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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Winds are still coming out of the SSE closer to the middle of the convection:



Edit: Center found it looks like....right where the newest blowup of convection is ongoing.
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Quoting Floodman:


Been lurking a wee bit; I'm actually finally caught up on work and feel like a good fight...I come in here and the only person being "officious" is Taz, and I like Taz...this place is no fun anymore. Where have all the good trolls gone?


Ill be your huckleberry.
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A
Quoting TomTaylor:
Whenever we see convection restart over the center again, or pick up in intensity, a storm is intensifying.

Convection is what lowers surface pressures. When you lower surface pressures, you raise winds. Therefore, increasing convection intensity, or the reappearance of convection signals intensification.

The reason convection lowers surface pressures is convection, in meteorology, is the vertical transport of air. When you have air rising, pressures at the surface will naturally fall. The key is, you need warm SSTs and abundant moisture to allow for instability for convection to occur. You need upper divergence and low level convergence to allow convection to continue. And finally, you need light shear values to allow convection to remain over the storm, rather than getting sheared off to the side.
Thanks for the info it was very informative
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Quoting tiggeriffic:


lol tired of people trolling...one says this another that then people make threats when they are just as guilty for making comments that are not topic related...over it...been lurking more than posting today...how bout you Flood?


Been lurking a wee bit; I'm actually finally caught up on work and feel like a good fight...I come in here and the only person being "officious" is Taz, and I like Taz...this place is no fun anymore. Where have all the good trolls gone?
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1004.9 is the lates pressure reading from the HH wich is 7 miles from the center so i think it could be around 1003mb
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366. DFWjc
Quoting uptxcoast:


I was right there as well (Ponderosa Forest) and even that far inland that was one scary storm.


OMG i remember PF...the memories are flooding back..okay maybe not the right word to use...
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Meh, not the most ideal structure, but at least it's got something going on under there again.

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This image implies that the surface center might be closer to the center of the convective mass than we thought. Notice the wind barbs...

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Quoting DFWjc:


That's what my parents did in '83 with Alicia, we lived off of 1960 and I-45...i had juice packs (was 7 at the time)


I was right there as well (Ponderosa Forest) and even that far inland that was one scary storm.
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taz i don't see the what big problem is, it was just one picture and im not seein' it as offensive, all im sayin is that yall need to chill, its your own option to ignore them if you want to, but the blog administrator will deal with the major infractions so leave it to the big dogs and just ignore it if you don't like it. No rudeness or meaness intended.
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Mid Level Center is under the convection according to HH report.
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Don has moved SW....

or, the midlevel and low level circulation is very discoupled.
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Quoting belizeit:
18z GFS finally develops Don then takes him into the border


I have very low confidence in the models this year. Call it budget cuts or lack of will to upgrade, or maybe they upgraded and screwed them up who knows, but they are not as accurate as previous years. Maybe is the configuration we find ourselves this year when it comes to cyclogenesis
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Quoting tiggeriffic:
wow...from what i am reading Don is under shear anywhere from 5 to 40 mph...it is strengthening while it is weakening, growing while it is shrinking, becoming better organized while it is dying, taking a northward jog as it goes south, stacking while it is collapsing...i think i got it all...

You forgot that it has a pinhole eye in it's naked swirl.
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active july after all could be one more in the deck.
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Quoting CorneliaMarie:
VA... my post was sarcastic.... relax... if my posts bother you, PLEASE ignore me, hon'.....


It really wasnt directed toward you, I hate when anybody does that. I dont ignore anyone anyway.
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354. DFWjc
Quoting miguel617:
TAZ, don't get the handbook out on me because of my post, just alluded to the fact that some folks ride out the storm with an ice chest full of beer and a few bottles of liquor.


That's what my parents did in '83 with Alicia, we lived off of 1960 and I-45...i had juice packs (was 7 at the time)
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So don is a wittle bitty storm, at least Texas will be getting some much needed rain. We have finally got some much needed rain her in Mobile,Al the past few weeks. But the rivers show we need more. I hear some folks talking about the wave out the between Africa and the Lesser Antilles it looks like a pretty nice size wave, does anyone think it might turn in to anything? I know it's a long ways off, just wondering that's all.

Also what is that next to the + - ! it has a + and a number?

sheri
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Quoting nigel20:
Is don now strenghtening?
Whenever we see convection restart over the center again, or pick up in intensity, a storm is intensifying.

Convection is what lowers surface pressures. When you lower surface pressures, you raise winds. Therefore, increasing convection intensity, or the reappearance of convection signals intensification.

The reason convection lowers surface pressures is convection, in meteorology, is the vertical transport of air. When you have air rising, pressures at the surface will naturally fall. The key is, you need warm SSTs and abundant moisture to allow for instability for convection to occur. You need upper divergence and low level convergence to allow convection to continue. And finally, you need light shear values to allow convection to remain over the storm, rather than getting sheared off to the side.
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Quoting belizeit:
18z GFS finally develops Don then takes him into the border



the GFS is a little late
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TAZ, don't get the handbook out on me because of my post, just alluded to the fact that some folks like to ride out storms with an ice chest full of beer and a few bottles of liquor.
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Quoting MiamiHurricanes09:
Don has been improving structurally throughout the last few hours, albeit still a disorganized tropical storm. The question is if it will be able to continue to fire thunderstorm activity throughout the diurnal minimum.


Since he is away from land, diurnal minimum will probably have less of an impact than it did last night when it was hovering along the Yucatan coastline.

One thing that I'm noticing on visible is that there are a lot less outflow boundaries flying around Don now. That tells me that he might have taken care of the dry air issue for now, at least.
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Quoting sarahjola:

i believe it. don looks a heck of a lot better than last night and this morning. is don on track still or is he going further north than expected? tia


im still thinking it will head toward rockport, tx but right now its making a odd west movement and we gotta see how long that lasts.
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Don seems to have exploded or something, or is this my imagination? Also, the shear has stopped, or is this also my imagination?

Also, the dry air seems to have abated, or is this my imagination?
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04L
on approach


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342. DFWjc
Quoting Tazmanian:



am giveing you guys a warning but hey if you dont want too take my warning and go a head get a 24hr ban and when i storm about too make land fall in the USA some where you wont get the info you need too no so the Risk is yours too take i saw a few last year that got banned for posting thing they sould have not so if you want too take that Risk go right a head but dont come crying too me


let me translate that....

I am giving you people a warning, but hey if you don't want to take my warnings, you might as well take a 24hr ban. When the storm makes a landfall, you won't be able to get the information you need. You need to not take the risk, because i saw a few people last year that got ban for posting things they shouldn't have been. So if you want to take the risk, go right ahead, but don't crying to me....
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18z GFS finally develops Don then takes him into the border
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Recon couldn't have timed their flight any better. Don is on the upswing and it will be interesting to view their data over the next couple of hours.
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In regards to wind shear:

I'm using weak wind shear as anything around (10mph-20mph)

You have a flame on a candle, and you try to blow the candle out. Since it's a small flame, it doesn't take a lot of air (or weak wind shear) to extinguish the flame.

Now, let's say you have an entire house on fire. Breathing on the fire will not impact the fire itself because it's much larger, and your exhalation of air (weak wind shear) won't cut it.

Tropical systems are very much like that. Don is like a flame on a candle. It doesn't take a lot of wind shear to weaken or disrupt the organization of the system.

Larger systems can handle weak wind shear.

Since Don is small and not as organized, weak wind shear can really cause issues with it because it's like a small flame on a candle.

Wind shear over 30mph can really disrupt any tropical cyclone trying to form.
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Quoting DFWjc:


And who gives out these "ban hammers"? just wondering...i didn't see it in the "rules"...



here are the rules

Keep it civil. Personal attacks, bickering, flaming, and general trollish behavior will not be tolerated. Disagreements are fine, but keep them civil and short.
Stay on topic.
No monomania.
No hot linking external or copyright images without the image owner's permission.
Do not circumvent a ban. Most bans last 24 hours or less, please accept the ban. If you create a new username to circumvent a ban, you will be blocked from the site completely.
No comments that contain only personal notes such as, "Good Morning!", or "You've got mail, X".
Do not "1st!", "1st post!", or any of the numerical/linguistic derivatives. This is a worthless use of blog space.
No spamming.
No spamming.
Seriously, no spamming. Spamming includes but is not limited to, trying to sell products, trying to solicit traffic for your own blog, trying to solicit traffic for other commercial entities, etc. Do not post links to your own site unless they are directly relevant and even then, use sparingly.



and the most of all this one would be rule # 11 in my books


During active periods of hurricane season, these rules will be strictly enforced. Violations will be met with a minimum 24 hour ban.
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Quoting P451:


Does 15kt of shear displace convection 100s of miles though? I don't think so - small storm or not. I think the shear values are incorrect.

Look at the storms far to the north east as well - they have been torn apart leaving trails several hundred miles long in just a few short hours.

That is some pretty intense shear.

The values on the maps must be understated/incorrect.

I will take what I can see on satellite imagery over maps that have been wrong before.

We're expecting CIMSS to be correct. We know what we see on satellite imagery. The two do _not_ correspond with each other. Given the satellite is real time imagery - and the shear maps are computer generated overlays - I think there is no reason to trust the CIMSS maps right now.
Quoting P451:


The center is on the northern edge of a convection plume that is stretched hundreds of miles southward.

This is beyond the idea that a small system is more susceptible.

The values are wrong.

The proof is right there in all the imagery.

P451, the shear map on the CIMSS analysis is usually pretty accurate, and whatever inaccuracies it does have are not very significant. Wind shear analysis from the CIMSS site reveals wind shear values around 5-15 knots around the storm. If you don't like the CIMSS map, you can try the less accurate, SSD site. Even that site is showing 5-15 knot wind shear values, however.

These kind of wind shear values are not really destructive. The main problem is this is a small system, and convection has been strongest and mostly confined over the South and West sides of the storm. This is partially to do with shear coming from the opposite direction, but also partially a result of greater convergence aloft on that side of the storm. Upper level wind analysis reveals air heading toward the system on the north and east sides, while air being pulled away on the south and west sides.




The strong ridge over the US and the ULL of NE Mexico are responsible for these upper level winds. When you get convergence aloft like this on one side, and divergence on the other, convection will naturally be favored on the divergent side.

Dry air and subsidence associated with the large ridge to the north and ULL to the west are partially limiting convection. Upper convergence to the north and east is limiting convection in these quadrants. Shear is keeping the current convection maybe slightly removed from the center, but it's not what is preventing convection from firing on the north and east sides.
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Quoting CorneliaMarie:
PcolaDan also gets a +1 for his detailed beyond all belief weather forecast.


okay you dont need to post +1... since now all you have to do is press the plus button and everybody will see how awesome you think posts are.
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Quoting Floodman:


That'd be too much like making sense, hon...how are you?


lol tired of people trolling...one says this another that then people make threats when they are just as guilty for making comments that are not topic related...over it...been lurking more than posting today...how bout you Flood?
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Don has been improving structurally throughout the last few hours, albeit still a disorganized tropical storm. The question is if it will be able to continue to fire thunderstorm activity throughout the diurnal minimum.
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you gotta have some fun on the blogs or else it would just be dry, i got on to this blog because i liked the mix of humor and weather talk, so i encourage the use of some but not alot of comic relief.
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Quoting cat5hurricane:

You da po po?



am giveing you guys a warning but hey if you dont want too take my warning and go a head get a 24hr ban and when i storm about too make land fall in the USA some where you wont get the info you need too no so the Risk is yours too take i saw a few last year that got ban for posting thing they sould have not so if you want too take that Risk go right a head but dont come crying too me
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Quoting GHOSTY1:


It appears so but others don't want to believe it

i believe it. don looks a heck of a lot better than last night and this morning. is don on track still or is he going further north than expected? tia
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Quoting tiggeriffic:


actually all of the opinions on here have Don as this: Don is under shear anywhere from 5 to 40 mph...it is strengthening while it is weakening, growing while it is shrinking, becoming better organized while it is dying, taking a northward jog as it goes south, stacking while it is collapsing...i think i got it all...

not trying to be sarcastic but for people that are new and do NOT understand the physical make up of a storm, all of this can be really confusing... i know enough to be able to read the charts, maps and know why the models see what they do and that there is error in the models...but to those of you who are screaming about what is going on...post a chart or map to prove what you are saying so the newbies can learn...


That'd be too much like making sense, hon...how are you?
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i can't tell is this just a slight movement to the west or is he headed for Mexico?
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Quoting tiggeriffic:
wow...from what i am reading Don is under shear anywhere from 5 to 40 mph...it is strengthening while it is weakening, growing while it is shrinking, becoming better organized while it is dying, taking a northward jog as it goes south, stacking while it is collapsing...i think i got it all...


With that information, I am now ready to give my prognostication:

Don will move () for the next () hours, then drift (). It will () in strength until it reaches () status. People in () should look for () as Don reaches shore as it () bringing () rain and even () wind.
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Almost to center, Pressure high, winds weak.

Time: 21:43:30Z
Coordinates: 24.6833N 90.9W
Acft. Static Air Press: 750.7 mb (~ 22.17 inHg)
Acft. Geopotential Hgt: 2,565 meters (~ 8,415 feet)
Extrap. Sfc. Press: 1006.0 mb (~ 29.71 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 134 at 20 knots (From the SE at ~ 23.0 mph)
Air Temp: 15.7C (~ 60.3F)
Dew Pt: 12.1C (~ 53.8F)
Peak (10s) Flt. Lvl. Wind: 21 knots (~ 24.1 mph)
SFMR Peak (10s) Sfc. Wind: 35 knots* (~ 40.2 mph*)

SFMR Rain Rate: 0 mm/hr* (~ 0 in/hr*)
(*) Denotes suspect data

And its moved almost due west since the 5pm advisory
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323. DFWjc
Quoting GHOSTY1:


It sort of looks like it but we gotta watch for maybe the next few hours to see for sure


Okay, maybe that frontal ridge is sliding West blocking it from hitting Texas, maybe?
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Quoting cat5hurricane:

You da po po?


yes
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Don has looked worse than he does now. There is actually some expansion of the convection towards the north and outflow has improved substantially. It's only limited on the northeast side of Don at this time:

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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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