070
FXUS63 KLOT 061206
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
606 AM CST SAT DEC 6 2025
KEY MESSAGES
- A CLIPPER SYSTEM WILL DELIVER A SWATH OF SNOW TO THE AREA
TONIGHT INTO SUNDAY MORNING. A WINTER WEATHER ADVISORY HAS
BEEN ISSUED FOR A PORTION OF NORTHERN ILLINOIS WHERE SNOW
TOTALS IN THE 3-5" RANGE ARE EXPECTED. A WINTRY MIX ALSO
LOOKS INCREASINGLY LIKELY TO BE SEEN IN LOCATIONS SOUTH OF
I-80.
- LIGHT LAKE EFFECT SNOW MAY DEVELOP INTO PORTIONS OF NORTHWEST
INDIANA AND NORTHEAST ILLINOIS LATE SUNDAY THROUGH MONDAY
MORNING.
DISCUSSION
ISSUED AT 419 AM CST SAT DEC 6 2025
THROUGH SUNDAY:
THE MAIN CONCERN IN THE SHORT TERM PORTION OF THE FORECAST IS
THE LIKELIHOOD OF ACCUMULATING SNOW FROM A CLIPPER SYSTEM
TONIGHT INTO TOMORROW. THIS SYSTEM'S SURFACE LOW (ANALYZED OVER
NORTHEASTERN WYOMING AT PRESS TIME) WILL TRACK EAST-
SOUTHEASTWARD TODAY ACROSS THE CENTRAL PLAINS AND INTO THE
MIDWEST IN TANDEM WITH AN UPPER-LEVEL SHORTWAVE, LIKELY
WEAKENING OVER THE COURSE OF THE DAY AS IT LOSES BETTER UPPER-
LEVEL JET SUPPORT. THE EARLIER CHASM BETWEEN THE HI-RES MODEL
GUIDANCE DEPICTING A STRONGER AND MORE NORTHERLY TRACKING SYSTEM
AND GLOBAL GUIDANCE FAVORING A WEAKER AND MORE SOUTHERLY
TRACKING SYSTEM HAS SHRUNK CONSIDERABLY, WITH THE BROADER SUITE
OF MODEL GUIDANCE HONING IN ON A HAPPY MEDIUM SOLUTION WITH A
MORE NORTHERLY TRACK TO THE SYSTEM, BUT A LESS-AMPED UP OR ILL-
DEFINED SURFACE LOW. THE LINGERING MODEL SPREAD REGARDING THE
STRENGTH OF THIS INCOMING SYSTEM IS STILL, HOWEVER, LEADING TO
PERSISTING FINER-SCALE UNCERTAINTIES IN OUR SNOWFALL AND
PRECIPITATION TYPE FORECASTS LESS THAN 24 HOURS AWAY FROM THE
ONSET OF PRECIPITATION TONIGHT.
THE LARGER-SCALE PICTURE WITH THIS INCOMING SYSTEM REMAINS
FAIRLY CLEAR, WITH ISENTROPIC ASCENT INDUCING A SWATH OF
PRECIPITATION THAT WILL SPREAD OVER THE AREA LATE THIS EVENING
AND INTO THE OVERNIGHT HOURS BEFORE A CANADIAN SURFACE HIGH
FILTERS IN BEHIND THE CLIPPER AND PUTS AND END TO MOST OF THE
PRECIPITATION SUNDAY MORNING (LAKE EFFECT SNOW MAY PERSIST
BEYOND THIS TIME NEAR LAKE MICHIGAN). THERMAL PROFILES ARE, FOR
THE MOST PART, EXPECTED TO REMAIN BELOW 0C THROUGH MOST OR ALL
OF THE EVENT IN MOST LOCATIONS, RESULTING IN SNOW BEING THE
DOMINANT PRECIPITATION TYPE. HOWEVER, WITH THE MORE NORTHERLY
TRACK TO THE SYSTEM NOW, PARTS OF OUR SOUTHERN CWA APPEAR
INCREASINGLY LIKELY TO BE AN EXCEPTION TO THIS AS WARMING LOW-
LEVEL AND SURFACE TEMPERATURES LOOK TO PUSH THE LOWEST PORTION
OF THE THERMAL PROFILES THERE ABOVE THE FREEZING MARK FOR AT
LEAST A BRIEF PERIOD OF TIME. THIS WOULD RESULT IN A WINTRY MIX
OR EVEN OUTRIGHT RAIN OCCURRING THERE FOR A GOOD CHUNK OF THIS
PRECIPITATION EVENT, ESPECIALLY IN A SCENARIO FEATURING A
SURFACE LOW THAT'S STILL SOMEWHAT WELL-DEFINED AS IT TRACKS
ACROSS ILLINOIS.
THE PRESENCE/STRENGTH OF THE SURFACE LOW WILL ALSO MODULATE THE
MAGNITUDE OF LOW- TO MID-LEVEL FRONTOGENESIS OCCURRING
COINCIDENT WITH THE SNOW SWATH. SOME OF THE MORE AGGRESSIVE
LATEST MODEL SOLUTIONS, SUCH AS THE 00Z HRRR AND RAP, ACTUALLY
DEPICT FAIRLY INTENSE ASCENT WITHIN THE HEART OF THEIR MODELED
FRONTOGENETICAL BANDING, WITH OMEGA PEAKING WITHIN OR JUST BELOW
THE DENDRITIC GROWTH ZONE (WHICH WOULD FEATURE STEEP LAPSE
RATES THAT WOULD FURTHER ENHANCE SNOWFALL RATES). IF SUCH AN
OUTCOME WERE TO BE REALIZED, THEN THERE COULD BE A NARROW
CORRIDOR SOMEWHERE THAT OVERPERFORMS ACCUMULATION-WISE. HOWEVER,
THESE MODELS HAVE A TENDENCY TO RUN A LITTLE HOT WITH WINTRY
SYSTEMS 24+ HOURS OUT, SO THERE ARE REASONS TO BE SKEPTICAL OF
THEIR SNOWFALL OUTPUT.
REGARDLESS, THE LATEST MULTI-MODEL CONSENSUS SUGGESTS THAT
THERE IS A FAIRLY GOOD LIKELIHOOD OF LOCATIONS NORTH OF I-80
PICKING UP BETWEEN 2 AND 5 INCHES OF SNOW ACCUMULATION (HIGHEST
WHEREVER THE F-GEN BANDING SETS UP) TONIGHT THROUGH SUNDAY
MORNING. TEMPERATURES ARE EXPECTED TO REMAIN IN THE 20S HERE
WHILE SNOW IS FALLING, WHICH WILL ALLOW THE SNOW TO ACCUMULATE
ON ROADWAYS AND MAKE FOR A MESSY DRIVE FOR ANYONE WHO IS
PLANNING TO HEAD OUT EARLY SUNDAY MORNING. HAVE ISSUED A WINTER
WEATHER ADVISORY FOR OUR ILLINOIS COUNTIES NORTH OF I-80 TO
HIGHLIGHT THIS.
SNOW TOTALS FARTHER TO THE SOUTH AND EASTWARD INTO NORTHWEST
INDIANA ARE LESS CERTAIN DUE TO THE EXPECTATION THAT THE SYSTEM
WILL BE WEAKENING AND THE POTENTIAL FOR WARMING TEMPERATURES TO
CUT INTO SNOW TOTALS AND REDUCE THE OVERALL MAGNITUDE OF THE
IMPACTS THAT THE SNOW MAY CAUSE THERE. THAT SAID, IT'S POSSIBLE
THAT A SOUTHWARD EXPANSION OF THE ADVISORY MAY EVENTUALLY BE
WARRANTED IF THE SYSTEM WIGGLES A LITTLE FARTHER SOUTH FROM ITS
PRESENTLY FAVORED TRACK OR IF THE LIKELIHOOD OF FREEZING RAIN
ICING UP ROADWAYS NORTH OF THE I-74 CORRIDOR INCREASES.
OGOREK
SUNDAY NIGHT THROUGH FRIDAY:
A PERIOD OF LAKE EFFECT SNOW MAY DEVELOP IN THE WAKE OF THE
WEEKEND WINTER SYSTEM LATE SUNDAY INTO EARLY MONDAY MORNING AS
SURFACE HIGH PRESSURE MOVES INTO THE REGION. THIS WILL LEAD TO A
PERIOD OF NORTH NORTHEAST WINDS WHICH WOULD BE FAVORABLE FOR
THE DEVELOPMENT OF A LAKE EFFECT SNOW BAND ORIENTED INTO THE
CHICAGO METRO COUNTIES OF NORTHEAST ILLINOIS AND NORTHWEST
INDIANA. THE COMBINATION OF COOLER LAKE TEMPERATURES, OVERALL
LOWER INVERSION HEIGHTS (BARELY TOUCHING THE DGZ AROUND 5-6KFT)
AND A SHALLOWER CLOUD LAYER, SUGGESTS THAT SNOW RATIOS WILL LEAN
LOWER AND LIMIT ACCUMULATIONS OVERALL. HAVE OPTED TO MAINTAIN A
ROUGHLY 20-40% CHANCE OF LAKE EFFECT SNOW SHOWERS FROM SUNDAY
EVENING THROUGH MONDAY MORNING BUT HAVE KEPT QPF UNDER 0.1"
WHICH RESULTS IN ADDITIONAL SNOWFALL ACCUMULATIONS OF A DUSTING
TO LOCALLY UP TO AROUND 1".
THE ADVERTISED ACTIVE CLIPPER PATTERN CONTINUES WITH THE NEXT
DISTURBANCE MOVING ACROSS WISCONSIN MONDAY NIGHT INTO TUESDAY
MORNING, WHICH MAY GRAZE FAR NORTHERN ILLINOIS WITH SOME LIGHT
SNOW. CURRENT INDICATIONS ARE THAT ACCUMULATIONS MAY BE MAINLY
LIMITED TO A DUSTING TOWARD THE WI/IL STATE LINE. A STRONGER
SYSTEM (SUB 1000MB LOW) THEN ARRIVES RIGHT ON ITS HEELS TUESDAY
NIGHT INTO WEDNESDAY. THE WARM ADVECTIVE REGIME THAT SETS UP
AHEAD OF IT WILL SUPPORT TEMPERATURES WARMING INTO THE 30S AND
LIKELY ABOVE FREEZING HERE LOCALLY IF THE CURRENT FORECAST
TRACK HOLDS. BEFORE THIS OCCURS A WINTRY MIX OF PRECIPITATION
WOULD BE POSSIBLE BEFORE SWITCHING OVER TO MAINLY RAIN FOR MOST
IF NOT THE ENTIRE AREA. TEMPERATURES THEN QUICKLY DROP BACK
BELOW FREEZING WEDNESDAY MORNING IN THE WAKE OF THE TUESDAY
NIGHT SYSTEM WITH A CONTINUED SIGNAL FOR GUSTY SNOW SHOWERS
POSSIBLE MID MORNING INTO EARLY AFTERNOON WEDNESDAY. SOMETHING
TO KEEP AN EYE ON OVER THE COMING DAYS.
PERIODIC SNOW CHANCES THEN CONTINUE THROUGH THE END OF THE WEEK
(20-30%) AS THE ACTIVE CLIPPER WAVE TRAIN CONTINUES.
TEMPERATURES TREND COOLER FOR THE LATTER HALF OF THE WEEK WITH
FORECAST HIGHS IN THE TEENS AND 20S AND LOWS IN THE SINGLE
DIGITS TO LOWER TEENS.
PETR
AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 605 AM CST SAT DEC 6 2025
- LOW (15% CHANCE) FOR PATCHY -FZDZ OR FLURRIES BETWEEN 21-01Z
- ACCUMULATING SNOW EXPECTED TONIGHT INTO SUNDAY AM.
- A PERIOD OF LOW-END IFR TO LIFR CIGS AND VSBYS APPEARS LIKELY
DURING HIGHEST SNOWFALL RATES.
WHILE VFR CIGS MAY LINGER FOR THE NEXT HOUR OR SO, MVFR STRATUS
IS EXPECTED TO GRADUALLY FILL IN THROUGH THE MORNING AND
CONTINUE THROUGH THE EVENING. OCCASIONAL BREAKS IN THE CLOUDS
ARE POSSIBLE THOUGH CONFIDENCE ON IDENTIFYING A PARTICULAR TIME
WINDOW REMAINS LOW.
A BRIEF PERIOD OF PATCHY FREEZING DRIZZLE OR FLURRIES CAN'T BE
FULLY RULED OUT MID AFTERNOON INTO EARLY EVENING (~21-01Z).
HOWEVER, CONFIDENCE IN THIS OCCURRING AT THE TERMINALS REMAINS
TOO LOW TO INCLUDE WITH THIS UPDATE AS IT WOULD BE VERY
LOCALIZED/PATCHY IF IT EVEN DEVELOPS. THIS WILL BE MONITORED
CLOSELY THROUGH THE DAY.
LOOKING AHEAD TO TONIGHT, WIDESPREAD SNOW MOVES INTO THE AREA
FROM WEST TO EAST LATE IN THE EVENING AND CONTINUES THROUGH
EARLY SUNDAY MORNING. MEANWHILE A WINTRY MIX IS POSSIBLE SOUTH
OF I-80 AND THE TERMINALS. HAVE TRENDED MORE PESSIMISTIC WITH
CIGS AND VSBYS WITH THIS UPDATE, INCLUDING TEMPOS FOR LOW-END
IFR CIGS AND LIFR VSBYS DURING THE PERIOD OF HIGHEST SNOWFALL
RATES EARLY SUNDAY AM. IT IS POSSIBLE THESE CONDITIONS LINGER A
BIT LATER INTO THE MORNING ON SUNDAY AT ORD/MDW/GYY PAIRED WITH
THE POTENTIAL FOR LAKE EFFECT SHOWERS DEVELOPING ON THE BACK
SIDE OF THE SYSTEM MID-LATE MORNING (CURRENTLY ACCOUNTED FOR
WITH A PROB30 GROUP). FORECAST ACCUMULATIONS ARE GENERALLY IN
THE 2-4" RANGE FOR THE CHICAGO AREA TERMINALS AND 3-5" TOWARD
RFD.
WIND TRENDS: WNW WINDS TODAY WILL TURN LIGHT AND VARIABLE IN
THE AFTERNOON/EARLY EVENING BEFORE SETTLING INTO A SOUTHEAST
DIRECTION AS THE SNOW MOVES IN OVERNIGHT. WINDS THEN TURN
NORTHEAST WITH INCREASING GUSTS BY SUNDAY MORNING.
PETR
LOT WATCHES/WARNINGS/ADVISORIES
IL...WINTER WEATHER ADVISORY FROM 9 PM THIS EVENING TO 6 AM CST
SUNDAY FOR ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-ILZ019.
WINTER WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 9 AM CST
SUNDAY FOR ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ103-
ILZ104-ILZ105-ILZ106.
IN...NONE.
LM...NONE.
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