04/29/05: TALK: OK/MO/AR/IL/KY/TN

[Glen Romine]

A quick update - additional rounds of convection noted in earlier forecast discussion continue to expand in coverage, as well as downstream across north-central AR. Thermal boundary clearly evident from the LZK reflectivity continues to slowly sag southward, recently crossing south of Hot Springs. Mesoscale analyses showing some weakening of the cap across southern AR, but with limited instability present (mostly high up), and as LLJ continues to veer, convection should remain well elevated next few hours. Narrow axis of heating just ahead of the front (narrow cloud band in vis sat) in northeast Texas appears will be kept at bay by increasingly strong capping until after 00Z. So, I think you have to be patient with central AR still where cap should be more easily broken despite cooler temps. Expecting currently elevated convection to eventually work closer to the surface boundary by late afternoon. Target axis from just south of HOT to PBF looks reasonable, after the LLJ has backed by later this afternoon leading to improved shear profiles (currently rather sad). Otherwise, the 18Z soundings may indicate models are out to lunch - hopefully in a good way for current chasers in the field.

[edit]

Originally posted by Scott Olson

14z ruc anaylsis yielded 1500 Mucape for the 21z-0z timeframe and 250 m/s of 0-3km SRH for the region I was speaking of. Hope that helps.

-Scott.

Ok, thanks Scott. The units of SRH are m^2/s^2, or J/Kg (both are equivalent), not m/s.
[/edit]
Glen

 

[Mike Johnston]

Well, if surface moisture convergence and first sign of dropping LFC's are any indication (both of which oriented on an axis from NE TX sww'd), initiation later this afternoon looks more like E TX / W LA than AR. Pocket of 200 m2s2 helicity at both 0-1km and 0-3km indicated over this area - so now all of a sudden we have a SigTorn index value of 1 popping up over E TX!

Still alot of variables in flux, but this setup is shaping up quite differently than derecho over southern IL we were talking about 2 days ago!

 

[nickgrillo]

Elevated severe convection continues north of the front... I'm thinking that with shear profiles/instability that is currently in place, convecton should become SFC based by 22Z across portions of Southern/Central Arkansas.

Amos, where you at? Kurt gave you a call earlier. We're in Stutgart, currently working with Windows 98 in the town's library, which I'd like to light on fire... :lol:

I'm still expecting plenty of discrete convection from late afternoon and into early evening, and with the strong shear in place, THUS I'm still calling for supercells/tornadoes...

 

[Tony Laubach]

Originally posted by nickgrillo
We're in Stutgart, currently working with Windows 98 in the town's library, which I'd like to light on fire... :lol:

Nick, probably the most intelligent thing you've said all year! :lol:

Glancing at the vis. sat.; it looks like Mississippi is clearing up a bit. Not exactly sure where Amos is or where he's heading, but I definately would think that you're better off south and east of Little Rock... my familiarity with Arkansas is pretty bleak, so I'm not sure how the terrain is down that way... one thing to keep in mind is that the best setup may not be in the best chasable terrain... this is a very unique situation cause terrain does play a HUGE factor in how you'll be chasing...

 

[nickgrillo]

BTW, read the discussion for the new blue box (AR/TN)... It indicates tornadoes will become more likely in the afternoon, and the southern portion of watch may be upgraded to a red box...

 

[nickgrillo]

Tony, LOL....

'Studgart' (did I spell it wrong? Well, I don't really care, anyways... :lol: ) is SE of Little Rock, and we'll probably continue going south as soon as we're done with data, which should be soon (library should kick us out soon, anyways... :lol: )

 

[Robert Dewey]

Originally posted by Scott Olson
14z ruc anaylsis yielded 1500 Mucape for the 21z-0z timeframe and 250 m/s of 0-3km SRH for the region I was speaking of. Hope that helps.

-Scott.

I think the confusion came in with the units you are using. Helicity is expressed as units m2/s2 (meters squared PER second squared).

 

[Glen Romine]

Outflow boundary persists along Arkadelphia-Sheridan-Stuttgart line, those has become increasingly difficult to identify via radar. Addtional trough/boundary extends from roughly Prescott-to Monticello, with warm and very moist conditions, along with veered flow, south of this boundary. Mesoanalyses from SPC indicate narrow weakly capped instability axis along I-30 corridor from Texarkana to Prescott. Weak convection has persisted now wnw of Arkadelphia - which could move close to northern boundary, but may stay just north. But, I'd watch for additional convective development in the Arkadelphia to Prescott area over the next few hours. Tornadic activity most probable with storms moving along the southern boundary, but can't be ruled out along the northern outflow either, which should see action first.

Also interesting to note the axis of poor visibility - Probably the smoke that Rich Thompson warned of - immediately ahead of the front.

[edit]
18z soundings now in. Little Rock looked horrible - but also had some bad data - so I don't know how much to trust it. Further south, Shreveport still showing a stout capping inversion. Radar has shown some enhanced reflectivities to the west, but based on this sounding it appears unlikely surface based storms will erupt there anytime soon.
[/edit]

Glen

 

[Mike Johnston]

Re: anticipation of convective mode, which is always a good little reminder at this time on a chase day, upper-level storm relative winds are 70kt+ generally across the northern 1/2 of AR, an indicator of LP mode, while 50-60kt across southern 1/2 of AR, an indicator of classic mode.

 

[Andrew Pritchard]

New tornado watch out for parts of Texas, Arkansas, Mississippi and Louisiana. If I were to be out, I'd still like to be around southern Arkansas, maybe around the southern ARK, northern MS border. But alas, since I'm not chasing, I havent paid a TON of attention to the day, that's just based on my quick glance.

 

[Mickey Ptak]

First signs of squall line setup is ever so present by the way this satellite image looks.

http://weather.cod.edu/satellite/1km/E_Texas.gif

I am sure with as much shear we have with this system some imbedded supercells could produce some tornadoes but it sure will not be easy to see. I am wondering if and discrete cells will fire ahead of the front?

Mick

 

[Glen Romine]

Well Mickey, it seems the question has been answered, with fairly widespread convection developing along the pair of boundaries discussed above. It seems the very weak and disorganized surface flow is not providing much focus for any particular convective region - whether a cluster or perhaps discrete cells can evolve out of the mass of cells is unclear. Even MUCAPE values are only in the 500-1000 J/Kg range for modestly elevated parcels. Strength of low-level jet still yielding acceptable helicity values, but low-level buoyancy is still limited. Meanwhile, narrow cold-frontal rainband getting started back across Texas - which could support a rogue tornado from extreme ne TX into sw AR. Prospects continue to look grim elsewhere - but perhaps something interesting eventually emerges out of the muck in central AR.

Glen

 

[Skip Talbot]

We're packing it up here in Forrest City, AR. The cloud cover here makes for terrible visibility, and it appears at this hour that all of the cells here in eastern AR are junk. We did catch some structure off a small cell that fired in northeast AR earlier this afternoon before it mushed out after the svr warning was issued. I don't think the day holds anymore chase potential... calling it a bust and heading home.

 

[Scott Olson]

Originally posted by rdewey+--><div class='quotetop'>QUOTE(rdewey)</div>

<!--QuoteBegin-Scott Olson

14z ruc anaylsis yielded 1500 Mucape for the 21z-0z timeframe and 250 m/s of 0-3km SRH for the region I was speaking of. Hope that helps.

-Scott.

I think the confusion came in with the units you are using. Helicity is expressed as units m2/s2 (meters squared PER second squared).[/b]

Right. m2/s2, it's been a long day and im sure an even longer day for those who made the drive. Well, hopefully there is at least some consolation prize in the convection after 0z.
-Scott.

 

[Alex Lamers]

Family of supercells beginning to form over extreme SE AR and into NW MS at the moment. Lapse rates steeper throughout the atmosphere in this area than were earlier found in Arkansas. Jet dynamics look to be quite favorable as well with an increasing LLJ and some slight coupling of upper level jet streaks.

More upscale development appears to be occurring. I think that a few nocturnal tornadoes are not out of the question over NE LA and into C/N MS if any of you are still out on the road.

...Alex Lamers...