Severe Storm Outlook Narrative (AC)
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000
ACUS03 KWNS 210823
SWODY3
SPC AC 210822

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0222 AM CST Thu Feb 21 2019

Valid 231200Z - 241200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SATURDAY
THROUGH SATURDAY EVENING ACROSS PARTS OF CENTRAL/EASTERN
ARKANSAS...THE MISSOURI BOOTHEEL...WESTERN/CENTRAL
KENTUCKY...WESTERN/MIDDLE TENNESSEE...NORTHERN MISSISSIPPI AND
NORTHWESTERN ALABAMA...

...SUMMARY...
An organized cluster of severe thunderstorms may evolve Saturday in
a corridor across the Ozark Plateau through the lower Ohio Valley,
with additional severe storms possible across the lower Mississippi
into Tennessee Valleys.  Considerable wind damage may accompany
these storms along with the risk for tornadoes, some of which could
be strong.

...Discussion...
In phase with the subtropical westerlies, a vigorous short wave
impulse of mid-latitude Pacific origins is forecast to accelerate
northeast of the southern Plains through the Great Lakes region
during this period.  Strong cyclogenesis may already be underway by
12Z Saturday near the Texas/Oklahoma Panhandle vicinity, and models
continue to indicate that the rapid evolution of a broad and deep
cyclone will proceed northeastward into the Great Lakes region by
the end of the period.

This likely will include the intensification of a cyclonic mid-level
jet, including speeds in excess of 100 kt at 500 mb, across the
southern Plains Red River Valley, through the middle Mississippi and
Ohio Valleys.  Across the evolving warm sector, models indicate that
south/southwesterly winds will strengthen to 50-70+ kt through the
850-700 mb layer.

Although the warm frontal zone probably will surge north of the Ohio
River, and into/through the lower Great Lakes region by late
Saturday evening, an initial position roughly along the Ohio River
west-southwestward into the Ozark Plateau may provide the main focus
for severe thunderstorm potential.  Early period convection,
associated with weak elevated destabilization above the front, may
inhibit, or at least slow, boundary-layer destabilization to the
north, while leaving a remnant surface boundary.

In association with the onset of stronger surface pressure falls,
surface dew points are expected to increase through the lower/mid
60s along and south of this boundary.  Coupled with strengthening
large-scale ascent, this is expected to contribute at least weak
boundary-layer destabilization supportive of an evolving organized
mesoscale convective system.  CAPE on the order of 500+ J/kg appears
possible.  Given the strength of the environmental wind fields (and
shear) within the convective layer, the convective system may be
accompanied by considerable potential for strong and damaging wind
gusts.

It appears that this may initiate over parts of central and eastern
Arkansas by midday, before progressing east-northeastward through
the lower Ohio Valley by Saturday evening.  A few tornadoes, some
strong, are also possible, particularly with discrete supercells
which may form near/just ahead of mainly the southern flank of the
evolving system.

More discrete storms, including supercells, may eventually develop
as far south as the lower Mississippi Valley through portions of the
southern Appalachians by late Saturday night.

..Kerr.. 02/21/2019

$$



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