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
This Hazardous Weather Outlook is for portions of North Central
Illinois…Northeast Illinois and Northwest Indiana.
.DAY ONE…This Afternoon and Tonight.
Weather hazards expected…
Significant Thunderstorm Risk…with an associated:
Elevated Tornado risk.
Limited damaging wind risk…up to 60 mph.
Limited Flooding Risk.
High Swim Risk
Thunder will be limited with most of the shower activity through early
afternoon. Expect occasional showers and thunderstorms this afternoon.
from northwest Illinois arcing southeast to portions of east central Illinois.
Conditions could become favorable for a couple of these storms to
potentially become tornadic from mid afternoon until around sunset.
There is also a low damaging wind threat. The greatest risk of severe
weather is south and west of a line from Dixon to Joliet to Fowler Indiana.