Day 1 Severe Weather Outlook

Valid 241300Z – 251200Z


Isolated marginally severe storms are possible from southern
Missouri into northern Mississippi during the afternoon and early

Through tomorrow morning, the mid/upper-level pattern over the
central CONUS will maintain broadly cyclonic flow, while changing
from split flow to phased troughing. This will occur as a strong
shortwave trough — now apparent in moisture-channel imagery over
the south-central Plains with embedded low over south-central KS —
ejects east-northeastward and deamplifies considerably. By 00Z, the
weakened 500-mb trough should be located near an STL-MEM-PBF axis,
with a low-amplitude extension southwestward past DRT. By 12Z
tomorrow, that southern extension will split off into a weak,
elongated, strongly positively tilted vorticity ribbon near an ATL-
MSY-MFE line. By then, the original/main trough will be a rapidly
weakening, small perturbation from Lake Huron across eastern OH.
This will occur as a northern-stream shortwave trough — initially
over the northern Rockies — digs southeastward and evolves into a
closed cyclone over the KS/NE line.

At the surface, 11Z analysis showed a low over northern OK, roughly
between END-PNC and somewhat south of the mid/upper vortex center.
A weak “Pacific” cold front — which may act effectively as a
dryline once diurnal heating/mixing intensifies behind it — was
drawn from the low across north-central/central TX to near LRD. The
low should move to near TBN by 00Z, with cold front arching
southward/southwestward across the Mid-South, central/southwestern
LA, and deep south TX. By 12Z, the low should reach southern IN,
with cold front south-southwestward across southeastern MS and the
northwestern Gulf.

Sufficient destabilization is expected between the cold front and an
early-day plume of clouds/precip to permit an arc of widely
scattered thunderstorms to form this afternoon, near the western
edge of the outlook area. This activity should progress across
portions of the Ozarks and eastward over the adjoining Mississippi
Valley, offering isolated, marginally severe gusts and hail. A
tornado cannot be ruled out, given some supercell potential in a
marginally but sufficiently unstable, low-LCL setting.

A narrow, eastward-shifting corridor of diabatically driven
boundary-layer destabilization is expected behind the early precip
plume, steepening low-level lapse rates and weakening already modest
MLCINH. This process will underlie midlevel cooling related to the
weakening mid/upper trough, with increases in midlevel lapse rates
being strongest across the Ozarks region to northeastern AR/
southeastern MO/western TN. The influence of the mid/upper
perturbation will diminish with southward extent across MS and
eastern AR. Despite cool surface temperatures, MLCAPE in the
500-1000 J/kg range is possible, especially over the Ozarks, amidst
30-45-kt effective/shear magnitudes and 150-300 J/kg effective SRH.
This should support a blend of multicells and supercells, with
enough cross-arc component to mean-wind and deep-shear vectors to
indicate discrete and semi-discrete mode.

Some southeastward backbuilding of the arc into at least northern MS
and possibly central MS should occur, but overall, convection should
be weaker with southward extent through MS amidst less mid/upper
forcing and smaller lapse rates aloft. The main convective arc
should outpace its supportive buoyant plume this evening, both in
terms of translating across it and magnitude/spatial shrinkage of
the favorable CAPE due to diabatic surface cooling.

…FL Panhandle…
Elevated, episodic areas of thunderstorms may become common over
parts of the swath from the FL Panhandle across southeastern AL and
southern GA overnight. This activity will be supported by moisture
advection/transport and isentropic lift to LFC, within a
strengthening low-level warm-advection regime. Boundary-layer
theta-e, within the partially modified/Gulf return-flow air mass,
may increase sufficiently late tonight to yield surface-based
effective-inflow parcels over portions of the FL Panhandle. The
bulk of convection should be displaced inland from sustained/
surface-based parcels, though deep shear and low-level hodographs
alone conditionally will support severe potential in any convection
that can remain within the plume. Forecast soundings indicate
pockets of relative stability and otherwise weak lapse rates in
low/middle levels, rendering horizontally and vertically fragmented
MLCAPE less than about 200-300 J/kg. For now, the threat appears
too isolated and uncertain to warrant a categorical/unconditional
risk area.

..Edwards/Broyles.. 02/24/2020


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