Day 1 Severe Weather Outlook

Valid 151300Z – 161200Z

…NO SEVERE THUNDERSTORM AREAS FORECAST…

…SUMMARY…
Severe weather is not anticipated today and tonight.

…Synopsis…
Mid/upper-level moisture-channel imagery indicates a strong
shortwave trough over parts of the Sierra Nevada and the
northwestern NV/northeastern CA state-line area. As this trough
proceeds east-southeastward and amplifies somewhat, height falls
will spread across the southern Rockies and (overnight) the
southern/central Great Plains. By 12Z, the 500-mb trough is
forecast near an axis from LBF-GLD-CAO-ELP.

At the surface, 11Z analysis showed a cold front over the Atlantic
northeast of the Bahamas, arching southwestward and becoming
quasistationary across northern Cuba, then curving northwestward as
a warm front over the north-central Gulf, to an inflection area
bending along the lower/middle TX coast southward past BRO. This
frontal zone is forecast to shift back northward while becoming more
diffuse through the period. This will permit increasingly moisture-
rich return-flow trajectories rooted in a reservoir of high
boundary-layer theta-e already over the western/southern Gulf,
well-modified since the last deep Gulf frontal penetration, and
relatively unaffected by the most recent FL frontal passage.

Another cold front was drawn from Maine across central PA and middle
TN, becoming wavy/quasistationary near MEM, LIT and TXK, and over
the Red River region of north TX/southern OK, to a weak low
southwest of SPS. The boundary became quasistationary from there
northwestward to northeastern NM near LVS. The low is expected to
ripple northeastward along the front, perhaps discontinuously given
its weakness, reaching central KY by the end of the period.
Another, initially weak frontal-wave low may form along the trailing
cold front over the LA/AR border region around 12Z. By that time,
the front should extend from the KY low across western TN, east TX,
and south-central TX.

…Mid-South region to Ohio Valley/central Appalachians…
The boundary layer across the region should remain capped for
surface-based development until after the period. Elevated,
thunderstorms, rooted in a layer of at least marginally favorable
moisture and buoyancy over and east of the surface front, are
expected to develop gradually overnight and spread from southwest to
northeast across the outlook area. While hail approaching severe
limits cannot be ruled out, activity should remain too disorganized,
atop a relatively stable boundary layer, to offer an unconditional
severe risk until day 2.

The aforementioned moistening process above the surface, and
accompanying isentropic lift, will result in progressively more
parcels reaching LFC through the night, amidst elevated MUCAPE
ranging from 500-1000 J/kg across the Mid-South to less than 350
J/kg over WV. Strong winds will be present through much of the
buoyant layer, supporting cell motions occasionally exceeding 50 kt,
though the greatest shear will be through the stable layer beneath.
Forecast soundings show effective-shear magnitudes commonly 35-45
kt. Where lapse rates aloft will be steepest, over the Mid-South
region, isolated hail will be possible from relatively sustained/
discrete storms. However, the potential for quick mergers and messy
modes renders the hail threat very conditional — too much so to
introduce severe probabilities on this outlook cycle.

..Edwards/Broyles.. 12/15/2019

$$

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