Well-defined surface low pressure now in the lee of the Rockies is forecast to move toward the Midwest, with NAM and GFS agreeing on its center in NE MO/SE IA tomorrow. Even warmer, more moist air than is already in place will continue to be advected into the Ohio Valley over the next 12-24 hours; all guidance indicates this will result in CAPE > 3000 J/kg across much of the Ohio Valley/Upper Midwest/southern Great Lakes. Aloft, a 500-mb speed max with 40-60 kt, generally WNWly flow should overspread the area of interest, from MO through IL, IN, and OH into central WV. NAM forecasts a significant shortwave trough moving into the area midday.
While forecast CAPE is not on the order of the 5000-6000 J/kg available in the high-end Appalachian derecho of 6/29/2012, the amount of instability is still unusually high for the region. Flow from mid-levels on up will be stronger in this event than in last year's, and the increased shear is somewhat concerning in terms of supercells--whether discrete or embedded in a damaging QLCS. It appears the main source of lift will be near the western end of SPC's Day 2 moderate risk area, in IL and IN, in the afternoon hours. However, the pronounced cap from last year's derecho is absent on forecast soundings, so smaller shortwaves or convergence along any surface boundary may result in discrete convection farther east. Supercell composite forecasts from the SREF are alarming for OH and west WV, should discrete storms develop.
Forecast: Supercells initiate from central IL across IN, possibly into OH and/or far western WV, mid-afternoon. Some of these will pose a tornado threat before growing upscale into one or more lines. These squall lines will be capable of widespread significant wind damage as they turn southeastward from IL, IN, and OH through much of WV, and possibly SW PA. Any organized line of storms will likely continue to produce damaging winds late into the night. Severe hail will also be possible with the strongest storms.
The area most at risk in WV should be from Huntington to Wheeling eastward to the I-79 corridor.
The area most at risk in WV should be from Huntington to Wheeling eastward to the I-79 corridor.
