Rockford Scanner™: Chance For Severe Weather Later Today



If any watches are put out, be sure to listen to the Northern Illinois SKYWARN Net on 147.195 

Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-DeKalb-Kane-DuPage-Cook-
La Salle-Kendall-Grundy-Will-Kankakee-Livingston-Iroquois-Ford-
Lake IN-Porter-Newton-Jasper-Benton-
256 PM CDT Mon Oct 1 2018 /356 PM EDT Mon Oct 1 2018/

This Hazardous Weather Outlook is for portions of North Central
Illinois…Northeast Illinois and Northwest Indiana.

.DAY ONE…Tonight.

Weather hazards expected…

Significant Thunderstorm Risk…with an associated:
Elevated Tornado risk.
Elevated hail risk…up to quarter size.
Elevated damaging wind risk…up to 70 mph.
Elevated Flooding Risk.


Widely scattered thunderstorms may form along a warm front south
of Interstate 88 late this afternoon or early evening. These
storms may produce large hail, damaging winds, and tornadoes.
Additional thunderstorms ahead of an approaching cold front will
enter north-central Illinois mid to late this evening. These
storms will be capable of producing damaging winds. In addition,
multiple rounds of storms could result in localized flooding.

A cluster of severe storms capable of all hazards may occur between
about 4 to 10 PM CDT across eastern Iowa into northern Illinois.

No changes were made to the severe aspect of the outlook at 20Z.

Elevated thunderstorms currently persist from northern IA into WI,
with the warm front situated from south-central IA into northern IL
as of 20Z. This boundary may shift slightly north toward the WI
border later today, as heating and mixing of the boundary layer
persist upstream. With time, storms are expected to become more
surface based over eastern IA, with strong westerly winds aloft
aiding wind gusts. Veering of the winds with height may also support
a corridor of supercell potential. Relatively weak instability will
remain a mitigating factor, but a brief tornado and hail will also
be possible.

..Jewell.. 10/01/2018

.PREV DISCUSSION… /ISSUED 1126 AM CDT Mon Oct 01 2018/

…Midwest including eastern Iowa/northern Illinois…
A relatively confined regional corridor of severe potential is
expected later this afternoon and evening. A warm front will
continue to make a slow northward shift across southern/eastern IA
and northern IL, but perhaps not reach quite as far north as
previously thought owing to persistent low clouds and scattered
precipitation to the north of the boundary. Regardless, a shortwave
trough over SD at mid-morning will continue eastward and modestly
amplify, which will influence weak surface wave development along
the front from northeast KS toward the IA/IL Quad Cities and
eventually Chicagoland tonight.

Strengthening southwesterly winds just above the surface will
continue to transport a relatively moist air mass northeastward,
with low to middle 60s F surface dewpoints becoming increasingly
common by evening within the slow-northward-developing warm sector.
While mid-level lapse rates will not be particularly steep, a plume
of somewhat steeper mid-level lapse rates noted in the 12Z Omaha
observed sounding will advect east-northeastward and coincide with
Midwest areas along/just north of the surface front.

Given some residual capping and limited forcing into the warm
sector, the main severe risk should be focused relatively close to
the advancing surface wave and front late this afternoon and
evening, with near-surface-based storm intensification most likely
to initially occur across east-central IA. Relatively strong
deep-layer/low-level shear and modest buoyancy (1000 J/kg MLCAPE or
less) will support the possibility of both supercells and bowing
segments. Severe hail may occur initially and the potential for a
tornado or two will exist given the increasingly enlarged low-level
hodographs. Ultimately, damaging winds could become the most common
hazard as bow development occurs this evening.