Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 221952
SWODY1
SPC AC 221950

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0250 PM CDT Thu Jun 22 2017

Valid 222000Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS PARTS OF THE CENTRAL GULF COAST
STATES...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL HIGH PLAINS...AND WITHIN A CORRIDOR ACROSS THE MID
MISSOURI VALLEY THROUGH THE SOUTHERN GREAT LAKES REGION...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
GULF COAST SLIGHT RISK AREA...INTO PORTIONS OF THE MID SOUTH...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS AREAS
SURROUNDING THE SLIGHT RISK ACROSS THE CENTRAL PLAINS AND MID
MISSOURI VALLEY INTO THE GREAT LAKES REGION...

...SUMMARY...
Thunderstorm activity, potentially capable of producing localized
damaging wind gusts and a couple of tornadoes, will persist across
parts of the central Gulf States into the Mid South late this
afternoon and evening.  Other widely scattered strong to severe
storm development is possible in a corridor from the central high
Plains through the mid Missouri Valley and southern portions of the
Great Lakes region.

...20Z Outlook Update...
The general idea of the preceding outlooks remains largely
unchanged.  However, adjustments have been made to the categorical
and probabilistic lines to account for ongoing trends, and attempt
to better define areas of severe weather potential for late this
afternoon and evening.  Higher (slight risk) severe probabilities
east northeast of the central high Plains are generally focused
within the narrow corridor of stronger boundary layer heating now
taking place just ahead of the cold front from the central
Plains/mid Missouri Valley into the upper Great Lakes region, and
along/south of a remnant outflow boundary across parts of the Upper
Midwest.  This is mostly south of the strongest westerlies, but
shear on the southern fringe of the westerlies, coupled with
thermodynamic profiles characterized by relatively steep mid-level
lapse rates and moderate CAPE, appears to provide an environment
conducive to strong/severe storm development.  For more specific
details concerning ongoing or more imminent severe weather
potential, please refer to the latest SPC mesoscale discussions.

..Kerr.. 06/22/2017

.PREV DISCUSSION... /ISSUED 1143 AM CDT Thu Jun 22 2017/

...Gulf Coast States/lower MS Valley into TN Valley...
Tropical Depression Cindy, centered over far west-central LA as of
mid/late morning, is expected to continue to drift slowly
north-northeastward through the afternoon into tonight. Even while
the system may begin to fill with a tendency for a weakening surface
low, strong low/mid-level southerly winds on the eastern periphery
of the system will continue to yield the possibility of low-topped
supercells and at least some tornado risk over a relatively broad
area of the region. A few weakly rotating cells have been noted this
morning across the western FL Panhandle as well as southeast LA into
southern MS, and this same scenario/supportive environment should
somewhat expand north-northwestward through the afternoon and
evening. The overall tornado risk, which appears relatively modest
overall across a broad region, should diurnally peak this afternoon
into early/mid-evening, although some tornado risk could continue
into the overnight and early morning hours of Friday.

...CO/WY Front Range and Central/southern High Plains...
Isolated high-based storms are expected to develop by late afternoon
along the south/southeastward-moving front spanning portions of KS
into southeast NE. Where storms develop, moderate amounts of
CAPE/vertical shear will allow for semi-organized clusters of storms
capable of isolated large hail aside from a locally damaging wind
risk with outflow dominant convection.

Other storms are likely to develop this afternoon within an evolving
post-frontal upslope flow regime, with initial development along and
just east of the CO/southeast WY Front Range. As an increasingly
moist environment develops into the region and the boundary layer
destabilizes, relatively long/straight hodographs will support the
possibility of splitting supercells capable of large hail.
Severe-caliber wind gusts will also possible given relatively
deep/well-mixed sub-cloud layers.

...Upper Midwest/Great Lakes...
Ahead of a southeastward-moving cold front, a fast-moving cluster of
storms will continue eastward from southeast MN into southern WI
today to the north of an outflow boundary and weakening convection
across eastern IA/far southern WI and far northern IL. Widespread
cloudiness across WI to the north of the outflow boundary should
largely curb surface-based severe potential, with storms otherwise
likely to develop by mid/late afternoon along the
southeastward-moving front across IA and possibly into far southeast
MN/southwest WI. While the strongest winds aloft will tend to lag
(to the west/northwest) the cold front, sufficient vertical shear
(30-40 kt) in the presence of moderate buoyancy (generally 1500-2500
J/kg MLCAPE) will yield multicells and some supercells capable of
large hail and damaging winds especially late this afternoon into
evening.

$$



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