Severe Storm Outlook Narrative (AC)
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118 ACUS01 KWNS 251250 SWODY1 SPC AC 251249 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Tue Jun 25 2024 Valid 251300Z - 261200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE CENTRAL PLAINS AND MIDWEST... ...SUMMARY... Damaging wind gusts and large hail are possible across parts of the central/southern Great Plains and Midwest, late this afternoon and evening. ...Synopsis... In mid/upper levels, an initially zonal pattern from the northern Rockies to the Great Lakes will amplify through the period. This will occur as ridging builds over the northern Rockies ahead of a strong synoptic-scale trough that will move ashore in the Northwest day 2. Downstream, a shortwave trough was evident in moisture- channel imagery aligned nearly west-east over the southern Canadian Prairie Provinces. The middle/western parts of this trough should cross the international border around 00Z, then assume more meridional tilt overnight. By 12Z, the trough should extend across portions of Lake Superior, northern WI, southern MN, and southeastern SD. A separate, smaller and weaker perturbation -- now apparent over southeastern WY -- should move slowly east- southeastward across NE to the TOP/STJ vicinity by the end of the period. Elsewhere, a 500-mb high should strengthen over NM, with an MCV from earlier Mexican convection moving northwestward across parts of southern/western AZ. At the surface, the 11Z analysis showed a low over southwestern WI, with warm front southeastward over southern Lake Michigan, then southward over western IN, and a cold front across central parts of IA/NE. By 00Z, the cold front should extend from parts of Lower MI across central IA to a weak frontal-wave low over eastern NE, then southwestward over southwestern KS. By 12Z, the front should move to southern parts of IL/IN, central MO, eastern KS, and southeastern CO, with weak low(s) over parts of the lower Missouri Valley region. ...Central/northern Plains to Midwest... Earlier separate complexes of convection have merged into a lengthy corridor of thunderstorms from parts of southern Lower MI across the warm front to northern IL. See SPC Severe Thunderstorm Watch 456 and related mesoscale discussions for near-term guidance on this activity, most of which generally should diminish into late morning as it moves into weaker magnitudes of both low-level theta-e and mid/upper support. The trailing/western part may persist into midday or early afternoon, as diurnal destabilization and moist advection boost buoyancy across eastern parts of the outlook area. The main concern should continue to be damaging gusts, with isolated large hail also possible mainly from any relatively discrete cells. Though mid/upper support should remain modest over the central Plains to mid Mississippi Valley today, with the Canadian shortwave trough still well north of the area, weak large-scale lift is possible ahead of the smaller WY perturbation as it approaches. Meanwhile, the airmass near the front, and near the western part of the outflow boundary from the morning MCS activity, should attain strong heating and evapotranspiratively aided surface dewpoints in the upper 60s to low 70s F. By mid/late afternoon, with steep midlevel lapse rates, a corridor of 3000-4500 J/kg MLCAPE should develop from parts of northern IL across IA and eastern NE, perhaps into southern SD. Lift east and northeast of the surface low, in the frontal zone, should contribute further to relatively weak MLCINH and convective potential. From late afternoon into early evening, favorable parameters should be relatively maximized, amid 35-40 kt effective-shear magnitudes. This will support organized multicells, a few supercells, and merging/clustering of convection into one or more southward to southeastward over the lower Missouri and mid Mississippi Valley regions. With time and upscale organization this evening, an early mixed-mode/mixed-hazard threat should evolve toward damaging gusts. Specific areas for development in the high-CAPE corridor still appear uncertain due to weak low-level flow and the subtlety of associated forcing. The threat is far from uniform across the 15%-severe probability area, but mesoscale uncertainties preclude finer-scale refinement at this time. More-isolated post-frontal convection may pose a marginal hail/wind threat over parts of the Dakotas in post-frontal, but still sufficiently moist/unstable flow as well, beneath a strip of ascent aloft that precedes the northern trough. ...Southern Plains... Ahead of the front, most likely along a surface trough from southern KS across northern/western OK to the TX Panhandle, isolated to widely scattered thunderstorms are possible late this afternoon through early evening. Isolated severe gusts and hail will be possible. Hot surface temperatures, a deeply well-mixed boundary layer, and still enough low-level moisture will remain to support uninhibited buoyancy, with MLCAPE in the 1000-2000 J/kg range. Despite veering of winds with height, weak low/middle-level winds will dampen vertical shear, except for some strong upper/anvil-level flow aiding ventilation high aloft. Being strongly tied to daytime heating, this activity should weaken and diminish considerably after about 03Z. ..Edwards/Mosier.. 06/25/2024 $$