Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 201940
SWODY1
SPC AC 201939

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0139 PM CST Tue Feb 20 2018

Valid 202000Z - 211200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
TEXAS ACROSS THE OZARKS...

...SUMMARY...
Thunderstorms are expected across a broad part of the south-central
U.S. into the Midwest, with the possibility of a few strong to
locally severe storms through this evening from central Texas into
parts of the Ozarks.

...Central TX into AR and southern MO...
Changes this outlook include an expansion of the Marginal Risk area
farther southwestward across central TX, and a bit farther east from
east Texas into south central AR. In addition, added a small 2%
tornado risk area for the possibility of a brief/weak tornado over
central TX.

A moist and sufficiently unstable air mass remains in place ahead of
the cold front, with strong heating leading to isolated severe
storms capable of hail across central TX. Other storms may form
along the I-35 corridor in TX as deep moisture convergence persists,
with some supercell potential. Extensive clouds persist in much of
the moist sector but pockets of heating may yield further isolated
storm activity developing across central TX where effective SRH
values are currently in excess of 300 m2/s2.  In addition, locally
damaging wind gusts cannot be ruled out especially later this
evening and overnight when linear structures form along the
advancing cold front, from northeast TX across southeast OK and into
AR.

..Jewell.. 02/20/2018

.PREV DISCUSSION... /ISSUED 1017 AM CST Tue Feb 20 2018/

...Synopsis...
A broad long wave trough is expected to persist over the western
CONUS as an initial short wave trough near the four-corners areas
lifts northeastward and deamplifies, and an upstream short wave
trough off the Pacific NW coast moves southward toward the central
California coast.  A strong cold front at the surface, currently
located from extreme northwest Illinois into southwest Missouri,
southwest Oklahoma into east central New Mexico, will continue
moving southeastward into across the middle Mississippi Valley and
the southern Plains.

...Southern Plains into the Ozarks...
Multiple bands of convection are occurring within a broad region of
moist southwest flow aloft from central Texas into the Midwest.  12Z
regional soundings ahead of the cold front indicate generally weak
deep layer lapse rates but FWD and OUN soundings exhibited layers of
7-8 C/km lapse rates that are enhancing zones of buoyancy.
Thunderstorms have organized into several lines over north central
Texas with radar depicting occasional bow echo signatures moving
toward the northeast.  Latest VAD wind profiles show winds of 40-50
kt in the lowest 1 km AGL suggesting potential for a few strong to
locally severe wind gusts to reach the surface as convective
downdrafts transfer the strong horizontal momentum downward.  In
addition, marginal hail may occur with a few of the stronger
updrafts, especially over parts of Texas, where steeper lapse rates
are likely to persist.

Although mid-level height changes over the southern Plains will
remain neutral or are slightly positive as the long wave trough
remains over the west, water vapor imagery indicates additional
perturbations will move northeast from the Big Bend region of Texas
over the southern Plains toward the Ozarks.  This suggests potential
for additional convection to develop this afternoon.  12Z HREF
guidance and latest HRRR hourly runs are consistent with this
scenario, showing several strong organized convective bands/clusters
spreading from Texas into Arkansas later this afternoon and evening.
CAM simulated reflectivity forecasts indicate transient QLCS
structures with embedded bow echoes, suggesting a continued
potential for a few strong to locally severe wind gusts in
association with stronger storms through the evening hours.

$$



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