJunkie:
Hey tigger :)

And Keith, that'd be a K for Krystal...And no thank-you! I have swore those things off for the rest of my life thanks to this crew...lol


Krystal? Blech. Five Guys. Or Smashburger. Or Moo Yah.
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Quoting angiest:


To be honest, I have yet to see the COC myself. Pray tell, where is it? :/


91.3W, 24.9N, On the edge of the convection on the northern side.
Member Since: July 6, 2010 Posts: 108 Comments: 30237
Boring.I hope Texas get's their rain without any concequences.Where are some of the other fun forecasters that I know of/talk to on here?
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Don is looking better and hopefully will continue to for the sake of us in Texas
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Hey tigger :)

And Keith, that'd be a K for Krystal...And no thank-you! I have swore those things off for the rest of my life thanks to this crew...lol
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Quoting StormJunkie:
Hey all ;)

I see Levi's here and some of the other usual suspects, so I know the fort is in good hands. Busy with school, Michael, and life...But I'll be around more when the season heats up.
stop buy anytime SJ...I'll buy you a Crystal burger..or 2.
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Quoting StormJunkie:
Hey all ;)

I see Levi's here and some of the other usual suspects, so I know the fort is in good hands. Busy with school, Michael, and life...But I'll be around more when the season heats up.


woo hoo...JUNKIE!!! long time no see
Member Since: September 16, 2007 Posts: 0 Comments: 3599


Floater - Rainbow Color Infrared Loop
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Hey all ;)

I see Levi's here and some of the other usual suspects, so I know the fort is in good hands. Busy with school, Michael, and life...But I'll be around more when the season heats up.
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Quoting jasonweatherman2011:
maybe a invest very soon


Notice all the stratocumulus clouds surrounding the entire wave to the north. Very indicative of a stable, dry environment.
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Quoting duajones78413:
What are the chances of Don pulling it back together tonight?


Don will probably stay in his current state until about 12-18 hours before landfall. He desperately needs to get away from the northerly shearing winds. I think once he gets closer to the Texas coastline, the shear looks to relax some and he will try to strengthen some as most tropical cyclones do when making landfall in Texas. The shape of the Texas coastline tends to do that, just like the shape of the Bay of Campeche. The limiting factor for intensification before landfall might be the arid-like airmass that is entrenched over Texas.

Edit: After looking at water vapor, the airmass doesn't appear to be terribly dry. However, I'd bet if we looked at soundings from different layers, they would be drier towards the mid and lower levels.
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TROPICAL STORM DON DISCUSSION NUMBER 5
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011
400 PM CDT THU JUL 28 2011

DON HAS A CLASSIC SHEAR PATTERN IN SATELLITE IMAGERY THIS AFTERNOON
WITH STRONG BURSTS OF CONVECTION OCCURRING NEAR AND SOUTH OF THE
CENTER. ANALYSES FROM THE SHIPS MODEL AND FROM CIMSS AT THE
UNIVERSITY OF WISCONSIN INDICATE 10-15 KT OF NORTHERLY VERTICAL
WIND SHEAR CONTINUING OVER THE STORM. IN ADDITION...A STREAM OF
ARC CLOUDS/GUSTS FRONTS ON THE EASTERN SIDE OF THE CIRCULATION
SUGGESTS THAT DRY AIR IS BEING ENTRAINED INTO THE CONVECTION. DATA
FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT SHOWED THE
CENTRAL PRESSURE RISING TO 1005 MB...WHILE THE AIRCRAFT WINDS
SUPPORTED AN INITIAL INTENSITY OF 40 KT.

THE INITIAL MOTION IS NOW 305/14. DON REMAINS 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 IN ABOUT 36 HR. THE TRACK GUIDANCE...WHICH SHIFTED NORTHWARD
BETWEEN 06Z AND 12Z...HAS SHIFTED SOUTHWARD FOR THIS ADVISORY. THE
NEW FORECAST TRACK HAS BEEN ONLY SLIGHTLY ADJUSTED TO THE SOUTH AND
LIES ON THE RIGHT SIDE OF THE GUIDANCE ENVELOPE. SOME ADJUSTMENT
TO THE FORECAST TRACK MAY BE NECESSARY IF THE TRACK GUIDANCE
STABILIZES AROUND A TRACK SOUTH OF THE CURRENT FORECAST.

THE LARGE-SCALE MODELS AGAIN FORECAST MODERATE SHEAR TO CONTINUE
UNTIL DON MAKES LANDFALL. THIS AND CONTINUED INTERACTION WITH DRY
AIR OVER THE WESTERN GULF OF MEXICO IS LIKELY TO IMPEDE
INTENSIFICATION...AND THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT
THAT DON WILL MAKE LANDFALL AS A TROPICAL STORM AND NOT AS A
HURRICANE. THE NEW INTENSITY FORECAST CALLS FOR A LITTLE LESS
STRENGTHENING DURING THE FIRST 12-24 HR DUE TO THE PRESENCE OF
SHEAR AND ARC CLOUDS...THEN CALLS FOR DON TO REACH A PEAK INTENSITY
OF 50 KT NEAR LANDFALL. AFTER LANDFALL...THE CYCLONE SHOULD WEAKEN
TO A TROPICAL DEPRESSION BY THE 48 HR POINT AND TO A REMNANT LOW BY
THE 72 HR POINT.

FORECAST POSITIONS AND MAX WINDS

INIT 28/2100Z 24.9N 91.3W 40 KT 45 MPH
12H 29/0600Z 25.8N 93.1W 40 KT 45 MPH
24H 29/1800Z 26.8N 95.2W 45 KT 50 MPH
36H 30/0600Z 27.8N 97.5W 50 KT 60 MPH...INLAND
48H 30/1800Z 28.8N 100.0W 30 KT 35 MPH...INLAND
72H 31/1800Z 30.5N 103.5W 20 KT 25 MPH...POST-TROP/REMNT LOW
96H 01/1800Z...DISSIPATED

$$
FORECASTER BEVEN
Member Since: July 6, 2010 Posts: 108 Comments: 30237
Quoting TropicalAnalystwx13:
Don looking better..





To be honest, I have yet to see the COC myself. Pray tell, where is it? :/
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Shear will be with Don until landfall and no Hurricane Don

THE LARGE-SCALE MODELS AGAIN FORECAST MODERATE SHEAR TO CONTINUE
UNTIL DON MAKES LANDFALL
. THIS AND CONTINUED INTERACTION WITH DRY
AIR OVER THE WESTERN GULF OF MEXICO IS LIKELY TO IMPEDE
INTENSIFICATION...AND THE INTENSITY GUIDANCE IS IN GOOD AGREEMENT
THAT DON WILL MAKE LANDFALL AS A TROPICAL STORM AND NOT AS A
HURRICANE.
THE NEW INTENSITY FORECAST CALLS FOR A LITTLE LESS
STRENGTHENING DURING THE FIRST 12-24 HR DUE TO THE PRESENCE OF
SHEAR AND ARC CLOUDS...THEN CALLS FOR DON TO REACH A PEAK INTENSITY
OF 50 KT NEAR LANDFALL. AFTER LANDFALL...THE CYCLONE SHOULD WEAKEN
TO A TROPICAL DEPRESSION BY THE 48 HR POINT AND TO A REMNANT LOW BY
THE 72 HR POINT.
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Quoting duajones78413:
What are the chances of Don pulling it back together tonight?


I give it a pretty good chance i just don't see things continuing to be hostile for don and im sure when he gets that first break he'll take to it and start, but everythings changes so you gotta keep a good eye on things, but for your question i give a high chance of developement.
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I think Don is starting to take advantage of that small window of opportunity to get it together. The next 12-36 hrs will tell the tale. Looks to me Don just got out of the worst shear. We shall see!
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Don looking better..



Member Since: July 6, 2010 Posts: 108 Comments: 30237
149. Daveg
Question... the models seem to be bouncing back and forth from more northward to more southward.

What would be causing them to do that, and it is likely that the NHC track will tend to stay more where it is unless the tracks stop that back and forth?
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Quoting Floodman:


You know, I hear Dawn is good for getting ULAC off of things
birds in particular :)
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We have dry areas of Texas like the Southeast but then we have dry areas in Central and West Texas that have also had 60 to 75 days of 100 degree heat for some areas, that is the area I am praying for.
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Quoting Floodman:


You know, I hear Dawn is good for getting ULAC off of things


woo hoo....FLOOD!
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What are the chances of Don pulling it back together tonight?
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Quoting CybrTeddy:
The African wave has a ULAC over it.


You know, I hear Dawn is good for getting ULAC off of things
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Quoting hcubed:


I'll go with option "!"


If you think the polls are wasting space, you are doing the exact same thing when announcing that you are reporting him. Not much difference, IMO.
Member Since: July 6, 2010 Posts: 108 Comments: 30237
Quoting Levi32:


You can't get that specific that far out yet. Step one is the track across the central Atlantic and its eventual latitude gain. My idea for step 1 right now is that this will hit the eastern Caribbean instead of moving north of Puerto Rico. After that it may try to lift north of the islands as a trough is forecasted to pass by to the north. How strong that trough will be can't be known yet. Remember that Don was forecasted to recurve into a similar trough, but it wasn't strong enough to do so because of the pattern. I have not done an in-depth analysis of the pattern ahead of the wave yet, as Don has been my primary focus. After Don makes landfall, I will fully switch gears to the next system in line.
Haven't looked at the models much today to see what they're hinting at now, but that's exactly what I've been thinking for the last few days now. Based off model runs from the last two days.
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Large moisture envelope associated with it:

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139. j2008
Quoting TropicalAnalystwx13:
RAMSDIS has a floater up on it:


Definatly looks like something to keep our eyes on.
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I gotta get back to work. Back later.
Member Since: November 24, 2005 Posts: 628 Comments: 26452
Quoting MississippiWx:


If you look at the 8-10 day 500mb anomalies from the Euro/GFS, it looks like a continuation of the current pattern. However, the ridge seems weaker.


Well, if you watch that product as much as I do, you will notice that the central U.S. ridge quite often does look weaker in the 12-15 day period, due to member variance. Usually, the pattern never ends up that close to "normal" with no anomalies.
Member Since: November 24, 2005 Posts: 628 Comments: 26452
Quoting TropicalAnalystwx13:
RAMSDIS has a floater up on it:


if it werent for the dry air it would be trying to become a td due to warm waters and weak shear and its good cyclone turning
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Quoting floridaboy14:
lets have a little vote for fun. will the central atlantic wave a: recurve b: similar track like don into the carribean or c: affect the carribean and SE US coast. im going with D i dont know :)


I'll go with option "!"
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Quoting angiest:


Almost looks like he recently exhaled a bunch of that dry air.
ockquote> Exhaled,well maybe is dead.
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RAMSDIS has a floater up on it:

Member Since: July 6, 2010 Posts: 108 Comments: 30237
Quoting TropicalAnalystwx13:


Nope. One of the products is wrong, but considering how the wave looks, I'm guessing that this image is wrong.

wind shear values are determined off the upper level wind product. When looking for a ulac, look at the upper level winds first. Or the 200mb vort map. Shear map should be the last thing you look at to determine upper level features as it doesn't depict upper level streamlines...it shows shear streamlines.
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Quoting floridaboy14:

i know you metioned in your video that a trough might come down and recurve that wave but if the trough missed it or never even came off the east coast, based on steering flow where would it be steered? like a don track maybe a little north?


You can't get that specific that far out yet. Step one is the track across the central Atlantic and its eventual latitude gain. My idea for step 1 right now is that this will hit the eastern Caribbean instead of moving north of Puerto Rico. After that it may try to lift north of the islands as a trough is forecasted to pass by to the north. How strong that trough will be can't be known yet. Remember that Don was forecasted to recurve into a similar trough, but it wasn't strong enough to do so because of the pattern. I have not done an in-depth analysis of the pattern ahead of the wave yet, as Don has been my primary focus. After Don makes landfall, I will fully switch gears to the next system in line.
Member Since: November 24, 2005 Posts: 628 Comments: 26452
Quoting Jedkins01:
Don isn't very impressive, and I don't think it will become much more impressive either. Sadly Don will probably be one of the lowest rain producing tropical cyclones possible, which isn't great news for Texas. Its small, fast moving, and lacks a large area of deep convective bands that most tropical cyclones have. Its moisture field isn't as impressive as most tropical cyclones either.

There will be some very heavy rain, but it will not be as widespread and will not last long enough to give much widespread accumulation.


That's really unfortunate. Hopefully we will see a quick spin-up, slow-moving, weak tropical storm that gives Texas some beneficial rainfall. Not too much, but not to low either.
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Quoting Levi32:


I don't have access to the ECMWF weeklies out to 4 weeks, which is the only product I know of that does forecasts in that time-frame, so I don't know. All I can really offer is the analog package that argues for what "should" happen if the course of events follows historical trends. Whether it actually does is pretty hard to say. Last year we had the same kind of analog pattern, but 2010 did not cooperate with that.


If you look at the 8-10 day 500mb anomalies from the Euro/GFS, it looks like a continuation of the current pattern. However, the ridge seems weaker.
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127. yoboi
Quoting cchsweatherman:
Here is exactly why I don't believe the tropical wave in the Central Atlantic will develop in the near future and may never develop:



1) Massive expanse of dry air and deep subsidence due to unfavorable MJO cycle (downward motion through atmosphere) and deep Saharan air exists all to the north and west of the tropical wave. This is some of the driest and most stable air to date over the Central Atlantic.

2) Well established TUTT over the Central Atlantic stretching into the Eastern Caribbean is creating hostile wind shear 10N to the North Atlantic essentially walling off the entire area. With no real signs of this easing up, upper level winds will become increasingly more unfavorable.


repeat of last yr season is a bust.........
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Quoting Levi32:


I don't have access to the ECMWF weeklies out to 4 weeks, which is the only product I know of that does forecasts in that time-frame, so I don't know. All I can really offer is the analog package that argues for what "should" happen if the course of events follows historical trends. Whether it actually does is pretty hard to say. Last year we had the same kind of analog pattern, but 2010 did not cooperate with that.

Levi the ensemble members are forecasting a very negative nao for the early half of august does that mean the persisntet east coast trough of last year will return?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Don isn't very impressive, and I don't think it will become much more impressive either. Sadly Don will probably be one of the lowest rain producing tropical cyclones possible, which isn't great news for Texas. Its small, fast moving, and lacks a large area of deep convective bands that most tropical cyclones have. Its moisture field isn't as impressive as most tropical cyclones either.

There will be some very heavy rain, but it will not be as widespread and will not last long enough to give much widespread accumulation.
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Quoting MississippiWx:


Do the 500mb anomalies still show the ridge retreating north into August?


I don't have access to the ECMWF weeklies out to 4 weeks, which is the only product I know of that does forecasts in that time-frame, so I don't know. All I can really offer is the analog package that argues for what "should" happen if the course of events follows historical trends. Whether it actually does is pretty hard to say. Last year we had the same kind of analog pattern, but 2010 did not cooperate with that.
Member Since: November 24, 2005 Posts: 628 Comments: 26452
Quoting cchsweatherman:


Interesting because the upper level winds per CIMSS does not support a true upper level anitcyclone over the wave.
just about to post that.

Shear map shows shear direction, not upper level winds. Usually shear direction follows the upper level winds. However, when you get light winds aloft and a broad surface circulation below it can give you the false impression of an anticyclone aloft
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Quoting Levi32:
TUTTs move and evolve a whole lot during the course of 10 days. Its current position and form is only that - current. Models have upper winds looking better later on once the wave nears the Caribbean, as the renewed upward motion over the central Atlantic will warm temperatures aloft a bit and cause the TUTT to shrink.
So Emily could be back for revenge?
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8-10 day 500mb Anomalies show mainly zonal flow across the US:

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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.