Don battling dry air and wind shear
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
Reader Comments
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jason ur right
laterz- -
Morning, WxLogic. What do you know about the MJO, Bermuda High and anything else that would influence steering 91L? Am sure you know, but NHC has it at 30% as of 4:35 a.m.
I didn't call ya a troll.. ;)
OSCAT 9:53 PM EDT last night
Link
Although I do think some warming is occuring.
I'm not sure of the averages for 1995 through current, but the 1966-2009 average says the fourth named storm normally appears on August 23, the first hurricane on August 10, and the first major on September 4. Lots of info here.
LOLZ!
Scientists aren't buying it. (In short: Spencer's wrong. Again.)
Looks like more of a threat in terms of winds than Don ever was.
depends on how you look at it...Don is approaching NOW, it is going to hit land, this we know...if the rains fall too hard then they wash out and cause flooding...what little is left for the farmers will be gone...91L is a 10-15 days away...don't know where it is going or if it will go...
Thanks, Nea. Of course, that works to 11-6-2 with the current AMO positive era at approximately 15-8-4, so it would be interesting to see how it fits in.
Especially with 4 storms prior to August 1 happening for the fifth time (97, 03, 05, 08 and now 11) in under fifteen years.
And then on to 91L.
After Tomas Last Year I Dont Think We Can Take Another One For Now, Hoping That Its Gonna Be A Fish, SLU Whats Up.
:) you have quite a bit on your mind there... here're the answers to best of my abilities:
MJO:
- Currently we're in a "neutral" MJO phase as we're exiting a downward period and entering and active MJO in this upcoming week. Whatever develops will have a better chance of surviving that what the conditions Don encountered when it first "became" last week when the downward MJO was around.
Steering:
- Currently the steering will be mainly W to WNW:
- The Bermuda High is currently getting itself better establish as we speak and should be E of CONUS within the next couple days and bridging with the Azores High. This should allow 91L to proceed on a W to WNW for the time being until the time it enters the Caribbean. By then... it is expected that a TROF develops across the CONUS E coast. The depth, strength, and timing of this TROF will dictate if 91L moves into the GOM, E/SE CONUS, or re-curves out to see.
Forecast:
- I'm expecting 91L to be in the 40% mark by the 8AM TWO. It currently has a well established low level circulation which correlates to the moderate to high convergence values:
Divergence is good too as well as a nice ULAC which validates the good outflow aloft:
- Hope this high level overview helps.
It sounds like good news.
However, when a piece of 'groundbreaking' science is first broken by Forbes, and not by something like NewScientist, as well as some of the suggestive semantics used, the alarm bells go off.
hey is don weakining currently ?
Coordinates: 27.5N 91.95W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,621 meters (~ 5,318 feet)
Extrap. Sfc. Press: 1016.0 mb (~ 30.00 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 148° at 25 knots (From the SSE at ~ 28.7 mph)
Air Temp: 17.5°C (~ 63.5°F)
Dew Pt: 7.2°C (~ 45.0°F)
Peak (10s) Flt. Lvl. Wind: 25 knots (~ 28.7 mph)
SFMR Peak (10s) Sfc. Wind: 24 knots (~ 27.6 mph)
SFMR Rain Rate: 0 mm/hr (~ 0 in/hr)!
Yeah, that is super.
Folks like you are one very good reason why this blog is so interesting and valuable.
Thanks for your time and expertise!
Of course the humor and the sorts of discussions that go on here regarding different opinions are also gratifying when having to work at the computer much of the day!
This system looks like a major threat to the Lesser Antilles. Could be a system of significant intensity by the time it reaches here.
The main WX associated with Don (Rain mainly) should be confined to the center and S of it as the upper level WNDS are not that optimal for equal expansion (or at least as equal as possible) on all sides of the TS.
Think they did that with Don originally, though it ended up a bit more south than that.
Wonder if it's a pattern.
It's one to watch, especially as you head into August.
SAL doesn't affect steering.
Here's a SAL Map:
LinkSouthAfricanDust
Looking spotty ahead of 91L.
Aww man. But at what time frame? Monday-Tuesday? Or later?
I'm awake.
But I'm not awake.
Coffee.
I need coffee.....
A.FFFIIIIIIISSSHHH!!!!!!!
B.Not Fish
For me, i pick B.
- 91L is doing a pretty good job to say the least at fending off the SAL. As it approaches 50W the moisture content in the environment has been and will be more conductive to dilute any further SAL in the Caribbean region and around 91L.
In the image above you'll see that SAL has being decreasing substantially over the past 6 to 9hrs and expect to to continue and making the environment more conductive for development.
Agreed. And that is why from today,I am preparing to avoid the last minute rush.
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