Day 4-8 Outlook
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0250 AM CST Tue Dec 21 2021
Valid 241200Z - 291200Z
...DISCUSSION...
The day-4 to day-8 period largely will be characterized by low
amplitude on the central/eastern CONUS subset of planetary to
synoptic scales. Several shortwaves will traverse the midlatitude
westerlies across the CONUS. ECMWF/GFS and their ensembles indicate
these perturbations' fields of large-scale ascent will be mostly
located over relatively low-theta-e boundary layers, poleward of the
oscillatory surface front, with a substantial component of flow
aloft parallel to the mean front.
A broad warm sector with at least marginally favorable Gulf moisture
trajectories should develop and occupy much of the south-central/
southeastern CONUS south of that front. However, with specific foci
for strong-severe convection strongly dependent on low-level mass
response to low-predictability shortwaves, overall predictability
appears low for the period. For example, even by day 4/Christmas
Eve, spread discussed in the day-3 outlook increases into day 4/
Christmas Eve and day 5/Christmas Day, regarding phase speed and
southern-branch amplitude of the trough moving eastward from the
Plains across the Mississippi Valley. Weak relative maxima in
convective potential appear day 4 around the lower/mid Mississippi
Valley near a possible frontal wave, and perhaps days 7-8 (27th-
29th) somewhere in the southeastern 1/4 of the CONUS, as overall
flow aloft trends more cyclonic. Still, these scenarios appear too
uncertain for any 15%+ unconditional severe areas at this time.
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