776
FXUS63 KLOT 121124
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
524 AM CST FRI DEC 12 2025
KEY MESSAGES
- ACCUMULATING SNOW IS EXPECTED FOR MUCH OF THE AREA ON
SATURDAY, WITH SEVERAL INCHES OF ACCUMULATION POSSIBLE SOUTH
OF INTERSTATE 80.
- VERY COLD TEMPERATURES AND BITTERLY COLD WIND CHILLS ARE
EXPECTED THIS WEEKEND WITH WIND CHILLS BELOW ZERO.
- THERE IS A GROWING SIGNAL FOR A PATTERN CHANGE TOWARD MILDER
(AND LESS SNOWY) CONDITIONS NEXT WEEK.
DISCUSSION
ISSUED AT 253 AM CST FRI DEC 12 2025
THROUGH SATURDAY EVENING:
ACCUMULATING SNOW IS QUICKLY DEPARTING TO THE ESE EARLY THIS
MORNING FOR AREAS SOUTH OF THE I-80 CORRIDOR. MEANWHILE, A
POCKET OF LIGHT LAKE EFFECT SNOW SHOWERS DEVELOPING OVER FAR
SOUTHERN LAKE MICHIGAN SHOULD REMAIN OFFSHORE AS LOW-LEVEL FLOW
QUICKLY TURNS WEST THIS MORNING. SOME CLEARING ACROSS THE
NORTHERN CWA CAN ALSO BE EXPECTED THIS MORNING, BUT THICKENING
MID AND UPPER-LEVEL CLOUDS WILL FILTER OVER THE AREA THROUGH THE
AFTERNOON.
A RAGGED MID-LEVEL WAVE WITH WEAK ASCENT WITHIN A RIBBON OF
MOISTURE WILL CROSS SOUTHERN WISCONSIN THIS AFTERNOON AND CLIP
NORTHEAST ILLINOIS THIS EVENING. WITH MARGINAL FORCING AND SOME
LOW-LEVEL DRY AIR IN PLACE, PRECIP SHOULD BE LIMITED TO ONLY
SOME SPORADIC FLURRIES.
THE ACTIVE UPPER JET ON THE SOUTHWEST EXTENT OF A LONGWAVE
TROUGH OVER EASTERN NORTH AMERICA WILL BRING YET ANOTHER QUICK-
MOVING SYSTEM ACROSS THE MID TO UPPER-MISSISSIPPI RIVER VALLEY
ON SATURDAY. 700-800 HPA FRONTOGENESIS BELOW THE RIGHT ENTRANCE
OF A 140 KNOT UPPER-JET STREAK WILL PROVIDE AMPLE FORCING WITHIN
AN AXIS OF PACIFIC-BASED MOISTURE. ACCUMULATING SNOW IS
EXPECTED FOR MUCH OF THE AREA, WITH SEVERAL INCHES OF
ACCUMULATION POSSIBLE FOR AREAS SOUTH OF I-80 AND ESPECIALLY
TOWARD CENTRAL ILLINOIS. WHILE THERMO PROFILES DEPICT A DEEP
LAYER WITHIN THE DGZ, MORE THAN HALF OF THAT DEPTH RESIDES
SOLIDLY BELOW THE LAYER OF MAXIMUM ASCENT. THE ENTIRE THERMO
PROFILE WILL ALSO BE SHIFTING COLDER WITH TIME, SO THE RESIDENCE
TIME OF MAXIMUM ASCENT WITHIN THE DEEPER DGZ WILL LAST ONLY A
FEW HOURS AT ANY GIVEN LOCATION. HOWEVER, WHERE THE UPPER JET
FORCING AND FRONTOGENESIS BEST ALIGN WILL LIKELY RESULT IN VERY
HIGH SLR VALUES OF 20:1.
LATEST INDICATIONS ARE THAT THE AXIS OF HIGHEST SNOWFALL OF 3-5
INCHES WILL SETTLE JUST SOUTHWEST OF THE CWA BUT CLIP AREAS
SOUTH OF A PONTIAC TO BENTON COUNTY LINE. MODEST LOW-LEVEL DRY
AIR ADVECTION COMBINED WITH DECREASING FORCING WITH NORTHWARD
EXTENT WILL LEAD TO A SOMEWHAT SHARP SNOWFALL GRADIENT OVER THE
CWA. IT IS FEASIBLE THAT AREAS NORTH OF I-88 REMAIN DRY, WITH
THE MAIN SNOWFALL GRADIENT ALONG OR NEAR THE KANKAKEE RIVER
VALLEY. HOWEVER, ONGOING CAA WITH TEMPS HOLDING STEADY IN THE
UPPER SINGLE DIGITS TO LOWER TEENS MEAN LOWER AMOUNTS WILL STILL
RESULT IN VERY SLIPPERY TRAVEL CONDITIONS. IN COORDINATION WITH
NEIGHBORING OFFICES ALONG WITH REMAINING UNCERTAINTY AS TO
WHETHER THE HIGHEST SNOWFALL AXIS AFFECTS THE SOUTHERN CWA, DID
NOT ISSUE A WINTER WEATHER ADVISORY AT THIS TIME. HOWEVER, IF
GUIDANCE REMAINS STEADY OVER THE NEXT COUPLE CYCLES, AN ADVISORY
WILL LIKELY BE NEEDED FOR AT LEAST THE SOUTHERN TIER OF
COUNTIES AND POSSIBLY AS FAR NORTH AS I-80.
SYNOPTIC-SCALE SNOW WILL QUICKLY EXIST TO THE SOUTHEAST SHORTLY
AFTER SUNSET ON SATURDAY AS STRONG CAA CONTINUES. THOUGH A 5KFT
INVERSION WILL LIMIT LES POTENTIAL WITH THE WNW WIND BELTS OF
NORTHERN PORTER COUNTY SATURDAY NIGHT, THE ENTIRE CLOUD DEPTH
RESIDING IN THE DGZ WILL BE QUITE EFFICIENT IN SNOWFALL
PRODUCTION. WOULD NOT BE SURPRISED TO SEE UP TO A FEW INCHES OF
HIGH SLR SNOW DURING THE NIGHTTIME HOURS.
KLUBER
LATE SATURDAY NIGHT THROUGH THURSDAY:
IN THE WAKE OF SATURDAY'S SYSTEM, AN ARCTIC AIRMASS WILL
DESCEND ON THE REGION AS A MID 1040S MB HIGH BUILDS ACROSS
MINNESOTA AND IOWA. WHILE IT'S ALWAYS DIFFICULT TO TRUST MODEL
GUIDANCE'S HANDLING OF LOW-LEVEL MOISTURE AND CLOUD COVER IN
THESE ARCTIC AIRMASSES, A SIGNIFICANT PUSH OF DRY AIR IS
ADVERTISED WITH THE INCOMING HIGH, SUGGESTING THINGS SHOULD
PRETTY READILY CLEAR OUT SATURDAY NIGHT AWAY FROM LINGERING LAKE
EFFECT PRECIPITATION ACROSS PARTS OF NW INDIANA. WITH THE CORE
OF THE SURFACE HIGH POSITIONED SOLIDLY TO OUR WEST, WE'LL
MAINTAIN PERSISTENT NORTHWESTERLY BREEZES (10-15 MPH WIND WITH
SOME GUSTS AROUND 20 MPH) INTO SUNDAY MORNING. THERE IS NOW
STRONG AGREEMENT ON WIND CHILLS DROPPING SOLIDLY INTO THE 15 TO
25 F BELOW ZERO RANGE OVERNIGHT SATURDAY INTO SUNDAY MORNING,
PERHAPS BRIEFLY A BIT COLDER IN SPOTS. AS A RESULT, WE'RE LIKELY
HEADED TOWARDS COLD WEATHER ADVISORIES FOR ALL OF THE FORECAST
AREA INTO SUNDAY MORNING.
IT LOOKS LIKE WE'LL STILL MAINTAIN A LITTLE WIND THROUGH THE
DAY ON SUNDAY AS TEMPERATURES STRUGGLE BACK THROUGH THE POSITIVE
SINGLE DIGITS. THIS WILL LIKELY MEAN WIND CHILLS WILL REMAIN
BELOW ZERO THROUGH THE DAY. THE CORE OF THE SURFACE HIGH IS NOW
FORECAST TO DRIFT JUST SOUTH OF THE REGION EARLY SUNDAY EVENING.
WHILE THIS SHOULD ALLOW TEMPERATURES TO PLUMMET INITIALLY,
SOUTHWESTERLY WINDS MAY GRADUALLY RETURN OVERNIGHT, POSSIBLY
RESULTING IN SLOWLY RISING TEMPERATURES INTO MONDAY MORNING. THE
NET RESULT OF THIS WILL LIKELY BE WIND CHILLS HOLDING MORE-OR-
LESS STEADY IN THE 10 TO 20 BELOW RANGE.
STRENGTHENING WARM ADVECTION ON MONDAY WILL HELP SEND TEMPERATURES
BACK INTO THE TEENS AND 20S. THANKFULLY, NOT SEEING ANY SIGNS OF
THIS INTENSIFYING WAA AND ATTENDANT ASCENT DRIVING ANY MEANINGFUL
MID-LEVEL SATURATION, WITH PRECIP-FREE CONDITIONS TO START THE WEEK.
MEDIUM RANGE GUIDANCE, TO VARYING DEGREES, DEPICTS A LOW-
AMPLITUDE DISTURBANCE IN THE GENERAL REGION TUESDAY NIGHT INTO
WEDNESDAY. INTENSIFYING SOUTHWESTERLY FLOW MAY HELP DRIVE AN
INCREASE IN LOW-LEVEL MOISTURE, WITH SOME GUIDANCE (GFS AND CMC
IN PARTICULAR) LOOKING LIKE THEY WANT TO SQUEEZE OUT SOME
DRIZZLE AT TIMES. AT THIS RANGE, THIS SIGNAL REMAINS A BIT
NEBULOUS, BUT AIR AND DEWPOINT TEMPERATURES LOOK TO BE WARMING
TO NEAR/ABOVE FREEZING.
AN ABRUPT TRANSITION TO QUASI-ZONAL FLOW IS ADVERTISED TOWARDS THE
END OF NEXT WEEK/NEXT WEEKEND WITH A STRONG SIGNAL THAT THE DEEP
ARCTIC AIRMASS WILL--AT LEAST BRIEFLY--GET SHOVED NORTH OF THE
REGION.
CARLAW
AVIATION /12Z TAFS THROUGH 18Z SATURDAY/
ISSUED AT 524 AM CST FRI DEC 12 2025
KEY MESSAGES:
- BRIEF PERIOD OF MVFR/IFR VSBYS/CIGS POSSIBLE AT RFD EARLY THIS
MORNING.
- MVFR CIGS DEVELOP THIS AFTERNOON.
- FROPA THIS EVENING WITH GUSTY NW WINDS. SMALL CHANCE (<30%
CURRENTLY) FOR SOME FLURRIES/LIGHT SNOW SHOWERS.
- POTENTIAL FOR -SN AT THE TAIL END OF THE EXTENDED ORD/MDW
TAFS.
POCKETS OF SHALLOW BR/FG HAVE DEVELOPED EARLY THIS MORNING
OUTSIDE OF CHICAGO. VSBY TRENDS AT RFD REMAIN A BIT UNCLEAR WITH
A SMALL PATCH OF 015-020 CIGS OVERHEAD ATTM. IF THESE SCATTER
OUT, VSBYS COULD DROP QUICKLY (FEP IS REPORTING FG CURRENTLY).
HAVE TEMPOD LOW-MVFR/NEAR IFR CONDITIONS AT RFD THROUGH 16Z, BUT
WILL CONTINUE TO MONITOR TRENDS. AT THE CHICAGO-AREA TERMINALS,
SOME SPOTTY HIGH-MVFR CIGS WILL LINGER FOR A BIT THIS MORNING.
MORE WIDESPREAD MVFR IS EXPECTED TO DEVELOP LATER THIS
AFTERNOON. A COLD FRONT WILL DELIVER A GUSTY NW WIND SHIFT THIS
EVENING. THERE IS A SMALL POTENTIAL FOR SOME FLURRIES OR -SHSN
DURING A BRIEF WINDOW ASSOCIATED WITH THE FROPA, BUT CHANCES
APPEAR TOO LOW TO WARRANT A MENTION IN THE TAFS AT THIS TIME.
THE STRONGEST GUSTS WILL EASE SATURDAY MORNING. ANOTHER
DISTURBANCE WILL BRING THE NEXT CHANCES FOR SNOW TO THE REGION
LATE SATURDAY MORNING AND THROUGH THE AFTERNOON. A SHARP
GRADIENT IN SNOWFALL APPEARS LIKELY, POSSIBLY IN THE VICINITY
OF THE CHICAGO-AREA TERMINALS. GIVEN UNCERTAINTIES ASSOCIATED
WITH SYSTEM TRACK AND DRY LOW-LEVEL AIR IN PLACE, HAVE ADDED A
PROB30 FOR MVFR -SN AT ORD, AND PREVAILED IT AT MDW AFTER 16Z.
LOW CIGS/VSBYS WILL LIKELY OCCUR JUST BEYOND THE END OF THE
EXTENDED TAF WINDOW.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO NOON CST SUNDAY
FOR WINTHROP HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY FROM 9 PM THIS EVENING TO 9 PM CST SUNDAY
FOR GARY TO BURNS HARBOR IN-BURNS HARBOR TO MICHIGAN CITY
IN.
VISIT US AT WEATHER.GOV/CHICAGO
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