NWS Chicago Decision Support Briefing - Forecast

227
FXUS63 KLOT 271716
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1216 PM CDT Sat Jun 27 2026

.KEY MESSAGES...

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

&&

.DISCUSSION...
Issued at 252 AM CDT Sat Jun 27 2026

Spotty light showers persist across northern Illinois early
this morning with a lingering low to mid-level deformation axis
wringing out a remaining layer of modest moisture. Expectations
are for remaining showers to erode by mid morning as dry air
advection increases over the area and the deformation axis exits
to the east. For areas south of the Kankakee/Illinois River,
the northern extent of a very moist airmass (PWATS 2+ or higher)
combined with limited low-level capping and diurnal influences
should yield isolated shallow showers with efficient warm rain
processes to develop this afternoon. Expect highs today to range
from the upper 70s inland to the upper 60s along the immediate
shore.

Steepening mid-level lapse rates above an incoming EML on
Sunday will provide at least a small chance for a NW to SE
oriented axis of convection to develop toward the Mississippi
River late tonight. However, limited moisture near the base of
the incoming EML and the lack of a decent LLJ directed toward
the area imply that any upstream convection will struggle to
continue with northeast extent over the area by Sunday morning.
Otherwise, will also need to monitor the track for a potential
MCS developing over South Dakota late this evening as the
notable instability gradient (albeit driven by the incoming EML)
will be focused across northern Illinois Sunday afternoon.

Heat and humidity will begin to build with heat index values
rising into the lower 90s inland during the afternoon, with
persistent onshore flow maintaining much cooler conditions along
the shore.

Focus remains on the potential for an extended period of
dangerous heat through most or all of next week as an
anomalously strong mid-level ridge (heights very near 600dam)
drifting along the Ohio River Valley. H850/H700 temps of over
25C/12C Monday through Wednesday will yield unseasonably warm
conditions with daytime highs well into the 90s while building
an incredibly strong cap over the local area. This should
deflect convection well to the northwest (Minnesota through
Upper Michigan) through at least Wednesday. Combined with
persistent southwest low-level flow, there looks to be no relief
from lake breezes or upstream convective outflow boundaries
during this time. Beyond that point, cooler temps aloft may
allow more robust convective complexes to near from the north
and provide some relief for northern portions of the area. If
this does not occur, similar heat will likely continue heading
into the Independence Day weekend.

For Cook County, current forecast conditions support the
eventual need for an Extreme Heat Watch/Warning for a longer
duration event with afternoon heat index values over 100 degrees
for three days or more in conjunction with nighttime lows
likely struggling to fall much below 80 in the core of the
metro.

For areas outside of Cook County, it appears likely that at
least a Heat Advisory (Heat Index of 105+) will be required for
some days or every day from Monday through Thursday. Whether
conditions meet Extreme Heat Warning criteria (110+) are less
clear and will hinge heavily on whether dew points rise into the
upper 70s versus low to mid 70s.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1216 PM CDT Sat Jun 27 2026

Northeasterly winds will prevail today and this evening, before
shifting out of the southeast on Sunday, likely becoming
sporadically gusty during the afternoon.

Largely VFR conditions are expected. Some MVFR cigs will be
possible near GYY this afternoon, followed by some shallow VFR
BR late tonight. Diurnal VFR cumulus will develop through the
day on Sunday. At ORD/MDW: low stratus continues to hug the
immediate lakeshore. There is a small chance this activity
attempts to ooze inland this evening, but currently not seeing a
significant signal for this and have maintained VFR conditions.

There is a very small (at this time less than 10 percent) chance
for a complex of thunderstorms originating out of Minnesota to
move into the region tomorrow afternoon. This does not appear to
be the most likely scenario as the environment will become
increasingly hostile to convection in the local area. Will
continue to monitor trends, but chances are much too low to
justify any precip mention in the extended ORD/MDW TAFs.

Carlaw

&&

.CLIMATE...
Issued at 252 AM CDT Sat Jun 27 2026

.Daily Records for the Upcoming Week........

Chicago (KORD)
Record High Record Warm Minimum
June 29 97 (1954) 80 (1931)
June 30 99 (1953) 80 (2018)
July 1 103 (1956) 78 (1931)
July 2 99 (1970) 79 (1911)
July 3 102 (1949) 78 (1911)
July 4 102 (2012) 80 (1921)


Rockford (KRFD)
Record High Record Warm Minimum
June 29 103 (1931) 75 (1931)
June 30 102 (1931) 76 (2018)
July 1 101 (1931) 76 (1970)
July 2 99 (1910) 74 (1970)
July 3 99 (1925) 75 (2012)
July 4 102 (2012) 77 (1977)

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Extreme Heat Watch from Monday afternoon through Wednesday
evening for ILZ103-ILZ104-ILZ105.

IN...None.
LM...None.

&&

$$

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