SPC Day 1 Convective Outlook — 06Z Forecast Discussion on 21 May 2024

913 ACUS01 KWNS 210555 SWODY1 SPC AC 210554 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1254 AM CDT Tue May 21 2024 Valid 211200Z - 221200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS IN PARTS OF IOWA...NORTHERN MISSOURI...NORTHWEST ILLINOIS...SOUTHWEST WISCONSIN...AND FAR SOUTHEAST MINNESOTA... ...SUMMARY... An outbreak of severe thunderstorms, including the potential for strong tornadoes, is probable today, peaking in the afternoon to early evening. The area most likely to be impacted will include the Mid to Lower Missouri Valley across the Mid to Upper Mississippi Valley to the Lake Michigan vicinity. ...Mid-Lower MO Valley into the western Great Lakes... An MCS and other remnant storm clusters are expected to be ongoing across parts of the Mid-MO Valley to Upper MS Valley at 12Z. How exactly this morning activity evolves eastward is somewhat uncertain. An initial bout of severe wind and hail may persist with this activity through midday. This will likely be over a focused mesoscale corridor, given a stout elevated mixed layer to its south and progressively limited instability to the north. Quick air mass recovery is expected in the wake of this early-day convection within a progressive/highly dynamic synoptic pattern. Pronounced surface cyclogenesis is expected with amplification of a vigorous shortwave trough from the central High Plains to the Upper MS Valley. This will yield substantial amplification of low to mid-level flow, with a belt of 70+ kt 500-mb winds extending from eastern KS and western MO through at least IA this afternoon. As the surface cyclone deepens, an attendant cold front will surge eastward into central IA through northwest MO by 00Z. Very steep mid-level lapse rates will overspread the increasingly moist warm sector, supporting aggressive diurnal destabilization especially on the southern fringe of lingering convection/cloud cover. Initial supercell development should occur near the deepening surface cyclone by midday to early afternoon over the Mid-MO Valley. Increasingly widespread thunderstorm development is expected through the afternoon from the Upper MS Valley southward to the Lower MO Valley. Deep-layer shear will support an initial mixed mode of semi-discrete supercells and embedded supercells in clusters. But by late afternoon, upscale growth into a fast-moving arc of bowing line segments and embedded supercells is expected. Additional supercells should also form downstream along the effective warm front/remnant large-scale outflow that advances north in southern to central WI. Very large hail potential will be most likely with the initial supercell development, and with southern extent where semi-discrete storms should persist longest. Tornadoes, some of which should be strong, appear most probable from northern MO northward into southeast MN and southwest to central WI from both supercell and mesovortex processes. How far north strong-tornado potential can be realized will be modulated by the degree of destabilization in the wake of the early-day convection, but low-level SRH will be quite large. Damaging winds will become increasingly likely later in the afternoon into the evening with gusts of 75-90 mph possible as a well-organized QLCS forms. This should tend to be maximized across parts of central/eastern IA into western IL and northern MO. QLCS intensity should subside after dusk, as convection outpaces larger buoyancy, but a damaging wind threat could persist into Lower MI through the Mid-MO Valley. ...Ozarks to central TX... Convective coverage will decrease with southward extent as large-scale ascent focuses northward toward the Upper MS Valley and western Great Lakes. Nevertheless, scattered severe storms are possible during the late afternoon/evening near where the cold front overtakes the dryline in the Ozarks. More isolated convective coverage is expected southwestward along the dryline in central TX. Large buoyancy and ample deep-layer shear will support a primary threat of very large hail in TX. While a mix of large hail, damaging winds, and a few tornadoes will be possible over the Ozarks. ...New England and northeast NY... A low-amplitude shortwave impulse/remnant MCV will move across southern QC and northern New England. This will result in a band of enhanced westerly deep-layer flow from northern NY into ME. A seasonally moist airmass will be in place with surface dewpoints into the low 60s F, with strong heating and steepening low-level lapse rates. Modest MLCAPE and effective shear from 30-35 kt will support a few organized cells. Moderately elongated hodographs suggest marginally severe hail will be possible along with locally damaging wind gusts. ..Grams/Barnes.. 05/21/2024 $$