SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1132 PM CST Tue Dec 22 2020 Valid 231200Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF SOUTHERN LOUISIANA AND FAR SOUTHERN MISSISSIPPI... ...SUMMARY... A few severe thunderstorms are expected from late afternoon through tonight from the upper Texas Coast across much of southern Louisiana and Mississippi, with isolated activity from southern Alabama into the western Florida Panhandle by Thursday morning. ...Synopsis... A large, positive-tilt upper trough will progress across the Plains today, and will take on a neutral orientation as it approaches the MS Valley by 12Z Thursday. Ahead of this trough, a broad belt of moderate southwesterlies aloft will develop across much of the eastern CONUS, with steady height falls/cooling aloft. The strongest lift is forecast to occur after 00Z from the Sabine River across MS and into AL and TN, where strengthening low-level convergence will increasing divergence aloft. At the surface, the primary surface low will develop over WI and Upper MI during the day, with a cold front across the MS Valley and upper TX Coast. A secondary low is forecast to develop overnight across MS and AL, coincident with the aforementioned large-scale ascent. Surface dewpoints will increase into the mid to upper 60s F along the TX Coast by 00Z, with mid 60s F across southern LA and coastal MS/AL/FL Panhandle. This front will provide the primary focus for severe weather, including damaging gusts, and a couple tornadoes. ...Southeast TX, southern LA, MS, AL, western Florida Panhandle... Scattered, non-severe storms are possible during the afternoon from southeast TX into LA preceding the cold front where heating and moisture advection will result in an uncapped air mass. Storms are expected to intensify after 00Z, near the TX/LA border, becoming numerous along the front overnight. The most favorable combination of surface-based effective parcels as well as low-level SRH should develop across southern LA during the late evening, then across far southern MS, AL and into the western FL Panhandle by 12Z Thursday. The main threat is expected to be damaging gusts with the linear structures along the front, but any isolated or embedded cells may produce a tornado as effective SRH increases to around 300 m2/s2, while maintaining modest midlevel lapse rates which will boost updraft intensity. Elsewhere, while lift will be strong across other parts of MS and AL, forecast soundings indicate the surface air mass may remain too cool for a severe threat. ..Jewell/Gleason.. 12/23/2020
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