Day 1 Severe Weather Outlook

Valid 271300Z – 281200Z


Isolated severe thunderstorms may affect the High Plains of west
Texas and eastern New Mexico this evening and tonight.

In mid/upper levels, the main synoptic feature for this period will
be a synoptic trough moving eastward across the western CONUS.
Initially, its dominant component is a cyclone covering the lower
Colorado River region from southern NV to northern Baja, centered
near EED. The cyclone is progged to shift northeastward to the Four
Corners by 12Z and devolve into an open wave, but increasingly in
phase with northern-stream shortwave troughing now somewhat offset
westward over BC/WA. By the end of the period, a major/high-
amplitude trough should extend along the length of SK, southward
over MT, WY, CO, NM and then south-southwestward across northwestern
MX to southern Baja.

At the surface, a cold front was analyzed from southwestern ON
across northwestern OH, southwestern IN, and southwestern AR,
becoming quasistationary west-southwestward to southeastern NM and a
weak low over southeastern AZ. A cold front extended from that low
southwestward across Sonora. The southern Plains limb of the front
is expected to move slowly northward and undergo gradual frontolysis
through the period. This will occur amidst a broad area of low-
level warm advection, strengthening during the period ahead of the
approaching synoptic trough. By 00Z, an elongated area of low
pressure should develop across eastern NM, with cold front across
southeastern NM, far west TX and northern Chihuahua. By 12Z, the
western cold front should extend from a broad cyclone centered over
the KS/CO border area to the eastern TX Panhandle, perhaps extreme
western/southwestern OK, then southward/southwestward across
west-central TX to northern Coahuila.

…Southern High Plains…
Widely scattered to scattered thunderstorms are expected to form
from late this afternoon through this evening, along/ahead of the
southwestern CONUS cold front. A few supercells are possible, with
isolated large hail. A tornado cannot be ruled out as well.

Despite modest lapse rates above the boundary layer, a combination
of muted diurnal heating and warm advection should increase low-
level theta-e enough to weaken MLCINH substantially by late
afternoon. Lift will increase at multiple scales, from the
large-scale effects of warm advection to frontal forcing, down to
localized buoyancy effects. Meanwhile, strengthening mid/upper
winds and pronounced veering with height in the profile will
contribute to favorable bulk shear. Forecast soundings reasonably
depict a corridor of weakly capped, 500-1000 J/kg preconvective
MLCAPE, with LCLs below 1 km, and effective-shear magnitudes 45-55
kt. Some veering of warm-sector surface winds with time, gaining
southerly component at the expense of easterly, will limit hodograph
size, though 100-200 J/kg effective SRH is possible.

The outlook area was expanded somewhat northward in deference to
potential for weak surface-based buoyancy to spread into more of the
extreme east-central NM/southwestern TX Panhandle area, amidst a
vertical wind profile favorable for supercells.

..Edwards/Smith.. 12/27/2019


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