279
FXUS63 KLOT 180547
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1247 AM CDT FRI SEP 18 2026
KEY MESSAGES
- SCATTERED SHOWERS AND THUNDERSTORMS SOUTH OF THE KANKAKEE
RIVER THIS EVENING.
- A BRIEF BREAK IN THE PATTERN IS EXPECTED TOMORROW AFTERNOON
THROUGH FRIDAY BEFORE ANOTHER PERIOD OF WET WEATHER
MATERIALIZES THIS WEEKEND. LOCALIZED FLASH FLOODING WILL BE
POSSIBLE WHERE THE HEAVIEST RAIN SETS UP.
DISCUSSION
ISSUED AT 158 PM CDT THU SEP 17 2026
A DIFFUSE FRONT/DIFFERENTIAL HEATING ZONE IN THE VICINITY OF
I-90/88 CONTINUES TO SLOWLY WORK ITS WAY SOUTHWARD EARLY THIS
AFTERNOON. TO THE NORTH OF THE BOUNDARY, PERSISTENT LOW STRATUS
HAS HELD TEMPERATURES IN THE UPPER 60S AND LOWER 70S, WHILE TO
THE SOUTH TEMPERATURES HAVE WARMED INTO THE UPPER 80S. THIS
BOUNDARY WILL CONTINUE TO OOZE SOUTHWARD THROUGH THE REST OF THE
AFTERNOON AND EVENING. AS IT DOES SO, LOW-LEVEL SATURATION
DEPTHS ARE FORECAST TO INCREASE A BIT ALONG AND NORTH OF I-80 AS
A MODEST TRAILING MID-LEVEL SHORTWAVE PASSES BY OVERHEAD. THIS
MAY BE SUFFICIENT TO CRANK OUT SOME SHOWERS OR DRIZZLE AT TIMES.
AHEAD OF THE FRONT, SOME SHOWERS OR STORMS MAY DEVELOP LATE
THIS AFTERNOON AND INTO THE EVENING AS LINGERING CONVECTIVE
INHIBITION ERODES FOLLOWING PEAK HEATING. LOW-LEVEL CONVERGENCE
ALONG THE BOUNDARY DOESN'T LOOK PARTICULARLY ROBUST HOWEVER, AND
THIS MAY END UP BEING A LIMITING FACTOR IN TERMS OF ANY
CONVECTIVE COVERAGE. AS MODEST LOW-LEVEL ISENTROPIC UPGLIDE
CONTINUES ATOP THE SAGGING FRONTAL ZONE, ADDITIONAL SHOWERS AND
A FEW STORMS MAY CONTINUE TO FESTER THROUGH THE EVENING SOUTH OF
I-80.
CONDITIONS LOOK TO REMAIN LARGELY DRY WITHIN THE POST-FRONTAL
AIRMASS TOMORROW. HOWEVER, SOME GUIDANCE DOES SUGGEST THE SURFACE
BAROCLINIC ZONE MAY SLOSH BACK NORTHWARD JUST FAR ENOUGH TO
PUSH THE ELEVATED INSTABILITY LAYER BACK ACROSS OUR FAR SOUTH
AND WEST LOCALES DURING THE AFTERNOON. HAVE INCLUDED A SLIGHT
CHANCE OF STORMS TO ACCOUNT FOR THIS POTENTIAL, BUT MOST
GUIDANCE SUGGESTS THIS BOUNDARY SHOULD REMAIN SOUTH OF THE CWA.
SOME ADDITIONAL SPOTTY SHOWERS CAN'T BE RULED OUT ACROSS
NORTHERN ILLINOIS AHEAD OF THE NEXT DEVELOPING STORM SYSTEM.
ATTENTION THEN SHIFTS TO WHAT LOOKS LIKE ANOTHER PERIOD OF
OCCASIONALLY STORMY WEATHER IN THE REGION OVER THE WEEKEND, WITH
AN INCREASING THREAT FOR HEAVY RAINFALL AND AT LEAST A
LOCALIZED FLASH FLOOD THREAT. SEVERAL MID-LEVEL IMPULSES WILL
SLINGSHOT AROUND THE EASTERN PERIPHERY OF A BROAD AND PERSISTENT
UPPER-LEVEL TROUGH ACROSS THE WESTERN CONUS FRIDAY NIGHT INTO
SATURDAY MORNING. THIS IN TURN WILL INDUCE CYCLOGENESIS ACROSS
THE CENTRAL PLAINS. AS THIS OCCURS, INTENSIFYING SOUTHWESTERLY
FLOW WILL OVERTOP THE SURFACE BAROCLINIC ZONE, RESULTING IN A
SIGNIFICANT EXPANSION OF RAIN AND EMBEDDED THUNDERSTORMS ACROSS
THE UPPER MIDWEST. INCREASING UPPER DIVERGENCE WITHIN THE RIGHT
ENTRANCE REGION OF A FAST ANTICYCLONIC JET STREAK, COUPLED WITH
BROAD AND FAIRLY INTENSE WAA/UPGLIDE WITHIN A RICHLY-MOIST
AIRMASS CHARACTERIZED BY ESSENTIALLY RECORD PWATS NORTH OF 2
INCHES WILL FACILITATE THE POTENTIAL FOR EMBEDDED HEAVY
TRAINING CONVECTIVE ELEMENTS WITH TORRENTIAL RAINFALL. WHILE
SOME UNCERTAINTY IN THE EXACT PLACEMENT OF THE SALIENT
BAROCLINIC ZONE AND LOW TRACK REMAINS, EXTENDED CAM AND COARSE-
RESOLUTION ENSEMBLE GUIDANCE CONTINUES TO PAIN THE GREATEST
HEAVY RAIN PROBS ROUGHLY NEAR/NORTH OF I-88 LATE FRIDAY NIGHT
THROUGH SATURDAY MORNING. CONCEPTUALLY, THE SET UP IS A
CONCERNING ONE FROM A FLASH FLOOD POTENTIAL, ALTHOUGH THE MOST
PERSISTENT JET DIVERGENCE/DEEPEST MOISTURE MAY SET UP JUST NORTH
AND WEST OF OUR CWA--STILL TO BE DETERMINED THOUGH.
WITH AM CONVECTION DECIDEDLY ELEVATED, THE SEVERE THREAT APPEARS
LOW. CAMS HINT AT THE POTENTIAL FOR SOME ELEVATED GRAVITY WAVE
ASSOCIATED CONVECTION (GWAC), BUT MEAGER MUCAPE/DCAPE SHOULD
CURTAIL THE THREAT FOR SEVERE GUSTS IN THE AREA.
IT'S A BIT UNCLEAR WHERE THE SURFACE BOUNDARY WILL RESIDE
SATURDAY AFTERNOON. WITH WIDESPREAD MORNING CONVECTION, THE
BOUNDARY MAY HAVE A DIFFICULT TIME MAKING IT MEANINGFULLY BACK
INTO OUR FORECAST AREA (SUSPECT THE LATEST ECMWF REMAINS A BIT
TOO FAR NORTH). IN THIS SCENARIO, ANY ADDITIONAL AFTERNOON
ACTIVITY SHOULD GENERALLY BE OF THE LIGHT-MODERATE SHOWER
VARIETY. HOWEVER, A SECOND WAVE IS FORECAST TO SHIFT OVERHEAD
SATURDAY NIGHT INTO SUNDAY, AND MAY RESULT IN ANOTHER ROUND OF
LOCALLY HEAVY RAINFALL NEARLY ACROSS THE SAME AREAS.
HAVE ISSUED A HYDROLOGIC OUTLOOK FOR AREAS ROUGHLY ALONG AND
NORTH OF I-80 GIVEN THE POTENTIAL FOR TWO ROUNDS OF HEAVY
RAINFALL. MAY NEED CONSIDER FLOOD WATCH(ES) FOR PARTS OF THE
AREA IF CONFIDENCE IN TRAINING CONVECTION CONTINUES TO INCREASE.
SOME SHOWER ACTIVITY MAY LINGER INTO MONDAY, BUT HIGH PRESSURE
IS FORECAST TO BUILD INTO THE REGION TOWARDS THE MIDDLE AND END
OF NEXT WEEK.
CARLAW
AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
ISSUED AT 1247 AM CDT FRI SEP 18 2026
KEY AVIATION FORECAST MESSAGES:
- IFR-MVFR CEILINGS GRADUALLY LIFTING TO VFR THIS MORNING,
LATEST AT RFD.
- SHRA/TSRA POTENTIAL RAMPS UP LATE TONIGHT INTO SATURDAY
MORNING, WITH MVFR/IFR CONDITIONS POSSIBLE IN HEAVY CONVECTIVE
SHRA/TS.
SURFACE HIGH PRESSURE BUILDING EAST-SOUTHEAST ACROSS ONTARIO AND
THE NORTHERN GREAT LAKES WAS BRINING SLIGHTLY DRIER NORTH-
NORTHEAST SURFACE WINDS TO THE REGION EARLY THIS MORNING, AND
HELPING TO PUSH THE DOWNSTATE FRONTAL ZONE FARTHER SOUTH. AREA
OF IFR CIGS WAS GRADUALLY IMPROVING TO LOW-MVFR FROM THE
NORTHEAST ACROSS THE TERMINALS AT 0530Z, AND SURFACE/SATELLITE
OBS AND HIGH-RES MODEL GUIDANCE SUPPORT GRADUAL CONTINUED
IMPROVEMENT TO HIGH-END MVFR/VFR THROUGH THIS MORNING WITH RFD
LIKELY SLOWEST TO IMPROVE TO VFR TOWARD MID-MORNING.
SURFACE WINDS SHOULD TURN MORE EAST-NORTHEAST DURING THE DAY AS
THE SURFACE HIGH CONTINUES TO MOVE OFF TO OUR NORTHEAST, AND
EVENTUALLY TO MORE OF AN EAST-SOUTHEAST DIRECTION TONIGHT AS AN
AREA OF LOW PRESSURE DEVELOPS INTO IA. AS THIS OCCURS, THE
FRONTAL BOUNDARY TO OUR SOUTH WILL GRADUALLY RETURN NORTHWARD
AS A WARM FRONT, WITH AN EXPANSION OF SHRA/TSRA DEVELOPING
ACROSS THE UPPER MISSISSIPPI VALLEY NEAR AND NORTH OF THE
SURFACE LOW AND WARM FRONT. WHILE A FEW SPOTTY SHOWERS CAN'T BE
COMPLETELY RULED OUT AS EARLY AS THIS AFTERNOON ESPECIALLY
ACROSS NORTHWEST IL/SOUTHERN WI, THE GREATEST POTENTIAL FOR MORE
WIDESPREAD AND IMPACTFUL HEAVY SHRA/TSRA ACROSS THE TERMINALS
APPEARS TO BE AFTER 06Z FOR RFD AND AFTER 08/09Z FOR THE
CHICAGO TERMINALS. A PROLONGED PERIOD OF SHRA/TSRA COULD THEN
PERSIST INTO MID-MORNING SATURDAY, WITH MVFR/IFR VIS/CIG
CONDITIONS.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...BEACH HAZARDS STATEMENT FROM 5 AM CDT EARLY THIS MORNING
THROUGH THIS AFTERNOON FOR ILZ006-ILZ103-ILZ104.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 7 AM THIS MORNING TO 3 PM CDT THIS
AFTERNOON FOR WINTHROP HARBOR TO WILMETTE HARBOR IL.
VISIT US AT WEATHER.GOV/CHICAGO
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