SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook NWS Storm Prediction Center Norman OK 0228 AM CST Fri Nov 27 2020 Valid 291200Z - 301200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE LOWER MISSISSIPPI VALLEY/SOUTHEAST INTO THE CAROLINAS... ...SUMMARY... Isolated strong to severe thunderstorms may occur Sunday across parts of the lower Mississippi Valley into the Southeast and Carolinas. ...Lower Mississippi Valley into the Southeast and Carolinas... A closed upper low over the southern Plains Sunday morning should move eastward across the lower MS Valley and Southeast through the day. This upper low should begin to merge with a northern-stream upper trough over the eastern CONUS Sunday evening into early Monday morning. A weak surface low initially near the coast of LA is forecast to develop northeastward across the Southeast through the day while gradually deepening. Additional deepening of this surface low should occur Sunday evening through the end of the period as it continues developing northeastward along/near the spine of the southern/central Appalachians. A warm front should lift northward across parts of the Southeast and Carolinas through the period, while a cold front attendant to the surface low sweeps eastward across these regions throughout the period. A prior frontal intrusion over the northern Gulf of Mexico may tend to limit the prospect for substantial inland destabilization across the Southeast through Sunday evening. Even so, low to mid 60s surface dewpoints should advance northward into parts of southern LA/MS/AL and the FL Panhandle in tandem with the developing surface low and warm front through the day. Forecast soundings from various models suggest weak instability may gradually develop with modest daytime heating across these areas along/south of the warm front. Strong low-level and deep-layer shear appear likely given the enhanced wind field associated with the approaching upper low. This high-shear, low-instability setup typically favors isolated strong to severe convection across the developing warm sector along/ahead of the eastward-moving cold front. Damaging winds a perhaps a couple tornadoes would be the main threats with both low-topped supercells and linear convection given the forecast strength of the low-level flow and shear. This isolated severe threat may continue across parts of GA into the Carolinas Sunday night and early Monday morning (end of the Day 3 period), as low-level moisture return occurs from the western Atlantic. However, even through the low-level flow across these areas should be quite strong, surface-based instability is forecast to remain rather weak. Given the potentially limiting factor of modest instability, have opted to introduce a broad Marginal Risk area from parts of the lower MS Valley/Southeast into the Carolinas. The northern extent of any damaging wind/tornado threat remains unclear, as there is uncertainty in how far north the low to mid 60s surface dewpoints (which will probably be needed for surface-based storms) will advance across the Southeast and Carolinas. If confidence increases in a narrow corridor of greater instability developing across the warm sector, a Slight Risk for damaging winds and/or tornadoes may need to be included across some portion of these regions in a later outlook update. ..Gleason.. 11/27/2020
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