Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 230446
SWODY2
SPC AC 230445

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1145 PM CDT Tue Aug 22 2017

Valid 241200Z - 251200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THURSDAY
AFTERNOON AND EVENING ACROSS THE NEAR THE FRONT RANGE OF THE
ROCKIES...THE BLACK HILLS AND ADJACENT CENTRAL HIGH PLAINS...

...SUMMARY...
Strong thunderstorms may impact the Front Range of the Rockies, the
Black Hills vicinity and adjacent areas of the 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 during
this period.  A significant short wave trough on the leading edge of
this regime is forecast to continue inland, and take on an
increasing negative tilt east northeast of the Canadian Rockies
through much of Alberta by late Thursday night.  As it does, it
appears that the mid-level cold core and stronger cyclonic flow will
largely remain north of the international border area.

Downstream, an axis of amplified cyclonic flow is still expected to
make only slow eastward progress across parts of the Canadian
Maritimes, Quebec and the Atlantic Seaboard.  However, it appears
that this troughing will begin to undergo considerable deformation.
At the same time, to the south, subtropical ridging may begin to
rebound across much of the eastern Gulf of Mexico and northern Gulf
Coast region, while a tropical wave and possible intensifying
embedded tropical cyclone approach the Mexican/lower Texas Gulf
coast.

High moisture content air is expected to remain largely confined to
areas along and south of a remnant front zone across the south
Atlantic and Gulf Coast region, which will be well south of the
mid-latitude westerlies.  It does appear, though, that a narrow
corridor of substantive moisture return could take place on
southerly low-level flow into the vicinity of surface troughing to
the lee of the Rockies.

...Parts of central high Plains into Black Hills region...
Models continue to indicate that the low-level moisture return
beneath modestly steep mid-level lapse rates will contribute to CAPE
on the order of 1000-2000 J/kg by late Thursday afternoon.  Deep
layer westerly mean wind fields will remain rather weak, but
pronounced turning of low through mid-level winds with height may
contribute to marginally sufficient vertical shear for organized
convective development.  This potential could be enhanced further by
southerly low-level jet strengthening (up to around 30 kt at 850 mb)
Thursday evening.  Given these conditions, the environment seems
likely to become conducive to the initiation of at least isolated to
widely scattered storms off the Front Range and in the vicinity of
the Black Hills, even in the absence of more readily apparent
large-scale forcing for ascent, late Thursday afternoon and evening.
Some of these could be accompanied by a risk for severe hail and
locally strong surface gusts.

...Lower Texas coastal areas...
Ensemble output of the various models suggest that it is possible
that a developing tropical cyclone could approach lower Texas
coastal areas by late Thursday afternoon.  Even if this becomes the
case (and this remains uncertain), any possible associated risk for
tornadoes will depend on many other uncertain factors, including the
extent of tropical cyclone intensification.  Regardless, any
appreciable tornado probabilities would seem most likely to await
the turning of low-level wind fields around to the southeast, with
the inland progression of the system, allowing for the advection of
somewhat more moist and buoyant boundary layer air into and inland
of coastal areas.  This currently appears unlikely during this
period.

..Kerr.. 08/23/2017

$$



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