Day 4-8 Outlook

Day 4-8 Outlook Image

Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0345 AM CST Sat Nov 21 2020

Valid 241200Z - 291200Z

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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