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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2170. NASA101
06Z HWRF and GFDL - definitely overdoing it on intensity with Cat-3 forecast near PR



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Are the hurricane hunters finding don stronger?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting floridaboy14:

Actually the canadain recurves 91L due to a VERY STRONG and i quote STRONG trough coming off the east coast which will suck it out of the carribean into the open atlantic. Thats one scenario..


Missed the trof issue (no coffee yet this morning) but yes.....Trof eroding the high is one scenario. Another is a weaker trof, high building back in, and keeping the storm on a track towards the Bahamas.....Time will tell but I don't like the look of this one so far. It's a classic CV feature in early August.
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TROPICAL STORM DON INTERMEDIATE ADVISORY NUMBER 7A
NWS NATIONAL HURRICANE CENTER MIAMI FL AL042011
700 AM CDT FRI JUL 29 2011

...DON CONTINUES MOVING TOWARD SOUTH TEXAS WITH LITTLE CHANGE IN
STRENGTH...


SUMMARY OF 700 AM CDT...1200 UTC...INFORMATION
----------------------------------------------
LOCATION...25.7N 94.3W
ABOUT 255 MI...395 KM SE OF CORPUS CHRISTI TEXAS
ABOUT 195 MI...315 KM E OF BROWNSVILLE TEXAS
MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H
PRESENT MOVEMENT...WNW OR 295 DEGREES AT 14 MPH...22 KM/H
MINIMUM CENTRAL PRESSURE...1000 MB...29.53 INCHES
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2165. Gearsts
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2164. Mixed
Quoting SLU:
...30 PERCENT... is rather conservative given the forecast models.


Lets Just See What Comes at 8 am
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Quoting SLU:
...30 PERCENT... is rather conservative given the forecast models.
Probably go up later on when they see how it is throughout the day.
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Click on image to enlarge
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2161. SLU
...30 PERCENT... is rather conservative given the forecast models.
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TWC just say 8am advisory the same
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting weathermanwannabe:
Time will tell how the models react to 91L. Latest CMC develops the storm, takes it right over Puerto Rico, then around the Bermumda High into the Atlantic. It's all about the "current" position of the A-B for a particular storm.

Link


Actually the canadain recurves 91L due to a VERY STRONG and i quote STRONG trough coming off the east coast which will suck it out of the carribean into the open atlantic. Thats one scenario..
Member Since: July 25, 2011 Posts: 0 Comments: 1102
2158. barbamz
Quoting Chicklit:
You're good Barb to keep track of Don!
I wonder when they'll put a floater on 91L.



LinkNAVYSITE


Hi, Chicklit. In the morning hours I'm of advantage, while the US is still sleeping, lol. I still hope for more rain to dry Texas from Don ... 91L can wait a lil.
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Time will tell how the models react to 91L. Latest CMC develops the storm, takes it right over Puerto Rico, then around the Bermumda High into the Atlantic. It's all about the "current" position of the A-B for a particular storm.

Link

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New advisory on Don come out? Also looks like don is picking up intensity
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Don has finally got his act together with flight level winds higher than surface winds.
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Quoting stormwatcherCI:
The tracks can and will change. 90L was "supposed" to go north of the islands. Whenever something is developing that far south 9 times out of 10 it tracks through the central Caribbean. Maybe a little more north but the Caribbean nonetheless.


I chose my words carefully - using 'currently' and 'at this point'.

The thing is, it is what you say, an invest. It looks a good candidate to form. The preliminary intensity indicators are looking strong. But, it is still an invest.

But, yeah, I'll leave it before I start boring people. :)

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Quoting Gearsts:
Estan poniendo las tormenteras y los paneles! LOL


Buena!!
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gfdl is on occasion right ernesto
Member Since: September 11, 2010 Posts: 1 Comments: 4891
2149. SLU
Quoting stoormfury:
hey SLU and mixed it seems that we might be in for it. only hope the radio stations start sensitising the people of what is to come. here again saint lucians are in party mood this holiday weekend. it was the same for TOMAS
Quoting nrtiwlnvragn:
Ugly 06Z GFDL on 91L....end of run


HOUR: 96.0 LONG: -60.92 LAT: 16.04 MIN PRESS (hPa): 975.01 MAX SURF WIND (KNOTS): 85.33
HOUR:102.0 LONG: -62.11 LAT: 16.48 MIN PRESS (hPa): 969.46 MAX SURF WIND (KNOTS): 96.73
HOUR:108.0 LONG: -63.29 LAT: 16.87 MIN PRESS (hPa): 965.14 MAX SURF WIND (KNOTS): 94.54
HOUR:114.0 LONG: -64.27 LAT: 17.10 MIN PRESS (hPa): 959.18 MAX SURF WIND (KNOTS):103.81
HOUR:120.0 LONG: -65.28 LAT: 17.38 MIN PRESS (hPa): 953.41 MAX SURF WIND (KNOTS):112.22
HOUR:126.0 LONG: -66.26 LAT: 17.78 MIN PRESS (hPa): 957.65 MAX SURF WIND (KNOTS):103.42


This is really starting to look very ugly.
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2148. Gearsts
Quoting nrtiwlnvragn:
Ugly 06Z GFDL on 91L....end of run


HOUR: 96.0 LONG: -60.92 LAT: 16.04 MIN PRESS (hPa): 975.01 MAX SURF WIND (KNOTS): 85.33
HOUR:102.0 LONG: -62.11 LAT: 16.48 MIN PRESS (hPa): 969.46 MAX SURF WIND (KNOTS): 96.73
HOUR:108.0 LONG: -63.29 LAT: 16.87 MIN PRESS (hPa): 965.14 MAX SURF WIND (KNOTS): 94.54
HOUR:114.0 LONG: -64.27 LAT: 17.10 MIN PRESS (hPa): 959.18 MAX SURF WIND (KNOTS):103.81
HOUR:120.0 LONG: -65.28 LAT: 17.38 MIN PRESS (hPa): 953.41 MAX SURF WIND (KNOTS):112.22
HOUR:126.0 LONG: -66.26 LAT: 17.78 MIN PRESS (hPa): 957.65 MAX SURF WIND (KNOTS):103.42
WOW i think a 40% in the next two.
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You're good Barb to keep track of Don!
I wonder when they'll put a floater on 91L.



LinkNAVYSITE
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whoops
Member Since: November 13, 2009 Posts: 2 Comments: 728
Ugly 06Z GFDL on 91L....end of run


HOUR: 96.0 LONG: -60.92 LAT: 16.04 MIN PRESS (hPa): 975.01 MAX SURF WIND (KNOTS): 85.33
HOUR:102.0 LONG: -62.11 LAT: 16.48 MIN PRESS (hPa): 969.46 MAX SURF WIND (KNOTS): 96.73
HOUR:108.0 LONG: -63.29 LAT: 16.87 MIN PRESS (hPa): 965.14 MAX SURF WIND (KNOTS): 94.54
HOUR:114.0 LONG: -64.27 LAT: 17.10 MIN PRESS (hPa): 959.18 MAX SURF WIND (KNOTS):103.81
HOUR:120.0 LONG: -65.28 LAT: 17.38 MIN PRESS (hPa): 953.41 MAX SURF WIND (KNOTS):112.22
HOUR:126.0 LONG: -66.26 LAT: 17.78 MIN PRESS (hPa): 957.65 MAX SURF WIND (KNOTS):103.42
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Any radar velocity image from brownsville other than this site? hard to find rotation from Don.
Member Since: November 13, 2009 Posts: 2 Comments: 728
Quoting barbamz:


Pressure was gradually dropping during the last HH flight to 1000mb. Now new Recon is on it, will reach the center soon.

ok so don is stregnthining.. thanks
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting Cotillion:


Perhaps, but I do raise an eyebrow when anyone jumps onto the Category 5 bandwagon anytime there's a perceived threat. Even now, there's only ever been two before middle of August - Allen and Emily. Neither of those storms match this one currently.

They are rare. Exceptionally rare at this time of year.

It happens every year. It is fine to speculate, but there's a time and a place.

This is not to discount the potential danger; the system is a real potential threat and should be watched like any other. Storms can be plenty dangerous without ever hitting that intensity.

Tirade over.



I hope you feel I'm not jumping on you too much. Just the track doesn't match up, and as said up above, the intensity at this point doesn't either.

Records are made to be broken, sure, but still.
The tracks can and will change. 90L was "supposed" to go north of the islands. Whenever something is developing that far south 9 times out of 10 it tracks through the central Caribbean. Maybe a little more north but the Caribbean nonetheless.
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2141. Gearsts
Quoting palmasdelrio:
Any idea why the National Weather Service in San Juan has no weather discussion today? Does this occur often?
Estan poniendo las tormenteras y los paneles! LOL
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2140. barbamz
Quoting floridaboy14:

yes im not really good at understanding this lol but does it show don pressure dropping or rising ?


Pressure was gradually dropping during the last HH flight to 1000mb. Now new Recon is on it, will reach the center soon.
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Puerto Rico NWS Discussion

ON THE MEDIUM RANGE...A LARGE TROPICAL WAVE ACCOMPANIED BY A
WELL-DEFINED LOW PRESSURE SYSTEM IS LOCATED BETWEEN 39W AND 41W.
THIS DISTURBANCE IS FORECAST TO MOVE WEST NORTHWEST BETWEEN 15 TO
20 KT DURING THE NEXT SEVERAL DAYS. REGARDLESS OF DEVELOPMENT...
THIS ACTIVE WAVE SHOULD APPROACH THE LOCAL REGION LATE MONDAY OR
TUESDAY...INCREASING THE CHANCES FOR ACTIVE WEATHER ACROSS THE
ISLANDS AND REGIONAL WATERS...AS WELL AS THE POTENTIAL FOR HEAVY
RAINFALL AND LOCALIZED FLOODING. THEREFORE...RESIDENTS AND
VISITORS SHOULD MONITOR THIS SYSTEM CLOSELY.
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hey SLU and mixed it seems that we might be in for it. only hope the radio stations start sensitising the people of what is to come. here again saint lucians are in party mood this holiday weekend. it was the same for TOMAS
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Quoting Chicklit:
Dean was more problematic, but it was a Cape Verde storm, which this isn't.
Cotillion

91L is a Cape Verde wave.


Many storms come from AEWs, but this one is currently situated in the middle of the Atlantic. Typically, I don't think you'd normally include these as classic CV storms. They tend to form more east.
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Don't forget that 91L is currently- AH forget it
Member Since: November 13, 2009 Posts: 2 Comments: 728
Quoting barbamz:


I hope it's this one (Vortex Data):

000
URNT12 KNHC 290607
VORTEX DATA MESSAGE AL042011
A. 29/05:35:40Z
B. 24 deg 59 min N
092 deg 56 min W
C. 850 mb 1455 m
D. 26 kt
E. 318 deg 17 nm
F. 081 deg 34 kt
G. 319 deg 13 nm
H. EXTRAP 1000 mb
I. 21 C / 1524 m
J. 27 C / 1527 m
K. 14 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 18
MAX OUTBOUND AND MAX FL WIND 46 KT NE QUAD 05:56:40Z
SLP EXTRAP FROM 850 MB
;


yes im not really good at understanding this lol but does it show don pressure dropping or rising ?
Member Since: July 25, 2011 Posts: 0 Comments: 1102
Quoting palmasdelrio:
Any idea why the National Weather Service in San Juan has no weather discussion today? Does this occur often?

Maybe they're waiting for the 8 AM update or maybe on a conference call.
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Good Morning Friends. If 91L does develop into a TD or Storm over the next 5-10 days (whether before reaching the Antilles and after moving into the Caribbean), I would say that we are off to one of the most active starts of the season that I have seen since 2005. Not suggesting an analog, just making the observation.
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Brownsville Radar
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2130. jpsb
Quoting yonzabam:


Yep. Flat earthers, creationists and global warming deniers are proof of that
Did the ski season end in Washington State yet? I think I read someplace that it was still snowing at higher altitudes there. I've always been curious why it is that when it's hot it's AGW but then it's cold it just weather. Why is that? FYI the last few winters have been very cold, I am expecting more of the same with the Sun in an inactive period. My thinking is maybe that 800,000 mile wide nuclear furnace that we see in our skies everyday might just have something to to with climate. I know, call me crazy but something tells me the Sun has something to do with it.
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2129. barbamz
Quoting floridaboy14:
sorry i know everyone one is talking about that wave (invest 91L) but does anyone have the latest info from the hurricane hunters in terms of pressure and wind speed? thanks


I hope it's this one (Vortex Data):

000
URNT12 KNHC 290607
VORTEX DATA MESSAGE AL042011
A. 29/05:35:40Z
B. 24 deg 59 min N
092 deg 56 min W
C. 850 mb 1455 m
D. 26 kt
E. 318 deg 17 nm
F. 081 deg 34 kt
G. 319 deg 13 nm
H. EXTRAP 1000 mb
I. 21 C / 1524 m
J. 27 C / 1527 m
K. 14 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 18
MAX OUTBOUND AND MAX FL WIND 46 KT NE QUAD 05:56:40Z
SLP EXTRAP FROM 850 MB
;

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Any idea why the National Weather Service in San Juan has no weather discussion today? Does this occur often?
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Quoting Chicklit:
Good point about Felix Cot.
That one went through RI after the wave passed the
Antilles and entered Caribbean.
But I think he was talking about intensity-wise.
Not saying anything else at this point.
It's just a big wave and NHC says conditions are favorable ahead of it.
This wave did come from Cape Verde originally.


Perhaps, but I do raise an eyebrow when anyone jumps onto the Category 5 bandwagon anytime there's a perceived threat. Even now, there's only ever been two before middle of August - Allen and Emily. Neither of those storms match this one currently.

They are rare. Exceptionally rare at this time of year.

It happens every year. It is fine to speculate, but there's a time and a place.

This is not to discount the potential danger; the system is a real potential threat and should be watched like any other. Storms can be plenty dangerous without ever hitting that intensity.

Tirade over.

Quoting stormwatcherCI:
I was only speaking in terms of strength and track.


I hope you feel I'm not jumping on you too much. Just the track doesn't match up, and as said up above, the intensity at this point doesn't either.

Records are made to be broken, sure, but still.
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Dean was more problematic, but it was a Cape Verde storm, which this isn't.
Cotillion

91L is a Cape Verde wave.
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Quoting Vincent4989:
2115
Oh my cow!


I figure the Blog will be hopping this long weekend :)
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2124. SLU
Quoting WxLogic:


Is unfortunate... but I have a feeling which I sure didn't got from Don that this could be quite troublesome.


Yes indeed. This could be the 1st big storm of 2011.
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91L seems to be made up of two parts, each of which appears to be rotating. I take it that it's the northern part we're interested in, or is 91L both parts?

Also there's a wave following behind it (30W)that seems to have some spin to it, too.

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At the rate the dry air is backing off it shouldn't be much of a problem for 91L in a couple days.
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2115
Oh my cow!
Member Since: November 13, 2009 Posts: 2 Comments: 728

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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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