Scattered severe thunderstorms with large hail, damaging winds, and a couple of tornadoes are possible across parts of the Corn Belt in Iowa, Minnesota, Nebraska, and South Dakota this evening into tonight. Scattered damaging winds are possible across parts of Virginia and North Carolina this afternoon.
The primary change to the Day 1 20Z Convective Outlook update was to make minor eastward expansions of wind probabilities into eastern IA, as some of the latest convection allowing guidance depicts an MCS persisting into this region. Otherwise, the previous forecast (see below) remains on track, with very small adjustments made to thunder/severe probabilities to account for observational trends and/or guidance consensus updates.
Late morning surface analysis places a stationary front draped from west to east from the mid MS Valley eastward into the Mid-Atlantic states. Visible satellite/radar mosaic imagery shows a couple of weak MCVs moving east over the northern VA/MD vicinity and over far eastern KY. The latter disturbance will probably prove instrumental in aiding storm development across VA into northern NC this afternoon.
South of the front, rich low-level moisture and heating will act tooffset weak mid-level lapse rates and yield moderate to strongbuoyancy from southeast MO eastward to the Mid-Atlantic coast.Early indications of a swelling cumulus field over the WV/VAmountains will gradually evolve into a cluster of storms asconvective inhibition continues to erode through early afternoon.Other clusters of storms will develop and move west to east givenmarginal deep-layer mean westerly flow. Although small hail0.50-1.00 inch in diameter is possible with the strongest cores, theprimary hazard with the stronger storms will be damaging gusts(50-65 mph) with the more intense water-loaded downdrafts. Thisactivity will likely diminish by the early to mid evening.
Gradual low-level moistening is forecast through this evening ahead of a weak front forecast to be in place over the eastern Dakotas into western NE. An initially appreciable capping inversion will inhibit surface-based storm development until heating gradually weakens inhibition by late afternoon. A very unstable airmass is forecast by late afternoon/early evening, but nebulous forcing lends uncertainty in terms of storm development/timing. Models suggest storms will primarily develop this evening associated with strengthening low-level theta-e advection. Supercells are possible early in the convective life cycle with a risk for all hazards, including the possibility for a couple of tornadoes, but large hail may be the main hazard. Upscale growth into a cluster or MCS is possible near the terminus of a southwesterly 850-mb LLJ with severe gusts the becoming the primary threat. Models vary considerably whether storm intensity is maintained through much of the overnight and beyond the Day-1 period, or if a gradual diminishing in severe occurs.
Initially high-based convection should develop this afternoon across parts of the Great Basin into WY as modest ascent associated with a weak/low-amplitude mid-level impulse spreads eastward across these areas. Steep low/mid-level lapse rates and a deep, very well-mixed boundary layer should support isolated severe gusts with the stronger thunderstorms. Somewhat greater low-level moisture should be present this afternoon across parts of southern MT into northern/eastern WY, generally along/east of the Bighorn/Laramie Mountains. Isolated severe hail may occur with marginal supercells given greater instability/shear forecast across these areas.