2011-04-19 FCST: MO/OK/IL/IN/AR

[Skip Talbot]

I agree with playing the triple point/just east of the surface low. I'd go so far to say that its prefrontal initiation or bust (at least in terms of having a decent shot at a tornado). Surface winds right on the cold front are going to be badly veered, and the forcing from the cold front should kick up a rather solid, linear line after a brief window of initial supercells. If we can get something out ahead of that line, however, I think its game on for tornadoes. Helicity across the warm sector is amazing and coupled with the strong instability, 1km EHI values are very impressive and more than support a strong tornado threat:
http://www.twisterdata.com/data/mod...4/18/18/NAM_221_2011041818_F30_EHI_1000_M.png

The NAM has been consistently showing this environment. What's much less certain is whether we'll see prefrontal initiation. The last few runs have had a stout cap out along the warm front, but a decent hole by the triple point/surface low, which still made the setup look promising. I have noticed a trend toward weakening that cap as the event approaches, however, and given the NAM's tendency to overdo the cap this year I can buy that trend. In fact, the 18z run erodes the CINH all the way to the Indiana border by 0z:
http://www.twisterdata.com/data/mod.../18/18/NAM_221_2011041818_F30_CIN_SURFACE.png

Lift is going to come from a variety of sources. Models have consistently shown an MCS lifting out of the area that morning which should put down some nice outflow boundaries. The nose of this impressive low level jet could kicking something off in eastern IL/western IN:
http://www.twisterdata.com/data/mod.../18/18/NAM_221_2011041818_F30_WSPD_850_MB.png
The shortwave over the IL warm front would help further west:
http://www.twisterdata.com/data/mod.../18/18/NAM_221_2011041818_F30_WSPD_500_MB.png
As well as convergence near the triple point/surface low center hanging out by St. Louis.

I'd love to see that warm front push a little further north into better terrain and where the MS river is further west, but right now I'm eying the IL I-70 corridor for discrete tornadic supercells ahead of the cold front... or at least I'd rather bust there than chasing linear junk through the jungles of Missouri.

 

[Jesse Risley]

It looks like the WF currently situated near the I-70 corridor will be the focal point for elevated convection over the next 12 hours or so north of that boundary. This will be quite the key to how tomorrow evolves, because if convection clears out sooner than forecast and ample destabilization takes place, I can see the WF moving north further than presently anticipated. The models may have a hard time picking up on this as usual, due partially to convective outflows, though given the the steep baroclinic zone in place (AOA 30 degree temperature differences attm), I can see the front being suppressed further south as well (per GFS and NAM). However, the local WRF has been trending the surface low from central MO - UIN - KIKK, with front nudging nearer the US-36 corridor, trending the 60 degree surface dewpoint line up to PPQ-DNV by 00z/20. Then again, this could end up being an outlier, too.

Erosion of the CINH should begin sometime after 21z as forcing for ascent courtesy the H5 perturbation aims towards the Middle Mississippi Valley region. Granted, surface winds are parallel to the cold front across a large portion of the warm sector, with the best 0-1 km SRH values AOA 250 m^2/s^2 across areas S/SE of the triple point. If storms can stay discrete and root in the boundary layer near the WF, then an enhanced TOR thread would exist due to the magnanimous shear profiles where 0-1 km vectors are oriented more normal to the WF. I'd expect everything to have congealed into more of a linear structure, perhaps extending from far NE TX to southern Lake Michigan by 03z, at the latest, as shear vectors really become quite parallel essentially all across the area of concern as the surface cyclone treks into NC IL. The best directional shear is still confined to areas nearest the WF, so that probably holds the best play for substantial tornadoes (besides INVO the triple point) between 21z/19 and 03z/20.

 

[John Farley]

Looks like this evening the NAM has come into line with the slightly faster solution of the GFS on the placement of the surface low/triple point. It does look to me like the surface winds are a little more veered on this NAM run than earlier ones also. I still think the warm front will be the best play if storms develop along it, but I am not sure how far east of the triple point storms will develop on the warm front. It does look like the cap is weak enough to be broken, but the models don't break out much precip along the WF much to the east of the triple point. The HHHR and RUC in the morning might give a better picture of that. I think the best place to chase will be between I-70 and I-72 in Illinois, but exactly where depends on the position of the warm front and how far east of the triple point storms are likely to fire. With the more veered surface winds, I am not sure storms in the warm sector away from the warm front will be a good play.

I have commitments until about 1 or 1:30 tomorrow, but that should give me plenty of time given the nearness of this setup. I think the question of whether to stick around Edwardsville and wait for initiation or position somewhere a little east and/or north might be clearer with the morning runs and looking at real-time data as the day unfolds. I think this setup has considerable potential, but I don't have the sense of this being quite as huge an event as it looked like on earlier runs of the NAM. Time will tell.

 

[Dan Robinson]

One item of interest is that so far, the expected LLJ intensification has yet to get going across MO/IL tonight. Winds in the low levels and resultant convergence on the boundary remain weak, and widespread strong convection that nearly all models had firing on the nose of the jet just after 00z has yet to materialize. This may have implications on the boundary placement tomorrow.

 

[Cody Pryor]

Watching the Cluster of Storms Moving out of the STL Metroplex. I am hoping this will leave the so called boundry layer to enhance shear. I am considering Salem, IL as a staging ground for today. I have a feeling things will go linear fairly quick along the cold front. Salem seems far enough south and far enough north, only concern is weather or not it will be far enough west. I like the possible cell that goes up south of Terra Haute right around 22z. SPC just updated and kinda down plays the tornado chances and pushes more towards the QLCS or possible Derecho event more likely.

Regardless today will have to be monitered for fine tuning. Also, I still pretty much a newbie. So definately open for input.

Cody Pryor

 

[Aaron Shaffer]

Interesting looking at the initial few frames of the 12Z RUC this morning. Looks like, and maybe this is just a pipe dream, it is dragging the low back a little bit further west. We're about to hit the road and head south to Mexico (MO, not the country ) so we'll see how this pans out. Definitely playing the western WF & triple point w/ this one, and hoping for some enhanced low-level shear w/ some backing near the WF.

 

[Tim Marshall]

will remain at the south end of this system knowing that there are better days ahead. I figure on chasing some high based hailers near the Red River this evening. Large T/Td spreads and veered winds ahead of the triple point should will make for a higher threat of hail and downburst winds, than tornadoes. Need to shoot some good HD hail video.

 

[Mike Johnston]

Well, instability certainly shouldn't be a problem today. RUC indicating upwards of 5,000 j/kg. Looks like some decent clearing now over southern Missouri. Hate to say it, but St. Louis is looking very at risk today. Looks like initiation should be around 21-22z. From a chasing standpoint, I suppose the most significant obstacle might be crossing the big river if need be. Correct me if I'm wrong, but from the maps it looks like the only crossing between Cape Girardeau and the St. Louis area is MO state highway 51 from Perryville, MO across to Chester, IL.

 

[Michael Riedl]

On the latest RUC runs as the night goes on the SFC winds start near the triple point, as I figured the would all along, therefore EHI numbers at 01z look pretty dang good. Winds at the Upper Levels still seem pretty weak (45KNTS at 300MB) so I am not too sure how that will affect the set up, but it can't be a good thing. Storm mode should be HP cells with low LCL's, so any discrete storm should have a decent chance at producing.

Central Illinois, around Effingham would be where I would set up if I could be chasing today. Very Interested to see how this plays out.

 

[Chris Koetters]

Latest meso analysis is already seeing 3000 cape, supercell composite of 28, and siggytor of 2 southwest of stl along the warm front. 75 over 65 just south of columbia, mo now

 

[Rob Gardner]

Im not too familiar with how each of the models trends towards reality over the past months, seems like there is conflict between the models on cap erosion. the 11z run of the ruc holds pretty firm in central illinois well through 21z, with the cap eroding in southern most part of Illinois around 20z and northward from there. Nam def erodes between 18z and 21z, and the gfs looks even more promising with some erosion by 18z. 12z Ruc doesn`t run past 19z yet(at least on the site I am on), there is a small notch at 19z northeast of stl. Will be curious how the next run looks. still trying to decide between heading south on 55 and staying just NE of st louis, or coming down 57 to 70 and stopping somewhere around vandalia.

Edit:

Just saw the 13z ruc run is in full, and most of the 14z is in tact on the ruc. Definitely looks like cap erosion right along I-70 between 19z and 21z. Targetting Vandalia, IL for now, will have 70 E and W and 51 N to run with.

 

[Jeff Duda]

Continuing to note what I mentioned in my post a few days ago. The RR and current RUC analyses show rather pathetic shear above about 850 mb with very little acceleration or turning. Thus, the vast majority of the 0-6 km shear shown is from shear in the lowest 1-2 km. I think this will favor a linear storm mode quite a bit over a cellular storm mode.

 

[Chris Gullikson]

I see a narrow window of tornado potential with such cold temps on the north side of the WF. Reminds me of an event in 05 around the Hastings NE area where an expected big tornado threat resulted in prolific hail producers as storms initiated along the WF but quickly crossed into the stable boundary layer.

 

[Glenn Schubert]

Well, instability certainly shouldn't be a problem today. RUC indicating upwards of 5,000 j/kg. Looks like some decent clearing now over southern Missouri. Hate to say it, but St. Louis is looking very at risk today. Looks like initiation should be around 21-22z. From a chasing standpoint, I suppose the most significant obstacle might be crossing the big river if need be. Correct me if I'm wrong, but from the maps it looks like the only crossing between Cape Girardeau and the St. Louis area is MO state highway 51 from Perryville, MO across to Chester, IL.

Mike, You are correct. The only place to cross the Mississippi between Cape Girardeau and St. Louis is at Chester, IL. Unfortunately it is not directly from I-55 and might be hard to find without some GPS system. I plan to head north from Carbondale, IL at 5pm after I get off work. Areas north of Rt 13 in southern Illinois toward St. Louis are pretty good chasing terrain.

 

[Chip Redmond]

I agree, some parts of Illinois there are good to chase in. A few concerns at the moment are: what was already stated, the unidirectional winds, with current analysis having veering winds from 850-500mb; the warm front is still fairly far south which would keep the best probability for tornadoes in river valleys and hilly terrain; and also, the cold front with the majority of forcing is already entering MO as the low is developing. Move that forward to around initiation in about 4 hours and it could prohibit discrete structures and promote linear/bowing storms. There is an area that seems to be relatively un-capped already near the low pressure in SW MO that has a cumulus field already in vicinity. The strongest vorticity is getting ready to move over this environment as it rotates about the low. I wonder if convection will get going in the next few hours and ruin the show for later.

As of my play, I am hanging back and hope to be around west central OH and east central IN around 0z to get some lightning shots later and hopefully a discrete storm out ahead of the linear mess. Should pan out to be an interesting day.

Oh and I almost forgot, boundaries. Looking at temperatures and satellite there doesn't seem to be any depictable at the moment. There is a region of convergence from St. Louis northeastward to Bloomington area. This is the biggest that I can find from last night/this mornings convection.

Chip