Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 251654
SWODY2
SPC AC 251653

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1153 AM CDT Mon Sep 25 2017

Valid 261200Z - 271200Z

...NO SEVERE THUNDERSTORM AREAS FORECAST...

...SUMMARY...
The risk of severe thunderstorms appears negligible across the U.S.
Tuesday through Tuesday night.

...Discussion...
The southern branch of split westerlies emanating from the
mid-latitude Pacific remains amplified, but a significant
perturbation appears likely to emerge from embedded large-scale
troughing over the western U.S., and accelerate
northeastward/eastward across the northern Plains through the upper
Great Lakes region during this period.  At the same time, it appears
that a digging upstream perturbation will contribute to the
development of another closed low within remnant upper troughing
near/east of the lower Colorado Valley.  Downstream of this latter
feature, a sharpening ridge is forecast to build from the lower Rio
Grande Valley through the Tennessee/Ohio Valleys, while Maria turns
northward to the east of the southern Mid Atlantic coast.  A weak
mid/upper low now digging into the northeastern Gulf of Mexico
appears likely to linger through this period and beyond.

Models do indicate that the perturbation emerging from the
intermountain region may provide support for cyclogenesis along a
front now quasi-stationary across the upper Great Lakes/Upper
Midwest region.  However, the more significant deepening and
strengthening of lower/mid tropospheric wind fields and shear are
expected northeast of the Great Lakes region, into the vicinity of
James Bay.  With the mid-level cold core of the upper impulse
lagging to the cool side of the surface frontal zone, and weak
mid-level lapse rates limiting destabilization within the warm
sector, the risk for severe storms still appears negligible at this
time.

Across the Southwest, a rather dry environment seems likely to
initially suppress/delay convective development, but forcing for
ascent downstream of the evolving closed low, coupled with moisture
advection, may eventually contribute to scattered thunderstorm
development across parts of the southern Rockies into the Colorado
Plateau.  It appears that this is most probable after dark, with
generally negligible risk for severe weather.

..Kerr.. 09/25/2017

$$



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