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ACUS11 KWNS 181049
SWOMCD
SPC MCD 181048
MNZ000-IAZ000-SDZ000-181315-

Mesoscale Discussion 1475
NWS Storm Prediction Center Norman OK
0548 AM CDT Tue Sep 18 2018

Areas affected...Extreme eastern SD...southwest MN...and adjacent
northwest IA

Concerning...Severe potential...Watch unlikely

Valid 181048Z - 181315Z

Probability of Watch Issuance...20 percent

SUMMARY...A locally strong/damaging wind gust is possible early this
morning across extreme east-central and southeast South Dakota into
southwest Minnesota and extreme northwest Iowa, as a line of storms
moves east through this region. Isolated hail will remain a threat
as well.

DISCUSSION...Trends in mosaic radar imagery showed a compact forward-propagating MCS had accelerated since 0830Z across eastern
SD, with current track toward the east-northeast at close to 50 kt.
At 907Z, HON recorded a wind gust of 46 kt and at 912Z, MHE recorded
a wind gust of 53 kt with this line of storms. This MCS is rooted
above a stable boundary layer, given the surface analysis showed a
synoptic boundary extended from southern IA northwest into northeast
NE and then westward, with temperatures dew points in the 50s across
much of eastern SD and southern MN. However, 00Z soundings at
RAP/ABR indicated and RAP analysis/forecast soundings suggested dry
air was located beneath 700 mb (sub-cloud layer). Despite the
presence of a stable boundary layer, higher-momentum air within
downdrafts could be enhanced as they are flung through the
aforementioned sub-cloud dry air, reaching the ground as a severe
wind gust. This scenario appears possible into southwest MN and
perhaps a row of counties in northwest IA. There is uncertainty
with how far east this potential wind threat will extend into
southern MN, as 1) forecast soundings suggest less of a sub-cloud
dry air layer that far east, and 2) recent surface observations at
FSD (45 kt at 1030Z) and BKX (43 kt at 1020Z) indicated sub-severe
wind gusts.

..Peters/Edwards.. 09/18/2018

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...MPX...DMX...FSD...ABR...

LAT...LON 43629695 44469676 44949689 44659518 44429410 44059385
43589401 43359422 43289492 43309597 43339702 43629695



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