(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 210818
SPC AC 210818
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0318 AM CDT Thu Oct 21 2021
Valid 241200Z - 291200Z
...DISCUSSION...
(Day 4) Models are trending toward better agreement and have
demonstrated some run-to-run consistency regarding the evolution of
a southern-stream shortwave trough forecast to amplify as it
approaches the southern and central High Plains Saturday night. This
feature will move through the central/southern Plains Sunday before
reaching the middle/lower MS Valley Sunday evening. There are still
some model differences, particularly with regard to the amplitude
and speed of this system.
Deepening lee low should be situated over central KS by 12Z Sunday
with a trailing cold front extending southwestward into western TX.
A warm front will stretch eastward from the low through northern KS
and into central MO. Partially modified Gulf air will advect through
the warm sector beneath steeper mid-level lapse rates resulting in
moderate instability during the afternoon. A capping inversion will
delay surface-based thunderstorm initiation until mid afternoon when
storms will likely develop along the cold front across central
eastern KS into eastern OK as well as farther east along the warm
conveyor belt into the lower MS Valley. Vertical wind profiles will
support organized storms including supercells with potential for all
severe hazards. Given some lingering model differences, will only
introduce 15% category for this outlook, but higher probabilities
might be warranted with better model agreement in later updates.
(Day 5) - A severe threat will probably persist as this feature
continues into the TN Valley region Monday and possibly the Middle
Atlantic region Tuesday. However, model difference become more
substantial at this time frame, so will defer introduction of any
categorical risk areas to possibly the next update.
(Days 6-7) - Models including most ensemble members are in
reasonable agreement regarding the approach of an upper trough into
the Plains with robust moisture return toward the early and middle
part of next week. If these trends continue, a severe risk area may
be introduced for portions of the central and southern Plains in the
next 4-8 outlook update.
..Dial.. 10/21/2021
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