663
FXUS63 KLOT 140223
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
923 PM CDT Sun Jul 13 2025

.KEY MESSAGES...

- Shower and thunderstorm chances will increase each day Tuesday
  through Thursday.

&&

.UPDATE...
Issued at 923 PM CDT Sun Jul 13 2025

Couple of minor additions to the forecast for the remainder of
the night. First is to had haze on account of wildfire smoke
that while mostly aloft, is resulting in some 3-6SM visibility
observations and an otherwise hazy abhorrence to the sky where
cloud cover is otherwise minimal. Second is to add areas of fog
mainly south of I- 80 after midnight tonight where there could
be some dense fog, as well patchy fog farther north outside of
the core of the Chicago metro.

Mid-evenig surface analysis depicts a nearly stationary cold
front across central IL/northern IN, in the vicinity of the
ILX/LOT border region. Rain along the baroclinic zone has come
to an end for the LOT forecast area, in association with a mid-
level short wave now moving east of the region across IN.
Surface dew points in the vicinity of the surface boundary
remain around 70F, which combined with light/variable winds
within a weak/baggy surface pressure gradient is likely to allow
for fog development overnight south of I- 80, and especially
south of the Kankakee/Illinois river valleys. Even farther
north, guidance is suggesting patchy fog development away from
the core of the Chicago urban area and have included patchy fog
there as well. Can`t rule out the eventual need for a dense fog
advisory by predawn Monday morning across our southern cwa.

Skies have been hazy today from wildfire smoke which has
returned to the region behind the cold front and recent mid-
level trough passage. Seeing some 3-6SM ASOS/AWOS obs across the
region here at mid-evening, along with a generally hazy sky
after thicker cirrus has moved out. Thus have added haze mention
overnight (outside of fog) with HRRR near-sfc smoke locally
focused. Concentrations are forecast to decrease somewhat on
Monday, though we`ll keep an eye on the hazy conditions.

Ratzer

&&

.DISCUSSION...
Issued at 303 PM CDT Sun Jul 13 2025

Through Monday Night:

A shortwave trough and its associated weak surface low continue
to traverse across eastern MO and central IL this afternoon
generating a broad area of scattered showers and embedded
thunderstorms as it does so. Thus far the greatest coverage of
showers and thunderstorms has remained south of our CWA where
the combination of better forcing, moisture, and instability
have come together. However, forecast soundings do show some
modest (600-800 J/kg of MLCAPE) instability in the presence of
upper 60 to near 70 degree dew points south of the Kankakee
River which may support at least some scattered showers and
perhaps an isolated storm or two. If any showers/storms do
materialize the greater coverage should remain along and south
of a Pontiac, IL to Rensselaer, IN line with shower/storm
coverage expected to gradually diminish after sunset.

Elsewhere, the main forecast concern for this evening will be
the partly to mostly cloudy skies caused by the cirrus blow off
from storms in central IL and some wildfire smoke aloft. Skies
are expected to gradually clear out overnight areawide as storms
move into IN and OH and the smoke aloft wanes. Therefore,
expect a rather pleasant evening with temperatures in the lower
80s to start before cooling into the mid-60s overnight.

As for Monday, mid-level height rises are expected to establish
overhead which will erode the cloud cover and give a mostly
sunny start to the work week. Highs on Monday will be a bit
warmer than today with readings in the mid to upper 80s, but the
mild humidity will keep heat indices on par with the
temperatures. Additionally, a weak lake breeze is forecast to
move onshore Monday afternoon which will result in some cooling
temperatures along the lakeshore capping highs more into the
upper 70s and lower 80s here. The tranquil conditions will
extend into Monday night with clear skies and mild temperatures
in the mid-60s.

Tuesday through Sunday:

The weather is expected to become more active heading into the
middle and later portions of the week as several disturbances
traverse through the Mississippi Valley and Great Lakes. The
first disturbance to impact the region will be the ongoing MCV
and its associated shortwave in central TX. This MCV and wave
are forecast to begin ejecting into the southern Plains on
Monday and then into the Mississippi Valley by Tuesday morning.
Given that the current guidance consensus is for this
disturbance to pivot more into central IL and IN Tuesday
afternoon, it seems that the moisture return into northern IL
and northwest IN could be more limited. Couple this with the
drier mid-level air forecast to be in place, suspect that shower
and thunderstorm coverage on Tuesday will likely be somewhat
spotty and mainly confined to our southern CWA (namely areas
south of I-80).

Another convectively enhanced shortwave trough is forecast to
develop across the central Plains Tuesday night and move into
northern IL on Wednesday. With better moisture expected to be in
place in addition to more focused forcing overhead, the
coverage of showers and thunderstorms on Wednesday does look to
be higher than Tuesday but still in a more scattered nature.
Though, the best chances for showers and thunderstorms this week
is not expected until late Wednesday night through Thursday as
a more amplified trough pivots out of the Mountain West and
Upper Midwest and drags a cold front through northern IL and
northwest IN. This front looks to keep a decent coverage of
showers and storms in play through Thursday night and possibly
into Friday pending how quickly the front exits to our south.
Furthermore, with PWATs forecast to peak in the 1.7-1.8 inch
range the showers/storms will likely be capable of producing
heavy rainfall which could result in a flooding threat if storms
train along the frontal boundary. While it is still a bit too
early to know exact rainfall amounts and location of heaviest
rain, this will be a period to monitor over the coming days.

Outside of the increasing rain chances, warmer temperatures and
increasing humidity will also return Tuesday through Thursday.
While forecast air temperatures look to be in the upper 80s to
around 90 (warmest Tuesday and Wednesday), the upper 60s to
lower 70 degree dew point will support heat indices in the mid
to upper 90s during this period.

Once the front pushes through the area on Friday, a period of
drier and more comfortable conditions is expected to establish
for Friday and Saturday with highs in the lower to mid-80s.
However, these conditions will be short lived as a northwest
flow pattern is forecast to set up late next weekend into the
following week which looks to bring another active period of
weather to the region.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 634 PM CDT Sun Jul 13 2025

Light W/WNW winds at TAF issuance will either become VRB or
settle in the SE quadrant this evening as a decaying weak lake
breeze shifts across NE Illinois. A surface ridge passing just
south of the terminals will then result in generally light winds
around 5 knots or less through much of the period. Wind
direction will favor SW late tonight through much of Monday, but
could become VRB at times given the light flow. A weak lake
breeze should then cross the terminals late afternoon or early
evening.

Upper-level clouds will clear to the east this evening, leaving
mostly clear skies through the period. Wildfire smoke aloft
tonight and some diurnal VFR cumulus Monday are both represented
in the TAF. Some of the smoke is mixing toward the surface, so
would not by surprised to see lower VFR visibility with HZ
tonight. For the outlying sites (RFD/DPA/GYY), BR/MIFG/BCFG is
possible for a few hours around sunrise and may produce MVFR
visibility.

Kluber

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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