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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Wednesday, December 15, 2021

SPC Dec 15, 2021 1630 UTC Day 1 Convective Outlook

SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1025 AM CST Wed Dec 15 2021 Valid 151630Z - 161200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO EARLY TONIGHT FROM EXTREME EASTERN NE ACROSS IA INTO SOUTHEASTERN MN AND EXTREME SOUTHWESTERN WI... ...SUMMARY... Widespread severe wind gusts of 60-75 mph along with at least a few tornadoes are likely from mid afternoon through early tonight across the Mid-Missouri Valley to the Upper Mississippi Valley. Embedded gusts of 75-100 mph and a strong tornado or two are also possible, particularly from extreme eastern Nebraska across western to northern Iowa and southeastern Minnesota. ...A rapidly evolving outbreak of severe storms with widespread damaging winds and a few tornadoes is expected to begin by mid afternoon across eastern NE and continue northeastward into the upper MS Valley by early tonight... ...Mid MO and upper MS Valleys this afternoon into early tonight... An intense midlevel shortwave trough and associated deepening surface cyclone will eject rapidly northeastward today from the central High Plains to the upper MS Valley by tonight. A developing dryline will surge east-northeastward across KS/NE into western IA/northwestern MO to the south of the cyclone, prior to being overtaken by a cold front tonight. An unseasonably moist air mass is spreading northward from the western Gulf of Mexico, with boundary-layer dewpoints near or above 60 F already into central IA. The moistening is occurring beneath a plume of steep midlevel lapse rates near 8 C/km per regional 12z soundings. Destabilization will continue northward through the day across eastern NE/IA into southern MN in advance of the deepening cyclone, with MLCAPE expected to reach or exceed 1000 J/kg. Though the elevated mixed layer plume will act to cap the moistening boundary layer, strong forcing for ascent with the surging dryline and within the left-exit region of the intense mid-upper jet streak will quickly remove convective inhibition in a narrow zone along the dryline by mid afternoon (20-21z), when convective initiation is likely across east central/southeast NE and northeast KS. Extremely strong wind profiles will accompany the ejecting midlevel trough, with speeds approaching 125 kt at 500 mb and 70-80 kt not far above the ground. Effective bulk shear will also be quite strong at 70-80 kt. The strong linear forcing for ascent and cap will support a narrow line of storms, though the very strong deep-layer shear vectors/long hodographs oriented across the boundary, and increasing low-level buoyancy by mid-late afternoon, should allow some supercell structures within the band of storms. Downward momentum transport by the convection will result in the potential for widespread wind damage with peak gusts potentially in the 75-100 mph range, and embedded circulations will be capable of producing a few fast-moving tornadoes (one or two of which could be strong) with both right-moving supercells and embedded QLCS mesovortices. Gradually weakening buoyancy with east/northeastward extent should result in a diminishing severe threat by 06z across WI. The severe threat will also diminish with southward extent, where forcing for ascent will be weaker and the orientation of the flow will be much more parallel to the cold front tonight. ..Thompson/Kerr.. 12/15/2021 Read more LIVE: