Day 1 Severe Weather Outlook

Valid 111300Z – 121200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
ARKANSAS TO PARTS OF ALABAMA…

…SUMMARY…
Severe hail, damaging thunderstorm wind and a few tornadoes are
possible today from parts of Arkansas to parts of Alabama.

…Synopsis…
In mid/upper levels, a split-flow pattern will remain prominent
across the West Coast region, related to a cut-off cyclone centered
off southern CA. Though reorientation/reorganization of the gyre’s
vorticity field will occur, the net effect should be to keep the
500-mb low offshore through the period, albeit with sharp troughing
northeastward toward the southern Sierra.

Downstream, a ridge will persist from north-central MX across parts
of the Great Basin, with its northern part flattening as a
northern-stream trough digs southeastward across parts of the
northern/central Rockies. A weak shortwave trough — evident in
moisture-channel imagery from southern WI to southern KS — will
split, with the northern part north of the OH Valley accelerating
eastward. The southern part will become dominated by convectively
induced/enhanced vorticity related to an ongoing, elevated MCS
discussed below.

At the surface, 11Z analysis showed a wavy, quasistationary front
from a weak low over southwestern WV across eastern/middle TN,
north-central MS, becoming a warm front near the LA/AR border to
another weak low near OKC. By 00Z, the synoptic front should extend
with little movement from a weak low over northern NC or southern VA
across mid TN to northern MS and east-central/southeastern AR, with
some modulation likely over the Mid-South into southern AR related
to convective outflow. The western part should return northward
across OK as a warm front, connecting by 12Z to a weak low over
southern KS.

…Ozarks, Mid-South to AL…
An ongoing small complex of thunderstorms over the Ozarks region,
along with some newer, more erratically organized convection
extending into central AR, are rooted in a layer of elevated
buoyancy with MUCAPE estimated around 1500 J/kg. That CAPE exists
atop a strongly stable layer located between 40s to 50s F surface
temperatures and an inversion top ranging from around the 935-mb
level in the 12Z LZK RAOB (east of the steepest midlevel lapse
rates, with little CAPE) to near 850 mb in the Ozarks. Steep
low/middle-level lapse rates just west/northwest of there will
spread southeastward just ahead of some of the convection, along
with lack of MUCINH above the stable layer. Expect moist advection/
transport around 850 mb and lift from the associated MCV.

These factors will continue to support an area of convection as it
moves southeastward across the outlook area. For a few more hours,
sporadic strong/isolated severe gusts and marginally severe hail
will continue to be the main threats. Uncertainty exists as to when
and to what areal extent any further upscale growth will take place;
therefore, no greater corridor of probabilities is introduced yet.
Still, the convective wind potential should increase through late
this morning into afternoon as activity gradually encounters more
near-surface buoyancy in an air mass that will be destabilizing from
two effects:
1. Theta-e advection with related moistening in the boundary layer,
and
2. Diabatic surface heating, with insolation tempered somewhat
through a plume of mid/high clouds.

Some of the convection ultimately will encounter purely surface-
based effective-inflow parcels across parts of the Mid-South into
MS. As that occurs, amidst favorable deep shear (e.g., effective-
shear vectors 40-55 kt) for supercells, the potential will exist for
either MCS-embedded mesocirculations or a dispersion of some of the
clusters into embedded to semi-discrete supercells. Forecast
soundings suggest MLCAPE 1000-2000 J/kg in the AR/northwestern MS
warm sector, along the southern rim of the complex, late this
morning into afternoon. Thereafter, the main convective area should
outrun the spatial pace of destabilization into AL, and any
remaining severe threat will become more conditional, dispersed
farther westward across southern AR, and dependent on
characteristics of the trailing outflow boundary.

..Edwards/Goss.. 03/11/2020

$$

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