Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0358 AM CDT Thu Apr 22 2021 Valid 251200Z - 301200Z ...DISCUSSION... An upper trough is forecast to move off the East Coast early on Day 4/Sunday, with rich low-level moisture and boundary-layer instability confined to parts of the FL Peninsula to the south of a cold front. Both low and mid-level flow should veer to westerly behind the upper trough passage. This should limit low-level convergence along the surface front, and organized severe thunderstorm potential appears low on Day 4/Sunday. Latest medium-range guidance remains in remarkably good agreement with the progression of an upper trough over the western CONUS from Day 4/Sunday into Day 5/Monday. If this upper trough evolves as forecast, then lee cyclogenesis across the southern/central High Plains should occur early next week. Rich Gulf moisture should advect northward across the southern/central Plains to the east of a sharpening dryline from Day 5/Monday into Day 6/Tuesday. The run-to-run consistency of the ECMWF continues, with the latest run indicating the development of moderate to strong instability east of the dryline by Tuesday afternoon across central TX into western/central OK and parts of KS. As the upper trough ejects across the Plains, an enhanced mid-level jet should overspread the dryline and warm sector. Large-scale ascent associated with the upper trough and low-level convergence along the dryline appear sufficient to initiate convection by Tuesday afternoon. If this occurs, then an organized severe thunderstorm threat will develop, with all severe hazards possible. The 15% severe area for Tuesday has been expanded to include most of the length of the dryline where severe thunderstorms appear possible. Expect adjustments to this area in the coming days, including the potential for greater severe probabilities, as additional model guidance comes in. For Day 7/Wednesday and Day 8/Thursday, confidence is not high enough to introduce 15% severe probabilities. Depending on Tuesday's convection and the quality of low-level moisture return ahead of the eastward-moving upper trough/low, some severe threat may exist on Day 7/Wednesday across parts of the lower/mid MS Valley. But, predictability is low at this extended time frame.
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