Kernan, Illinois Weather Forecast Discussion

101
FXUS63 KLOT 121705
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1205 PM CDT Fri Sep 12 2025
.KEY MESSAGES...
- Shower and thunderstorm chances are increasing for late
  tonight through Saturday night (20-30%).
- A conditional threat for heavy rain and localized flooding
  exists Saturday mainly east of a Rockford to Rensselaer line.
- A generally dry and warm pattern will prevail for much of the
  upcoming week.
&&
.DISCUSSION...
Issued at 333 AM CDT Fri Sep 12 2025
Through Saturday Night:
An upper ridge continues to reside from the south-central
Plains to the western Great Lakes. As a result light winds and
mostly clear skies late in the evening across northern IL and
northwest IN which has lead to the development of patchy fog
early this morning in places where temperature-dew point
spreads have fallen below 2 degrees. While visibilities have
generally been in the 2-5 mile range, some locally lower
visibilities under 1 mile have been observed particularly near
the IL-WI line. With moisture depths rather shallow suspect fog
will remain patchy in coverage and mainly confined to river
valleys and near the IL-WI line where some lower clouds are
mixing down, but cannot rule out most of the area seeing at
least some fog as we head towards daybreak. That said, be sure
to use caution when heading out for the morning commute.
Fog is expected to erode by mid-morning which will set the
stage for another mainly sunny and warm afternoon. Temperatures
today will top out in the mid-80s for most though a lake breeze
this afternoon will keep readings near the lake in the upper 70s
to lower 80s.
The weather is forecast to become a bit more active late
tonight through Saturday night as a shortwave trough (currently
over eastern CO and southwest NE) rounds the aforementioned
ridge and tracks across southern WI towards southern Lower MI.
As this occurs, the warm front in west-central MO is forecast to
pivot into northeast IL and generate a focus for shower and
thunderstorm activity. While the 00z guidance has trended into
much better agreement on the front setting up across northeast
IL and showers/storms developing along it, there remains some
uncertainty as to whether or not the axis of better instability
will pivot into the area before the wave arrives during the
predawn hours Saturday morning. If sufficient instability can
materialize, the 1.5-1.70 inch PWATs (near the 90th percentile
based on ILX Sounding Climatology for this time of year) and
front parallel steering flow will make for a favorable
environment for training and heavy rain producing thunderstorms
Saturday morning into the afternoon. Despite the recent period
of dry weather, the storms could yield localized rainfall totals
in excess of an inch over the period of about 6-8 hours which
may result in a localized flash flooding threat especially if
the amounts fall over urbanized areas.
Given the uncertainty in the magnitude of instability, this
heavy rain/flooding threat remains highly conditional (around
10% chance of occurrence) at this time. That said, POPs were
increased into the 20-30% range with this update with the
highest POPs focused along and east of a Rockford, IL to
Rensselaer, IN line where guidance currently favors the heavy
rain threat setting up. Obviously we will be keeping a close eye
on trends with future forecasts today so be sure to stay tuned.
The shortwave trough and front are expected to move to the east
Saturday night as the ridge axis builds into northern IL.
Therefore, showers and storms should gradually taper from
northwest to southeast during this period with dry conditions
expected by Sunday morning. Outside of the showers/storms,
temperatures will remain summer like through Saturday with highs
in the mid to upper 80s and possibly even nearing 90 in spots.
However, if storm coverage is greater than currently forecast
then temperatures may be several degrees cooler especially near
the lake. Though overnight lows should remain in the low to
mid-60s regardless.
Yack
Sunday through Thursday:
Dry and warm conditions will prevail through much of the
upcoming week as high amplitude upper level ridging sets up
across the region.
While generally warmer conditions are expected to continue,
model guidance has trended toward onshore flow potentially
continuing through much of the day on Sunday in the wake of the
Saturday system paired with a reinforcing lake breeze in the
afternoon. This could result in less warming into portions of
northeast Illinois and northwest Indiana. While upper ridging
will be building over the area (supporting broad low-to-mid
level warm advection), opted to trend cooler with daytime
temperatures with this update with upper 70s to mid 80s expected
across far northeast Illinois and northwest Indiana (coolest
along the lakeshore). With that said, confidence in how far
inland the cooler air reaches remains lower and have accordingly
maintained upper 80s to lower 90s for areas south and west of a
Rockford to Joliet to Valparaiso line for now.
Long-range guidance begins to diverge a bit heading into the
work week on the position and strength of the various upper
level features. The EPS indicates a pseudo-Rex Block pattern may
set up across the eastern half of the CONUS (upper high over
the Great Lakes and upper low over the Southeast) which favors
warm and dry conditions prevailing in the local area for the
majority of the week. The GEFS also has high amplitude ridging
across the region but without a closed upper low feature which
allows the eastern periphery of the western trough and any
embedded shortwaves to get closer to the region which could lead
to an earlier return to shower chances. For now this update
leans toward the EPS solutions and maintains a dry forecast
Sunday through Thursday with low shower chances then returning
Thursday night onward (20-30%). High temperatures will be mainly
in the mid 80s to lower 90s the rest of the week for much of
the area. Overall weak surface flow should allow daily lake
breezes to push inland which should keep temperatures cooler
along the lakeshore in the mid 70s to lower 80s.
Petr
&&
.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1205 PM CDT Fri Sep 12 2025
Main Concerns:
- Potential for 1 or 2 rounds of thunderstorms on Saturday
- Wind directions and speeds behind potential TS on Saturday
A lake breeze will slowly push inland this afternoon and shift
winds at ORD from current southerly (varying from SSE to SSW) to
east/east-southeast. Aside from this item of note, quiet VFR
conditions are expected until early to mid Saturday morning.
We`re monitoring ongoing showers and thunderstorms over the
broader northern Plains and upper Midwest region (from western
IA to northern MN). While there`s plenty of uncertainty in the
exact details, the disturbances driving the current activity are
favored to yield SHRA/TSRA nearby or overhead on Saturday
morning. Depending on how this all plays out, some TS footprint
may linger into the early to even mid afternoon. Finally, can`t
completely rule out an overall later time of arrival of
upstream convection, but this scenario appears more unlikely
than the other possibilities. Given the current large range of
plausible outcomes, continued with PROB30 mention in the TAFs,
earliest start at RFD, and latest start at GYY.
In the wake of the morning into afternoon activity, the surface
wind field is likely to be affected, even if the TS themselves
focus to the north and east of the terminals. If winds end up
east of south (as in the latest ORD and MDW TAFs), speeds may
approach or exceed 10 kt.
Castro
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion