Day 1 Severe Weather Outlook

Valid 221300Z – 231200Z


Severe thunderstorms are not forecast across the contiguous United
States this period.

A progressive, increasingly amplified upper-air pattern will shift
eastward across much of the CONUS through tomorrow morning. The
principally pertinent feature will be a building synoptic trough
shifting eastward from the Rockies and High Plains across the
central states. Several embedded/transitory shortwaves will
contribute to this process, even after the leading perturbation now
over portions of KS/OK ejects toward the upper Mississippi Valley.
The strongest of these shortwave troughs — evident in moisture-
channel imagery from the northern Rockies of the U.S. to
southwestern BC — will pivot southeastward out of the mean trough
position, reaching the northern/central Great Plains by 12Z. At
that time, the synoptic trough still will be slightly farther east:
near an axis from FAR-OMA-DAL-LRD.

At the surface, troughing now analyzed from eastern ND to the
southern High Plains will shift eastward across the Plains States
through the period, overtaken on the north end (Dakotas, WY, then
eastern CO tonight) by a seasonally weak cold front. A 1034-mb high
was analyzed over the central Appalachians, with pronounced ridging
southwestward to the northwestern Gulf. The high and ridging will
shift very slowly eastward through the period. Related short
trajectories of continental/polar air over the Gulf are keeping 40s
F surface dew points common over much of the north-central/
northwestern Gulf. Marine modification into the 50s will become
widespread across the northwestern Gulf through the period, with 60s
offshore from the TX coast tonight, as a surface wave develops along
a marine frontal zone just offshore from the mid/upper TX Coast.

As low-level trajectories veer with longer over-water fetches, and a
40-50-kt LLJ develops amidst strong warm advection this evening and
tonight, moisture will increase inland above a relatively stable
boundary layer. This will boost buoyancy, enabling inland 500-1000
J/kg MUCAPE in support of increasing thunderstorm potential.
Isolated to widely scattered thunderstorms may form as early as this
afternoon near the western edge of the outlook area and move
eastward, becoming widely scattered to scattered overnight across
east TX/western LA and southward to the open Gulf. Present
indications are that sustained surface-based inflow parcels will
remain just offshore, along with a CAPE/shear parameter space better
favoring severe potential. As such, no unconditional severe
probabilities are being introduced at this time, though severe
thunderstorms (including supercells) may affect the northwestern
Gulf late tonight into tomorrow morning, south of the upper TX and
southwestern LA coasts.

..Edwards/Gleason.. 01/22/2020


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