549
FXUS63 KLOT 271126
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
626 AM 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 /12Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 626 AM CDT SAT JUN 27 2026
AVIATION FORECAST CONCERNS FOR THE 12Z TAFS:
- SOME POTENTIAL FOR MVFR OR LOW-END VFR CEILINGS ACROSS THE
TERMINALS TODAY, WITH THE GREATEST LIKELIHOOD AT KGYY.
A WEAK SURFACE LOW PRESSURE WAS NOTED NEAR KSTL THIS MORNING,
AND WILL TRACK EAST ALONG A NEARLY STATIONARY FRONTAL BOUNDARY
STRETCHING WEST TO EAST ACROSS SOUTHERN IL/IN. RELATIVELY MOIST
LOW-LEVELS NORTH OF THE FRONT WAS SUPPORTING A VARIETY OF CLOUD
LAYERS FROM VLIFR ACROSS CENTRAL IL TO 3500-6000 FOOT VFR BASES
FARTHER NORTH ACROSS OUR TERMINALS. CAN'T RULE OUT SOME PATCHY
MVFR CEILINGS DEVELOPING THIS MORNING WITH DIURNAL WARMING, WITH
THE GREATEST POTENTIAL MAINLY SOUTH AND SOUTHWEST OF THE
CHICAGO AREA. KGYY IS LIKELY MOST SUSCEPTIBLE TO MVFR
DEVELOPMENT, WITH DECREASING PROBABILITIES FOR TERMINALS TO THE
NORTH. ONSHORE FLOW OFF OF LAKE MICHIGAN MAY RESULT IN
SCATTERING OF LOWER CLOUDS THIS AFTERNOON, THOUGH VARIOUS
GUIDANCE REMAINS IN GOOD AGREEMENT IN VFR CEILINGS IN THE
4000-5000 FOOT RANGE REDEVELOPING TONIGHT BEFORE EVENTUALLY
SCATTERING OUT MORE COMPLETELY SUNDAY MORNING.
SURFACE WINDS WILL REMAIN EAST-NORTHEAST THROUGH MOST OF THE
FORECAST PERIOD WITH SPEEDS AROUND 10 KT TODAY. WINDS WILL
EVENTUALLY TURN MORE EAST-SOUTHEAST SUNDAY MORNING.
RATZER
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...NONE.
IN...NONE.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
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