Day 1 Severe Weather Outlook

Valid 211300Z – 221200Z


Isolated thunderstorms are possible near coastal Washington and
Oregon today.

The synoptic pattern in mid/upper levels will remain active through
the period, with rich texture in 500-mb isallohypsic fields. A
strong shortwave trough — currently located over the southern
Appalachians and Tennessee Valley regions — will move southeastward
and offshore from the southern Atlantic Coast through the period,
forming a well-defined, deep, closed cyclone north/northeast of the
Bahamas by 12Z. Meanwhile, a series of variably amplified shortwave
perturbations will occupy the cyclonic-flow region of a synoptic
trough crossing the western CONUS. Of these, two are potentially
important convectively, by region affected:

…Coastal Pacific Northwest…
A negatively tilted cyclone now apparent in moisture-channel imagery
west of WA and Vancouver Island, with trough southeastward to near
Cape Mendocino. The leading/basal portion of the trough will move
inland through the afternoon, before the closed cyclone devolves to
a shortwave trough and crosses coastal BC this evening. Satellite
imagery shows that the offshore cyclone/trough has been profusely
suffused with shallow, yet often glaciated, convective elements.
These have included isolated, sporadic thunderstorms, based on
animated lightning plots.

As the associated field of large-scale DCVA lift and cooling
approaches the coastline, 500-mb temperatures should cool below -30
C. Low/middle-level lapse rates will steepen atop a favorably moist
marine layer, with minimal MLCINH. This should result in peak
MLCAPE in the 200-400 J/kg range, commonly 100-200 J/kg, with the
buoyant layer extending well above the height of the -20 C isotherm.
The most intense cells may produce small hail; however, severe
potential appears too low and isolated for any categorical-level
unconditional probabilities.

…Southern Plains…
A southern-stream, mid/upper-level perturbation is crossing parts of
NV, southern CA and northwestern Baja. This feature should move
eastward to eastern NM, far west TX and Chihuahua by 00Z, with a
loosely connected northern lobe over eastern CO. By 12Z, its
vorticity pattern should weaken and eject eastward/northeastward
across the southern Plains, with the 500-mb height trough lagging
across the central/southern High Plains and Coahuila. Low-level lee
troughing and return flow will strengthen across the central/
southern Great Plains in response, but with trajectories mostly from
the antecedent continental/polar high with only weak marine
modification at best. Still, an area of large-scale ascent
supporting both convective and stratiform precip will spread across
the southern/central Plains today and tonight.

Some midlevel CAPE may develop tonight over parts of northwest/
north-central TX and OK, and a few lightning strikes cannot be
definitively ruled out in either rain or winter-precip areas.
However, forecast soundings from multiple progs show relatively
stable lapse rates in ideal thermal layers for lightning generation.
Thunder potential currently appears too sparse, uncertain and
conditional for a 10% line.

..Edwards/Gleason.. 01/21/2020


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