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Severe Storm Outlook Narrative (AC)
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ACUS03 KWNS 220704
SWODY3
SPC AC 220703

Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0203 AM CDT Tue Aug 22 2017

Valid 241200Z - 251200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THURSDAY
AFTERNOON AND EVENING ACROSS THE BLACK HILLS REGION AND CENTRAL HIGH
PLAINS...

...SUMMARY...
Strong thunderstorms may impact the Black Hills region and central
high Plains late Thursday afternoon and evening, perhaps accompanied
by some risk for severe weather.

...Synopsis...
Models indicate that the westerlies will remain most progressive
across the northern mid-latitude Pacific into western Canada through
this period.  A significant short wave impulse on the leading edge
of this regime is forecast to continue inland, east northeast of the
Canadian Rockies, through much of Alberta by late Thursday night.
Downstream, an axis of amplified troughing may make only slow
eastward progress across parts of the Canadian Maritimes, Quebec and
the Atlantic Seaboard.  However, it appears that this feature will
begin to undergo considerable deformation, and subtropical ridging
may begin to rebound across the eastern Gulf of Mexico/Gulf Coast
region, in the wake of a tropical system which may approach the
lower Texas coast.

High moisture content air will largely remain confined to areas
along and south of a remnant frontal zone (and the mid-latitude
westerlies) across the Gulf and south Atlantic Coast region.
However, a narrow corridor of substantive low-level moisture return
is forecast into the vicinity of surface troughing to the lee of the
Rockies.

...Parts of central high Plains into Black Hills region...
Models suggest that low-level moistening beneath at least modestly
steep mid-level lapse rates will contribute to CAPE on the order of
1000-2000 J/kg by late Thursday afternoon.  Although deep layer
westerly mean wind fields are expected to remain rather weak,
veering from south/southeasterly to westerly in the lower to
mid-levels may contribute to at least marginally sufficient shear
for organized convective development.  This potential could be
further enhanced by modest southerly low-level jet strengthening (to
around 30 kt at 850 mb) Thursday evening.  While large-scale forcing
for ascent to support convective development remains unclear, the
environment probably will become conducive to the initiation of at
least isolated to widely scattered strong storms, some of which
could be accompanied by a risk for severe hail and locally strong
surface gusts.

..Kerr.. 08/22/2017

$$
National Weather Service Weather Forecast Office
