Day 1 Severe Weather Outlook

Valid 122000Z – 131200Z


Severe thunderstorm development is possible late this afternoon into
tonight across parts of the lower Mississippi into Tennessee
Valleys, some of which may become capable of producing damaging wind
gusts and perhaps a couple of tornadoes.

…20Z Outlook Update…
Latest model output and observational data offer little to support
substantive change to the outlook.

To this point, convective development capable of producing lightning
has generally remained focused above the frontal inversion, to the
cool side of the cold front advancing eastward into the lower
Mississippi Valley. Strongest 2-hourly surface pressure falls (on
the order of 2-6 mb) have become focused along an axis from western
Kentucky and Tennessee through northwest and west central
Mississippi. While this is largely to cool side of a retreating
warm frontal zone, to the northeast of a developing frontal wave,
this does extend southward into the destabilizing warm sector,
roughly across Natchez and McComb MS into the Greenwood MS vicinity.

Through 13/00Z, the southern edge of the stronger 2-hourly pressure
falls is forecast to shift north of the I-20 corridor into
northeastern Mississippi, where at least weak boundary-layer
destabilization may occur in the presence of wind profiles
characterized by strong deep-layer shear and large low-level
hodographs with sizable clockwise curvature. A short-lived window
of opportunity for the initiation of discrete supercells may
develop, mostly within a narrow corridor just ahead of the eastward
advancing front, before the frontal wave shifts into the lower
Ohio/Tennessee Valleys this evening, and low-level hodographs become
more unidirectional.

..Kerr.. 02/12/2020

.PREV DISCUSSION… /ISSUED 1023 AM CST Wed Feb 12 2020/

…MS this afternoon to TN this evening…
No changes to the previous outlook area. A midlevel shortwave
trough over the southern Plains this morning will accelerate
east-northeastward and lose amplitude through the period, as a
primary northern stream trough amplifies over the upper
Midwest/Great Lakes, and a subtropical ridge persists over FL. The
ejecting trough will glance the northwest edge of the surface warm
sector today, with weak cyclogenesis expected along a baroclinic
zone from the ArkLaMiss this afternoon to the TN Valley this evening
and the Appalachians overnight.

Low-level moisture will spread inland rather quickly today in
advance of the developing surface cyclone, with boundary-layer
dewpoints in the 60s expected as far north and southern middle TN by
this evening. However, regional 12z soundings revealed poor lapse
rates below relatively warm temperatures near 600 mb (associated
with the subtropical ridge). Surface heating will also be limited
by clouds (aside from possibly southeast LA into southwest MS).
Thus, narrow CAPE profiles (MLCAPE 250-500 J/kg) are expected in an
environment with otherwise increasing low-level and deep-layer
vertical shear beginning this afternoon across MS and spreading into
the TN Valley this evening. The poor lapse rates/weak buoyancy
suggest it will be difficult to form and sustain warm-sector
supercells this afternoon, and the majority of the deeper convection
should be confined to the track of the surface low and trailing cold
front from mid afternoon into early tonight. Since convection will
tend toward a linear mode along the front, the main threat should be
damaging winds and perhaps a couple of tornadoes with embedded
circulations. Lingering frontal convection will likely weaken
overnight toward the southern Appalachians.


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