Day 4-8 Severe Weather Outlook Issued on Dec 21, 2021
Updated: Tue Dec 21 08:53:03 UTC 2021
Day 4 Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 5 Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 6 Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 7 Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Day 8 Risk
Area (sq. mi.)
Area Pop.
Some Larger Population Centers in Risk Area
No Risk Areas Forecast
D4
Fri, Dec 24, 2021 - Sat, Dec 25, 2021
D7
Mon, Dec 27, 2021 - Tue, Dec 28, 2021
D5
Sat, Dec 25, 2021 - Sun, Dec 26, 2021
D8
Tue, Dec 28, 2021 - Wed, Dec 29, 2021
D6
Sun, Dec 26, 2021 - Mon, Dec 27, 2021
(All days are valid from 12 UTC - 12 UTC the following day)
Note: A severe weather area depicted in the Day 4-8 period indicates 15%, 30% or higher probability for severe thunderstorms within 25 miles of any point.
PREDICTABILITY TOO LOW is used to indicate severe storms may be possible based on some model scenarios. However, the location or occurrence of severe storms are in doubt due to: 1) large differences in the deterministic model solutions, 2) large spread in the ensemble guidance, and/or 3) minimal run-to-run continuity.
POTENTIAL TOO LOW means the threat for a regional area of organized severe storms appears unlikely (i.e., less than 15%) for the forecast day.
Forecast Discussion
ZCZC SPCSWOD48 ALL
ACUS48 KWNS 210850
SPC AC 210850
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.
..Edwards.. 12/21/2021
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