Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 211957
SPC MCD 211957

Mesoscale Discussion 1723
NWS Storm Prediction Center Norman OK
0257 PM CDT Sat Oct 21 2017

Areas affected...portions of western and central Iowa into southern

Concerning...Severe potential...Watch possible

Valid 211957Z - 212200Z

Probability of Watch Issuance...40 percent

SUMMARY...A line of thunderstorms along an eastward advancing cold
front may pose a sporadic damaging wind threat into the evening
hours across central Iowa into parts of far southern Minnesota.
Some small hail also may be possible with the stronger cells. Trends
will be monitored for possible watch issuance later this afternoon.

DISCUSSION...Thunderstorms have developed along the eastward surging
cold front across western MN/IA. While the area has been under
mostly cloudy skies today, breaks ahead of the front have allowed
temperatures to warm into the upper 60s and low 70s with dewpoints
in the low 60s. This has resulted in a narrow corridor of weak
instability with MLCAPE values around 500-1000 J/kg. Strong deep
layer shear oriented generally parallel to the front will maintain
linear storm mode as storms quickly track eastward. Steep midlevel
lapse rates in the presence of weak instability and strong shear
will allow for some cells to approach severe limits with damaging
wind gusts the main concern.

Given the fast, progressive nature of the cold front and
aforementioned shear vector orientation with respect to the front,
storms will have difficulty remaining ahead of the boundary. This
may temper the overall threat as storms become undercut by the
front, and any severe potential may be brief/sporadic. Trends will
be monitored and a severe thunderstorm watch may be needed at some
point this afternoon.

..Leitman/Thompson.. 10/21/2017

...Please see for graphic product...


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