SPC AC 041255
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0655 AM CST Sat Dec 04 2021
Valid 041300Z - 051200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
...SUMMARY...
Isolated thunderstorms are possible from north Texas and southern
Oklahoma to parts of Alabama.
...Synopsis...
A split-flow pattern will continue in mid/upper levels over the
CONUS, with the northern stream being less amplified but much
faster, and a more-amplified yet weaker southern stream. In the
northern branch of westerlies, a nearly phased set of vorticity
maxima and related shortwave perturbations are evident in moisture-
channel imagery approaching coastal BC and the southern AK
Panhandle. A stronger, better-defined trough will evolve from the
southern part of the current nearshore perturbations and progress
east-southeastward, reaching the Dakotas and eastern MT late tonight
into early tomorrow morning.
In the southern stream, a small, weak, cutoff cyclone now over AZ is
expected to weaken further. Its remnants will be absorbed into the
broader cyclonic-flow field around a more-progressive 500-mb low now
located west of northern Baja. By 12Z tomorrow, the latter low
should be over the northern Gulf of California, along a positively
tilted trough extending northeastward to central NM and west-
southwestward for at least 1000 nm over the open Pacific.
Meanwhile, two somewhat phased shortwave perturbations are apparent
in moisture-channel imagery in the south-central CONUS:
1. Over southern AR into central MS, forecast to move slowly
eastward to MS/AL overnight, and
2. Over southern OK and north TX near the Red River, forecast to
drift eastward to eastern OK and AR, while weakening.
The 11Z surface chart showed a wavy, quasistationary frontal zone
from southern WV southwestward over southern KY, west-central AR, to
a weak frontal-wave low between MWL-ABI, then across west-central/
southwest TX. The low should migrate slowly and erratically across
north-central TX to south-central OK through 06Z, weakening
overnight along with the front to which it is attached. Substantial
cyclogenesis is expected overnight in the post-cold-frontal air mass
across eastern MT and western ND, related to the approaching
Pacific/northern-stream perturbation. Related mass response will
increase baroclinicity there, while rendering a broad but weak field
of surface southerlies over the southern/central Plains. This will
contribute to frontolysis in the latter region.
...Southern Plains to portions of AL...
Sporadic, isolated thunderstorms are possible through tonight over
the outlook area -- initially over parts of north TX, but with
potential expending northward and eastward as the period progresses.
Activity largely will remain southeast through northeast of the
surface low and its remnant frontal wave, where low-level moisture
and warm advection should remain most favorable. A window for
surface based convection exists today over northeast TX,
southeastern OK to the ArkLaMiss region, where diurnal heating will
reduce MLCINH and boost MLCAPE to around 1000-1500 J/kg. However,
near-surface winds and convergence will be weak, acting in tandem
with weak deep shear (effective-shear magnitudes only around 15-30
kt) to limit coverage and intensity.
Weakening of the mid/upper perturbations and of related large-scale
ascent, combined with decreasing low-level baroclinic forcing in the
frontolytic regime, will combine to counterbalance the favorable
ingredient of increasing low-level moisture with time this evening
and overnight. As such, thunderstorm coverage should remain
isolated, and much of the outlook area may not experience one.
Pockets of relatively steep, EML-related low/middle-level lapse
rates (apparent in morning RAOBs) will linger over the region, and
brief pulse-storm hail approaching severe limits cannot be ruled
out. However, severe potential appears too poorly focused,
disorganized and isolated to warrant an outlook area.
..Edwards.. 12/04/2021
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
|
| 
)
