Howard Colman Library, Illinois Weather Forecast Discussion

864
FXUS63 KLOT 061950
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
250 PM CDT Mon Oct 6 2025
.KEY MESSAGES...
- Much cooler conditions will settle in tomorrow and remain into
  the end of the week.
- Expect a few waves of showers and some embedded thunderstorms
  this afternoon through early Tuesday morning.
&&
.DISCUSSION...
Issued at 250 PM CDT Mon Oct 6 2025
Early this afternoon, we`re seeing a handful of showers, some
heavier, and a couple of embedded thunderstorms popping up
around our CWA. These are forming just ahead of a cold front
that`s pushing across northwest IL as of 2 PM. Guidance does not
have a good handle on this activity as it develops out ahead of
the more effective boundary, but model soundings do show plenty
of support for such activity to continue through the afternoon
here ahead of the front as it makes headway into the CWA.
Thus far, storm coverage has been rather isolated amid these
showers. Expectations are that shower and storm coverage will
expand into the late afternoon and evening. Guidance is in good
agreement that moisture pooling ahead of the front will result
in a degree or two jump in dewpoints over the next couple of
hours. This should give parcels uncapped access to over 1,000
Joules of MLCAPE by late this afternoon. A couple of stronger
cells can`t even be ruled out with latest RAP mesoanalysis
resolving 35 to 40 kt of effective shear near the boundary. The
front will gradually progress southeastward into the evening,
but looks to get hung up and become qusi-stationary somewhere
across the Chicago metro overnight before pushing onward early
tomorrow. The highest thunder coverage and greatest strong storm
potential will be found near and just ahead of the front
through the evening hours. Diurnal cooling will chip away at
SBCAPE into the night, but additional storms and heavier showers
will remain possible through the night near and southeast of
the front.
Behind the front, densely scattered to widespread showers are
anticipated through the night. A few storms will be possible
here and there behind the front with some elevated instability
still in place and support from a departing upper jet max, but
forecast soundings are more reminiscent of just heavier showers
vs widespread embedded thunder. These showers are likely to
still be festering around the metro into early tomorrow, but
should push east of the metro area by mid-morning and out of our
CWA altogether by the end of the morning.
A big push of cold low level air will feed in behind the front
resulting in a much cooler day tomorrow. 925mb temps will drop
nearly 10C (18F) between this afternoon and tomorrow afternoon.
Highs are forecast to only reach the middle and upper 60s,
possibly nearing 70 in spots. While this sounds quite cool
compared to recently, these sorts of values are much closer to
climatological norms for early October. The day will begin
beneath mostly cloudy skies with those showers still working out
of the area, but cloud cover should thin out from NW to SE
through the day and should allow much of the area to see some
good sunshine during the latter part of the day.
After the upper trough axis swings across on Tuesday, we`ll
find ourselves in a quiet split flow regime through the end of
the week. High pressure will meander about the Great Lakes
keeping the cooler air pumping into the region from the north
through the middle of the week. Widespread 60s are likely
through at least Thursday. Some milder return flow should then
warm things up a bit closer to this weekend. Ensemble PoP
guidance begins to light up again this weekend after the high
ejects to the NE US and another wave looks to track across the
region, but there is even stronger support for dry conditions
beyond today to prevail into early next week.
Doom
&&
.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1226 PM CDT Mon Oct 6 2025
Key Aviation Messages for the current TAF period:
- Periods of rain/showers mid-late afternoon onward, along with
  a chance of thunderstorms into or through the evening
- Winds shift to northerly early evening
- Deteriorating CIGs late tonight-early Tuesday, with reduced
  VSBY also possible in BR and any drizzle
Pre-frontal winds out of the southwest will continue through the
afternoon with gusts around 20 knots. As the cold moves through,
winds will switch to the north. Some recent models are
suggesting that the wind shift at Chicago terminals could be at
01Z or even 02Z. With lower confidence in the exact timing, the
TAFs pushed the wind shift at ORD back to 00Z to at least trend
toward a later front arrival.
There is a chance for isolated storms to develop out ahead of
the frontal movement. With lower confidence in thunder at an
individual terminal, VCTS was added for the earliest chances at
22Z. Better coverage of showers is expected after 00Z with the
fronts arrival and maintained the PROB30 for -TSRA during this
period. No changes were made on timing with showers and thunder
chances at KRFD.
Cigs will are expected to deteriorate tonight. MVFR cigs will
become IFR after midnight, with the possibility of LIFR cigs
closer to the lake as showers and drizzle linger into Tuesday
morning. Rain will taper off from west to east after 12Z. MVFR
cigs may linger through the middle of the morning, but
eventually return to VFR conditions. Winds will remain out of
the northeast with gusts around 20 knots for terminals closer to
the lake.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 AM Tuesday to 3 PM CDT Wednesday
     for the IL nearshore waters.
Small Craft Advisory from 4 AM Tuesday to 10 AM CDT Wednesday
     for the IN nearshore waters.
&&
$$
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