Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
722 ACUS02 KWNS 180557 SWODY2 SPC AC 180555 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1255 AM CDT Wed Sep 18 2024 Valid 191200Z - 201200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN A PART OF THE UPPER MIDWEST... ...SUMMARY... Scattered severe thunderstorms are possible from mid-afternoon to mid-evening on Thursday across central/eastern Minnesota, northern Iowa, and western Wisconsin. A few tornadoes, isolated very large hail, and damaging winds may occur. ...Upper Midwest... A vertically stacked cyclone near the ND/SK/MB border area on Thursday morning will advance northeast across MB through the period. Mid-level west-southwesterlies will remain enhanced (35-50 kt from 700-500 mb) through the afternoon to the south-southeast of this cyclone, before the system as a whole weakens and progresses farther away from the Upper Midwest. At the surface, a cold front will arc south, and progress east over MN/IA. Ahead of the front, a confined plume of mid to upper 60s surface dew points will be maintained, but will decrease within a drier boundary layer deeper into WI and IL. The aforementioned flow regime will yield initially steep mid-level lapse rates from 7-8 C/km through early afternoon. Coupled with the rich buoyancy plume, moderate to large MLCAPE of 2000-3000 J/kg appears plausible, northeast of the Mid-MO Valley. Guidance spread does exist with the timing and related spatial placement of initial thunderstorm development along the front. Some models suggest scattered convection should form during the early to mid afternoon, while others are delayed until the late afternoon. This may be related to the strength of the elevated mixed layer and timing of weakened MLCIN. This uncertainty within a limited longitudinal extent of greater threat, precludes a level 3-ENH risk. The conditional large to very large hail threat appears favorable amid a vertically veering wind profile with height that will support initial supercells. While some clustering is possible owing to frontal convergence, the bulk of severe convection may remain tied to semi-discrete supercells given the nearly perpendicular mid-level wind profile to the front and moderately enlarged low-level hodograph. This should support potential for a few tornadoes as well. Clustering should become more prominent after sunset, but the severe threat is expected to diminish soon after dusk as activity outpaces the nocturnally shrinking buoyancy plume. ..Grams.. 09/18/2024 $$