2014-08-19 FCST: IL/IN

[Skip Talbot]

Interesting shear/instability combos over IL/IN on the last couple runs of the NAM. 12z NAM has an interesting setup with modest diverging northwest flow aloft and what looks like a weakly defined NW to SE orientated warm front extending from central IL into southern Indiana/KY. It looks like a lobe of the EML is pushing into southwest Illinois with temps in the 90s and dews in the 60s. On the cool side of this, surface winds back from southwest to southerly or even southeasterly with moisture pooling into low 70s dews, resulting in a strong to extremely unstable airmass: 3000-4000 j/kg mlcape. Coupled with 40 knots of effective shear and 200 m2/s2 effective storm relative helicity we should have a favorable environment for supercells and perhaps a tornado on the cool side of that boundary where the surface winds back and hodos have small loops. I'd expect initiation probably north of the boundary in eastern Illinois towards a weak, sloppy area of low pressure by mid to late afternoon and for cells to track southeast into Indiana morphing into an MCS. Air mass looks capped to surface based convection south of this boundary:

 

[Skip Talbot]

Woah, what happened to that surface pattern? Looks like the NAM and I were wishcasting a bit. That closed surface low and boundary is not showing up on this morning's runs, and flow at the surface and aloft looks quite a bit weaker. We've still got some decent instability and shear combos for severe weather, but I'm not seeing any real focal point or the hodos for supercells. Weak southwest winds at the surface and 20-30 knots of WNW flow aloft is making for some small, disorganized hodographs and lackluster directional shear. I've got the van ready to go home, but I'll probably keep it in the garage today unless something blows up nearby. There could still be some short lived supercells embedded in this MCS tracking west across IL right now, and perhaps along the I-57 corridor by mid afternoon when the parameters are maximized. Looks like a pretty meager crapshoot from a chasing standpoint though.