Day 1 Severe Weather Outlook

Valid 141300Z – 151200Z


Marginally severe hail or thunderstorm wind is possible from the
lower Mississippi Valley into northwest Georgia.

In mid/upper levels, nearly zonal flow will characterize the pattern
across most of the CONUS, except where perturbed by substantial
shortwaves. The leading shortwave trough — initially located from
MN southeastward across portions of IN/KY — will deamplify and
eject east-northeastward, crossing southern QC and New England by
12Z. The trailing shortwave trough — now apparent in moisture-
channel imagery from offshore WA/OR southeastward over the northern
Sierra — will pivot eastward across the north-central Rockies
tonight, reaching the Dakotas and NE by the end of the period.

In the southern stream, a broad belt of weakly confluent mid/upper-
level flow will prevail from the Pacific (offshore Baja) across
northern MX to the Southern States. Weak vorticity lobes will
accelerate into the confluent flow, preceding a better-defined
perturbation over the Pacific that mostly will remain offshore this

At the surface, 11Z analysis showed a wavy, quasistationary frontal
zone from eastern NC across northwestern SC, north-central GA,
central MS, and the TX coastal plain. The segment from the
Mississippi Delta area to AL effectively will be shunted southward
somewhat by convection. The western part of this boundary should
lose definition through the period. A weak cold front initially
from eastern MN to western KS will undergo frontolysis as it moves
slowly eastward. Meanwhile, cold frontogenesis is forecast this
evening and overnight across the central High Plains. By 12Z this
front should overtake the weak/preexisting frontal zone and extend
from a low over IA across eastern KS, northwestern OK, and
northeastern NM.

The bulk of substantial convective and (marginal) severe potential
this period will be tied to the indirect and fringe influences of
the leading northern shortwave trough. A corridor of strong
low-level warm advection and moisture transport, mostly above the
surface and related to that perturbation, is ongoing in
southwesterly low-level flow from the western Gulf across the
Mississippi Delta region, MS, AL, and TN. A related corridor of
precip, with scattered to widely scattered embedded thunderstorms in
clusters, extends from east TX to eastern TN.

The strongest convection has become (and should remain) concentrated
near the southern rim of this convective belt, especially from the
Delta region northeastward. In that swath, persistent convective
precip has stabilized the elevated air mass with northward distance
from the southern edge. Meanwhile, the southern part will continue
to experience relatively undisturbed access to elevated buoyancy
along obliquely directed inflow trajectories. The northern bounds
of the outlook area have been adjusted to conform to these trends,
given that the affected southern part is not expected to retreat
northward nor intensify once the shortwave trough eject away from
the area. Instead, the related LLJ and storm-relative winds in the
inflow layer will diminish gradually, lessening both severe
potential and convective coverage through the latter half of the

In the meantime, rich low-level moisture (e.g., with dew points
commonly mid-60s to near 70 F) and areas of sustained insolation
will offset modest midlevel lapse rates enough to increasing deep
buoyancy through mid-late afternoon. Forecast sounding show
1000-1500 MLCAPE over southern portions of AL/MS and central LA,
amidst roughly 30-40-kt effective-shear magnitudes. This should
support organized multicells and transient supercell processes, with
marginally severe hail and isolated gusts near severe limits being
the main concerns. Though a brief tornado cannot be definitively
ruled out, the potential appears to conditional on a cascade of
low-predictability meso-gamma to storm-scale effects to assign
unconditional/categorical probabilities, given the modest
environmental deep shear and lack of large low-level hodographs.

..Edwards/Mosier.. 01/14/2020


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