Day 4-8 Outlook

Day 4-8 Outlook Image

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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