Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 211620
SWODY1
SPC AC 211619

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1119 AM CDT Mon Aug 21 2017

Valid 211630Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS MID-MO VALLEY AND
UPPER MIDWEST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS CENTRAL GREAT
PLAINS TO THE GREAT LAKES AND MID-ATLANTIC STATES...

...SUMMARY...
Scattered severe hail and wind is anticipated across parts of the
Mid-Missouri Valley and Upper Midwest this afternoon and evening.
Isolated severe storms are also possible across a broad region from
the central Great Plains to the Mid-Atlantic States.

...Mid-MO Valley and Upper Midwest...
A small forward-propagating MCS over south-central SD is the primary
concern for severe potential this afternoon. Robust insolation is
occurring downstream across eastern NE into western IA. This will
likely result in a strongly buoyant air mass developing by peak
heating amid very steep mid-level lapse rates near 8.5 C/km from
700-500 mb per 12Z Rapid City and North Platte soundings. On the
fringe of faster mid-level westerlies over the Upper Midwest, this
MCS should remain organized with a threat for severe hail/wind and
expand in coverage as it likely evolves east-southeast. The overall
threat with this MCS should diminish during the evening over parts
of IA given a large swath of ongoing non-severe convection across
central IA to northern IL hindering diabatic heating.

...Midwest to Mid-Atlantic States...
Outside of the aforementioned scenario, confidence is low in
highlighting probabilities beyond a Marginal risk. Pockets of
moderate to large buoyancy should develop across parts of southern
IL into IN and northern Lower MI, as well as east of the lee trough
in the Mid-Atlantic States. A broad belt of moderate westerlies
southeast of the amplifying short wave trough over the southern
Canadian Prairies will support multicells and a few transient
supercell structures with risks for isolated damaging winds and
hail.

..Grams/Picca.. 08/21/2017

$$



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