Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 261251
SWODY1
SPC AC 261250

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Tue Sep 26 2017

Valid 261300Z - 271200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
WEST TEXAS AND SOUTHERN NEW MEXICO...

...SUMMARY...
A few strong to severe storms with hail and gusty winds are possible
this afternoon and evening over far west Texas and adjacent New
Mexico.

...Synopsis...
A high-amplitude pattern will continue to feature mean upper
troughing over the western U.S., with two main/strong shortwave
components becoming increasingly separated from each other.  The
first shortwave trough is located over the central/northern Plains,
with a closed 500-mb low now evident over the MT/ND border region.
This perturbation will eject northeastward through 12Z, by which
time it will assume positive tilt from a cyclone over northwestern
ON to a col region in western WY.  Meanwhile, a pronounced speed max
and amplifying shortwave trough -- now evident in moisture-channel
imagery from northwestern NV over the central/southern Sierras,
should continue digging southward and southeastward, forming a
closed mid/upper-level cyclone tonight over the lower Colorado River
Valley.

At the surface, 11Z analysis showed a low over northwestern WI, cold
front southwestward over central OK, to near DYS, arching
southwestward across the Big Bend region of west TX.  The northern
low should move northeastward across north-central/northeastern ON
through the period with the approach of the ejecting shortwave
trough.  The cold front should progress to southeastern Lower MI,
western KY, and the Arklatex region by 12Z, while stalling or moving
only slowly southward over the Chihuahuan Desert.

...West TX/southern NM...
Episodic isolated to widely scattered thunderstorms are expected
throughout the period, the great majority being sub-severe.
Somewhat greater thunderstorm coverage -- widely scattered to
scattered -- is expected this afternoon into early evening,
resulting from pockets of diabatically destabilized boundary-layer
air with relatively weak MLCINH.  This air mass will be moistened by
post-frontal flow with easterly and upslope components aiding both
lift and directional shear, and the potential for isolated severe
hail/gusts.

Forecast soundings and modified 12Z EPZ sounding show well-mixed
subcloud layers when adjusted for expected heating and moist
advection, along with 45-50 kt effective-shear magnitudes.  Lack of
more robust low-level winds will limit hodograph size, though deep
shear is still supercell-favorable.  500-1000 J/kg MLCAPE should
predominate in and near the outlook area, locally higher especially
near the Mexican border.  These conditions support the potential for
messy, occasionally discrete to clustered storm modes with sporadic
and generally short-lived storm-scale rotation.  As such, and given
the modest midlevel lapse rates and lack of greater buoyancy, hail
potential appears marginal, both in size and coverage terms.

..Edwards/Jewell.. 09/26/2017

$$



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