SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0232 PM CDT Fri Oct 23 2020 Valid 232000Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF THE GREAT LAKES AND OHIO VALLEY... ...SUMMARY... Threat for a few severe storms capable of damaging wind, hail and perhaps a tornado will continue this afternoon into the early evening over a portion of the Great Lakes and Ohio Valley. ...Discussion... Forecast reasoning as laid out in prior outlooks remains reflective of current convective/severe weather evolution across the outlook area. Some minor line adjustments have been made to account for the western edge of the outlook areas, to reflect progression of advancing convection. Tweaks have also been made to the slight risk area to account for counties currently within WW #501. Otherwise however, Local severe risk will continue across the outlook area through early evening. ..Goss.. 10/23/2020 .PREV DISCUSSION... /ISSUED 1120 AM CDT Fri Oct 23 2020/ ...Great Lakes/OH Valley this afternoon/evening... Within the northern stream, a shortwave trough will progress from the upper MS Valley to the upper Great Lakes by this evening. An associated surface cyclone in lower MI will gradually deepen and move northeastward toward Lake Huron, as a trailing cold front translates southeastward across Lower MI, the mid MS Valley, and the southern Plains. 12z regional soundings support the possibility of both low-level moisture advection from southwest-to-northeast toward Lower MI in advance of the cold front, and downward mixing of moisture from aloft with daytime heating. The net result should be boundary-layer dewpoints increasing into the low-mid 60s in southeast Lower MI and the upper 60s across the lower OH Valley. Some weak elevated convection is ongoing in the warm sector across the lower OH Valley, along the corridor of the low-level jet and where convective inhibition is weak for parcels originating near 850 mb. Additional warm sector convection will be possible through the day as the low levels destabilize with heating/mixing. The primary focus for thunderstorm development will be the zone of ascent along and just ahead of the cold front through the afternoon. MLCAPE is expected to range from 1000-2000 J/kg (greatest across the lower OH Valley) in advance of the cold front, while low-midlevel flow/shear will be stronger toward lower MI. The net result will be an environment favoring bands of storms capable of producing damaging winds with sufficiently strong downdrafts and momentum transfer. A tornado cannot be ruled out with embedded rotating storms given effective SRH close to 150 m2/s2 and effective bulk shear near 35 kt, and marginally severe hail may also occur with midlevel lapse rates of 7-7.5 C/km. The severe threat will diminish by late evening across OH as the boundary layer stabilizes and the primary shortwave trough passes farther to the north. ...East TX area through this evening... Convection has organized as an MCS across north TX this morning with a leading convective line and weak MCV. The downstream environment across southeast TX appears conducive to maintaining some form of this convection through the afternoon, with the southwest extent of the convection limited by more substantial midlevel capping.
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