Nachusa Illinois Weather Forecast Discussion

869
FXUS63 KLOT 070555
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1255 AM CDT Tue Oct 7 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.
&&
.UPDATE...
Issued at 1000 PM CDT Mon Oct 6 2025
I made some forecast changes to bump up POPs and chances for
thunder overnight, particularly across northern IL, to the
north of the surface cold front. As of 9:30 PM, the primary
surface cold front had settled near the I-55 corridor. This has
and will likely continue to be a focus for some scattered
showers and thunderstorms late this evening. However, farther
north and northwest, a second area of showers and thunderstorms
is expected to continue increasing in areal coverage into the
overnight hours across northern IL as a result of enhanced
moisture convergence and frontogenesis along the approaching
850 mb front. The main threat from this activity overnight will
be occasional lightning and instances of locally heavy
downpours.
Showers are likely to linger through the early to mid morning
hours of Tuesday before the rain ends.
KJB
&&
.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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1255 AM CDT Tue Oct 7 2025
Main Concerns:
- Showers and occasional thunderstorms through the predawn hours,
  with showers and BR lingering past sunrise
- Low CIGs scattering by midday northern IL and by mid
  afternoon at GYY
The highest coverage of SHRA and embedded TS (VSBY as low as
2-3SM) will initially focus north of I-88/290, and then should
encompass MDW and perhaps GYY eventually. Continued on and off
SHRA are expected to improve from west to east after daybreak.
Low CIGs currently ranging from LIFR at DPA, IFR at ORD and RFD,
and low MVFR at MDW and GYY may build down a bit at current
MVFR sites through dawn, and then lift back to MVFR by the mid
to late morning. Once VFR conditions return, they`re expected to
prevail the rest of the period.
Northerly winds ranging from 350-020 degrees will increase to
~10-12 kt this morning, along with gusts near/around 20 kt near
the lake. Winds will diminish with sunset this evening and
eventually shift to north-northwest.
Castro
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 PM CDT Wednesday for the IL
     nearshore waters.
Small Craft Advisory from 4 AM early this morning to 10 AM CDT
     Wednesday for the IN nearshore waters.
&&
$$
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NWS LOT Office Area Forecast Discussion