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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Outflow to the NW from Don has improved significantly in the last hour. Watch for convection to build over northern semicircle soon.
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Clear strengthening with Don now, 5 to 10 mph increase is possible at next advisory.
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569. red0
Meanwhile in Fort Smith, Arkansas...

This is the 27th straight day over 100, but I could feel something different in the air today from the beginning. The clouds were looking rather ominous and, it was nearly 1:00 before the signs around town started registering triple digits. I was thinking maybe today's high would be only like 102 instead of a more typical for this heat wave 107.

As I walked out of the theater this afternoon the humidity hit me in the face and I spotted what looked suspiciously like the aftermath of a brief shower on the pavement. The little spots of quickly evaporating wetness were no match for the high pressure ridge of doom though, and were fading fast. I put my headphones on and headed towards my bike when it started raining again, and I took shelter under an awning. As the humidity went from ridiculous to hard-to-breath I stood watching this piece of pavement in front of me get rained on for 18 minutes (2.5 plays of Radiohead's Street Spirit heh) without it getting wet. It was as if the pavement didn't quite remember what to do when it got rained on. 30 minutes later though, the sky opened up and we got our first measurable rain of July and the temperature is now in the 80's. Quite nice!
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Quoting Walnut:
StormW also seems to have Poofed himself.


Lol...I'd be careful mentioning that name around this blog. While I like StormW personally, some on here do not like him. He has his own site. If you want it, I'll give you the link through e-mail. Can't give it to you over the public blog.
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exploding convection just south of the center
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WOW, the Tropical Wave in the Atlantic is getting better organized and very quick !!! I think 91L is coming soon !!!
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Quoting JeffM:
Does Kman still participate around here? Loved following his observations that past several years.


He hasn't been on in a few weeks. Hope he's alright.

I'd certainly love to know his input on the Central Atlantic wave.
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Im goin for now be back later!
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Quoting JeffM:
Does Kman still participate around here? Loved following his observations that past several years.
StormW also seems to have Poofed himself.
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Quoting GiovannaDatoli:
CosmicEvents. I was thinking the exact same thing! Why does it always seem we so eye to eye on everything??
idk....if you're a woman maybe we were meant for each other...tropically speaking:)
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560. JeffM
Does Kman still participate around here? Loved following his observations that past several years.
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Quoting angiest:
when don was named, the HH kept getting surface winds higher than flight. Is this the same plane as yesterday?


No this is a NOAA flight, all others have been AFR.
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Quoting washingtonian115:
Well I'm out.Hope don doesn't pull no surprising stunts on us or Texas for that matter.Good night everyone.


I think TX will take some surprising stunts if it means more rain.
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Quoting HurricaneHunterJoe:
The Don seems to be expanding his territory
He needs to get to Texas-sized - and slow down the forward motion.
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04L
on approach


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when don was named, the HH kept getting surface winds higher than flight. Is this the same plane as yesterday?
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Quoting HurricaneHunterJoe:
thats a bit odd, wouldnt you say?


Look at the rain-rates, there's your explanation. However there is a lot of 45kt+ winds that aren't embedded within the higher rains, so I would think 45kts at the next advisory.
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Well I'm out.Hope don doesn't pull no surprising stunts on us or Texas for that matter.Good night everyone.
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Quoting Patrap:
The Don seems to be expanding his territory
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are some of yall RIP's recognizing that Don is no dead storm or weak system? he is never done and only when landfall happens death is immenint (i think i mispelled immenint) unless TS Allison scenario in order (unlikely)
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Don totally has tiger blood in him.He's playing tricks with us.I don't like tricky storms.But tricky storms are more fun to forecast.Becuase you never know what they're gonna do next.
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Quoting KEEPEROFTHEGATE:
04L
on approach


An intense updraft developed around 5:30p.m EDT (21:30UTC) almost directly above the circulation. Pretty impressive.
Member Since: September 2, 2009 Posts: 130 Comments: 21193
Quoting CosmicEvents:
There's a few of us, who blog privately away from the fray, who've had our eye on this one since it was spinning over Burkina Faso.
Everyone was focused on the central Atlantic wave.So I thought they ignored what was going on with the potential storm off of the Yucatan.
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Back after work.
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Quoting tiggeriffic:


wait till noon tomorrow ganny...sun hot and blaring today...48 hours ahead of shedule...that high is gonna lift more to the north faster than they think...


Well if it does and if it strengthens to more than a tropical storm a lot of people are going to be caught off guard I would think.
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04L
on approach


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Quoting MississippiWx:
Don continues to organize nicely. Intense thunderstorms continue to blow up over the center:

i want more padding on the north side so Donnie can go to town
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Quoting catastropheadjuster:


Washingtonian, Thanks I'll go over there and see what i can find.
sheri
Your welcomed.
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Quoting SevereWeatherAddict:
As you can see I very rarely post. But what is the changes of Don becoming a cat 1 ? And thanks in advanced.


The chances of that are fairly low. Small systems like Don have surprised folks before, like when Bret recently made a run at hurricane status despite unfavorable conditions. However, conditions are worse for Don here, and hurricane intensity is unlikely.
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Quoting cat5hurricane:

Models are garbage beyond 5 days. Makes you wonder why so many are skeptical of Climate Change.


Because they don't understand that they are 2 completely different mathematical object? The dynamic models are deterministic and because of mathematical non-linearity, indeed garbage after 5 days because of uncertainty in initial conditions. Climate models are statistical and look at long term averages and probabilities...
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Quoting washingtonian115:
I'm glad someone else noticed besides me.....
There's a few of us, who blog privately away from the fray, who've had our eye on this one since it was spinning over Burkina Faso.
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Quoting HurricaneHunterJoe:
thats a bit odd, wouldnt you say?


Flt lvl winds don't match, contamination likely.
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Quoting P451:


Still going to wait until 8pm for better continuity purposes.

Also each convective burst will at some time push over the center but if still subject to the same northerly winds that have been plaguing the system will eventually retreat southward.

So looking moment to moment probably won't give a clear picture one way or another. At one moment it would appear to be organizing better and at the next disorganizing. So I don't want to do that.



Yeah, fair enough.
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Don right now is quoting Charile Sheen...he's saying"Winner,Winning,Duh".If this turns out to be a Dolly situation i totally called it first.
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long time member here, i just dont post much anymore with all the bickering. mostly just lurk

i just wanted to state my opinion that don is strengthening pretty quickly. as a smaller system, don is more prone to things that could weaken or destroy him (shear, dry air, water temps). that being said, as a smaller system, it is also more likely to intensify quicker as well. good examples of this would be TS marco in 2008, Cyclone tracy in 1974, and hurricane humberto in 2007.

it looks like the center is under that nice uniform ball of convention in the NW part of the syatem. but thats just my untrained eye. im also having a hard time figuring out where it will head. my gut is telling me a mid texas landfall, but it looks like a southern texas landfall based on current movement.
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18z Early Cycle NHC model tracks
Don
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)





Dynamic Models (More sophisticated models)

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Quoting weatherganny:



See you were right with your "gut" feeling. We should never ignore them.


wait till noon tomorrow ganny...sun hot and blaring today...48 hours ahead of shedule...that high is gonna lift more to the north faster than they think...
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527. JLPR2
That's the strongest convection I have seen on Don.



But I just came back, so I missed almost a whole day of satellites images.
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i dont know what some of yall are seeing but i see some convection forming to the north and more convection connecting to the south, you gotta give time with constant hating.
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NOAA HH about to turn back towards the center, AFR HH should provide more detailed wind measurements in the next couple hours.
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Quoting MrstormX:
Just noticed this data wasn't flagged as corrupt:

Time: 21:53:00Z
Coordinates: 24.2333N 91.3833W
Acft. Static Air Press: 751.7 mb* (~ 22.20 inHg*)
Acft. Geopotential Hgt: 2,533 meters* (~ 8,310 feet*)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 211° at 4 knots (From the SSW at ~ 4.6 mph)
Air Temp: 14.1°C* (~ 57.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 6 knots (~ 6.9 mph)
SFMR Peak (10s) Sfc. Wind: 63 knots (~ 72.4 mph)
SFMR Rain Rate: 37 mm/hr (~ 1.46 in/hr)
(*) Denotes suspect data



thats a bit odd, wouldnt you say?
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Quoting Levi32:
I haven't seen the high-density obs, but from the quotes on here a couple pages back, 58kts SFMR wind with 11kts at flight-level has to raise eyebrows. That is more than likely an inaccurate reading.


This was the message in question.
Time: 21:53:00Z
Coordinates: 24.2333N 91.3833W
Acft. Static Air Press: 751.7 mb* (~ 22.20 inHg*)
Acft. Geopotential Hgt: 2,533 meters* (~ 8,310 feet*)
Extrap. Sfc. Press: -
D-value: -
Flt. Lvl. Wind (30s): From 211° at 4 knots (From the SSW at ~ 4.6 mph)
Air Temp: 14.1°C* (~ 57.4°F*)
Dew Pt: -*
Peak (10s) Flt. Lvl. Wind: 6 knots (~ 6.9 mph)
SFMR Peak (10s) Sfc. Wind: 63 knots (~ 72.4 mph)
SFMR Rain Rate: 37 mm/hr (~ 1.46 in/hr)
(*) Denotes suspect data
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Don's circulation is located at 24.23N 91.38W.
Member Since: September 2, 2009 Posts: 130 Comments: 21193

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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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