Severe Storm Outlook Narrative (AC)
Issued by NWS

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ACUS01 KWNS 201938
SWODY1
SPC AC 201937

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0237 PM CDT Wed Sep 20 2017

Valid 202000Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
WI...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM NORTHERN MO
TO THE U.P. OF MI...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF THE SOUTHERN PLAINS...

...SUMMARY...
A few strong to severe storms with locally damaging wind and hail
remain possible from the Upper Peninsula of Michigan to northern
Missouri between about 4 to 11 pm CDT. A few strong storms are also
possible from southwest through north central Texas.

...Discussion...

No changes warranted to 1630z outlook.

..Darrow.. 09/20/2017

.PREV DISCUSSION... /ISSUED 1129 AM CDT Wed Sep 20 2017/

...Wisconsin through northern Missouri...

Boundary layer dewpoints in the mid to upper 60s F are advecting
northward through the pre-frontal warm sector, and this will result
in moderate to strong instability with 1500-2500 J/kg MLCAPE this
afternoon. However, mid-level lapse rates will remain modest due to
relatively warm temperatures aloft with -6 to -7C at 500 mb. The
stronger winds aloft will remain in post frontal region but a belt
of moderate flow with 40-50 kt in the 700-400 mb layer will exist in
the frontal zone. Height will rise slightly during the day in wake
of shortwave trough currently lifting into southern Manitoba.
Destabilization of the boundary layer and low-level convergence
should be sufficient to initiate storms along the cold front. Have
introduced a 15% categorical risk area over WI where the best
combination of stronger winds aloft and instability exists.

...Southwest through northwest Texas...

Subsidence aloft associated with low-amplitude shortwave ridging
remains a limiting factor for the development of storms today.
However, strong diabatic heating and mixing in the vicinity of
dryline should result in at least isolated storms from southwest
through northwest TX this afternoon. Temperatures aloft are warm due
to mid-upper subsidence, and this will mitigate mid-level lapse
rates and overall updraft strength despite 2000-3000 J/kg MLCAPE.
Nevertheless, some storms could pose a risk for a few instances of
downburst winds from late afternoon into early evening.

$$



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