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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1321. Patrap
G'night wunder folk

Don Tejas...

Maybe google the term "astroturfing" for a Heads up on a trend.
Member Since: July 3, 2005 Posts: 419 Comments: 127367
1320. GHOSTY1
Like somebody said in the past on this blog said the closer it heads to the coast it will probably gain strength rapidly. Question: What are the chances that Don will impact Houston since it seemed like it made a more northwest turn?
Member Since: July 25, 2011 Posts: 0 Comments: 453
Cindy was prettier
Member Since: July 14, 2008 Posts: 1 Comments: 9628
1318. Patrap
Quoting atmoaggie:
July 27 at 21 UTC ?


U r very retentive Ben,,I dont control a server burp.

This is the posted Image,





Member Since: July 3, 2005 Posts: 419 Comments: 127367
How does 999.5 = 1003mb?
Member Since: Posts: Comments:
Don is pathetic even comparing him to Bret...
Member Since: October 3, 2010 Posts: 40 Comments: 4129
Quoting Patrap:
Note how Dons envelope goes North/South up against that drier Air its Bumping up against on his West side/

July 27 at 21 UTC ?

Current on a refresh. Odd.
Member Since: Posts: Comments:
Quoting Levi32:


It came up slower than it went down....this is classic pulsing in a sheared system.


Did you notice the ULAC that formed just to the NW of the system? Once it gets closer, shear shouldn't be a problem.
Member Since: July 6, 2010 Posts: 112 Comments: 31327
1313. Patrap
1003mb confirms what we been seeing for the last 1.5 hours..

A weakening trend
Member Since: July 3, 2005 Posts: 419 Comments: 127367
1312. j2008
ATCF officialy saying Don has 45Knt (50MPH) wind speed with a min pressure of 998MB.
AL, 04, 2011072900, , BEST, 0, 246N, 919W, 45, 998, TS,
Member Since: December 19, 2008 Posts: 1 Comments: 224
1311. angiest
Quoting Levi32:
Weakening has already ensued again.

000
URNT12 KNHC 290220
VORTEX DATA MESSAGE AL042011
A. 29/01:54:10Z
B. 24 deg 33 min N
092 deg 08 min W
C. 850 mb 1451 m
D. 40 kt
E. 125 deg 18 nm
F. 192 deg 27 kt
G. 129 deg 37 nm
H. 1003 mb
I. 19 C / 1526 m
J. 27 C / 1518 m
K. 15 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 10
MAX FL WIND 42 KT NE QUAD 00:07:10Z
MAX OUTBOUND FL WIND 33 KT NW QUAD 01:57:00Z


OK, is the Google Earth feed slow? They are nowhere near the center.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Don's gonna have to muster up some convergence to be able to sustain these bursts of energy. Kinda like pinching off the gas line in a car. It will run for a little bit then fade fast.

Member Since: Posts: Comments:
Quoting Max1023:
That 1003 doesn't make sense - the pressure shouldn't come up that fast in a naked swirl.


Makes perfect sense, vort systems such as Don spin up and down rapidly,
Member Since: July 14, 2008 Posts: 1 Comments: 9628
Quoting Levi32:
Weakening has already ensued again.

000
URNT12 KNHC 290220
VORTEX DATA MESSAGE AL042011
A. 29/01:54:10Z
B. 24 deg 33 min N
092 deg 08 min W
C. 850 mb 1451 m
D. 40 kt
E. 125 deg 18 nm
F. 192 deg 27 kt
G. 129 deg 37 nm
H. 1003 mb
I. 19 C / 1526 m
J. 27 C / 1518 m
K. 15 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 10
MAX FL WIND 42 KT NE QUAD 00:07:10Z
MAX OUTBOUND FL WIND 33 KT NW QUAD 01:57:00Z


1000, 1001, 1004, 1005, 1004, 999, 996, 995, 999, 1003.

arghhhhh...So hard to forecast.
Member Since: July 6, 2010 Posts: 112 Comments: 31327
Well, if Don's convection falls apart tonight, it will be just following the trend of its lifetime. This has been one odd tropical system in that it gains organization during peak heating/diurnal minimum, then falls apart late at night when it should be gaining more convection/more organization.

The environment just doesn't want Don to become a hurricane. On the latest IR loop, you can see the shear starting to take its toll once more on the NE side.
Member Since: Posts: Comments:
1306. Levi32
Quoting Max1023:
That 1003 doesn't make sense - the pressure shouldn't come up that fast in a naked swirl.


It came up slower than it went down. This is classic pulsing in a sheared system.
Member Since: November 24, 2005 Posts: 635 Comments: 26543
11 PM Advisory for DON:
Tropical Storm Don:
45 mph
1003 MB
moving WNW
Member Since: October 3, 2010 Posts: 40 Comments: 4129
1304. KEEPEROFTHEGATE (Mod)
Quoting Tazmanian:



if it dos not slow down i am thinking some time late late tonight or friday PM
by the dawns early light it makes landfall tex/mex border region possible 3 miles inside the mex side just enough to make it a non US hit
Member Since: July 15, 2006 Posts: 168 Comments: 53285
1303. Max1023
That 1003 doesn't make sense - the pressure shouldn't come up that fast in a naked swirl.
Member Since: Posts: Comments:
1302. Levi32
Weakening has already ensued again.

000
URNT12 KNHC 290220
VORTEX DATA MESSAGE AL042011
A. 29/01:54:10Z
B. 24 deg 33 min N
092 deg 08 min W
C. 850 mb 1451 m
D. 40 kt
E. 125 deg 18 nm
F. 192 deg 27 kt
G. 129 deg 37 nm
H. 1003 mb
I. 19 C / 1526 m
J. 27 C / 1518 m
K. 15 C / NA
L. NA
M. NA
N. 1345 / 8
O. 0.02 / 2 nm
P. AF307 0404A DON OB 10
MAX FL WIND 42 KT NE QUAD 00:07:10Z
MAX OUTBOUND FL WIND 33 KT NW QUAD 01:57:00Z
Member Since: November 24, 2005 Posts: 635 Comments: 26543
1301. Patrap
2:24 AM GMT on July 29, 2011
Floater - Water Vapor Loop
Member Since: July 3, 2005 Posts: 419 Comments: 127367
1300. TampaSpin
2:24 AM GMT on July 29, 2011



You can see the Other High in the West a coming to build in over Don.....he won't make it much north of Corpus Cristi...
Member Since: September 2, 2007 Posts: 178 Comments: 20439
1299. TropicalAnalystwx13
2:23 AM GMT on July 29, 2011
Quoting floridaboy14:
well its going through dmin now isnt it


Yes.
Quoting Hurricanes12:
When is Don estimated to make landfall? It looks like it's already close to the Texas coast.


During the pre-dawn hours of Saturday morning.
Member Since: July 6, 2010 Posts: 112 Comments: 31327
1298. RitaEvac
2:23 AM GMT on July 29, 2011
Since my area is not gonna get much rain, it's time to say NEXT, whats the future 91L doing
Member Since: July 14, 2008 Posts: 1 Comments: 9628
1297. angiest
2:23 AM GMT on July 29, 2011
I think I am confident enough to commit to something I mentioned earlier today, and trim the eastern edge of my landfall area back from High Island to Freeport (which is a tad west of the TS warning's eastern edge). Not ready to extend down the coast yet, I want some more data before then. I still read the steering as allowing the storm to move further north closer to landfall, and I am, not too sure the earlier westward motion was real. Looks to me more like the center relocated but the last two vortex fixes look like a NW motion.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1296. Patrap
2:23 AM GMT on July 29, 2011
The Tops continue to warm and contract

Member Since: July 3, 2005 Posts: 419 Comments: 127367
1295. Tazmanian
2:23 AM GMT on July 29, 2011
Quoting Hurricanes12:
When is Don estimated to make landfall? It looks like it's already close to the Texas coast.



if it dos not slow down i am thinking some time late late tonight or friday PM
Member Since: May 21, 2006 Posts: 5091 Comments: 114649
1294. j2008
2:23 AM GMT on July 29, 2011
Quoting Hurricanes12:
When is Don estimated to make landfall? It looks like it's already close to the Texas coast.

Saturday Morning most likely.
Member Since: December 19, 2008 Posts: 1 Comments: 224
1293. floridaboy14
2:22 AM GMT on July 29, 2011
well its going through dmin now isnt it
Member Since: July 25, 2011 Posts: 0 Comments: 1102
1292. Hurricanes12
2:22 AM GMT on July 29, 2011
When is Don estimated to make landfall? It looks like it's already close to the Texas coast.
Member Since: June 21, 2010 Posts: 0 Comments: 528
1291. RitaEvac
2:22 AM GMT on July 29, 2011
Convective blowup last night in the upper midwest near Chicago was bigger than Don
Member Since: July 14, 2008 Posts: 1 Comments: 9628
1290. FrankZapper
2:22 AM GMT on July 29, 2011
Quoting Levi32:
I'm still expecting around 50mph at landfall, with the potential for 60mph if we get a convective burst right before Don moves onshore.
Finally a voice of reason.
Member Since: May 26, 2011 Posts: 0 Comments: 1193
1289. Tazmanian
2:21 AM GMT on July 29, 2011
by the way if DON dos not slow down it will likey move make land fall in the AM or PM on friday
Member Since: May 21, 2006 Posts: 5091 Comments: 114649
1288. RitaEvac
2:20 AM GMT on July 29, 2011
I've seen MCS over the plains bigger than this thing
Member Since: July 14, 2008 Posts: 1 Comments: 9628
1287. angiest
2:20 AM GMT on July 29, 2011
Quoting KEEPEROFTHEGATE:
i think it may have been an ef5 over water tornado in a super cell now its done show over


LOL

Now now, his is not Marco.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
1286. TampaSpin
2:20 AM GMT on July 29, 2011
Quoting Patrap:
Note the Showers wane offshore





You can see how the showers is absorbed up by this dry air........



Member Since: September 2, 2007 Posts: 178 Comments: 20439
1285. Levi32
2:19 AM GMT on July 29, 2011
I'm still expecting around 50mph at landfall, with the potential for 60mph if we get a convective burst right before Don moves onshore.
Member Since: November 24, 2005 Posts: 635 Comments: 26543
1284. xcool
2:19 AM GMT on July 29, 2011
scott39 i seeing nw move
Member Since: September 26, 2009 Posts: 2 Comments: 15618
1283. KEEPEROFTHEGATE (Mod)
2:19 AM GMT on July 29, 2011
Quoting Levi32:
Don's convection is waning again. The environment cannot support anything more than periodic pulsing, not a full-on sustained rapid intensification. Recon winds only support a 40-45kt TS at maximum right now.
i think it may have been an ef5 over water tornado in a super cell now its done show over
Member Since: July 15, 2006 Posts: 168 Comments: 53285
1282. TropicalAnalystwx13
2:18 AM GMT on July 29, 2011
Don is still struggling with a little dry air, but the environment should allow this system to make it to 60 mph by Saturday morning when it makes landfall. Not only that, but the Texas coast is like the BOC, which tends to ramp up systems right before landfall. His convection is weakening right now because of DMIN.

This will probably be a strong TS/minimal hurricane at landfall.
Member Since: July 6, 2010 Posts: 112 Comments: 31327
1281. Jax82
2:18 AM GMT on July 29, 2011
Don is like the little engine that could, he just has a really small engine.
Member Since: September 2, 2008 Posts: 1 Comments: 1261
1280. emcf30
2:18 AM GMT on July 29, 2011
Quoting floridaboy14:

is this by the center and does this indicate don weakining?

He is just taking a deep breath
Member Since: August 7, 2010 Posts: 0 Comments: 1933
1279. RitaEvac
2:18 AM GMT on July 29, 2011
Got more activity on Houston radar than Brownsville
Member Since: July 14, 2008 Posts: 1 Comments: 9628
1278. scott39
2:17 AM GMT on July 29, 2011
Looks like Don is starting to turn more to the NW again.
Member Since: June 13, 2009 Posts: 0 Comments: 6728
1277. CaicosRetiredSailor
2:17 AM GMT on July 29, 2011
hmmmmmm....

Has he come

unDONe ?
Member Since: July 12, 2007 Posts: 0 Comments: 5995
1275. j2008
2:16 AM GMT on July 29, 2011
Quoting HurricaneDean07:
j2008, they did pass the center, dunno if they went through or missed it...

I think they missed it, but I could be wrong.
Member Since: December 19, 2008 Posts: 1 Comments: 224
1274. Patrap
2:16 AM GMT on July 29, 2011
Note how Dons envelope goes North/South up against that drier Air its Bumping up against on his West side/

Member Since: July 3, 2005 Posts: 419 Comments: 127367
1273. want2lrn
2:15 AM GMT on July 29, 2011
Quoting jpsb:
You can expect hurricane force winds for a few hours. It looks to me like you will be on the dirty side of Don. I would definitely tie the boat down if it is high enough above water that waves will not hit it. If waves are a problem then move it. Cat 1 will bring 5-7' tide.


I appreciate your input jp. Boat sits on a bar lift about three feet above water, i don't foresee the wind being the problem but the rise in water levels. We need the rain but i can do without the wind. Unfortunately the local mets are downplaying this a little according to what i am reading on here. A lot of really good thoughts and explanations on here from most. Thank you.
Member Since: July 28, 2010 Posts: 0 Comments: 276
1272. Patrap
2:15 AM GMT on July 29, 2011
Note the Showers wane offshore


Member Since: July 3, 2005 Posts: 419 Comments: 127367
1271. GHOSTY1
2:15 AM GMT on July 29, 2011
Don is kinda big when you look at, Texas is big and got a pretty long coast and Don is pretty wide storm compared to the coastline
Member Since: July 25, 2011 Posts: 0 Comments: 453

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About

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