SPC Oct 22, 2017 2000 UTC Day 1 Convective Outlook

By Newsroom America Feeds at 22 Oct 2017

SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0257 PM CDT Sun Oct 22 2017 Valid 222000Z - 231200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN/CENTRAL ALABAMA AND THE WESTERN FLORIDA PANHANDLE... ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS NORTHEAST MONTANA AND NORTHWEST NORTH DAKOTA... ...SUMMARY... Isolated damaging gusts and a brief tornado or two will remain possible into tonight across southern Alabama and the western Florida Panhandle. Isolated strong/damaging gusts may also be possible through this evening across the northern High Plains. ...Discussion... Only noteworthy change is the removal of Louisiana and Mississippi from severe probabilities, as the convective cold pool has already advanced east through these areas. Ahead of the cold pool, moist southeasterly low-level flow will maintain sufficient shear and surface-based buoyancy for isolated damaging gusts and a tornado or two into the overnight hours. ..Picca.. 10/22/2017 .PREV DISCUSSION... /ISSUED 1126 AM CDT Sun Oct 22 2017/ ...Southeast LA today and southern AL through tonight... A midlevel trough approaching the MS Valley could briefly evolve into a closed low over the Mid South, prior to beginning to accelerate north-northeastward as a more open wave Monday. An associated surface cold front will continue eastward across LA/MS/AL, to the south of weak cyclogenesis across the TN and lower OH Valleys. Outflow from overnight convection has become the effective cold front across LA/MS this morning, and this boundary will continue to move eastward/southeastward through the day. Farther east, a zone of low-level warm advection, on the nose of the richer low-level moisture and highest PW, has focused convection from the MS coast into southwest AL. Several storms with supercell structures have been observed, per the Mobile VWP hodograph, though the storms thus far have weakened while encountering a more stable environment inland. Some low-end damaging wind potential will accompany the outflow across southeast LA today, and there will continue to be a low-end threat for rotating storms and a brief tornado along the southeast MS and southwest AL coasts. Gradual destabilization is underway to the east of the ongoing warm advection storms, and it appears there will be enough inland warming and moistening to support some expansion of the marginal risk area in AL through tonight. ...Northern MT/northwestern ND this afternoon/evening... A strong mid-upper jet accompanying a shortwave trough will progress eastward from the northern Rockies to the northern High Plains, before turning southeastward around the northeastern periphery of a ridge along the Pacific coast. Surface heating and residual low-level moisture should be sufficient to support very weak surface-based CAPE this afternoon/evening from northern MT into northwestern ND, along and immediately south of the path of the associated surface cyclone. Given steepening low-level lapse rates with the weak buoyancy, some low-topped convection could result in downward transport of high-momentum flow just above the surface. Read more



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