Author Topic: April 12-17, 2007 US Severe Weather  (Read 17497 times)

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Offline Macca

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April 12-17, 2007 US Severe Weather
« on: 11 April 2007, 11:18:15 AM »
Hi all,

Ok – with an active upper trough forecast to move across the US later this week I figured it was time for some warm-up forecasting given that some of us will be heading over relatively soon to start chasing. I’ve just been having a look at some charts for this coming Thursday/Friday’s event in the US. (Sorry bit distracted – laughing hysterically at Thank God You’re Here – first episode of the US version).

Now that we are inside the 84hr mark, there are a few more model options to have a peek at. Moisture has backed off a bit since the last time I checked (this time yesterday or so). I’m going to focus mostly on Thursday as I think Friday is an easier day to forecast – there are more variables which may or may not come off on Thursday so I think its worth a closer look.

SPC have recognized the potential for scattered severe convection late on Thursday evening but they also comment (in their Day 4-8 outlook) that the main show will be Friday (as noted in the previous paragraph).

ETA/NAM (what is it officially called?) only goes out to 84hrs and the action time for Thursday is in the 72hr forecast period (so a fair way out still). Note: the following comments are based on the 00z output which is for 7pm Thursday. This model shows a 991mb surface low sitting over north eastern New Mexico with the dryline extending along the eastern boundary of the Texas panhandle from about Laverne, OK down to Childress, TX and down to around San Angelo, TX.

A weak-moderate frontal system passing over the western plains at the moment is set to clear most of the moisture out of the area until southerly winds set up ahead of the significant upper trough associated with the Thurs/Fri system. The 65f DP line sits right along the TX coastline by 7pm Wednesday night and the southerly return flow looks to commence in the early hours of Thursday morning as the LLJ starts to set up. DP’s of 50f (10C) nosing up towards the south eastern TX panhandle with the 60f (16C) line sitting in southern TX around Austin. Both GFS and ETA/NAM have this moisture rapidly moving northwards during the day (although note that GFS has the dryline sitting slightly further east and moves the whole system through faster than the ETA/NAM) and by 7pm, the 60f line is sitting just N of the Red River (ie the southern border of TX and OK) – actually, GFS has 65f (18C) DP’s sitting over the Red River into south central Oklahoma. This is thanks to the southerly low level jet (LLJ) of about 40knts transporting this moisture northwards.

The surface temps look to be just nudging into the 80f (27C) mark so we are looking at temp/dp spreads of about 20f (11C) which is an arbitrary line in the sand for tornadic/non-tornadic storms (due to the relatively high bases of storms resulting from such spreads – the lower the LCL, typically the more likely for tornadic activity). The other thing to note about the surface progs is the further increase in moisture overnight Thursday. By 7am Friday morning, DPs in the southern plains (ie across most of OK) are looking at being pretty well into the low 60’s (16-17C)- its actually probable that this will happen later on Thursday evening which could/will help lower LCL’s as the surface temperature drops as well…interesting [Smile] .

There will be quite a steady cap in place on Thursday with the 850 temps across the area being relatively warm. GFS indicates they could be into the 18-19C range, whereas the ETA/NAM drops back to 16C. This could play a bit of a part in making or breaking the day (pun somewhat intended). Forcing along the dryline doesn’t look overly strong and with the main upper level feature is sitting quite a way back to the west on Thursday evening, a cap on the stronger side of these figures could see one of those lovely clear sky busts. Interestingly enough though, both GFS and ETA/NAM are progging a weak upper level short wave trough ahead of the main upper level low to move over the SW Texas panhandle. This feature is barely noticeable on these models but its there and could be enough to help break the cap *late* in the afternoon. Furthermore both models are indicating precipitation along the Red River east of Childress, TX and both models erode all the convective inhibition (CIN) over the south eastern TX panhandle and eastwards along the Red River. Given the above, I think if 850 temps remain under 17C, there will be a show.

Instability indices are showing up some reasonable numbers with CAPE in the 1500-1750j/kg range (with a tiny patch of higher CAPE – getting up to around 2000j/kg) and LI’s are around -6C or so in far SW Oklahoma/SE Tx Panhandle and eastwards along the Red River. Shear is quite nice with ETA/NAM suggesting a 30-40knt S’ly LLJ (850mb), 40-50knt SSW-SW’ly winds at 700mb, 60-70knt SW’ly mid-level jet (500mb) and a 90-100knt upper level jet (200mb) – GFS has each level about 10knts higher and the upper levels have a slightly more westerly component. ETA/NAM also shows a 20knt SSE’ly at the surface over the region. Certainly nothing to sneeze and should storms develop, they would have a decent chance at becoming supercellular.

Given all of the above, I think there is a reasonable chance of some nice isolated convection along the dryline on Thursday. My current thoughts would put me somewhere S of the Red River (trying to make the most of the moisture coming up from the south) near the SE corner of the TX panhandle. With GFS and ETA/NAM differing slightly on the position of the dryline, I’d probably set up somewhere like Vernon or Quanah along Hwy 287. The problem with picking a target close to the Red River is that there is limited locations to cross over into Oklahoma should the need arise. Assuming storms initiate somewhere between Vernon and Childress (TX), the shear profile indicates that storms are going to initially move in a NE’ly direction and anything that really gets going is probably going to move ENE and eventually cross over the Red River into southern Oklahoma. So in picking Vernon and Quanah as starting points, both have options of roosting north across the Red River into southern OK should things initiate further N or should storms develop and move towards the Red River.

I’m probably going to have time to refine this forecast on Wednesday night. This will be much more useful as it will allow us to look at the surface observations to see how accurate the models are in forecasting the moisture return (which could play quite a big part in whether storms develop and whether there will be tornado potential…if the DP’s can get up into the low 60’s, there would be at least a chance of a tornado in the early evening hours as surface temps drop slightly and the LCL gets lower). With 27/17 and -15C at 500mb and the shear like it is, we’d all be pretty happy with this set up at home [Smile] .

Cheers,

Macca

Offline nzstorm

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RE: April 12-17, 2007 US Severe Weather
« Reply #1 on: 11 April 2007, 02:23:35 PM »
Quote
  ETA/NAM (what is it officially called?)

I believe NAM replaced ETA.

Yes, good to see something happening over there soon after a quiet spell. 
Steven Williams
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #2 on: 11 April 2007, 02:37:50 PM »
Macca,

Thanks for the in depth analysis - I won't elaborate further haha. I have been following the long range models from well out. I actually planned my departure based on this scenario. Hopefully it would have been worth while!

Although mositure can be a concern at times, if storms rotate with impressive wind shear, I would not be surprised tornadoes can still occur. My first day last year (Easter Saturday 15th April?), the storm was reasonably high based but still produced tornado after the 10 hour marathon for me and Ray! Of course we were dealing with impressive deep layer shear!

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Jimmy Deguara
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #3 on: 15 April 2007, 10:15:22 PM »
Hi all,

I arrived after a relatively smooth flight on Wednesday local time:) I decided to await the Friday action.

Some of you may have been following this system development on stormtrack. I chased with Tim Marshall and he targetted Seymour where the tornado occurred (I had no way of accessing high speed internet but from what I saw I believed it was a valid target). I liked the planned move as well.

We were onto the storm early and it became tornado warned. There was only a narrow corridor of opportunity and the relatively short life of the tornadic supercell proves that. We followed the storm for as long as we could but the green impending hail core was not as inviting for vehicles! We ran out of road options as the hook echo swung around to our north and north - northeast! We saw what is likely one of the tornado reported as well as the developing mesocyclone being wrapped as the second wedge tornado! Anyone who got this wedge did so inside the hook! Base ball sized hail was falling at the time! Very nice indeed but you have to be the type of person who does not mind replacing windshields! Tim suggested he would rather miss a tornado even the wedge than to head on inside and get banged by hail base ball and larger!

We then made our way northeast but the storm had weakened. Next we headed to route 287 back to Fort Worth. We photographed and filmed the hail to 5cm as it came across only to find that a gust front tornado had spun up 5 miles away! This killed one person unfortunately after being crushed with something heavy.

So yes, we were close but unfortunately unable to intercept without being in harms way! The second tornado would have also meant that we would have had to remain in 2inch hail becoming larger - this HP supercell was dropping hail to 3 inches! The supercell was very nice and there is nice timelapse as well as images of this. I will attempt to find a way to post images at some point over the coming week!

Regards,

Jimmy Deguara
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #4 on: 17 April 2007, 01:40:43 AM »
Hi,

Here are the latest pictures:

http://australiansevereweather.com.au/photography/photos/2007/jd20070413.html

Serpentine funnel or should we call it a rope funnel

The mesocyclone that most likely produced the wedge tornado about 10 miles further northeast


Hail west of Fort Worth falling from the second HP supercell we chased - the storm moved east and generated one or two tornadoes as well as hail to 3 inches in diameter - one person killed.


Regards,

Jimmy Deguara
« Last Edit: 17 April 2007, 03:04:53 AM by Jimmy Deguara »
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Offline David C

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RE: April 12-17, 2007 US Severe Weather
« Reply #5 on: 17 April 2007, 05:13:18 AM »
Nice post macca, good stuff Jimmy.  Shame you missed the wedge but it looks as though you wont have too wait too long for redemption (21st still looks the goods).

I've just arrived back (in Sydney) from LA this morning and only watched this event unfold briefly via the weather channel. Yes, it was a narrow corridor of instablity and the tornadic storm motion did appear to take it north of the warm front somewhat quickly but the wind shear was great and sure worked nicely on the main cell. Were you expecting a squall line to develop so quickly ?
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #6 on: 17 April 2007, 07:42:23 AM »
Were we expecting a squall line to develop so quickly?

I guess I have to draw upon Chuck Doswell's comments from the past; that outbreaks are difficult to predict. When I saw a linear mode evolving ahead of the dryline, I was getting worried. However, it remained detached and ahead of the dryline allowing this supercell to develop. The cap to the south supressed convection for long enough and for sufficient time to allow a window of opportunity (a one hour long intensity of tornadic supercell structure behaviour is short lived given the wind shear in place).

The good thing is that we did manage to stay ahead of the developing squall line and end up near the next supercell and tornado producer - preliminary report here:

http://www.srh.noaa.gov/fwd/tor041307.htm

The warm front not lifting far enough saved Wichita Falls from a catastrophie!

Regards,

Jimmy Deguara
« Last Edit: 14 January 2008, 06:01:31 AM by Jimmy Deguara »
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #7 on: 18 April 2007, 07:45:25 PM »
Hi,

There is a small chance for tornadoes today 17th April 2007 here in north central Texas so I thought I would extend the period for this thread.

Although I don't like the direction of the 850hPa winds, nor the warm temperatures at the 300hPa layer though I am not going to be choosy on my annual vacation. My target is in the vicinity of 32N 98W and move east of there - 2 hour drive from Dallas. Storms move into a slightly better wind profile but cooler surface temperatures. I do agree based on the soundings that low topped supercells are in order. I'll be headed out with Tim perhaps around midday.

Enjoying my time here in Texas!

Regards,

Jimmy Deguara
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Offline Mike

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RE: April 12-17, 2007 US Severe Weather
« Reply #8 on: 19 April 2007, 08:40:16 AM »
Yeehar!

 Hi there Jimmy, good to see you made it safely.  Certainly some nice reports coming out of the US from those from the forum who are chasing.

Look forward to checking each day for updates - happy chasing.

Mike
« Last Edit: 19 April 2007, 04:46:07 PM by Jimmy Deguara »
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #9 on: 19 April 2007, 04:48:41 PM »
We didn't end up heading out. It turns out that 5 miles from where we live there was an F0 tornado along the gust front! Goes to show just what can occur at any time here in strong wind shear scenarios and steep lapse rates.

Regards,

Jimmy Deguara
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Offline Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #10 on: 19 April 2007, 05:31:46 PM »
Hi,

I am referring back to the 13th here and post a few links to the event chased thanks to Amos and Eric:

http://www.mesoscale.ws/07-documents/070413.htm   video of supercell and wedge tornado Seymour

http://www.mesoscale.ws/07-documents/070414.htm   aftermath of baseball sized hail Tarrant County in Dallas.

Sam Barricklow's report on the event:

http://www.k5kj.net/20070413.htm

Regards,

Jimmy Deguara

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RE: April 12-17, 2007 US Severe Weather
« Reply #11 on: 22 April 2007, 11:54:12 AM »
Damn Jimmy, Thats 1 fat wedge tornado but amazing how tornadoes develop quickly, must be very good storm season over there. :)


Cheers,

Michael Mark Rampton