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ACUS11 KWNS 202147
SWOMCD
SPC MCD 202146
ILZ000-WIZ000-IAZ000-202345-

Mesoscale Discussion 1329
NWS Storm Prediction Center Norman OK
0446 PM CDT Mon Aug 20 2018

Areas affected...Portions of far eastern IA...northern IL...and far
southern WI

Concerning...Severe potential...Watch unlikely

Valid 202146Z - 202345Z

Probability of Watch Issuance...20 percent

SUMMARY...A narrow spatial window remains for a brief tornado or two
with a band of thunderstorms developing northeastward late this
afternoon into early evening. Watch issuance is unlikely.

DISCUSSION...A broken line of thunderstorms has recently developed
across far eastern IA into parts of northern IL along an occluding
warm front. Limited diurnal heating to the north of this boundary
has allowed for weak instability to develop late this afternoon,
with 21Z RAP mesoanalysis estimating MLCAPE up to 1000 J/kg is
present. Both the low and mid-level flow is not overly strong across
this region as an occluding upper low continues to develop slowly
eastward across the Upper Midwest this evening. Still, VWPs from
several radars just to the north of the warm front (KMKX/KLOT) do
show a veering and modestly strengthening wind profile in the 0-3 km
AGL layer. Coupled with a north-northeasterly storm motion, this
low-level veering is supporting around 100 to locally 150 m2/s2 of
effective SRH. A couple of cells within the broken line have
exhibited low-level rotation, with one report of a tornado in
Clinton County IA at 2049Z. Current expectations are for a brief
tornado or two to remain possible with these low-topped
thunderstorms for the next couple of hours. However, the airmass
with northward extent (into southern WI) quickly becomes less
unstable due to persistent cloud cover. Therefore, the overall
limited spatial and temporal nature of the severe threat will
probably remain too marginal to justify watch issuance, although
radar and mesoscale trends will be closely monitored.

..Gleason/Edwards.. 08/20/2018

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

ATTN...WFO...LOT...MKX...DVN...ARX...

LAT...LON 41999091 42349101 42839067 42958960 42808847 42558792
41718753 41028759 40938845 41378879 41618914 41839019
41999091



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