NWS Chicago Decision Support Briefing - Forecast

130
FXUS63 KLOT 270533
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1233 AM CDT Sat Jun 27 2026

.KEY MESSAGES...

- A few thunderstorms are possible Saturday night into Sunday
morning, mainly west of the Fox Valley.

- Dangerous heat will build across the area next week, with
afternoon heat index values over 100 degrees each day
beginning Monday.

&&

.DISCUSSION...
Issued at 205 PM CDT Fri Jun 26 2026

Ongoing showers will continue through the evening and
overnight. Most of the rain thus far has been fairly minimal.
Light rain can be expected as far north as Interstate 90, but
the better coverage will remain closer to the higher moisture
plume south of Interstate 80 with localized heavier downpours
possible along and south of US 24. The better instability
remains well to the south and out west near the area of surface
low pressure. Thus, pulled the chances for thunder for the rest
of today for much of the area, with any lingering (less than 24
percent) chance around US 24.

The surface low, currently in western Missouri, will slowly
move northeast and pass just south of the forecast area
overnight. The models slowing the progression of the low gave
confidence in maintaining chance PoPs for areas southeast of
I-57 Saturday morning, though overall drier conditions are
expected tomorrow. Winds will remain out of the northeast
through the day on Saturday. High temperatures are expected in
the 70s to low 80s inland with cooler upper 60s to low 70s along
the lakeshore.

While Saturday is looking to shape up as a nice day locally,
the overall atmospheric pattern will be changing tomorrow. An
upper level low out over the Rocky Mountains will continue to
deepen and sink farther south over the western CONUS.
Subsequently, an upper level ridge will amplify northward from
the Gulf and deepen. This amplified pattern will set up broad
southwest flow over the Plains and Mississippi River Valley. A
weak wave is expected to eject out of the upper level low and
move toward Wisconsin. Decent mid-level lapse rates could
support thunder and the presence of a slug of moisture with
warm air advection and isentropic upglide could support showers
and a few thunderstorms in the overnight hours. However, this
will be moving in toward a drier and less favorable air mass,
and models are already responding giving confidence to lower
PoPs. Perhaps a few showers fester into Sunday morning, but
generally drier (and more humid conditions) are expected during
the day.

More concerning, starting on Sunday, is the potential for
dangerous heat next week. A strong upper level ridge will sit
east of the Mississippi River through the week. Southwest flow
will continue to advect in warmer air as 850 mb temps increase
over the area from 15 to 20C on Sunday to around 25C on Tuesday
(the current expected highest temperature forecast of the
stretch). Temperatures are expected to climb into the 90s for
most of the forecast area, and with overnight lows only in the
mid to upper 70s, little relief can be expected at night.
Additionally, the southwest winds will bring the afternoon
temperatures all the way to the traditionally cooler lakeshore.
Dew points climbing into the 70s will make for humid conditions
and drive heat index values up to around 105 F.

Setting a record high temperatures is not a guarantee
(especially for any records that are 100+, like much of KRFD),
but a Climate section has been created below for interested
individuals. Two big questions for this heat event will focus
on the center of the upper level high embedded in the ridge: 1.)
where will its location be? And 2.) how strong will it remain?
Monday and Tuesday look to be a good chance for these hot
temperatures to set up, but there is greater uncertainty for
mid-week and after. If the high were to weaken or even set up
along and south of the Ohio River Valley, there could be a
convective complex that could ride over the top of the ridge and
bring not just showers and storm chances to the area, but also
some relief from the temperatures. So no changes were made to
the splotchy chance PoPs that the NBM provided for the middle
and end of the week. However, recent models have trended towards
a slightly stronger high. If that were to materialize, the
region could be set up for not just drier conditions, but an
extended period of heat indices over 100F. It`s still far too
early for any headline (e.g. Watch, Warning, Advisory), but
there is growing confidence in the first heat wave of the year
leading into the holiday weekend.

DK

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1233 AM CDT Sat Jun 27 2026

Aviation forecast concerns for the 06Z TAFs:

- Some potential for lowering of VFR ceilings to MVFR Saturday,
with the greatest likelihood at KGYY. Otherwise, VFR
conditions expected to prevail.


A west-east oriented, nearly stationary frontal boundary remains
in place to the south of the terminals from central MO into
southern IL/IN early this morning. Aloft, a series of low-
amplitude mid-level disturbances continue to ripple eastward
along the elevated baroclinic zone north of the surface front.
The main effect on weather across the terminals is to maintain
persistent mainly VFR cloud layers in the 4-7 kft range, though
some lower MVFR/IFR bases have been noted along the Lake
Michigan shore and north of KORD where some weak but persistent
sprinkles/drizzle has been occurring since last evening.
Persistent warm/moist advection aloft is produce a gradual
northward shift in the lower VFR deck, resulting in a gradual
lowering of VFR cigs across the terminals. Can`t rule out patchy
MVFR cigs developing as far north as KORD during the day, though
the greatest likelihood of MVFR ceilings appears to be at KGYY
and locations west and south. Lack of any significant
precipitation suggests locations farther north (KRFD/KDPA/KORD
and KMDW) are more likely to remain VFR. High-res guidance
indicates VFR ceilings will persist Saturday night, though may
rise into the ~5 kft range or even scatter out at times.

Surface winds are expected to remain modest (6-11 kts) from the
east-northeast to northeast through the period.

Ratzer

&&

.CLIMATE...
Issued at 143 PM CDT Fri Jun 26 2026

....Daily Record High Temperatures........

Chicago (KORD) Rockford (KRFD)
June 28 101F (1951) 103F (1934)
June 29 97F (1954) 103F (1931)
June 30 99F (1913) 102F (1954)
July 1 103F (1956) 101F (1931)
July 2 99F (1970) 99F (1910)

DK

&&

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

&&

$$

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