Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0337 AM CST Thu Nov 19 2020 Valid 221200Z - 271200Z ...DISCUSSION... A progressive upper level pattern is expected during the Day 4-8 period. An upper trough over the Plains on Day 4/Sun will shift eastward to the Atlantic coast through Day 5/Mon. At the same time, a surface cold front will sweep east/southeast from the southern Plains through the Midwest and eastern U.S., moving well offshore the Atlantic coast by Day 6/Tue. Showers, and perhaps isolated thunderstorms, are expected to accompany the frontal passage, but mostly stable boundary-layer conditions will preclude severe thunderstorm concerns through Day 5/Mon. As this initial upper trough shifts east of the MS River on Day 5/Mon, an upper ridge will briefly amplify over the Plains. Meanwhile, a western upper trough will deepen and shift eastward toward the southern Rockies on Monday, and the central/southern Plains on Day 6/Tue. Strong deep layer southwesterly flow will overspread the southern Plains vicinity, and height falls will result in strengthening lee cyclogenesis developing eastward across the central/southern Plains Monday night through Tuesday. Increasing southerly low level flow will transport somewhat modest Gulf moisture northward. However, medium range guidance is in fairly good agreement that surface dewpoints as high as the low to mid 60s F are possible as far north as southern OK. How much destabilization will ultimately occur remains somewhat uncertain, as the cold frontal passage on Sunday/Monday could cut-off Gulf return flow more than guidance currently indicates. Additionally, cloudiness and showers in a strong warm advection regime could limit destabilization ahead of the surface low/front. Nevertheless, overall pattern favors at least some concern for isolated strong/severe thunderstorm potential across the southern Plains vicinity on Day 6/Tue, but uncertainty remains too high at this time to introduce probabilities. Beyond Tuesday, spread among various guidance increases dramatically, and predictability is low.
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