Day 1 Severe Weather Outlook

Valid 081300Z – 091200Z


Thunderstorms are expected over parts of the Southwest, and isolated
storms are possible late near the Carolina coast.

In mid/upper levels, height falls are expected across much of the
Rockies, Desert Southwest and Great Plains through the period, as a
series of variably amplified shortwave perturbations contributes to
larger-scale troughing. By the end of the period, this process will
lead to a positively tilted synoptic trough, extending from a
persistent cyclone over northern Hudson Bay southwestward across the
Dakotas, Four Corners, northern Baja, and Pacific waters west of
Baja. The most influential of those perturbations for this forecast
will be a trough now apparent in moisture-channel imagery over OR,
northwestern CA and adjacent Pacific waters. This trough is
forecast to dig southeastward to northern/western AZ and northern
Baja through the period.

At the surface, 11Z analysis showed a strong cold front from the
northwestern shore area of Lake Superior across central MN and
central SD, becoming quasistationary over northwestern SD and
south-central through northwestern MT. A weak low was analyzed
along the front near BIL. The front may move slightly southward
today with a southward shift of the location of the low into
northern WY. Largely in response to a northern-stream mid/upper
trough now located over the Canadian Rockies, further cyclogenesis
is expected along the frontal zone today over the northern WY/
southwestern SD area, shifting/reorganizing eastward across NE and
IA through 12Z tomorrow. By that time, the cold front should extend
from a low over eastern IA southwestward across eastern KS, the TX
Panhandle and northeastern NM.

…Southwestern CONUS…
Isolated to widely scattered thunderstorms may occur through the
period across the general-thunder areas presently outlined, but with
greatest potential in the afternoon as patches of diabatic surface
heating contribute weak low-level buoyancy. Forecast soundings
suggest midlevel buoyancy may reach well into icing layers suitable
for lightning generation. Associated peak MLCAPE values may reach
near 500 J/kg across parts of the CA Central Valley, western/
northern AZ, southeastern NV, and southwestern UT.

Given the strength of the mid/upper winds atop areas of surface-
based inflow, and related strong cloud-layer shear, small hail is
possible, along with and strong gusts may occur from the most
intense cells that can transfer momentum to surface. However, given
the limited buoyancy, overall coolness of surface temperatures, and
related lack of deeply mixed boundary layers with large dewpoint
depressions, the severe-wind threat appears too conditional,
disorganized and isolated to warrant a categorical outlook at this

..Edwards/Broyles.. 12/08/2019


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