Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 180504
SWODY2
SPC AC 180504

Day 2 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
1204 AM CDT Fri Aug 18 2017

Valid 191200Z - 201200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SATURDAY
AFTERNOON AND EVENING ACROSS PARTS OF THE UPPER OHIO VALLEY AND
LOWER GREAT LAKES REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL PLAINS...

CORRECTED 5 PERCENT SEVERE LINE ACROSS OHIO VALLEY/LOWER GREAT LAKES
REGION

...SUMMARY...
Strong thunderstorms may impact parts of the upper Ohio Valley and
lower Great Lakes region, as well as portions of the central Plains,
on Saturday, accompanied by some risk for severe weather.

...Synopsis...
The mid-latitude westerlies are in the process of retreating to the
general vicinity of the Canadian/U.S. border area.  As this occurs,
subtropical ridging appears likely to become a bit more prominent
over an expanding portion of the central and southern U.S. by
Saturday, particularly to the east of the Rockies.  Lingering upper
troughing within the westerlies is forecast to accelerate east
northeast of the Great Lakes/Ohio Valley region, into Quebec and the
Northeast by late Saturday night.  Within this regime, models
indicate that an appreciable embedded short wave impulse will pivot
into and through the upper Ohio Valley during and shortly after the
peak heating hours Saturday afternoon and evening.  The most
significant upstream short wave trough is expected to mainly
progress across Saskatchewan and Manitoba, but a minor perturbation
trailing to the southwest of this feature may migrate across the
northern U.S. intermountain region.

...Upper Ohio Valley/lower Great Lakes region...
Although seasonably high moisture content is expected to be
spreading east of the Atlantic Seaboard by early Saturday, guidance
indicates that residual low-level moisture may be sufficient to
contribute to modest boundary layer destabilization (CAPE up to
around 1000 J/kg) by Saturday afternoon.  This is expected in
response to insolation, beneath a relatively cool mid-level air mass
forecast to overspread the region (500 mb temps of -12 to -14 C).
Aided by mid-level forcing for ascent, scattered thunderstorm
activity is expected to initiate by early to mid afternoon, in the
presence of modest vertical shear associated with 30-40 kt cyclonic
mid-level flow.  At least marginally severe hail and localized
potentially damaging wind gusts appear possible in stronger storms,
which could include isolated short-lived supercell structures,
before activity diminishes Saturday evening.

...Central Plains...
In the presence of modestly steep mid-level lapse rates, models
continue to suggest that a corridor of moderate mixed layer CAPE
(1000-2000+ J/kg) may develop within the lee surface trough, across
parts of western Kansas into western and central Nebraska by late
Saturday afternoon.  With sharp turning of low through mid-level
wind fields perhaps contributing to modest vertical shear within
otherwise weak deep layer mean flow, the environment appears at
least potentially supportive of scattered severe storm development,
including isolated supercells.  Forcing to support the initiation of
storms is more unclear, but models indicate that a very weak
mid-level perturbation may cross the region during the late
afternoon/early evening hours.

..Kerr.. 08/18/2017

$$



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