Fair Oaks, Indiana Weather Forecast Discussion

921
FXUS63 KLOT 191754
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1254 PM CDT Fri Sep 19 2025
.KEY MESSAGES...
- Periodic chances for showers and some thunderstorms will
  return this afternoon into early next week, though there will
  continue to be plenty of dry hours too.
&&
.DISCUSSION...
Issued at 315 AM CDT Fri Sep 19 2025
Today will begin a transition into a somewhat more active weather
pattern as broad troughing that`s been a persistent feature
across the Great Plains pivots eastward and settles across the
Great Lakes. Numerous embedded vorticity maxima within the
expansive cyclonic flow aloft will meander across the region
into early next week, and this will result in the return of
periodic shower and thunderstorm chances.
Through this morning and afternoon, a plume of increased mid-
level moisture will push into the region. At the same time, a
series of compact vorticity maxima (associated with ongoing
convection across eastern Missouri) are forecast to push
northward out of central Illinois as they slingshot around the
eastern periphery of the main upper gyre near Sioux Falls.
Uncertainty in the coverage of afternoon showers and storms
revolves around the strength of these incoming vort lobes
(possibly convectively augmented), and the degree of boundary
layer mixing and its effects on late-day dewpoint trends. With
the bulk of the attendant forcing expected to focus west of the
Fox Valley, continued to focus the highest late-day PoPs (mid-
high end chance) mainly across parts of NW Illinois, tapering
downwards with east and south extent. Somewhat deeply-mixed
profiles suggest a potential for gusty winds should
stronger/taller cores develop with marginally-supportive
thermodynamic profiles (MLCAPE values ~500-750 J/kg). Some
convection may also attempt to develop on the sharpening lake
breeze boundary near/along the I-90 corridor.
Some diurnal decrease in precipitation coverage may occur this
evening with the loss of daytime, but expecting to see gradual
redevelopment take place through the late evening and overnight
hours as modest warm/moist upglide develops in response to
increasing mid and upper level flow. A little difficult to say
exactly where the favored precipitation corridor will develop
given run-to-run inconsistencies, but there`s now a decent
signal for scattered to numerous showers and embedded storms
across most of our Illinois counties overnight into early
Saturday morning. While the parameter space doesn`t look too
concerning from a flash flood perspective, southerly cloud-
bearing flow paralleling the main moist axis suggests some
threat for localized corridors of training storms.
Showers and storms should tend to diminish through Saturday
morning as the main upper low begins to lift into northern
Wisconsin. Increased boundary layer moisture with surface
dewpoints in the mid 60s, coupled with seasonably cool 500 mb
temperatures and at least filtered insolation will yield
increasing afternoon instability, with MLCAPE values possibly
pushing 1000-1500 J/kg in spots. While the bulk of the large
scale forcing for ascent will push north of the region, several
trailing disturbances are forecast to remain in the vicinity,
and upper divergence is slated to increase during the afternoon
as an upper jet streak slides overhead. This setup looks
conducive for scattered afternoon showers and storms across the
region.
Additional showers and some thunderstorms will be possible
Saturday night with the nocturnally-increasing low-level jet.
Then during the day on Sunday, forcing mechanisms look a bit
more nebulous, with the main large scale height falls and DCVA
focusing north of the Wisconsin state line. The potential for
lingering convectively-augmented MCVs/flow-enhancement remains
(the latest 00z NAM output, for example), but there`s little/no
ability to tell if this will be the case at this range. Deepest
boundary layer moisture may tend to shift off to the east a bit
as well, and the latest blended PoPs match this, with the
highest chance PoPs confined to areas along/east of I-57.
Finally, some recent runs of the GFS (in particular) had looked
a little interesting from a kinematic perspective with a notable
700 mb southwesterly jet nosing into the region during the
afternoon, but recent runs seem to have lost this feature.
Something to keep an eye on though, as the presence of stronger
shear could result in a threat for some stronger storms, mainly
into parts of NW Indiana.
Intermittent shower and thunderstorm chances will continue
Sunday night into Tuesday as additional disturbances ripple
through the west to northwesterly flow aloft. A chaotic upper
pattern into the middle and end of next week will likely result
in lower-than-typical forecast confidence. There`s some modest
signal in the extended guidance for at least a brief return to
drier conditions in the wake of a cold front towards midweek,
but depending on the influence of another upper low towards the
end of the week, this could end up being short-lived.
Carlaw
&&
.AVIATION /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1254 PM CDT Fri Sep 19 2025
- SCTD -SHRA and ISOLD TSRA expected in and around the terminals
  from late this afternoon/early this evening at times through
  Saturday.
Scattered showers and isolated thunderstorms are expected to
develop this afternoon across western and central Illinois.
While this activity is will be moving mostly northerly, it is
expected to slowly spread east this evening into the immediate
Chicago area. While there will likely be some ISOLD TSRA around,
confidence in a TSRA directly impacting one of the Chicago area
terminals is a bit too low to justify going with a TEMPO. The
PROB30 nicely covers the time period with the highest
probability for a thunderstorm at the terminals. There will
likely be a break in the rain developing Saturday morning into
the early afternoon, but additional SCTD SHRA/TSRA could develop
later Saturday afternoon.
Cloud bases should remain fairly high, so largely VFR conditions
are expected this TAF cycle. Any heavier shower or storm could
briefly reduce VSBY to MVFR and possibly even IFR.
- Izzi
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion