706
FXUS63 KLOT 162340
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
540 PM CST MON DEC 16 2024
KEY MESSAGES
- POTENTIAL FOR LIGHT SNOW ACCUMULATIONS AND TRAVEL IMPACTS
TUESDAY NIGHT INTO EARLY WEDNESDAY MORNING, PRIMARILY NORTH
OF I-80.
- ANOTHER ROUND OF SNOW AND TRAVEL IMPACTS POSSIBLE THURSDAY
INTO EARLY FRIDAY.
DISCUSSION
ISSUED AT 230 PM CST MON DEC 16 2024
THROUGH WEDNESDAY NIGHT:
A COLD FRONT IS SLOWLY PASSING THROUGH THE FORECAST AREA THIS
AFTERNOON. WINDS ARE OUT OF THE SOUTHWEST WITH GUSTS 20 TO 25
MPH, THOUGH THEY ARE EXPECTED TO BECOME MORE WESTERLY IF NOT
NORTHWESTERLY INTO THE EVENING AS THE FRONT EXITS TO THE EAST.
THERE IS A WEAK BOUNDARY TRAVELING EAST OVER SOUTHWESTERN
WISCONSIN. MOST HIGH RES MODELS KEEP IT NORTH OF THE STATE
LINE. IT WOULD NOT BE SURPRISING IF SOME LIGHT DRIZZLE/RAIN
OCCURS ON AREAS JUST SOUTH OF THE STATELINE, BUT CONFIDENCE WAS
LOW ENOUGH TO NOT ADD IT TO THE OFFICIAL FORECAST AND KEEP POPS
AT LESS THAN 15 PERCENT.
QUIETER WEATHER CONDITIONS ARE EXPECTED THROUGH THE OVERNIGHT
AND INTO TUESDAY MORNING. A CLEAR SLOT IN THE CLOUD COVER IS
EXPECTED TO MOVE NORTHEASTWARD AND EXPAND OVER THE FORECAST AREA
REDUCING CLOUD COVER. HOWEVER, IT WILL BE SHORT LIVED AS CLOUDS
FILL BACK IN FROM THE NORTHWEST TUESDAY AFTERNOON AHEAD OF THE
NEXT SYSTEM.
A POSITIVELY TILTED TROUGH WILL DESCEND OUT OF CANADA TUESDAY
NIGHT THROUGH WEDNESDAY MORNING WITH A STRONG UPPER-LEVEL JET
SUPPORTING LARGE SCALE FORCING FOR ASCENT. COLD AIR ADVECTION
WITH THIS WAVE SHOULD ALLOW FOR A COLD ENOUGH THERMAL PROFILE
TO SUPPORT MAINLY SNOW, ESPECIALLY NORTH OF I-80. AS THE CAA
TIGHTENS THE LOW AND MID-LEVEL THERMAL GRADIENT, THE ASSOCIATED
F-GEN MAY POTENTIALLY LOCALLY ENHANCE PRECIP RATES FOR A TIME
LATE TUESDAY NIGHT.
GIVEN THE RUN-TO-RUN CONSISTENCY, POPS WERE MAINTAINED TO
MENTION THE "LIKELY" EXPECTATION OF PRECIPITATION. THE POTENTIAL
FOR FAIRLY NARROW BANDING COMPONENT CERTAINLY POINTS TOWARD
NORTH-SOUTH WIGGLE ROOM IN PLACEMENT OF AXIS OF LIGHT SNOW
ACCUMULATIONS. THAT SAID, THE LIKELY POP PLACEMENT ALIGNS WITH
THE HIGHEST ENSEMBLE POPS. LIMITING FACTORS WILL BE:
RELATIVELY PALTRY COLUMN MOISTURE, THE OVERALL PROGRESSIVE
PATTERN, AND NEAR TO SLIGHTLY ABOVE FREEZING SURFACE TEMPS (SLR
GENERALLY <10:1). ENSEMBLE PROBABILITIES OF >1" SNOW ACCUMS ARE
LESS THAN 10%, SO A COATING TO A HALF INCH IS THE CURRENT
APPARENT UPPER LIMIT, UNLESS MESOSCALE BANDING AIDS IN RELATIVE
OVER- PERFORMANCE. THE SNOW BAND(S) SHOULD SLIDE EAST AND EXIT
FAIRLY EARLY WEDNESDAY MORNING. MAIN TAKEAWAY HERE IS THAT WE'RE
MONITORING FOR SOME WINTRY TRAVEL IMPACTS INTO THE WEDNESDAY
MORNING COMMUTE NEAR AND NORTH OF I-80 AND PRIMARILY INLAND OF
THE LAKE IN NORTHERN ILLINOIS. DRIER CONDITIONS ARE PROJECTED
WEDNESDAY AFTERNOON AND EVENING AS TEMPERATURES BECOME COLDER
BEHIND THE SYSTEM.
DK
THURSDAY THROUGH MONDAY:
NEAR NORMAL HIGH TEMPERATURES ON WEDNESDAY WILL STEP DOWN TO
BELOW NORMAL ON THURSDAY FOLLOWING A RELATIVELY CHILLY WEDNESDAY
NIGHT. ATTENTION IN THIS PERIOD TURNS TO A VIGOROUS ALBERTA
CLIPPER TYPE WAVE THAT WILL LIKELY IMPACT PORTIONS OF THE REGION
THURSDAY AFTERNOON INTO EARLY FRIDAY. AS IS COMMON FOR THESE
LOW-AMPLITUDE FEATURES, PREDICTABILITY IS ON THE LOWER END OF
THE SPECTRUM GIVEN SUBSTANTIAL RUN TO RUN VARIABILITY AND INTRA-
ENSEMBLE MEMBER VARIANCE OF THE ALL IMPORTANT TRACK OF THE
CLIPPER'S SURFACE LOW PRESSURE REFLECTION. LITTLE TO NO SNOW
(TRACK TOO FAR NORTH) TO LIGHT-MODERATE SNOW ACCUMS AND TRAVEL
IMPACTS (INCLUDING TO THE FRIDAY AM COMMUTE) ARE BOTH PLAUSIBLE
OUTCOMES. CHANCE POPS IN THE 30-50% RANGE, HIGHEST NORTHEAST
2/3 OR SO OF THE CWA, APPEAR REASONABLE FOR NOW.
FOLLOWING THE PASSAGE OF THIS WAVE, A STRETCH OF BELOW NORMAL
TEMPERATURES IS IN STORE FRIDAY THROUGH THE WEEKEND AS A DEEP
TROUGH TEMPORARILY ESTABLISHES ITSELF OVER EASTERN NORTH
AMERICA. CHANCES FOR LAKE EFFECT SNOW MAY TAKE AIM AT PORTIONS
OF OUR NORTHWEST INDIANA COUNTIES, HOWEVER CONFIDENCE IN THIS
REMAINS LOW AT BEST CURRENTLY.
BAKER/CASTRO
AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 540 PM CST MON DEC 16 2024
LOW MVFR CIGS THIS EVENING.
GUSTY WESTERLY WINDS THIS EVENING.
AN AREA OF LOW MVFR CIGS WILL MOVE ACROSS THE TERMINALS THIS
EVENING, SCATTERING OUT FROM WEST TO EAST DURING THE MID TO LATE
EVENING. FEW/SCT HIGH MVFR LEVEL CLOUD COVER MAY CONTINUE
OVERNIGHT, ESPECIALLY NEAR THE IL/WI STATE LINE BUT CURRENT
TRENDS SUGGEST CIGS WILL SCATTER OUT. A PERIOD OF MID CLOUDS IS
EXPECTED TUESDAY AND THEN INCREASING MID CLOUDS ARE EXPECTED
TUESDAY NIGHT.
WESTERLY WINDS MAY GUST INTO THE 25-30KT RANGE EARLY THIS
EVENING AND THEN STEADILY DIMINISH WITH GUSTS LIKELY ENDING FOR
MOST LOCATIONS OVERNIGHT. WESTERLY WINDS WILL CONTINUE TUESDAY
WITH SPEEDS AROUND 10KTS. SPEEDS WILL DIMINISH QUICKLY WITH
SUNSET TUESDAY EVENING AS HIGH PRESSURE MOVES ACROSS THE AREA.
WINDS MAY BECOME LIGHT AND VARIABLE TUESDAY EVENING AND THEN
SHIFT LIGHT NORTHERLY LATE TUESDAY EVENING INTO EARLY WEDNESDAY
MORNING.
A WEAK SYSTEM MAY BRING SOME LIGHT PRECIPITATION TO NORTHERN IL
EARLY WEDNESDAY MORNING, PERHAPS LIGHT SNOW OR A RAIN/SNOW MIX.
TIMING APPEARS TO BE AFTER 00Z AT RFD AND AFTER 06Z AT ORD/MDW,
IF IT MATERIALIZES. WILL TAKE A CLOSER LOOK AT THIS FOR
POSSIBLE MENTION WITH THE 06Z TAFS. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 AM CST TUESDAY FOR WINTHROP
HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY UNTIL 9 AM CST TUESDAY FOR GARY TO BURNS
HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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