(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 210919
SPC AC 210919
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0319 AM CST Thu Jan 21 2021
Valid 241200Z - 291200Z
...DISCUSSION...
Medium-range models are exhibiting reasonable agreement Day 4
(Sunday) and into Day 5 (Monday), after which divergence in
solutions begins to increase markedly.
On Day 4 (Sunday), the western U.S. trough is progged to begin
ejecting -- and deamplifying as it does -- across the Desert
Southwest and southern Rockies, and finally into the southern
Plains, as it acquires negative tilt through latter stages of the
period.
As this occurs, height falls will overspread northern and central
Texas and Oklahoma, atop southerly low-level flow providing theta-e
advection into the region. While a capping inversion -- remnants of
prior anticyclonic flow/subsidence aloft -- will likely hinder
potential for surface-based convection through much of the day,
isolated to scattered storms may evolve by early evening across
parts of central and northern Texas, and possibly into southeastern
Oklahoma. While questions persist with this scenario, including
degree of surface-based instability which may be present, shear
profiles will be quite supportive of organized/rotating storms.
Therefore, though details remain sketchy, enough consistency exists
within tonight's model runs, and those from prior days, to warrant
inclusion of a conditional 15% risk area for severe storms.
By Monday, differences begin to become apparent with
evolution/progression of the weakening mid-level system as it shifts
eastward toward/into the Midwest/Ohio Valley. Likewise, similar
differences are manifest in the strength/advance of the associated
surface low. This uncertainty, combined with likelihood for weaker
instability with eastward extent, suggests lesser/decreasing severe
potential, and diminishing confidence -- both therefore mitigating
against risk area inclusion at this time.
Beyond Day 5, model differences -- suggestive of an inherent lack of
predictability -- preclude any assessment of convective potential
through the remainder of the medium-range period.
..Goss.. 01/21/2021
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