SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1240 AM CDT Sat Aug 28 2021 Valid 281200Z - 291200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER GREAT LAKES/UPPER MISSISSIPPI VALLEY REGION... ...SUMMARY... Scattered severe storms are expected from the western Upper Great Lakes/Upper Mississippi Valley region, southwestward to the central High Plains. Damaging winds should be the primary severe risk. ...Upper Great Lakes/Upper MS Valley... Northern Rockies upper trough is forecast to shift east into the northern High Plains by late afternoon as one weak disturbance translates across the eastern Dakotas early in the period. Primary surface low will advance slowly east across southern SD in response to the main 500mb speed max that will eject into southwestern SD by 29/00z. This early-day feature will likely be responsible in part for ongoing convection at the beginning of the period. Isolated thunderstorms are beginning to develop west of MBG at 05z this morning, and this activity should continue to expand in areal coverage prior to the start of the period as it spreads east/northeast. Low-level warm advection will also aid this activity as a well-defined warm front orients itself northeast from the surface low-southern MN-northern WI. While the morning convection may generate isolated severe wind/hail, the primary concern will be later in the day as strong boundary-layer heating develops across southeastern SD into southwestern MN. Forecast soundings for RWF/MML/FSD suggest surface-based parcels will be uninhibited by 22-23z as temperatures rise to near 95F. Lower 90s may be observed as far east as the MSP area. This will result in abundant buoyancy within a favorably sheared environment for organized updrafts. It appears frontal forcing will ultimately be instrumental in storm-mode evolution; however, initial activity may be discrete and a few supercells should be observed, in addition to organized clusters/line segments. With time there is reason to believe one or more elongated linear MCSs could emerge. This activity should spread east-northeast across a reservoir of buoyancy that should easily support potentially damaging downbursts with bow-type structures. Forecast shear also suggests a threat for a few tornadoes, perhaps even along the advancing squall line. Isolated severe thunderstorms are expected to develop southwest along the front/dryline into NE where surface temperatures will warm well into the 90s. ...Western PA to Western VA... Upper ridging will hold across the eastern US through the day1 period. Seasonally high PW values are currently observed beneath this feature and strong boundary-layer heating is once again anticipated from western VA into southwestern PA. Isolated thunderstorms should develop across the higher terrain but very weak flow does not suggest individual updrafts will be particularly long lived. While gusty winds may be noted with a few of these storms severe probabilities are not currently warranted. ..Darrow/Wendt.. 08/28/2021 Read more LIVE: spc.noaa.gov
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