Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 231256
SWODY1
SPC AC 231254

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 AM CDT Tue May 23 2017

Valid 231300Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS
SOUTH/SOUTH-CENTRAL TEXAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHEAST STATES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM TEXAS TO
THE COASTAL CAROLINAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
ILLINOIS...

...SUMMARY...
Severe storms are expected across southern and eastern Texas, in
addition to portions of the Southeast States, mainly this afternoon
and evening.

...Synopsis...
An amplified large-scale pattern will continue over the CONUS, with
a prevalent longwave trough between the Rockies and Appalachians.
Multiple shortwave disturbances will move over a broad part of the
south-central/southeast CONUS and Midwest on the southern and
eastern peripheries of this longwave trough.

...South-Southeast Texas to Louisiana...
Embedded within west/northwesterly cyclonic flow aloft, water vapor
imagery early this morning features an eastward-moving low-amplitude
impulse over east TX in advance of a more substantial amplifying
shortwave trough over northwest TX. A cold front across parts of
west/northwest TX early today will gradually accelerate
southeastward, reaching the TX coast by evening. While the richest
boundary layer moisture (70s F surface dewpoints) will be relegated
to Deep South TX near a residual front, steep lapse rates and 60s F
surface dewpoints will lead to moderate surface-based
destabilization this afternoon in areas as far north as the TX Hill
Country.

Increasingly strong west/northwesterly winds aloft (50-75 kt 500 mb
and above) will support the development of supercells capable of
large hail. Gradual storm mergers may lead to upscale growth into
one or more southeastward-spreading bands of storms capable of
severe-caliber wind gusts as they spread toward parts of the TX
coastal plain through early/mid-evening.

...Southeast States...
Widespread showers and thunderstorms are ongoing early this morning
mainly across southern AL into GA and far north FL. The
eastward-moving precipitation should effectively delineate the
northward extent of appreciable surface-based destabilization later
today, although considerable cloud cover even exists on the
periphery of this early-day convection. Regardless, the relatively
strongest destabilization is expected to be relegated to south
GA/north FL to the coastal Carolinas, as well as across parts of AL
in the wake of the early-day precipitation.

Considerably strengthening low/mid-level winds above the surface are
expected today with deep-layer shear increasing to 35-45 kt in most
areas of the coastal Southeast. As such, the potential will exist
for organized severe thunderstorms including a few supercells and
organized linear clusters where more appreciable destabilization
occurs. Damaging winds and some tornado risk will exist, perhaps
especially southern/eastern GA into coastal SC where low-level
shear/SRH is expected to be strongest coincident with the general
southern edge of more prevalent early-day precipitation.

...Illinois...
As a closed upper low digs southward from MN to IA by late
afternoon, a southern-peripheral vort max will move northeastward
from MO into IL and gradually weaken. Modest moisture and cool
thermal profiles aloft/steep lapse rates may sufficient to yield a
few strong/locally severe storms as storms develop near the weak
front. This would be prior to the onset of more thunderstorms and
some southeastward acceleration of the front with some potential for
strong gusty winds where somewhat stronger heating and a more mixed
boundary layer exists.

..Guyer/Jewell.. 05/23/2017

$$


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