Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 261714
SWODY2
SPC AC 261713

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1213 PM CDT Tue Sep 26 2017

Valid 271200Z - 281200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
EVENING ACROSS PARTS OF NORTHERN NEW ENGLAND...

...SUMMARY...
Strong thunderstorms may impact parts of northern New England early
Wednesday evening, accompanied by at least some risk for severe
weather.

...Discussion...
The northern branch of split westerlies emanating from the
mid-latitude Pacific may continue to become more prominent across
North America during this period.  Within this regime, amplifying
ridging off the Pacific coast is forecast to build inland across the
Pacific Northwest/northern U.S. Rockies and western Canadian
provinces by late Wednesday night, to the north of a closed low now
evolving within the southern branch, near/east of the lower Colorado
Valley.  As this occurs, models do indicate that the center of the
closed low may undergo considerable deformation while accelerating
northeastward through the Four Corners region Wednesday through
Wednesday night, as a downstream southern branch ridge maintains
considerable strength across the western Gulf Coast/southern
Plains/lower Mississippi Valley region.

Across eastern North America into the western Atlantic, Maria is
expected to continue to weaken, with some eastward acceleration away
from the Mid Atlantic coast, as large-scale northern branch
troughing broadens across eastern Canada and the Great Lakes region,
toward the north Atlantic coast.  Within this regime, one vigorous
short wave impulse appears likely to accelerate eastward across
Quebec, accompanied by a significant surface cyclone, which may
undergo further deepening while occluding.

...Parts of northern New England...
Within the warm sector of the surface cyclone, low-level moisture
(characterized by mid 60s+ F surface dew points) will support the
development of weak to moderate CAPE (500-1000+ J/kg) with
insolation across southern Quebec into portions of northern New
England by Wednesday afternoon.  This is expected to coincide with
strengthening lower/mid tropospheric flow and vertical shear (30-50+
kt in the 850-500 mb layer), which appears likely to contribute to
an environment at least marginally conducive to the development of
severe storms.  However, forcing to support the initiation of storms
may be generally focused across southern Quebec during the hours
near/shortly after peak boundary layer heating and destabilization.
It may not be until the 28/00-03Z time frame before activity spreads
across the international border into portions of the northern New
England, but the environment may still be conducive to a continuing
risk for mainly potentially damaging wind gusts, before instability
and convection wane further later Wednesday evening.

...Four Corners states into southern Plains...
Moisture return to the west of the Continental Divide may remain
limited, but destabilization beneath the mid-level cold pool
associated with the upper low, coupled with orographic forcing, is
expected to become sufficient to support scattered thunderstorm
activity Wednesday afternoon and evening.  Farther east, despite the
continuing influx of moist air on easterly/southerly lower through
mid-level flow, thunderstorm probabilities remain unclear north of
the Rio Grande River through the southern  Plains, as weak mid-level
lapse rates may result in thermodynamic profiles only marginally
conducive to the generation of sufficient charge separation for
lightning.

..Kerr.. 09/26/2017

$$



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