SPC 0830Z Day 3 Outlook
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0151 AM CST Wed Dec 22 2021
Valid 241200Z - 251200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
...SUMMARY...
Isolated thunder is possible near the Pacific Coast, as well as over
portions of the southern Great Basin and Colorado Plateau.
...Synopsis/Discussion...
A series of shortwave perturbations will contribute to maintenance
of mean mid/upper-level troughing near the West Coast through
day-3/Christmas Eve. In the northern stream, a strong shortwave
trough is evident in moisture-channel imagery east of the Kamchatka
Peninsula. The leading part of this perturbation is forecast, with
reasonable consistency among various deterministic progs and most
ensemble members, to reach the WA coastline early this period then
weaken, while its western lobe digs southeastward across the Pacific
offshore from OR and northern CA. Associated DCVA/cooling aloft
should pivot ashore over OR/CA during the latter half of the period.
Progressively colder air aloft will spread across the inland
Northwest, Great Basin and northern Intermountain regions, but
predominantly atop cool surface conditions. Very isolated flashes
cannot be ruled out across a vast area of the CONUS between the
Rockies and Pacific coastal thunder potential. A relatively focused
(but still isolated) thunder potential may spread eastward across
portions of the southern Great Basin and Colorado Plateau during the
latter half of the period, as a Pacific southern-stream shortwave
trough discussed in the day-2 outlook ejects east-northeastward out
of the mean trough and across this region. Accompanying DCVA/
cooling aloft and low/middle-level moist advection above the surface
may support sporadic flashes within a broad plume of precip/showers.
This perturbation should weaken considerably as it moves past the
south-central Rockies.
Broad lee cyclogenesis/frontogenesis is expected over parts of the
central/northern Rockies and adjoining High Plains beneath strong
west-southwesterlies aloft, and as heights fall to the west.
However, a well-developed EML should cover most of the southern/
central Plains into lower/mid Mississippi Valley. Any substantial
moisture return from the Gulf should be too displaced from the areas
with strongest deep-layer forcing -- and from a weakly convergent
cold front extending from the upper Great Lakes across the central
Plains -- to support 10%+ thunder potential east of the Rockies.
..Edwards.. 12/22/2021
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