Kernan, Illinois Weather Forecast Discussion

219
FXUS63 KLOT 210802
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
302 AM CDT Sun Sep 21 2025
.KEY MESSAGES...
- Another potential for scattered showers and thunderstorms
  this afternoon.
- While there will be many dry hours, the threat of periodic
  showers and thunderstorms will continue through at least the
  first half of the upcoming work week.
&&
.DISCUSSION...
Issued at 302 AM CDT Sun Sep 21 2025
Through Monday night:
Through early/mid morning, broad but modest isentropic ascent
will continue to support the development of scattered showers.
Upglide does not appear to be as robust or through as deep a
layer as last night, and instability is also a bit more sparse,
but sufficient perhaps for a few isolated storms. Thereafter,
it looks like we may see a lull in activity into the early
afternoon. Patchy, shallow fog will be possible in spots where
the mid-level cloud deck erodes as well.
Forcing mechanisms later today are a bit nebulous. While broad
cyclonic flow will persist in the region, generally neutral to
even slight positive 500 mb height tendencies are forecast this
afternoon. Close inspection of recent moisture channel loops
seems to reveal some sort of low-amplitude disturbance pushing
across the Missouri/Iowa border, and this generally aligns with
recent model analyses of a 700 mb shortwave in the vicinity.
This feature is forecast to shift east into the region this
afternoon, and appears to be the main mechanism (along with
a late-day increase in upper jet divergence) that could focus
additional diurnal shower and storm development.
The amount of insolation today remains a bit in question with
the potential for a bit more low-mid deck. That said, with
surface dewpoints expected to remain in the mid to locally upper
60s, it won`t take too much heating to result in eroding CIN.
Given all of this, continue to focus general mid range chance
PoPs across about the eastern half of the forecast area this
afternoon and early evening. 500 mb flow is not forecast to be
all that significant, perhaps increasing to near 30 knots, which
would support similar 0-6 km/deep layer shear magnitudes. If
sufficient destabilization can be realized, there would be a
threat for a few stronger/gustier storms. If forcing is less
significant, an even lower coverage of storms would be possible.
Later this evening and overnight, an upper jet streak is
forecast to intensify overhead and across Wisconsin as the next
shortwave (currently emanating across western Nebraska) slides
towards the area. This will place our region under a zone of
increasing divergence aloft, as an attendant zone of loosely-
organized mid-level f-gen develops. Given how much guidance has
been struggling with the handling of these features even over
the last 12 hours, it`s hard to put too much stock in individual
solutions at this point. The GFS and its ensemble has an
incredibly muted precipitation response to the increasing
forcing for ascent, while the signal for more widespread steady
rainfall with embedded storms is higher in the ECMWF/EPS and
CMC/GEPS suite. Ultimately aligned the PoP forecast with the
latter solutions given the more impressive looking dynamics at
play, although suppose it`s also possible this axis shifts south
in future model forecasts. Finally, while the severe threat
tonight is low, there is a signal hinting at the potential for a
thunderstorm complex to push out of central Illinois late this
evening. This would occur coincident with an increasing 700 mb
wind field, so not entirely out of the question this supports
some gustier storms, but far from a certainty at this point.
Precipitation chances on Monday look like they should tend to
focus near and south of I-80 as the deepest moisture shifts
south and eastward. The low-level flow is forecast to weaken
Monday night, and if skies manage to clear out, forecast
soundings suggest a conducive environment for fog development.
No mention in the gridded forecast at this point though, but
something we`ll keep an eye on.
Tuesday through Saturday:
Forecast confidence heading into the middle of next week
remains decidedly lower than average with a pretty chaotic and
energetic upper pattern in place. Numerous upper waves may begin
to interact with one another as a Rex-type blocking pattern
across the western CONUS starts to break down. At some point
during the Tuesday-Wednesday time frame, there`s a decent signal
that a cold front will push down the lake, potentially bringing
with it a renewed chance for showers.
Thereafter, broad troughing and the potential for cutoff low
development could end up keeping intermittent precipitation
chances going through the end of the week, although an equally
plausible scenario is that we get into the trailing subsident
regime with most precipitation chances shunted to our south.
Altogether, too much uncertainty at this point to make any
changes to the blended 20-40 percent PoPs into Tuesday through
Friday.
Carlaw
&&
.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1232 AM CDT Sun Sep 21 2025
Key Aviation Messages for the current TAF period:
- Multiple rounds of showers with the chance for embedded
  thunder
Loosely scattered showers remain dotted around the region. With
current coverage being somewhat low, VCSH was prevailed at all
the TAF sites through the morning. With the amount of rain on
Saturday, there is a slight chance for some lower vis to crop up
in outlying areas away from Chicago but dense fog is not
expected. Another thing that would prevent widespread fog is
another chance for showers to move over the area from Central
Illinois. PROB30s were maintained for the chance for showers
overnight with higher rain rates that could reduce vsbys down to
MVFR conditions. Technically, the chance for thunder is non-
zero, but instability looks too meager and thus did not include
it in the TAF presently though trends will be monitored through
the night.
The overnight showers are expected to dissipate after sunrise
for a break in the rain through the early afternoon. Winds will
be out of the southwest through the day, and under 10 knots.
Another wave will pass over the area late in the afternoon that
could develop another round of showers with a better chance for
thunder with it. There is some uncertainty regarding the amount
of moisture available which kept a PROB30 mention rather than a
TEMPO. However, the RFD TAF was kept dry as the better chances
are closer to Chicago and southeastward.
Showers may linger into the early evening and overnight. Models
are suggesting yet another wave of showers will move over the
region overnight into Monday morning. Similar to tonight,
instability in model soundings does not look sufficient enough
to add TS to the present TAF, but PROB30s were added for a
renewed chance for showers.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion