802  
FXUS63 KLOT 162030  
AFDLOT  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL  
230 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 /18Z TAFS THROUGH 00Z WEDNESDAY/
 
 
ISSUED AT 1116 AM CST MON DEC 16 2024  
 
MAIN AVIATION CONCERNS FOR THE PERIOD:  
 
- CEILINGS AND VISBY'S IMPROVING INTO MIDDAY. SLOW LIFTING AND  
SFC CLEARING THROUGH THE AFTERNOON.  
 
- WINDS VEER FROM SW TO W/WNW BEHIND FROPA THIS AFTERNOON, WITH  
INCREASING GUSTS IN THE 20-25KT RANGE THROUGH THE EVENING.  
 
CEILINGS AND VISBY'S ACROSS THE MAIN METRO TERMINALS HAVE  
FINALLY BEGUN TO IMPROVE HEADING INTO MIDDAY. EXPECTING RURAL  
TERMINALS TO HOLD IFR CONDITIONS INTO THE AFTERNOON. AS THE  
MAIN FRONTAL PUSH OCCURS HERE OVER THE NEXT FEW HOURS, THERE IS  
THE POTENTIAL FOR A BRIEF DROP BACK TO LIFR BEHIND THE FRONT,  
THEREFORE CONTINUING THE TEMPO GROUP THROUGH 20Z. AFTER THAT,  
FORECAST CALLS FOR A LIFT OUT OF IFR CONDITIONS TO MVFR THIS  
AFTERNOON, ALONG WITH A RAMP UP IN W'RLY WIND GUSTS TO 25KTS.  
 
GUSTS FALL OFF OVERNIGHT, WHILE THE W'RLY WINDS PREVAIL  
THROUGHOUT THE REMAINDER OF THE TAF PERIOD. CEILINGS OVERNIGHT  
WILL SCATTER AND LIFT, WITH ADDITIONAL CLEARING THROUGH  
TUESDAY. ANTICIPATING SOME RETURN OF MID AND HIGH CLOUDS  
TUESDAY AFTERNOON, HOWEVER NOT EXPECTED TO BE IMPACTFUL.  
 
BAKER/KMD  
 

 
   
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.  
 

 
 

 
 
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