<pre>ACUS11 KWNS 202147 SWOMCD
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 </pre>
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