Union Hill, Illinois Weather Forecast Discussion

694
FXUS63 KLOT 231721
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
Issued by National Weather Service Lincoln IL
1221 PM CDT Tue Sep 23 2025
.KEY MESSAGES...
- Scattered showers and some storms possible this afternoon.
  Additional areas of showers and embedded storms tonight, but
  the greatest potential currently looks to focus mainly south
  of the forecast area.
- Intermittent showers may linger across the region Wednesday
  and overnight, but a trend towards drier conditions is
  expected through the end of the week and upcoming weekend.
&&
.UPDATE...
Issued at 1153 AM CDT Tue Sep 23 2025
Current storm activity from Watseka north-northeast to Lake
County IN associated with a gravity wave is anticipated to
gradually move east with time this afternoon. Lightning has not
been observed along this line, but we could see a handful of
strikes before the storms move out of northwest IN.
Precip chances have been updated through the afternoon to
account for this activity.
NWS ILX
&&
.DISCUSSION...
Issued at 249 AM CDT Tue Sep 23 2025
Through early this morning: ongoing disorganized convective
clusters currently across McHenry/Lake counties in northeast
Illinois, and any additional spotty convection that develops
along incoming outflows should gradually weaken and diminish
through daybreak. In an unperturbed environment, fog development
appears likely given weak near-surface flow and low/minimal
dewpoint depressions. However, the presence of the
aforementioned outflows/gravity waves and debris cloud cover may
end up shunting the greatest fog threat south and east of the
region. At the current time, the main fog corridor looks to be
east of about a Gibson City to Valparaiso line. If the cirrus
canopy diminishes across northern Illinois, the fog threat would
increase commensurately there as well.
Things looks generally quiet this morning. The one exception
might be along and lingering outflow which may continue pushing
east towards the I-57/65 corridors. Additionally, this may serve
as a mesoscale focus for isolated/widely scattered midday and
afternoon convection.
This afternoon and evening, a cold front will shift south out of
Wisconsin and down the lake. Dewpoints ahead of this boundary
are forecast to remain in the low to mid 60s, with temperatures
slated to rise through the upper 70s/near 80 degrees (assuming
ongoing convective debris cloud cover thins through the
morning). Forecast soundings reveal minimal inhibition to
surface-based parcels with a fairly deep (~200 mb) thick low-
level moist layer in place. Interestingly, most model guidance
fails to convect along this convergent axis, potentially owing
to lingering subsidence in the wake of the ongoing convection
and generally limited large scale forcing mechanisms upstairs.
Still, given the progged instability, elected to maintain 30-40
percent chances for showers/storms along the southward-advancing
boundary this afternoon (the ECMWF in particular depicts quite
a bit more coverage of afternoon convection). Finally, while 0-6
km shear is forecast to remain under 20-25 kts (owing to weak
500 mb flow), winds will increase towards 400 mb as a jet streak
intensifies overhead. Based on forecast parcel profiles and ELs
generally under 10 km, this flow should remain immaterial
today, but if warmer temperatures/more instability is realized,
we`d need to keep an eye on the threat for some stronger and
more organized cores.
This evening and overnight, forcing will increase atop the
near-surface frontal zone. This should lead to a general
increase in the coverage of showers and embedded thunderstorms.
As has been the case in this stretch, continue to see
significant run-to-run changes in the handling of the pertinent
larger scale features. A recent signal additionally shows a
compact and potentially fairly deep surface low developing
tonight into early Wednesday morning across downstate Illinois
into southern Indiana. Latest ensemble output indicates this
potential low track remaining south of the region, which would
tend to focus the greatest precipitation chances across central
Illinois and Indiana. Based on all of this, ended up confining
the highest PoPs (low-end likelies) mainly well south of the
Kankakee River. If a solution like the current 00z NAM, UKMET,
ECMWF materializes, a locally heavy rain threat could be
realized immediately to the north of the surface low track into
early Wednesday morning as a coupled upper jet interacts with a
compact zone of low-mid level convergence/f-gen. Presently, the
most concerning signal for this remains just south of the
forecast area.
Carlaw
Our region will be solidly post-frontal on Wednesday, with
breezy north to northeast winds in place. Aloft, several
shortwaves will transit the region, embedded within broad
cyclonic flow in place across the Great Lakes. The broad forcing
generated by these features in combination with ample moisture
will allow scattered showers and perhaps a few thunderstorms to
persist through the day on Wednesday with the highest coverage
(30-40%) focused generally of I-80/east of I-55. While the
highest rain chances will gradually shift southward with the
cold front Wednesday night, the cooler air associated with the
upper-low over the Great Lakes does look to develop some modest
lake induced instability. Thus allowing some lake effect showers
(and perhaps a storm or two) to linger into Thursday along the
northeast IL and northwest IN shores before the low pivots east
of the area Thursday night.
Temperatures on Wednesday and Thursday are forecast to be the
coolest of the week due to the expected cloud cover and
lingering rain chances. Therefore, expect highs during this
period to top out in the low to mid-70s areawide with lows in
the lower to mid-50s.
Heading into Friday, upper ridging is forecast to develop back
into the Great Lakes region which will set up another period of
dry and above average temperatures to close out September. While
another cold front is forecast to dive through our area on
Saturday/Sunday, the cold air reservoir behind the front does
not appear to be sufficient enough to alter temperatures
significantly, except perhaps near the lake. Thus highs should
remain in the upper 70s to lower 80s through the weekend with
overnight lows in the mid-50s. However, conditions will become
more favorable for daily lake breezes towards the later half of
the weekend which should keep temperatures cooler (low to
mid-70s) at the lakeshore.
Yack/Carlaw
&&
.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1221 PM CDT Tue Sep 23 2025
Key Aviation Messages for the current TAF period:
- Another round of showers possible this evening overnight
  though better coverage expected to the south in central
  Illinois
- MVFR, and potentially even IFR, cigs expected tonight
Winds turned northeast with a cold front moving south out of
Wisconsin and is being reinforced by a lake breeze. There is a
chance for occasional gusts this afternoon and evening between
15 to 25 knots, but being more isolated in nature it was
left out of the TAF for now. Stratus will move southward
behind the front, increasing the chance for MVFR cigs into
Tuesday evening. Guidance is starting to lean toward IFR cigs
being possible, but confidence is still too low to add it in
the TAF. MVFR cigs and northeast winds are expected to remain
into Wednesday morning. PROB30 for showers later this afternoon
and evening for all sites but KRFD has been included, but best
chances remain south of the forecast areas.
As the front moves southward overnight, models are suggesting
that there will be renewed chances of showers, and maybe
isolated storms, along a convergent axis that develops. The
better chances remain in central Illinois. However, there was
enough model agreement to keep a PROB30 in for KGYY.
NWS Lincoln
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion