Markham, Illinois Weather Forecast Discussion

808
FXUS63 KLOT 211634
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1134 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.
&&
.UPDATE...
Issued at 1134 AM CDT Sun Sep 21 2025
Made a few tweaks to forecast for this afternoon to decrease
cloud cover, nudge temps upward a bit, and lower precip chances
a smidge.
Confidence remains fairly low in coverage of showers and
thunderstorms this afternoon into the early evening. Fairly well
defined shortwave on water vapor imagery over northeast IL late
this morning will move east of the area by early afternoon. Some
subsidence and drying aloft is likely in the wake of this
shortwave. Morning soundings from ILX and DVN both already
depicted mid level (600-400mb) drying and subsidence with a 1-2C
inversion at the base.
Low and mid level cloudiness has cleared out nicely this morning
and will allow for much more sunshine than previously expected.
This should allow temps to warm into the low-mid 80s across all
but far northern IL this afternoon and have bumped up forecast
high temps accordingly.
Low levels remain pretty moist for mid-late September with mid-
upper 60 degree dewpoints across the region. Modifying morning
soundings for expected afternoon T/Td yields MLCAPE values at
or just above 1000 J/kg with little/no inhibition. The mid-level
drying and subsidence inversion around 600mb could lead to any
possible convective attempts struggling.
Near term model guidance has initialized a 700mb shortwave over
southern IA this morning and brings this wave across northern IL
later this afternoon. Not seeing any evidence of this feature on
satellite imagery or VWPs, so if it is there, it is likely
pretty subtle/weak. There has been some cumulus developing over
eastern IA late this morning, but that seems to be tied more to
a stronger/better defined shortwave moving east-northeast into
west central/northwest Wisconsin. Strongest forcing with that
wave should remain mostly north of our CWA.
Air mass should be unstable and uncapped, so certainly can`t
rule out some isolated to widely scattered convection developing
this afternoon. Based on the lack of meaningful forcing and
mid-level drying subsidence in wake of shortwave over NE IL,
felt comfortable lowering pop this afternoon/early this evening
to lower end chance category.
- Izzi
&&
.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 /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 630 AM CDT Sun Sep 21 2025
Key Aviation Messages for the current TAF period:
- Localized lower vis from BR this morning
- Multiple rounds of showers with the chance for embedded thunder
Scattered VFR showers prevail with the highest coverage south of a
KVYS to KMDW line. Most TAFs have VCSH prevail for the next few
hours, with KGYY the exception with a TEMPO given the better
coverage and better chances for MVFR vis reductions. Thunder cannot
be completely ruled out, but the chance was too low for any formal
mention in the TAF. Lastly, lower vis down to 1 SM has cropped up
overnight in rural areas away from Chicago. Lower vis may linger for
another couple hours, but vis improvement is expected with the
rising sun.
Showers are expected to dissipate through the morning 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 and potentially
brief stronger winds gusts. There is some uncertainty regarding the
amount of moisture available which kept a PROB30 mention rather than
a TEMPO.
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 last night/this morning,
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.
Around KRFD: VFR conditions with light winds that will become
southwesterly (if not a brief period of westerly) before
returning to the south tonight. A stray shower cannot be
completely ruled out, but the probability of it occurring was
too low for formal TAF mention and impacts are expected to be
minimal/non-existent.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion