2011-04-15 FCST: IL/IN/KY/TN/MS/AL/LA

[John Farley]

I know everybody is excited about today, but I'm surprised there is not even a thread for tomorrow. In addition to the obvious MS/AL target area, I think there is some potential for a cold-core event in southern and possibly central Illinois. The low-level shear looks good, and while there is not a lot of instability, you don't need a lot for this kind of setup. I like that the low-level directional shear is good, though I wish the upper winds were veered a little more. Another positive is that the 500 mb low is close to the surface low. I think it may come down to whether the atmosphere in the afternoon can recover enough from earlier clouds and precipitation to get strong storms. If it does, and if they can stay discrete for a while, then I think we could have an interesting show in IL east of I-55 and south of I-74. For me, another positive is that although I have morning and mid-day commitments, I should be able to break away by about 1 p.m., which will be plenty of time if things develop, given where the models are now showing the storm. The NAM has the low a bit farther northwest than the GFS, but both bring the front into IL around the time of peak diurnal heating.

 

[Bryan Wood]

I think I will be heading to that area west of Terre Haute, IN along I-70 after I leave work tomorrow morning around 10-11am. This set-up seems to line up with some of the research that Davies has done on tornado events with cold core 500 mb lows...as long as there is enough diurnal heating.

 

[Drew.Gardonia]

I was looking at the NAM and the GFS for tomorrow afternoon, and I was not at all impressed with the CAPE, or the 0-1 KM EHI CIN and the 0-3KM EHI CIN for Middle TN area. Central MS/AL may see more action, but overall not really impressive looking to me. I think the SPC may be overhyping this one a bit unless they're seeing something I'm not, and not only that but until now, there has been very limited interest/discussion for this event on here.

however, as is usually the case with the weather, things can change in a hurry and become very unstable, so you never know.

Additionally outreaching cloud cover well ahead of the storm front will limit diurnal heating (as is usually the case here in the Southeast) which I believe will be a limiting factor in the initiation of these storms here in the Middle TN, North AL/MS area.

 

[Jeff Russell]

Initial target: Emelle, AL (out of the jungle) Time frame: 18-20z NAM & GFS in pretty good agreement, moisture will not be a problem with mid 60s dew points, CAPE around 1000-2500 J/kg depending on which model you believe, LI's around -4 to -6 for the NAM & -8 GFS, Helicity 300-500 m2s2, crossover 0-500mb looks good. Only drawback I see is storm motions of ~35kts, but thats typical for the SE. Will be leaving Atlanta around 6am eastern.

Suggestions and corrections of my forecast will be greatly appreciated.

 

[Bryan Wood]

I'll be headed to Effingham, IL, which gives me plenty of choices. I'm a little bit concerned that the 500mb low seems decently stacked on the surface low and that things might get squally, although the latest SPC outlook mentions broken line segments.

 

[Rob Gardner]

Leaving soon for Mt Vernon, IL. Only question when I get there is whether to make a race for SW Missouri or wait there. If this system were 3 hours north I wouldn`t hesitate on this setup, but with the terrain SE of St. Louis and especially mostly south of I-70 I`m a little bit torn. Storm motions look to be decently slow, and it looks like there's a dry punch on the satellites that should be enough to keep the storms somewhat discrete, and allow for some day time heating behind the convective mess thats already moved through the area.

 

[John Farley]

Looking at this morning's model runs, NAM has backed off a little on instability in the northern target area compared to yesterday (and the RUC is similar, but a little better), but the HHHR has CAPE as high as 1500 in parts of SW IL. The models this morning also look a little slower on the progression of the front, so I think the storms could go severe as far west as the STL area and points south/southeast along the Mississippi River. So setting up somewhere near or just south of the Metro-East area looks like a good bet. I was a little surprised to see hatched on the SPC tornado outlook as far north as I-70, but if the HHHR is right that could verify. Sun has been out a good bit of the time since the initial rain band moved through, so I think decent de-stabilization could occur. One concern is that the storms could go linear pretty fast, but with good low-level directional shear, I think embedded supercells are possible even if that happens. Individual cells should move almost directly north, and pretty fast, given the direction and speed of the mid-level winds.

 

[Dan Robinson]

The four main concerns I have with this setup are the relatively stacked low, potential for an early line-out, too brief of an insolation period and unfavorable storm-relative winds in the low levels. The quick northerly storm motions will work against the southeasterly surface winds, a red flag on an otherwise nice-looking sounding: ( RUC forecast sounding for 21z ).

Nonetheless, one should never ignore a surface low that has any instability to work with - so it's a chase day regardless.