Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 241746
SWODY2
SPC AC 241746

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1246 PM CDT Fri Mar 24 2017

Valid 251200Z - 261200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST STATES TO
WESTERN AND MIDDLE TN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA TO THE MIDDLE MS VALLEY...

...SUMMARY...
Severe storms will be possible across parts of the lower Mississippi
Valley into the central Gulf Coast States and western and middle
Tennessee on Saturday.  A marginal risk for a strong to severe storm
is also possible into the mid Mississippi Valley.

...Synopsis...
Latest models are in general agreement with the evolution of the
closed low and attendant trough undergoing weakening as the 500-mb
low tracks northeastward from northeast OK/southwest MO region to
the mid Mississippi Valley by 12Z Sunday.  This trend in the upper
pattern should result in gradual weakening of 500-mb winds across
the eastern periphery of the synoptic trough, and the southern
extent of a southerly low-level jet shifting away from the southern
extent of the lower Mississippi Valley.  At the surface, the
attendant low will track from southwest MO to west-central IL area
while it gradually weakens.  A cold front trailing
south-southwestward will advance east across the lower and middle MS
Valleys.

Meanwhile, a progressive Pacific trough will track from CA through
the Southwest States to the southern Rockies on Saturday.

...Lower MS Valley/Central Gulf Coast States to western/middle TN...

A band of showers and thunderstorms (some severe) should be ongoing
at the beginning of the forecast period (early Saturday morning)
from the middle MS Valley region to western MS.  The greatest
severe-weather threat with the early morning storms should be across
the lower MS Valley where instability will be stronger combined with
strong bulk shear and a 40-60 kt southerly low-level jet will
enhance the damaging wind threat and some tornado threat.  This
activity may weaken as it advances east into an environment that is
initially more stable with eastward extent toward western AL.
However, the warm sector will moisten into Saturday afternoon, given
southerly low-level flow, while diabatic heating and some steepening
of midlevel lapse rates support moderate instability across much of
the Slight Risk area.  MLCAPE across western and middle TN should be
weaker, generally less than 1000 J/kg.

Models are in general agreement with new thunderstorm development
occurring along much of the cold front as it advances east into the
destabilizing environment.  All severe hazards will be possible,
given sufficient bulk shear for storm organization, with damaging
winds and a tornado threat being the primary severe threats, as
modest lapse rates may tend to limit a hail threat.  Despite weaker
instability with northward extent, the tornado threat may tend to be
greater across the northern extent of the Slight Risk area as the
low-level jet shifts away from the southern extent of the lower MS
Valley.

Although low-level winds will tend to veer across the southern
extent of the lower MS Valley Saturday afternoon, the exit region of
a strong west-northwesterly midlevel jet reaching LA and southern MS
may result in a few robust updrafts where the environment is more
unstable.  Damaging winds and hail should be the primary threats
with the afternoon to early evening storms across this region.

...Middle MS Valley...
Cold 500-mb temperatures/steepening midlevel lapse rates and forcing
for ascent ahead of the closed low track suggest a few stronger
storms will be possible Saturday afternoon.

...Northern AZ...
A few thunderstorms will be possible across northern AZ and far
southern UT Saturday afternoon as lapse rates steepen and forcing
for ascent increases with the eastward progression of the CA trough.
 Cold 500-mb temperatures and steep lapse rates suggest small hail
and/or locally strong wind gusts will be possible, though overall
coverage is not expected to be great enough to warrant a categorical
risk area, at this time.

..Peters.. 03/24/2017

$$


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