Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0345 AM CST Sat Nov 21 2020 Valid 241200Z - 291200Z ...DISCUSSION... At least an isolated/marginal severe threat should exist on Day 4/Tuesday across parts of the southern Plains into the Ozarks and lower MS Valley as an upper trough/low ejects eastward over these regions. Low-level moisture return ahead of a surface low and related cold front may be sufficient to support surface-based storms from Tuesday afternoon onward. Deep-layer shear appears more than sufficient for organized severe storms, but some uncertainty exists regarding how much instability will ultimately develop. Given this uncertainty, will hold off for now on including a 15% severe area on Day 4/Tuesday across any portion of the southern Plains vicinity, but will reevaluate this possibility in future outlooks. As the upper trough/low continues east-northeastward across the central/eastern CONUS on Day 5/Wednesday, a low-end/marginal severe threat may continue across parts of the lower MS Valley and Southeast. However, medium-range guidance begins to diverge in the placement and amplitude of the upper trough/low by Wednesday evening. It also appears possible that any storms ongoing Wednesday morning across the lower MS Valley/Mid-South may outpace the low-level moisture return across the Southeast through the day. Given these large-scale pattern predictability issues and concerns about sufficient instability across the warm sector, a 15% severe area has not been included across the lower MS Valley into the Southeast for Day 5/Wednesday. Substantial differences in the synoptic-scale pattern across the CONUS continue from Day 6/Thursday through the end of the forecast period. Even so, there appears to be some signal for another large-scale upper trough to move from the western CONUS to the Plains in the Day 7/Friday to Day 8/Saturday time frame. Depending on the evolution of this upper trough/low and how much low-level moisture return occurs ahead of it, there may be an increase in severe potential across portions of the southern Plains and lower MS Valley from Day 7/Friday into Day 8/Saturday. Regardless, predictability remains far too low to delineate any severe areas at this extended time frame.
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