778
FXUS63 KLOT 212322
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
622 PM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Areas of fog (dense in spots) will develop outside of Chicago
  late tonight and linger into Friday morning.

- Little to no impactful weather is expected through the
  upcoming weekend and early to mid next week.

- An early Fall temperature preview remains in store Sunday-
  Wednesday.

&&

.DISCUSSION...
Issued at 219 PM CDT Thu Aug 21 2025

Through Friday:

The main forecast concern in the near term continues to center
around the threat for fog development late tonight into early
Friday morning.

Light winds and mainly clear skies are expected tonight as a
surface high becomes centered overhead. This will foster
efficient radiational cooling and result in a rather cool night
as temperatures bottom out in the mid 50s to around 60 outside
of the Chicago urban heat island. This is problematic, as
afternoon dewpoints remain in the low to mid 60s. With little
change in these values expected through the remainder of the
afternoon, it appears that most areas outside of Chicago will
fall several degrees below their cross over temperatures late
tonight. Accordingly, fog development appears likely (70%+
chance) overnight into early Friday morning, and the fog could
become dense (visibility under a 1/4 mile) and impact early
morning travel in parts of the area. With this in mind, I
continued the trend of hitting the fog a bit harder in the grids
and in our weather story.

Following the early morning fog on Friday, expect another
rather pleasant late August day across the area. High pressure
will remain overhead during the day and will continue to promote
light winds. Temperatures on Friday will be similar to today,
with afternoons readings topping out around 80 (slightly cooler
lakeside). Dry weather is expected, though partly cloudy skies
are expected to develop into the afternoon as some fair weather
Strato Cu develops.

KJB


Friday Night through Thursday:

A strong upper low and an associated surface low will shift
east from Manitoba into Ontario Friday night. As it does, the
associated cold front will be approaching our region from the
Upper Midwest by early Saturday morning. Showers and
thunderstorms are likely to accompany this frontal boundary as
it moves across the Upper Midwest late Friday into Friday night.
However, with the front looking to move into our area into
early Saturday morning during a diurnally unfavorable time, the
activity looks to weaken considerably, if not diminish
altogether, as the front moves into northern IL Saturday
morning. Accordingly, the threat for showers and storms in our
area is low (20% or less) at this point, and could become more
focused southeast of I-55 Saturday afternoon. Otherwise, expect
another mild day Saturday with highs again topping out around
80.

The aforementioned Canadian upper low will carve out deep mid-
upper level trough across the Great Lakes Sunday into the first
half of next week. This will favor a period of below average
temperatures next week as a much cooler Autumn-like airmass
shifts into the area on breezy west-northwesterly winds. High
temperatures in this airmass will be favored to only be in the
low to mid 70s. Overnight lows will be in the upper 40s (a few
localized mid 40s?) to lower 50s outside of Chicago! In
addition, an early season lake effect rain (and even
thunderstorm) episode may unfold primarily to our east and
northeast over western Lower Michigan and north central Indiana
Sunday through Monday night. Some lake effect showers may sneak
into Porter County Indiana at times dependent upon low-level
flow/boundary layer convergence orientation shifting more
northerly.

Below normal, but comfortable/pleasant, temperatures should
persist through at least mid next week before readings return
back to near to slightly above seasonal norms heading into Labor
Day Weekend. Also importantly, primarily dry conditions should
prevail through next work week, giving more time to dry out from
the recent heavy rainfall.

KJB/Castro

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 622 PM CDT Thu Aug 21 2025

Aviation forecast concerns for the 00Z TAFs:

- Potential for IFR/LIFR/VLIFR fog across interior northern IL
  sites late tonight/early Friday morning.

- Low confidence in timing of lake breeze at ORD/MDW Friday
  afternoon.

Surface high pressure ridge axis will continue to settle across
the forecast area overnight into Friday, resulting in current
modest northeast winds becoming generally light/variable. Weak
gradient conditions would tend to support a lake breeze
developing Friday afternoon and pushing west toward ORD/MDW,
though a considerable amount of available model guidance doesn`t
depict the boundary making it to either airport. Would think it
would come close if not move across both fields, though have
pushed inherited timing of an easterly wind shift an hour later
with the 00Z issuance.

Prior to that however, tonight looks to be a decent setup for
fog development across interior northern IL (DPA and RFD
especially), with the potential for IFR/LIFR/VLIFR visibilities
by sunrise. Light winds, generally clear skies and lingering low
level moisture all support this. Have indicated fog in terminals
ranging from a shorter period of MVFR at ORD/MDW, to IFR at
DPA/RFD and can`t rule out a period of LIFR/VLIFR especially at
RFD toward sunrise. Conditions should gradually improve to VFR
through mid-morning.

Ratzer

&&

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

&&

$$

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