SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0246 PM CDT Sun Oct 11 2020 Valid 112000Z - 121200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO EARLY TONIGHT FROM SOUTHWEST MN TO EXTREME NORTHEAST KS... ...SUMMARY... Severe thunderstorms are possible late this afternoon and evening across the Upper Midwest and Middle Missouri Valley. A brief/weak tornado or two and isolated damaging gusts will be possible this afternoon across parts of the Carolinas. ...20Z Update.. ...MN/IA/eastern NE this evening through early tonight... Overall expectations outlined in the previous forecast (appended below) remain unchanged. A narrow corridor of upper 50s/low 60s dewpoints currently extends thorough eastern SD and into far southeast ND and southwest MN. Cold front currently extends from south-central Manitoba south-southwestward through central SD and into northeast CO. Convective inhibition is beginning to erode within the corridor of higher low-level moisture, with further decrease in this inhibition anticipated over the next few hours. Primary initiation zone still appears to be near the MN/SD border south-southwestward into central NE around 22-00Z. Large hail is possible initially, before storms evolve quickly into a line capable of damaging outflow gusts. ...Carolinas... Severe threat associated with the remnants of Delta is expected to continue for the next several hours amid modest instability and vertical shear. Primary threat is damaging wind gusts and a tornado or two. Greatest threat potential exists in the vicinity of the warm front, which currently extends from near LUX (in western SC) northeastward to BBP (near the central NC/SC border) and then back southeastward off the far southeast NC coast. ..Mosier.. 10/11/2020 .PREV DISCUSSION... /ISSUED 1122 AM CDT Sun Oct 11 2020/ ...MN/IA/eastern NE this evening through early tonight... In association with a midlevel shortwave trough moving eastward from WY/MT, a surface cold front will progress east-southeastward across the northern/central Plains today and reach the upper MS Valley overnight. A narrow corridor of low-level moistening is underway ahead of the cold front (boundary-layer dewpoints of 56-60 F), though the breadth and quality of the moisture return has been limited by the influence of tropical cyclone Delta across the Southeast. The modest low-level moistening is occurring beneath a plume of steep midlevel lapse rates near 9 C/km, which is also contributing to a strong cap. It appears that surface heating/mixing, continued moisture advection, and focused ascent along the front will all be necessary for thunderstorm initiation this evening, starting around 23z near the MN/ND border and developing southward toward eastern NE/western IA by late evening. Strong/linear forcing for ascent along the front and storm motions undercut by the front support rather rapid upscale growth into a squall line, while the narrow corridor of buoyancy and cap will limit the potential for pre-frontal storm development. The steep midlevel lapse rates and MLCAPE of 1000-1500 J/kg will support some potential for large hail, especially with any embedded supercells given effective bulk shear in excess of 40 kt. Otherwise, convection evolving quickly into a line suggests that damaging outflow gusts will become the main threat this evening. The severe threat will diminish gradually overnight as buoyancy diminishes and convective inhibition increases with eastward extent. ...Carolinas this afternoon... The remnants of tropical cyclone Delta will continue to move east-northeastward from the southern Appalachians and weaken. Vertical shear will remain at least marginally favorable for supercells given lingering 30-40 kt southwesterly midlevel flow and backed low-level winds/stronger low-level shear along a warm front approaching the SC/NC border. The threat for a brief tornado or two will be focused where the convection in central SC (along the primary confluence band) moves northeastward and interacts with the zone of somewhat stronger low-level shear along the warm front this afternoon.
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