Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 210545
SWODY1
SPC AC 210544

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1144 PM CST Tue Feb 20 2018

Valid 211200Z - 221200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN
TEXAS TO WESTERN MISSISSIPPI...

...SUMMARY...
Thunderstorms are forecast from Texas to the Ohio Valley and
Appalachians today into tonight, and a few of these storms may be
strong to severe from the Sabine Valley to western Mississippi.
Locally damaging winds and perhaps a tornado will be the main
threats.

...Synopsis...
The mid/upper pattern across the country will witness little
large-scale change today. An impressive ridge will persist
along/offshore the East Coast, while a western-US trough is
reinforced by a jet maximum advancing southward along the California
coast. Between the two, a broad corridor of strong southwesterly
flow will persist from the southern Plains to the Great Lakes
region. Meanwhile, surface high pressure will build over the
northern Plains and upper Midwest, helping push a cold front
southeast across southern/eastern Texas, the lower Mississippi
Valley, and the Ohio Valley.

...Eastern Texas to Mississippi...
Aided by widespread precipitation across the cool sector, a cold
front is forecast to continue progressing towards the Texas coast
and Sabine Valley through the early morning hours. However, as one
or more weak shortwave impulses lifts northeast within the
sub-tropical channel, a slowly amplifying southerly low-level jet
should slow the southeastern advance of the combined front/outflow
boundary over northern Louisiana. Furthermore, broad forcing for
ascent may induce a weak surface low along this baroclinic zone
during the afternoon and evening hours. Within the warm sector
along/ahead of this frontal wave, surface dew points in the upper
60s are expected to foster upwards of 500-1000 J/kg of MLCAPE,
despite modest 700-500mb lapse rates.

Thunderstorms should organize along the front, with some enhancement
in eastward propagation supported by any mergers from development
within moist/confluent flow in the open warm sector. A few small,
northeastward-advancing bowing segments may organize through the
afternoon and evening, owing to primarily uni-directional southerly
flow within the cloud-bearing layer. These cells will chiefly pose a
localized damaging wind threat. However, subtle backing of surface
flow near the front, combined with any northwest-southeast oriented
bands, may support a tornado or two as well.

..Picca/Leitman.. 02/21/2018

$$



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