Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 231301
SWODY1
SPC AC 231259

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0759 AM CDT Sun Jul 23 2017

Valid 231300Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE GREAT LAKES TO THE CENTRAL APPALACHIANS...MID-ATLANTIC AND
TIDEWATER REGIONS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING WESTWARD  TO MUCH OF OKLAHOMA...

...SUMMARY...
A broad zone of widely scattered to scattered thunderstorms and at
least isolated severe risk will reside over portions of the Great
Lakes, Ohio Valley and central Appalachians area today.  Isolated
strong to severe storms also may occur from Oklahoma eastward to the
Mid-South this afternoon.

...Synopsis...
The upper-air pattern will be characterized by continued/persistent
ridging over the Southwest, southern Plains and Deep South.
However, northern-stream amplification is underway (and will
continue) in relation to two major shortwave troughs now apparent in
moisture-channel imagery:
1.  From western Lake Superior to northern MO, with an
intermittently closed 500-mb low crossing Lake Superior.  This
perturbation is expected to move east-southeastward, reaching Lower
MI, IN and western KY by 00Z.  By 12Z, the trough should reach
southern ON, the upper Ohio Valley and eastern parts of KY/TN.
Associated cyclonic flow will extend from the upper Great Lakes and
upper Mississippi Valley to the northern/central Appalachians,
spreading into the Mid-Atlantic and New England late in the period.
One or more convectively induced/enhanced vorticity maxima may
traverse the cyclonic-flow belt ahead of the shortwave trough as
well, crossing parts of the Ohio Valley and central Appalachians.
2.  Over coastal BC, forecast to progress eastward over the Canadian
Rockies and parts of AB by the end of the period.

At the surface, a broad/weak cyclone initially over central/eastern
Upper MI is expected to move erratically southeastward then eastward
across Lake Huron and southern ON, reaching the Lake Ontario/western
NY vicinity by 12Z.  The trailing cold front is preceded by outflow
boundaries along much of its length, and as such isn`t very
well-defined, but was estimated at 11Z to extend from the U.P. low
southwestward across central IA, central KS and east-central NM.
The front should move southeastward across the Great lakes region
through the period, while decelerating/stalling over the southern
Plains.  By 12Z, it should extend from the western NY low
southwestward over OH, southern IL, the MO Ozarks and
central/western OK.

...Ohio Valley, Great Lakes to Mid-Atlantic/Tidewater...
Scattered thunderstorms are expected to develop across this region
in patchwork fashion, along and ahead of the surface cold front
today into this evening.  Activity will offer mainly damaging wind,
with isolated severe hail also possible.  The risk area remains
rather broad-brushed and generalized at this time due to the messy,
nuanced character and mesoscale dependence of potential convective
foci.

Scattered to numerous ongoing thunderstorms over the Ohio Valley,
mainly around southern IN and northwestern/north-central KY, are
related to the merger of
1.  A long-lived, severe, damaging MCS that affected the
MKC-COU-STL-SLO corridor overnight and
2.  Persistent, somewhat elevated storms forming continually for
several hours over an outflow pool left behind by a prior convective
complex near the IN/KY border.

This has resulted in an elongated banner of convectively generated
vorticity, evident in composite-reflectivity animations and
satellite imagery from south-central IL to south-central IN, with a
couple of embedded vorticity maxima evident.  The distorted MCV-like
feature should shift generally eastward across the central
Appalachians today, enhancing large-scale ascent and deep shear on
the mesoscale atop a very moist air mass from there to the coastal
Mid-Atlantic and Tidewater regions.  While convective cloud debris
and anvil blow-off will impede insolation somewhat, weak MLCINH and
sufficient diabatic heating still should support additional
convective development, predominately multicellular, with damaging
winds the main threat.  Convection along associated
mean-flow-parallel outflow boundaries across parts of KY also may
produce strong-severe gusts today.

Other areas of potential convective development include the frontal
region itself and the zone of large-scale ascent across the Great
Lakes.  These areas will reside ahead of the progressive shortwave
trough, in a regime of weaker boundary-layer theta-e but also colder
air aloft and somewhat stronger deep shear than farther south.

...Mid-South to southern Plains...
Widely scattered to scattered thunderstorms are forecast to develop
this afternoon near the frontal zone, as well as nearby warm-sector
outflows and differential-heating boundaries.  Hot, deeply mixed
subcloud layers will support a few hours of strong-severe downburst
potential in the most vigorous convection.  Deep-layer flow and
shear will be weak, indicating the predominance of outflow-dominant
multicells.  Activity should diminish markedly after sunset due to
the collective stabilizing effects of outflows and surface diabatic
cooling.

..Edwards/Goss.. 07/23/2017

$$



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