Shelby, Indiana Weather Forecast Discussion

005
FXUS63 KLOT 120833
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 AM CDT Fri Sep 12 2025
.KEY MESSAGES...
- Patchy fog is expected early this morning, some locally dense
  with visibilities under 1 mile.
- 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 my 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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1259 AM CDT Fri Sep 12 2025
- IFR to LIFR CIGs/VSBYs overnight at RFD/GYY and possibly DPA
An area of LIFR to IFR stratus that developed across southeast
Wisconsin late in the evening has since expanded into northern
Illinois between Rockford and Lake Michigan. It continues to
expand toward RFD and DPA and have accordingly bumped up the
timing of the TEMPO groups to 7Z and lowered the ceilings to
IFR. It is possible it arrives a bit earlier and lower (LIFR than
this. Visibilities beneath the stratus have remained mainly
MVFR but have occasionally dipped to IFR and lower. Confidence
in the stratus building down with greater coverage of IFR and
lower VSBYs remains low and will have to monitor trends through
the overnight hours.
Have kept ORD/MDW both VFR with this update with the
expectation that the stratus/fog remains outside of Chicago.
However, will continue to monitor satellite and observational
trends closely in case an unexpected southeastward expansion
occurs.
VSBYs at GYY may sporadically dip to as low as 1-2SM due to
shallow fog development near the southern Lake Michigan shore.
MGC has occasionally dropped below 1SM at times and this can`t
be ruled out near GYY as well overnight.
Otherwise VFR conditions are expected through the rest of the
TAF period. Winds will remain generally light out of the east
to southeast through this evening with a reinforcing lake
breeze expected at ORD/MDW/GYY during the afternoon. Winds then
begin to turn south southwesterly by early Saturday morning.
Petr
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion