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Wednesday, June 23, 2021

SPC Jun 23, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Wed Jun 23 2021 Valid 231300Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF NEBRASKA...NORTHERN MINNESOTA...NORTHERN WISCONSIN...AND WESTERN UPPER MICHIGAN... ...SUMMARY... Strong to severe storms are expected late this afternoon into tonight across parts of the central Plains and Upper Midwest. Damaging winds and large hail should be the main threats. ...Synopsis... In mid/upper levels, height rises are expected across much of the central/eastern CONUS as synoptic troughing deamplifies, while a low lingers quasistationary offshore from the Bay Area in CA. In between, large-scale ridging will persist from northern MX north- northwestward through the Rockies. A shortwave trough -- apparent in moisture-channel imagery near the Grand Canyon -- is expected to move northeastward through the mean ridge and cross the central High Plains tonight. Farther north, a positively tilted shortwave trough now over central parts of AB/SK should move southeastward across much of southern MB and into extreme northwestern ON by 00Z. This perturbation then should pivot eastward across northern ON and weaken overnight. The surface cold front associated with that northern-stream trough is expected to move southeastward from Canada across much of the northern Plains today. By 00Z, the boundary should reach northwestern MN, eastern ND, northwestern SD, and central WY. By 12Z, it should be located from western Lake Superior to southwestern MN to the NE Panhandle. A moist axis was drawn from the middle TX Coast across western OK to western KS, between the dryline and a decaying warm front over northeast TX and central OK. 60s F surface dew points reached west-central/southwestern KS, with low 70s as far north as the Red River region near SPS. As moisture returns northward through the warm sector today, a dryline will become better-defined from south to north near a lee trough, located over western parts of NE, KS and TX. Along the trough, a surface low should develop this evening over eastern CO and migrate to western KS by the end of the period. ...Central Plains... Isolated to scattered, high-based thunderstorms are expected to develop from late this afternoon and evening -- most likely over portions of northern and western NE, but perhaps as far as southern SD or northwestern KS. Damaging gusts and large hail will be noted from the most sustained/intense convection, the probability of which remains greatest across central and perhaps parts of eastern NE through the evening. Activity should diminish overnight across eastern parts of the outlook area as it encounters strengthening CINH. Although height rises and a general lack of substantial mid/upper support are expected through most or all of this afternoon, returning moisture and very strong surface heating should combine with lift on the trough and a prefrontal confluence axis to support development. Deeply mixed subcloud layers and steep low/middle- level lapse rates will combine with returning moisture (some offset by vertical mixing) for to peak MLCAPE in the 2000-3000 J/kg range, atop 1000-2000 J/kg DCAPE. With time into evening, the approaching central Rockies/High Plains perturbation aloft may provide some modest mid/upper lift as well, helping to maintain a favorable convective environment, in concert with the outer left portion of a 40-50-kt southwesterly LLJ. While low/middle-level winds will be modest outside the nocturnal LLJ, strong upper/anvil-level flow will contribute enough deep shear for some storm organization. ...Upper Midwest... Isolated to widely scattered thunderstorms should develop from late afternoon through evening, at first near the front over portions of southeastern MB, northwestern MN, adjoining portions of ON, and perhaps northeastern ND. Convection should move southeastward across the outlook area this evening into tonight, increasing in coverage, while offering at least isolated severe gusts/hail. The southern fringe of DCVA aloft (preceding the northwest-flow shortwave trough) will support a favorable mid/upper-level environment for convection near the cold front. As the front impinges upon a diurnally destabilized boundary layer, low-level warm/moist advection from the southwest will help to maintain minimal MLCINH into the evening while supplying increasing low-level theta-e to the inflow region. These processes will provide ample lift and favorable buoyancy, with steep midlevel lapse rates helping to support MUCAPE in the 2000-3000 J/kg range. Forecast soundings show effective-shear magnitudes in the 35-45-kt range, possibly reaching 50 kt in some locales, which favors a supercell/multicell modal mix. Upscale clustering/growth of activity is possible overnight, which would augment the damaging-wind threat, to the extent severe downdrafts can penetrate a nocturnally cooling near-surface layer. ..Edwards/Broyles.. 06/23/2021 Read more LIVE: spc.noaa.gov
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