From the NWS: Funnel clouds are possible late this afternoon associated with a
line of clouds and showers developing along the cold front as it
moves southward through the area. These funnels will be short
lived and likely will not touch down. Any funnels that would reach
the ground will be weak and brief.
Winnebago-Boone-McHenry-Lake Illinois-Ogle-Lee-De Kalb-Kane-
420 PM CDT Sat Jun 17 2017 /520 PM EDT Sat Jun 17 2017/
THIS HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL ILLINOIS…
NORTHEAST ILLINOIS AND NORTHWEST INDIANA.
WEATHER HAZARDS EXPECTED:
Significant Thunderstorm Risk…with an associated:
Limited Tornado Risk.
Significant Hail Risk…up to half dollar size.
Significant Thunderstorm Wind Damage Risk…up to 70 MPH.
Elevated Flooding Risk.
Scattered thunderstorms are likely late this afternoon into the
early evening across the entire outlook area. Severe threats of
hail, damaging winds, and heavy rainfall will be possible with any
storm. More widespread thunderstorms are expected later this
evening into the overnight hours, once again, across all of
northern Illinois and northwest Indiana. Damaging winds and heavy
rainfall will be the main threats, along with possible flash
Mesoscale Discussion 1071
NWS Storm Prediction Center Norman OK
0244 PM CDT Sat Jun 17 2017
Areas affected…northeast Missouri…southeast Iowa…northern
Illinois…far southeast Wisconsin…and far northwest Indiana
Concerning…Severe potential…Watch possible
Valid 171944Z – 172145Z
Probability of Watch Issuance…60 percent
SUMMARY…Deepening cumulus/showers developing at this time across
eastern Iowa and northern Illinois are expected to continue evolving
into isolated thunderstorms. Accompanying risk for locally damaging
winds and hail may require watch issuance.
DISCUSSION…Latest visible satellite loop shows a west-to-east band
of towering cumulus/small Cb developing from just north of MLI to
near ORD, and other deepening in the cu field across far northern
Missouri. This increase in cumulus development is occurring just
ahead of the slowly advancing cold front (extending from Lake
Michigan southwest across southern Wisconsin/eastern Iowa into
northwest Missouri), where gradual airmass destabilization
Early afternoon RAOBs from DVN — and particularly from ILX — show
a warm troposphere/very weak lapse rates in the 850 to 500 mb layer,
which will limit CAPE development over the next few hours despite a
moist boundary layer and continued heating through broken cloud
cover. Still, expect continued/gradual increase in the cu field to
result in isolated thunderstorm development over the next 1-2 hours.
With ample shear indicated by area VWPs and the aforementioned 18z
DVN RAOB, owing to mid-level westerly flow currently in the 30 to 40
kt range, a few of the developing cells may organize, and become
capable of producing hail and locally gusty/damaging winds. While
this development may require WW issuance, greater severe risk is
expected to evolve later this afternoon/early this evening, as a
mid-level short-wave trough/jet streak approaches from the west and
eventually overspreads the area — yielding an increase in
large-scale ascent and available shear.