Flossmoor, Illinois Weather Forecast Discussion

871
FXUS63 KLOT 220758
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 AM CDT Mon Sep 22 2025
.KEY MESSAGES...
- 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 258 AM CDT Mon Sep 22 2025
Through Tuesday:
Increasing upper jet divergence currently across downstate
Illinois will gradually expand northeastward into central
Indiana through mid morning. The bulk of the associated forcing
looks like it will focus just off to the south and east of our
forecast area, but mid-level moisture looks sufficient to
justify maintaining chance PoPs south of about the Kankakee
River through this morning. Across northwest Illinois, clearing
skies and light winds may yield the development of patchy fog
through daybreak.
The forecast for this afternoon and this evening remains a bit
uncertain. At first glance, precipitation chances look like they
should remain low to near nil, but closer inspection reveals a
few additional details. While mid-level moisture is progged to
push off to the east, surface moisture will remain in place,
with dewpoints forecast to remain in the mid to possibly locally
upper 60s F. With some mid-late morning clearing expected
across the northwest third/half of the CWA, it looks like we
should build a fairly healthy cumulus deck north and west of
about I-55. At the same time, persistent upper-level cloud cover
to the south and east will likely result in a modest
differential heating zone, with temperatures holding in the mid
70s to the east/south of I-80/I-57, and rising in the upper
70s/lower 80s farther to the northwest--a possible source of
local surface convergence. Aloft, there are hints of a low-
amplitude 700 mb shortwave near the Mississippi River which may
drift overhead through midday and afternoon, although whatever
feature is out there is pretty subtle.
Instability is forecast to increase through midday and this
afternoon, with MLCAPE values potentially pushing 1500 J/kg NW
of I-55 along with diminishing CIN. While forcing is on the
weaker side, it doesn`t look like it`ll be missing in action
entirely. With this in mind, it`s certainly not out of the
question for spotty airmass convection to develop through this
afternoon. Additionally, ongoing storm clusters across the Twin
Cities vicinity currently look a little more organized than
previously expected, and mid-level water vapor loops clearly
reveal a better-defined shortwave extending across southwest
Minnesota. Further intensification and upwind development could
allow a weak cold pool to encourage activity southeastward,
pushing outflow/storms into our far north late this afternoon.
As a result of all of this, didn`t feel comfortable with a "dry"
forecast today, and added some slight chances for showers and
storms. This evening and overnight, there`s a signal for
additional convection potentially pushing into northern
Illinois. Decreasing instability and lackluster low to mid-level
flow suggests activity should tend to diminish in intensity
through the evening.
Have also introduced a mention of fog to the grids tonight into
Tuesday morning, mainly near and south of I-80 where light flow
and the highest potential for clearing skies will overlap
lingering near-surface moisture. Not out of the question this
mention needs to be expanded farther north, but elected to
withhold a mention from northern Illinois for now given the
potential for convection.
On Tuesday, a cold front is forecast to push southward towards
the region. This feature, combined with a notable follow-up
shortwave aloft will serve as the next foci for shower/storm
development in the region. Given the poor handling of embedded
vort maxes/general timing of large scale features thus far, the
blended 40-50 percent PoPs area-wide Tuesday afternoon looked
fine at this range.
Carlaw
Tuesday night through Sunday:
Synoptically speaking, the weather pattern is expected to start
this period with an upper level low over the western Great
Lakes which will eventually move east by the weekend before an
upper level ridge potentially builds early next week. However,
Tuesday night through Wednesday, two upper level troughs will
pivot and phase with each other before becoming a vertically
stacked low for the end of the week. With plenty of moisture
available and a few whirlydoodles/vort maxes pivoting around
the main low, there will be periods of showers and storms
through the end of the week. While there is still model
disagreement in the path and timing that the upper level trough
will move east, there is lower confidence in thunder chances
later in the week as the trough becomes located more east of the
area. For now, the best chances for thunder are really through
Wednesday and maybe lingering into Thursday with better forcing.
This weather pattern is expected to have more northerly winds
through the remainder of the week. Winds should be mostly light,
though there is a chance for breezier conditions Wednesday
afternoon through Thursday. Nevertheless, these winds will help
usher in cooler temperatures, compared to recent temps. High
temperatures are expected in the low to mid 70s and overnight
lows in the upper 50s and low 60s.
For now, ensemble models are showing somewhat decent agreement
in an upper level ridge building over the weekend and into early
next week. With better height rises and a drier air mass,
chances for precipitation would finally leave the forecast and
high temperatures will creep back into the upper 70s and
possibly low 80s. However, models have been struggling recently
with how these systems have been developing. It is possible that
the upper level low does not move out of the area at the end of
the week which would impact the temperature and precipitation
forecast. There seems no need change the forecast currently from
what the NBM provided, but will certainly be monitored through
the week.
DK
&&
.AVIATION /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1230 AM CDT Mon Sep 22 2025
Key Aviation Messages for the current TAF period:
- MVFR BR possible outside of Chicago overnight, with chances of
  IFR or even LIFR around KRFD
Showers and storms continue tonight over central Illinois and
northern Indiana, far enough away from any of the TAF sites.
While an isolated shower may crop up over the Chicago terminals
overnight to around day break: a.) best chances are around KGYY
and b.) though a shower may reduce vis down around 4 SM, it is
expected to be a brief shower. Given confidence being lower,
the TAFs were kept dry.
While there is a thick cirrus shield on satellite imagery
currently, there are clearer skies to the west moving in which
will help with cooling temps down toward the dew point
temperature and potentially lower visibilities. Best chances for
MVFR vis is outside of Chicago, but (being farthest west) KRFD
is already at 6SM at this hour and has the best chance for IFR,
or even potentially LIFR conditions, early Monday morning.
VFR and dry conditions are expected through Monday. Winds are
expected out of the southwest at or below 10 knots. With
clearing skies through the day and evening, there may be a
better chance for fog development Monday night into Tuesday.
However, at this time range confidence was not there to add to
the 30 hour TAFs presently but will be looked into during the
12Z TAF package.
DK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&
$$
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NWS LOT Office Area Forecast Discussion