SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1249 AM CDT Wed Jul 28 2021 Valid 281200Z - 291200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM EXTREME EAST-CENTRAL MINNESOTA INTO SOUTHEAST WISCONSIN THIS AFTERNOON INTO TONIGHT... ...SUMMARY... Severe thunderstorms, capable of widespread damaging wind gusts, very large hail and several tornadoes, are forecast across portions of Minnesota and Wisconsin late this afternoon into tonight. Isolated strong storms also are possible across parts of the Mid-Atlantic and may produce locally damaging wind gusts. ...Upper Midwest... A favorable pattern will exist over MN/WI toward Lake Michigan today, supporting intense severe thunderstorms and possible derecho development later this afternoon/evening. As a result, a Moderate risk has been included from extreme east-central MN into southeast WI with the initial Day 1 Outlook. An upper shortwave impulse is forecast to migrate atop the Plains upper ridge and amplify as it traverses the Upper Midwest/Great Lakes region this afternoon into tonight. In response to midlevel height falls, and low level cyclogenesis, 850-700 mb flow is forecast to increase to 45-70 kt. At the surface, a reservoir of substantial moisture is in place, with dewpoints from the mid 60s to mid 70s F. While cloudiness and some early morning elevated convection are possible across parts of MN/WI in a strong warm advection regime, steepening midlevel lapse rates and at least pockets of strong heating will result in strong to extreme instability by afternoon. A surface low is forecast to shift east/southeast from eastern ND into southeast MI by early Thursday morning, sweeping a cold front southeast across the region. The cold front is forecast to extend from northeast MN into northern SD late this afternoon, while a warm front extends southeastward from northeast MN into central Lower MI. Capping will limit convection initially, however increasing large scale ascent associated with the upper shortwave impulse as well as frontal forcing/low level convergence should overcome inhibition. Any initial semi-discrete convection is likely to be short-lived, with rather fast upscale development anticipated. Nevertheless, strong vertical shear and enlarged low-level hodographs within an extremely unstable airmass will support supercell structures (both initially and embedded within developing bow echo/derecho). Very large hail (greater than 2 inches in diameter) will be possible early in convective evolution. As upscale development occurs, a transition toward intense, damaging wind gusts is expected, with a corridor of 65+ kt wind gusts possible. Tornadoes also are possible both early on near the triple point/warm front and once an organized bow develops via mesovortex processes. ...Mid-Atlantic Vicinity... Modest deep-layer northwesterly flow will be in place today as an upper trough pivots eastward off the Atlantic coast. A very moist airmass is in place with dewpoints generally from the upper 60s to mid 70s F. Strong heating will result in strong destabilization during the afternoon. Nebulous large-scale ascent is forecast, and should limit coverage, with isolated thunderstorm development expected during the afternoon/evening. While low-level winds will be light, southerly flow beneath northwesterly winds aloft will result in moderate effective shear magnitudes, aiding in at least briefly organized cells/clusters. Steep low-level lapse rates and high PW values will support isolated damaging gusts through early evening. ..Leitman/Bentley.. 07/28/2021 Read more LIVE: spc.noaa.gov
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