3/10/09 FCST: IL/MO/AR/IN/OK/OH

[Ryan Wichman]

MODS: Feel free to add other states to this as necessary.

The last few runs of the GFS and NAM have shown Tuesday afternoon in western IL could be interesting. The CAPE is modest in the 400-700 range, but strong sheer should more than make up for it. DPs are forecasted in the upper 50's with surface temps in the mid 60's. After the past few weeks I would say the models are overdoing the moisture; but with the heavy rainfall this weekend I think these DPs are very realistic. The dry slot looks to come through in the late morning/early afternoon to give the area some strong sunshine (even if it is march) and destabilize the atmosphere. This set up could be similar to what is seen across southern IL today, with descrete low topped supercells along the front. One problem right now looks to be the fast storm motion, moving NE AOA 45mph. The event is still 48hrs+ out, but right now I am excited about the possibility of a chase in my backyard.

 

[V. Gensini]

I always like chasing these early season set-ups to get back in severe weather mode...This is certainly one of "those" early-season dynamical high shear / low instability events that are always worth watching, but will ultimately make you frustrated due to the storm motion speed. This particular system (like most others of its type) will be completely dependent on the amount of mid level drying that takes place after the initial wave of showers in the early AM. If we dry slotted and can get some sunshine, watch out for some low-topped pearls in the warm sector. After 4/20/04, I will always keep an eye on these systems!

EDIT: I have blogged about this set-up for those interested. Enjoy...

 

[Scott Weberpal]

Yowsers, SigTor numbers are quite nice around the Peoria vicinity at 00z. Not so keen on the unidirectional flow above the surface, but with a nice low-level profile and good 0-3km CAPE, could be a nice outbreak of low-topped tornadic sups tomorrow in IL.

http://wxcaster4.com/nam/CENTRAL_ETA212_ATMOS_STP_36HR.gif (time sensetive link)

 

[Adam Lucio]

Im thinking of a target of El Paso tomorrow.

Main concern will be nasty storm speeds again, like this past weekend. I think initiation will be further west along the CF. I want to put myself east of there to give these storms time to utilize what little instability there is (both GFS and NAM had barely over 500.) Nothing worse than being on a tornadic storm when its born and then having it scream away from you only to produce 2hrs later.

I also think with the advancing warm front, that IF there is a shot at some locally backed winds it would be in Central or Eastern IL, im not banking on this however...but I noticed it played a role in Saturdays storms.

Later initiation will work in favor though, once the dry slot moves in and allows for some heating to commence. Surface temps in the 60s and dewpoints in the 50s advecting all the way north look favorable as well.

Not ideal by a long shot, but may be the last chance for a week or more so might as well jump on it.

 

[V. Gensini]

I think initiation will be further west along the CF. I want to put myself east of there to give these storms time to utilize what little instability there is (both GFS and NAM had barely over 500.) Nothing worse than being on a tornadic storm when its born and then having it scream away from you only to produce 2hrs later.

I would argue that in a situation like tomorrow, you would want to be on the storm as soon as it is born. Updrafts tomorrow will be rotating as soon as they are born, and after storms fire you will have to deal with shadowing from anvils (further inhibiting any instability present). This looks to go linear fairly quickly (as did Sunday's convection)... I agree with the storm motion comments, but IMO initiation will probably yield the best chance for tors...

 

[Adam Lucio]

I would argue that in a situation like tomorrow, you would want to be on the storm as soon as it is born. Updrafts tomorrow will be rotating as soon as they are born, and after storms fire you will have to deal with shadowing from anvils (further inhibiting any instability present). This looks to go linear fairly quickly (as did Sunday's convection)... I agree with the storm motion comments, but IMO initiation will probably yield the best chance for tors...

I see what you mean, but the storms [if theyre moving at those speeds] will be gone within minutes. So id rather position myself 15 mins east and then head west to intercept. If the storm forms right where Im at...I may lose it within a couple minutes...if it tornadoes 10 minutes after its born and I lose it within 2 minutes, it still does me no good, thats what I was trying to get at. Either way its a gamble.

i do agree on the storm mode though. Wind profiles are largely unidrectional with some good speed shear, so the chances will be early.

 

[Skip Talbot]

I think we're going to have a similar play tomorrow as we did on Sunday: low topped, semi discrete cells, racing northeast. The unidirectional winds aloft may not be so conducive to a linear mode given that these will be low topped storms. The low level veering is probably going to give us mini supercells. I also agree that initiation is where you want to be for this event. However, that looks like its going to be northern MO. I expect a line to go up southwest of the low at 21z on the trailing cold front. I'm going to position myself in western IL to get one of the northern cells as it comes across the river, and hopefully have a good highway to use so I can watch the base for a few minutes before it leaves me in the dust. The north end of the line might fair a little better with the better forecasted instability, directional shear, and less chance of getting undercut by the cold front. I'm thinking of a 21z initiation near Keokuk and a mature line over Peoria by 0z. I'm shooting for somewhere in between.

 

[sebenste]

I've been watching this for several days now. One of things you've got to remember is that the low is still intensifying, and it will REALLY start intensifying between 22Z-4Z. By the laws of physics, that HAS to back the winds ahead of the cold front. That's the reason why the NAM model has 0-1 KM shear of 300 m^2/s^2. Who cares about anything above 700 mb, folks, in terms of directional shear! Many of these storms won't see the light of day of 500 mb. If they hit 30kft, they'll be good ones given the atmospheric profile. So if all the shear is in the lowest kilometer, these things will spin like a top.

What I really like is the 70+ knot low level jet tonight with a source region of southeastern Texas. 70 dews have been pouring off the Gulf of Mexico all day...and that isn't shallow. Furthermore, the ground is wet---to say the very least---in central Illinois. Despite mixing, and as long as tonight's MCS doesn't wipe everything out, we'll probably see 70+ temps and mid-upper 50 dews. With strong lift from a negatively tilted trough and strong forcing from below with the cold front, we should be good to go.

I agree with Vic---get on a storm as early as you can, because even a cluster of supercells moving at 50-60 MPH puts you at risk of getting into a bad spot really fast.

 

[Jesse Risley]

The GFS is a bit more bullish on the shear, but the forecast moisture return isn't too bad for early March. Obviously the models are not taking much potential daytime heating into consideration, given the universally low CAPE values on both the GFS and the NAM. If the morning crapvection doesn't clear out, 500 might be an achievement. The low level wind fields could be stronger in central IL, but it's sufficient for an early TOR threat before everything dives into the linear end of the pool.

The latest NAM (27 hr.) shows a rather narrow tongue of more noticable instability into far NE Missouri, as I would expect storms to initiate 20 miles either side of the Mississippi River by 2 PM. Forecast soundings indicate that storm speeds should be a bit more manageable than Sunday (app. 45 knots), but still faster than grandma drives.

I think I'll hang fairly close to home at this time, keeping the Illinois River roadblock into consideration for a chase target.

 

[Andrew Pritchard]

I wrote this in my blog earlier this morning...

The models have been fluctuating back and forth a little bit on this one, but it appears a potentially volatile setup will exist across portions of Illinois on Tuesday afternoon and evening. A deepening negatively tilted trough with push into the midwest during the afternoon in conjunction with a very moist and potentially unstable (enough) air mass.

The day will begin with a large area of rain and thunderstorms across the central part of the state in advance of the warm front which can be a very sketchy way to begin a chase day. However, if played right this could aid in deepening the moisture in the area, and even lay out a boundary or two before peak heating sets in and enhance the warm front. Should the dry slot work in strong enough we could see substantial 0-3 km cape values to support low topped supercells in advance of the low and along the warm front in the warm sector across central and western Illinois. More severe storms will be possible along the cold front further south into eastern Missouri.

The extent of destabilization in the warm sector at this point is very unclear, but the potential for a potent afternoon is there. I'll be watching the area from Peoria to Quincy for now.

My main concern is back-building precip along the warm front and boundary laid down by the overnight convection. If we see substantial deepening of the low and quick NE lifting of the precip and a substantial dry slot works into the area it shouldn't take long for substantial 0-3 km cape to set in for minisupercells. However, with abundant moisture I'm weary of back-building on the rain shield keeping us socked in.

I also don't think we are going to see nearly the storm motions we did on Sunday. Yesterday was seriously a league of it's own for motions as those things were moving around 60-70 mph. Tuesday we should be looking at approximately 45 mph I'd guess, and with the central Illinois road network and terrain that should be very manageable for March.

 

[Andrew Pritchard]

Still looks like a decent minisupercell setup, just shifted a little south on the 0Z run which is understandable with such high qpf values early in the day north of the warm front. Even though the low should deepen through the day, the large area of convection should help resist quite as fast a northward surge of the warm front early in the day. However, we should still see a good surge in the afternoon with temperatures again reaching the 70F mark with at least upper 50's dew points towards Interstate 72 along a UIN to SPI to CMI line by early afternoon. Surface cape values will be fairly low, likely around the 500-750 mark, but it's the lowest 0-3 km that we'll want to be watching closely. Subsidence in wake of the precip could allow for good solar heating south of the warm front and resist early development allowing for sufficient destabilization for robust convection. Positive vorticity at 18Z and negative at 0Z implies some deep convection in the area, and the 850 jet will be streaking right through this area. Winds may also tend to want to back slightly, or at least be southerly south of the wf allow for sufficient low level turning for discrete low topped supercells. Right now I'll be watching the Interstate 55 corridor from Springfield to Litchfield, Illinois around 3 PM.

 

[Jared Farrer]

I agree with andrew I think hes pretty much nailed the setup. The latest NAM run really backs the surface winds as compared to previous runs. Im curious though because there looks to be a better instability axis further south from where you guys are talking. Id be more inclined to see these areas play out more favorably. And with the slightly weaker winds (not much but enough) storm motions could be more manageable down there at around 30 to 35 knots!

I am almost certain boundrys will be laid out from late night/morning convection. Very much like sunday, and these boundrys should be discearnable via satellite. I would not be surprised to see a line of mini- sups blow up on any such boundrys. I think seeding will be somewhat of an issue like sunday. But any tailend storms could be promising. This of course will all be amplified if we can get the dry sloting. Which at this point might mean the difference between tornadoes. With that slotting we might get the low level lapse rates we need. Itll be interesting! Yaaaa!

I think this is shaping up into a nice setup guys depending on if we can get some insolation or not? Depending on boundry setup we could get a sort of training setup with cells taking similar paths?

 

[Bill Schintler]

Chase Target for Tuesday, March 10

Chase target:
Havana, IL

Timing and storm mode:
Elevated storms and rain will move north and east of the area by mid-morning. Surface-based convection will fire along an advancing cold front by 3 PM CDT, with low-topped supercells likely during the daylight hours. An isolated weak tornado or two cannot be ruled out.

Synopsis:
A shortwave will eject from the NRN stream trough and phase with the subtropical jet over the Upper-Midwest. Attendant SFC low will track through DVN by 18Z-00Z while a trailing CF will serve as a focus for SVR WX in IL.

Discussion:
Convection, associated with a zone of WAA N of the WF, will be ongoing over NRN IL through the mid-morning hours. By early afternoon, an E/W-oriented WF will surge rapidly NWD; reaching I-80 by 00Z. Warm-sector dewpoints will increase to 60F beneath partially cloudy skies S of I-74; resulting in modest instability with MLCAPE’s AOB 500K/kg. SFC-based storms should rapidly redevelop in the warm sector after 21Z. SRH AOA 250m2/s2 will result as a SWLY 45kt LLJ races over slightly veered SFC flow, while SFC-6km deep-layer shear will likely exceed 70kts beneath a 85kt H5 streak.

- Bill
10:50 PM CST, 03/09/09

 

[Adam Lucio]

0z NAM has bumped up CAPE values a bit....but dragged everything else further south. WF should reach IL/WI border by 21Z and also shows some backing winds under the WF as it surges northward. If this pans out it will certainly help localized shear.

18z GFS is ugly though in all areas, especially cape. Im hoping the NAM solution verifies here. Not liking the idea of a target further south, but if this means the instability will be better in the dry slot then it may be worth the extra drive.

Currently thinking anywhere from Bloomington to Quincy and points south. If the mini sups cant do anything then ill settle for a nice shelf with some hail/wind from the line.

 

[V. Gensini]

18z GFS is ugly though in all areas, especially cape.

Totally agree. I kinda threw it out as I saw the run come in, but so far its QPF has been very good and radar trends show it on the right track. I suspect the lack of CAPE is coming from the enhanced RH's all the way through 21Z which would probably mean cloud cover for a majority of the warm sector, which would have drastic consequences on svr potential.

I just saw Dr. Forbes on the weather channel use "mini tornado outbreak" in Illinois tomorrow