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Thursday, June 17, 2021

SPC Jun 17, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0739 AM CDT Thu Jun 17 2021 Valid 171300Z - 181200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO TONIGHT FOR PARTS OF THE UPPER MS VALLEY... ...SUMMARY... Severe storms are expected late this afternoon into tonight for parts of the upper Mississippi Valley, from northern Iowa/southern Minnesota into southern Wisconsin and northern Illinois. Wind gusts near or exceeding 75 mph, hail up to the size of baseballs, and a couple of tornadoes will be possible. ...Upper MS Valley this afternoon into tonight... A midlevel shortwave trough over the northern Plains/southern Prairie Provinces has crested the ridge, and heights will be suppressed east-southeastward toward the upper MS Valley and upper Great lakes by tonight. An associated, weak surface cold front/trough will likewise move slowly southeastward across NE/IA/MN/WI through tonight. The front and any lingering outflow boundaries from overnight convection will likely focus afternoon and evening thunderstorm development across southeast MN and along the IA/MN border, in phase with an embedded speed max moving eastward from SD (as evidence by the weak midlevel convection over SD this morning). The thermodynamic environment will feature steep midlevel lapse rates with a warm elevated mixed layer extending eastward from the Rockies, atop boundary-layer dewpoints in the 65-70 F range. Strong surface heating in the wake of morning elevated convection will drive large buoyancy (MLCAPE in excess of 4000 J/kg) and weakening convective inhibition as surface temperatures warm into the 90s. Effective bulk shear of 40-50 kt and the large CAPE will support a threat for supercells initially, with very large hail (up to baseball size) and damaging winds. Any semi-discrete storms in the zone of stronger low-level shear/warm advection closer to the MS River will also pose a threat for a couple of tornadoes. Otherwise, storms are expected to coalesce and grow upscale into one or more bowing clusters with the potential to produce significant damaging winds (greater than 75 mph) prior to the convection spreading east of the larger buoyancy late tonight. ...AR this afternoon/evening... A remnant MCV over west central MO this morning will continue southward over AR by later this afternoon. Strong surface heating (temperatures warming into the lower 90s) and boundary-layer dewpoints in the lower 70s will support MLCAPE in excess of 3000 J/kg, and isolated to widely scattered thunderstorms are expected to form by mid-late afternoon. Deep-layer, north-northeasterly shear of 20-30 kt and DCAPE in excess of 1200 J/kg suggest that any cells/small clusters will move toward the south-southwest with the potential to produce isolated damaging downbursts and large hail for a few hours this late afternoon/evening. ..Thompson/Leitman.. 06/17/2021 Read more LIVE: spc.noaa.gov
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