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

[Scott Lincoln]

I remember specifically another discussion on another board about how the SPC has been putting out a larger number of high risks in recent years. I agree. In past years, there was only a small number of highs each year, maybe 1-3. Now it seems like they are becoming a little more numerous. The SPC has also been over-estimating the extent of severe weather so far this year, especially that high risk for the lower MS valley that was a total bust. They are trying to be sure of the data before they rely on it. The models have been kinda unreliable off and on this year, so that is probably why the SPC has been slow to jump on this system with a moderate risk.

I would agree that many factors are in place to at least warrant the possibility of a good severe weather day. We'll see what they say tomorrow morning I guess.....

 

[Robert Dewey]

Originally posted by Scott Lincoln
I remember specifically another discussion on another board about how the SPC has been putting out a larger number of high risks in recent years. I agree. In past years, there was only a small number of highs each year, maybe 1-3. Now it seems like they are becoming a little more numerous. The SPC has also been over-estimating the extent of severe weather so far this year, especially that high risk for the lower MS valley that was a total bust.

The only thing I can say is that you must go with the data you have. When the models and SFC data show an outbreak, then that's what you go with. The requirements for a MDT risk really aren't all that high, and most MDT risks this year HAVE verified. You can't win 'em all...

 

[Gabe Garfield]

As far as unidirectional shear is concerned, it's not necessarily all that bad for helicity, provided you have an easterly storm motion at a reasonable rate. Also, climatologically speaking, you don't need all that much directional shear in the southeast United States to get big tornadoes (although directional shear doesn't hurt), as long as you have good moisture/instability.

The strong kinematics have me thinking significant tornado outbreak...provided that the model progs of moisture/instability are correct (and storms can remain disrcrete--which might take some doing).

The other thing that has me intrigued is the potential for an outflow boundary. Strong warm air advection tonight will likely induce a number of elevated/surface based storms over AR/LA/MS tonight. The cold pools from this activity may allow several outflow boundaries to form. Not only do outflow boundaries enhance horizontal vorticity (through baroclinic generation), but they also would serve as a possible mechanism for convective initiation that would help to keep to keep a storm discrete for longer (ahead of the dryline/cold front where most of the forcing for convection will be). At any rate, if there is an outflow boundary tomorrow in the outlooked area, watch out.

Gabe

 

[Robert Dewey]

I'll probably get a more detailed outlook up in the FCST section as the 0Z NAM rolls in, that way all of the energy and whatnot is well sampled, and models begin to really converge on a solution. I think I'm also gonna run the Workstation ETA tonight on a 7km grid over the general target area and see what that comes up with...

 

[Ben Prusia]

I am right now making plans to leave for Little Rock, AR. The severe weather threat has shifted south and as of the 18Z GFS/NAM, it appears that the best chances stand for along the Interstate 40 corridor. I will leave at 1:00-3:00 tonight, depending on the Day 1 SPC Outlook at 6Z. Chance right now of myself chasing is at 70%, it all hinges on tonight's model runs and SPC Day 1 Outlook.

 

[Dick McGowan]

If anyone wants to head out of the KC metro area tonight......pm me .........

 

[Andrew Pritchard]

This is definetly a no go for me now. Looks like the models are pushing it further south every run...now, it looks like the best junk might be in southern Arkansas, and I'm just not commiting to that. Now with the uncertainties, and possibly bad terrain. I'll just be patient and wait for Illinois' season to kick off.

 

[Robert Dewey]

Yeah, I thought the 12Z NAM was bad, but this system is getting weaker and weaker with every run. I think the MDT risk is in pretty good shape,and I would be surprised to see a HIGH. Haven't looked at anything more than the theta-e fields and wind fields, but this really looks like a squall line event. Still, the degree of instabiliy, shear, and forcing would all suggest that a few tornadoes are possible, but I really don't think *widespread* tornadoes are likely.

As Andrew mentioned above, the NAM is going further and further south/east with each run, though it's probably pretty close to the "truth" now.

 

[Ben Prusia]

I also thought the 12Z NAM was bad, 0Z is way worse! It shows no upper level system at 0Z Saturday anywhere in Arkansas. I still believe that the system is there, just the NAM might be doing a hiccup (and not showing it, because it is there at 18Z Friday and at 6Z Saturday), but I will look at the 6Z NAM run when I wake up around 2:00-2:30. I still believe there is a risk for significant severe weather along the Interstate 40 corridor between Little Rock and Memphis, but what goes on tomorrow and what the models say will happen are two different things.

 

[Rich Thompson]

The SPC has lowered the threshold for MDT risks a little from the last few years, but that hasn't been why more have been issued this spring. Unfortunately, several of the forecasts (especially the TOR probs) have been a little too optimistic and just haven't worked out well.

The past several model runs should drive one thing home...it's almost never easy to get any details right beyond day 1-2! When you look at the larger scale flow regime, the flow is confluent across much of the area from the Rockies to the Plains. In a confluent flow regime, you normally expect waves to *lose* amplitude, thus the strong deepening forecast in previous runs looked a bit suspect and premature to me. Now I see the Eta continuing to trend toward a weaker solution, in agreement with the background pattern. That doesn't mean there won't be a few chaseable tornadic storms in ern AR, but sure suggests a little caution in approaching the outlook.

Also, remember that tornado outbreaks rarely ever happen. I tend to approach forecasts from the standpoint of failure modes, of which there are usually a few. In tomorrow's case, the weaker system may actually help focus the chase area to E of the low and along the warm front. A potential failure mode is air that's too cool/stable N of the boundary, and stronger low-level shear remains confined to this region of cool air. I don't think this will be a show stopper in this case, but it's something to consider. Also, farther S pushes everything closer to that awful stretch of the MS River where there's ONE river crossing between MEM and Greenville (~140 miles). We'd better all hope it stays in AR during daylight, or the threat clearly shifts into NW MS before dark.

Finally, if we happen to get the maritime tropical air back from the western Gulf, be prepared for a substantial drop in boundary layer visibility with the Mexican smoke plume. At least an early transition to a squall line appears unlikely to me given the low amplitude of the system and a fairly warm plume in the 850-700 mb layer spreading newd from TX :^)

Rich T.

 

[nickgrillo]

Originally posted by Ben Prusia
I also thought the 12Z NAM was bad, 0Z is way worse! It shows no upper level system at 0Z Saturday anywhere in Arkansas. I still believe that the system is there, just the NAM might be doing a hiccup (and not showing it, because it is there at 18Z Friday and at 6Z Saturday)

Agreed... I think something is wrong with this run's NAM. I'm looking towards SE AR tomorrow, BTW...

 

[Mikel Shively]

Originally posted by APritchard
This is definetly a no go for me now. Looks like the models are pushing it further south every run...now, it looks like the best junk might be in southern Arkansas, and I'm just not commiting to that. Now with the uncertainties, and possibly bad terrain. I'll just be patient and wait for Illinois' season to kick off.

Agreed. I dont want to drive 8 hours for a questionable event. I will stay at home and armchair chase and wait till closer to home later on.

 

[Amos Magliocco]

I still have hopes for this setup and believe it will produce a few tornado reports and a large number of wind and hail reports as well.

I agree with Rich (and Glen earlier today) that the slower, weaker shortwave is a great help in our case, allowing more time for the warm sector to juice up and destabilize (though the part that counts is really only the swath of chaseable turf on the eastern side of the state), and reducing the massive forcing we saw in earlier runs that had us all guessing about how quickly we'd transition to linear. I think that because of the less vigorous kinematics, stronger cap, and some indication of subsidence aloft (sometimes a bad thing, sometimes just what you need!), that we have a much higher chance of discrete convection for more than the two hours I was hoping for earlier. On top of that, our storm motions have slowed a little, which means the storms could potentially fire, mature, and produce tornadoes before they leave the eastern seaboard of the CONUS.

I like the I-40 corridor from Little Rock to West Memphis, and fifty miles either side of that line.

Anyway, the warm front tomorrow is progged further south, and winds in and around it remain backed longer than on previous runs yielding 0-1k ambient SRH values over 300 m2/s2 in areas where one could reasonably argue storms might reach. 3k SRH is even higher, around 350 m2/s2, a nice signal that supercells could be the favored mode for longer than expected. We weren't looking at those sort of values before because of the speed and strength of the various features aloft and surface.

There's all sorts of downside potential, too, like cloud cover associated with an advancing 850mb warm front that outpaces the surface boundary, as pointed out by a friend of mine, the Mexican smoke Rich mentioned, or the simple unraveling of the whole show if the shortwave comes out even slower and weaker than the 0z NAM depicts. Who knows?

But it seems that tornadoes aren't always about bombogenesis and negative tilt crashing shortwaves with MDT risks and dancing bears. I think there' s a chance again early next week that we could see more setups where a subtle balance of ingredients yields chaser joy in small doses.

 

[Todd Beal]

One thing that is of concern to me is that the Beale Street Music Festival kickoffs Friday afternoon and evening here in Memphis.

There could be upwards of 15-20k people downtown tomorrowing evening.

 

[George Tincher]

Good points Amos. While this may not be the high risk some envisioned earlier, it may actually be a better chase event. Bigger doesn't necessarily mean better. I think the likilihood of catching and videoing tornadic sups has gone up, while the overall threat may have slightly gone down.