Day 1 Severe Weather Outlook

Valid 162000Z – 171200Z

…THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
CENTRAL LOUISIANA INTO CENTRAL MISSISSIPPI…

…SUMMARY…
Numerous severe thunderstorms will continue to affect parts of the
Lower Mississippi Valley region this afternoon and evening, and into
portions of Alabama and the Tennessee Valley. Several tornadoes —
including a couple of long-track/significant ones — along with
damaging winds, remain the primary hazards. The peak tornado risk is
expected through early evening, centered on northern and central
Louisiana into central Mississippi.

…Discussion…
Overall forecast reasoning and expectations as laid out in the prior
outlook remain valid, as an area of thunderstorms — including
several severe/supercell storms — continues to move eastward across
the Sabine and lower Mississippi Valleys. The most significant
severe weather is ongoing across portions of central Louisiana and
vicinity at this time — i.e. across western portions of the ENH and
MDT risk areas — where a couple of long-track/damaging tornadoes
have occurred over the past couple of hours. Given a similarly
favorable downstream environment, expect this risk to continue over
the next several hours, before diminishing later tonight as storms
— and the parent cold front — shift eastward into a less unstable
environment.

For additional short-term details, please refer to ongoing mesoscale
discussion issuances — including recently issued MCD #2208.

..Goss.. 12/16/2019

.PREV DISCUSSION… /ISSUED 1020 AM CST Mon Dec 16 2019/

…Lower MS Valley and Deep South…
Primary changes this outlook are to increase tornado and wind
probabilities across parts of LA/MS. In addition, the SLGT risk has
been expanded north for wind into middle TN near the expected
surface cyclone track.

Gradual expansion of surface-based thunderstorms is expected across
the Sabine and Lower MS Valleys, along/ahead of the surface cold
front. Activity should grow in coverage and move across the
Mid-South and MS/AL regions through this evening into tonight.
Several tornadoes will be possible across the entire region, but
with greatest potential in a corridor from northern/central LA
across central MS where semi-discrete supercells are most probable,
and should access the most favorable CAPE/shear parameter space.
Damaging gusts should be the most common convective event area-wide.
Severe hail is most probable over southwestern parts of the outlook
area, essentially collocated with greatest tornado probabilities.

12Z soundings sampled the pronounced elevated mixed layer overlying
the rich low-level moisture plume (characterized by upper 60s to low
70s surface dew points) emanating northeast from the western Gulf.
Abundant cloud coverage will be a limiting factor, but moderate
surface-based buoyancy characterized by MLCAPE of 1000-2000 J/kg
will persist in the ambient environment ahead of the front across LA
through parts of MS. Relatively backed surface winds, beneath a
40-60-kt southwesterly LLJ, will contribute to enlarged hodographs
and 300 effective SRH.

Given potential for sustained warm-sector supercells, such as
progged by the HRRR, HRRR-P and WRF-ARW, such an environment will
support a risk for multiple significant tornadoes. Otherwise,
regardless of warm-sector convective density, a band of
thunderstorms should evolve close to the front and shift across the
remainder of the outlook area through the period. This activity will
offer a risk for numerous to widespread damaging winds and embedded
QLCS tornadoes. It will eventually encounter gradually weaker
low-level and mid-level lapse rates and less inflow-layer moisture
content with northeast/eastern extent. This should result in a
somewhat lesser threat for damaging winds and tornadoes tonight.

$$

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