561
FXUS63 KLOT 171858
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
158 PM CDT THU SEP 17 2026
KEY MESSAGES
- 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 /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1236 PM CDT THU SEP 17 2026
MVFR/BORDERLINE IFR CIGS REMAIN IN PLACE FROM RPJ - DPA - ORD
AND NORTHWARD EARLY THIS AFTERNOON, WITH VFR TO THE SOUTH. A
DIFFUSE COLD FRONT WILL CONTINUE TO SHIFT SOUTHWARD THIS
AFTERNOON. AS IT DOES SO, ANTICIPATE ADDITIONAL LOW-MVFR TO NEAR
IFR CIGS TO SPREAD SOUTHWARD. WHILE SOME HOLES IN THIS LOW
STRATUS MAY DEVELOP LATER THIS AFTERNOON, IT LOOKS LIKE THESE
SHOULD FILL BACK IN THROUGH THE EVENING. CONFIDENCE IN IFR
REMAINS A BIT TOO LOW TO WARRANT INCLUSION, BUT MAY BECOME
NECESSARY, ESPECIALLY NEAR ORD, DPA AND POSSIBLY GYY THIS
EVENING PENDING OBSERVATION AND GUIDANCE TRENDS.
ADDITIONALLY, A DEEPER LAYER OF LOW-LEVEL MOISTURE IS FORECAST
TO DEVELOP NORTH OF ABOUT I-88 THIS AFTERNOON AND INTO THE
EVENING AS A SECONDARY DISTURBANCE PIVOTS OVERHEAD. THIS MAY
RESULT IN THE DEVELOPMENT OF SOME DRIZZLE. THE BETTER PARAMETER
SPACE IS FORECAST TO REMAIN JUST NORTH OF ORD, SO HAVE REFRAINED
FROM A MENTION IN THE OUTGOING 18Z TAFS BUT WILL CONTINUE TO
MONITOR TRENDS. CHANCES ARE A BIT GREATER FOR BR AND/OR SOME DZ
AT RFD, AND MAY NEED TO CONSIDER TEMPO GROUPS AS TRENDS BECOME A
BIT MORE CLEAR.
DRIER AIR WILL EVENTUALLY SHIFT ACROSS THE TERMINALS LATE
TONIGHT/EARLY FRIDAY MORNING RESULTING IN A RETURN TO VFR. NE TO
E BREEZES WILL DEVELOP AND PERSIST THROUGH THE DAY.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...DENSE FOG ADVISORY UNTIL 3 PM CDT THIS AFTERNOON FOR WINTHROP
HARBOR TO WILMETTE HARBOR IL.
VISIT US AT WEATHER.GOV/CHICAGO
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