647
FXUS63 KLOT 180550
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1150 PM CST TUE DEC 17 2024
KEY MESSAGES
- BRIEF PERIOD OF SNOW EXPECTED OVERNIGHT INTO EARLY WEDNESDAY
MORNING WITH POSSIBLE TRAVEL IMPACTS FOR THE EARLY WEDNESDAY
MORNING COMMUTE, MAINLY ALONG AND NORTH OF I-88.
- ANOTHER ROUND OF SNOW AND TRAVEL IMPACTS POSSIBLE THURSDAY
INTO FRIDAY MORNING, PARTICULARLY FOR AREAS NORTH OF I-80.
- PERIODS OF LAKE EFFECT SNOW POSSIBLE FRIDAY AFTERNOON THROUGH
SATURDAY NIGHT MAY INCLUDE PARTS OF NORTHEAST IL AT TIMES.
UPDATE
ISSUED AT 936 PM CST TUE DEC 17 2024
THE MAIN FORECAST CONCERN INTO EARLY WEDNESDAY MORNING COMMUTE
CONTINUES TO BE THE THREAT FOR QUICK HITTING BAND OF ACCUMULATING
SNOW ACROSS FAR NORTHERN IL. THIS BAND IS QUICKLY ORGANIZING
JUST WEST AND NORTHWEST OF OUR AREA EARLY THIS EVENING, RIGHT
ALONG A STRENGTHENING BAND OF FRONTOGENESIS CENTERED NEAR 700 MB.
THE EXPECTATION IS FOR THIS BAND OF SNOW TO CONTINUE DEVELOPING
EASTWARD INTO FAR NORTHERN IL (AREAS MAINLY RIGHT ALONG AND NEAR
THE WI STATE LINE) THROUGH MIDNIGHT. THEREAFTER, WE SHOULD SEE THE
SOUTHWEST-TO-NORTHEAST ORIENTED PORTION OF THE BAND BEGIN TO
SLIDE EASTWARD ACROSS FAR NORTHERN IL OVERNIGHT (ESPECIALLY AREAS
NORTH OF THE I-88 CORRIDOR). THIS IS STILL EXPECTED TO BE A FAST
MOVING BAND OF SNOW, LIKELY ONLY LASTING 2 TO 4 HOURS AT ANY GIVEN
LOCATION. WHILE THE PROGRESSIVE NATURE OF THE BAND WILL LIMIT
TOTAL SNOW ACCUMULATIONS TO GENERALLY UNDER AN INCH, THE RATE AT
WHICH IT FALLS OVERNIGHT MAY RESULT IN ROADS QUICKLY BECOMING SNOW
COVERED AND SLICK ACROSS PARTS OF FAR NORTHERN IL.
THE ONLY REAL CHANGE TO THE GOING FORECAST WAS TO PUSH THE SNOW
AMOUNTS (OF UP TO AN INCH) A BIT FARTHER NORTH, ESPECIALLY
NORTH OF I-88. THIS IS WHERE THE PRECIPITATION IS EXPECTED TO
REMAIN ALL SNOW. FARTHER SOUTH, LIGHTER PRECIPITATION AND
SLIGHTLY WARMER LOW-LEVEL TEMPERATURES COULD SUPPORT A MIX OF
LIGHT SNOW AND RAIN/FREEZING RAIN, THUS LIMITING AMOUNTS TO A
LIGHT COATING. THE BAND OF PRECIPITATION IS EXPECTED TO QUICKLY
SWEEP EAST OF THE AREA EARLY WEDNESDAY MORNING.
KJB
DISCUSSION
ISSUED AT 256 PM CST TUE DEC 17 2024
THROUGH WEDNESDAY:
EARLY THIS AFTERNOON, WE FIND A TIGHTENING AREA OF SURFACE LOW
PRESSURE COMING OFF THE LEE OF THE ROCKIES. ALONG ITS NORTHERN
FRINGES IS A LARGE REGION OF LOW-MID LEVEL FRONTOGENESIS AS A
SPEEDY MERIDIONAL LOW LEVEL JET/WARM CONVEYOR BELT UNDERCUTS
DIVERGENT ZONAL FLOW ON THE TAIL END OF A MID LEVEL JET MAX. A
LARGE SWATH OF MOSTLY LIGHT, SOME MODERATE, SNOW COINCIDES WITH
THE REGION OF F-GEN. THE BULK OF THE FRONTOGENETIC FORCING EXISTS
NEAR THE IA/NE/MN TRIPOINT AT THE TIME THIS WAS WRITTEN (ROUGHLY 1
PM), BUT EXTENDS AS FAR EAST AS EASTERN IA WITH SOME LIGHT SNOW
BEING REPORTED ACROSS PORTIONS OF NORTH-CENTRAL IA. THIS REGION OF
F-GEN WILL TRANSLATE EASTWARD AND MOVE OVER NORTHERN IL LATE THIS
EVENING AND OVERNIGHT.
THERE REMAINS A FAIR DEAL OF DISCREPANCY AMONG GUIDANCE
REGARDING THE STRENGTH OF THE F-GEN FORCING TONIGHT. THE
CANADIAN AND ESPECIALLY GFS HAVE CONSISTENTLY BEEN THE LEAST
EXCITED ABOUT THIS FEATURE AND OFFER UP THE LEAST QPF ACROSS
GUIDANCE. EVEN THEIR RESPECTIVE ENSEMBLE SYSTEMS ARE HEAVILY
CLUSTERED AROUND GENERALLY LOWER QPF AND SNOW AMOUNTS THAN OTHER
GUIDANCE. MOST OF THE HIGH-RES CAMPS, HOWEVER, ARE MUCH MORE
AGGRESSIVE WITH THE FORCING AND, IN TURN, THE SNOWFALL. THIS IS
A TYPICAL FINDING IN SUCH ENVIRONMENTS WITH THE COARSER MEDIUM-
RANGE GUIDANCE OFTEN WINDING UP UNDERPLAYING THE MAGNITUDE OF
THE F-GEN. THEREFORE, MORE EGGS WERE THROWN INTO THE HIGH-RES
MODEL BASKET WHEN BUILDING THIS FORECAST.
WE SHOULD FIND PRECIP STARTING TO MOVE INTO OUR WEST LATE THIS
EVENING AND MOVE ACROSS THE CWA THROUGH THE NIGHT AND INTO EARLY
WEDNESDAY, WHICH LEAVES CONCERNS REGARDING THE MORNING COMMUTE.
A BIG MAJORITY OF THE PRECIP WE SEE TONIGHT WILL BE IN THE FORM
OF SNOW, BUT IT'S POSSIBLE THAT A LITTLE RAIN MIXES IN DURING
ONSET AS WE WAIT FOR THE LOW LEVELS TOO COOL OFF SOME. THE
SNOWFALL LOOKS TO BE GENERALLY ON THE LIGHTER SIDE, BUT A MORE
BONAFIDE BAND OF MODERATE SNOW WITH RATES NEAR 0.5"/HR CERTAINLY
SEEMS PLAUSIBLE WITH THIS F-GEN SIGNATURE. THIS WOULD BE ENOUGH
TO COAT EVEN THE WARMEST OF ROADS AND LEAD TO ADVERSE TRAVEL
CONDITIONS DURING THE MORNING COMMUTE. LUCKILY, THE BULK OF THE
SNOW AND ESPECIALLY THE HEAVIER SNOW POTENTIAL SHOULD BE MOVING
OUT OF THE CHICAGO METRO RIGHT AS THE MORNING COMMUTE REALLY
TAKES OFF BETWEEN 5 AND 6 AM. NONETHELESS, SOME ROADS MAY REMAIN
SLICK WELL INTO THE MORNING. SNOW TOTALS ARE LOOKING LIKE
ANYWHERE FROM A FEW TENTHS TO AROUND AN INCH, ALTHOUGH WOULD NOT
BE SHOCKED TO SEE A SWATH OF 1-2" TOTALS IF THIS BAND PERFORMS
TO ITS POTENTIAL. MODELS ARE GENERALLY FAVORING HIGHER
ACCUMULATIONS NORTH OF I-88 AND TOWARD THE WI STATE LINE.
FORECAST SOUNDINGS SUGGEST LOCALIZED POCKETS OF FREEZING RAIN
MAY ALSO BE IN THE CARDS, ESPECIALLY TOWARD THE END OF THE
EVENT AS THE MID LEVELS START TO DRY WHICH COULD EAT AWAY AT
CLOUD GLACIATION. SURFACE TEMPERATURES ARE FORECAST TO BOTTOM
OUT JUST BELOW FREEZING FOR AREAS GENERALLY NORTH OF I-80 AND
WEST OF I-355 AND NEAR OR ABOVE FREEZING ELSEWHERE. HOWEVER, IT
LOOKS AS THOUGH MOST OF THE PRECIP SHOULD COME AND GO PRIOR TO
TEMPERATURES DROPPING BELOW FREEZING AND, COMING OFF A SOMEWHAT
MILD TUESDAY, ROAD TEMPERATURES SHOULD LAG THE AMBIENT AIR
TEMPERATURE IN FALLING BELOW FREEZING. BASICALLY, THIS IS TO SAY
THAT FREEZING RAIN IMPACTS, IF ANY, LOOK TO BE VERY LOCALIZED
AND LIKELY TO RESTRICTED TO RURAL AND LESS- TRAVELED ROADS
OUTSIDE OF THE CHICAGO METRO. THIS POTENTIAL MAY JUST END UP
BEING A PERIOD OF DRIZZLE TO WRAP THINGS UP IN THE MORNING.
HOWEVER, WITH THE LOW LEVELS EXPECTED TO DRY OUT QUICKLY DURING
THE EARLY MORNING, THIS SHOULD BE A SHORT-LIVED POTENTIAL.
WE MAY SEE A TRANSITION TO MORE OF A MIX OR TO ALL RAIN NEAR AND
OVER NORTHWEST INDIANA AFTER SUNRISE AS THIS STUFF STILL WORKS
OUT OF THE AREA. WE SHOULD BE PRECIP-FREE BY THE END OF THE
MORNING WITH HIGH PRESSURE MOVING IN FROM THE WEST EATING AWAY
AT CLOUD COVER DURING THE AFTERNOON. TOMORROW WILL BE SEVERAL
DEGREES COOLER THAN TODAY WITH HIGHS PROGGED AT OR JUST ABOVE
FREEZING.
DOOM
WEDNESDAY NIGHT THROUGH TUESDAY:
THURSDAY THROUGH TUESDAY...
DISCUSSION LARGELY UNCHANGED FROM THE OVERNIGHT SHIFT. THE FOCUS
REMAINS ON SEVERAL PERIODS OF SNOW LATE THURSDAY INTO FRIDAY
AND AGAIN ON SATURDAY, WITH NO MAJOR SHIFTS IN FORECAST
GUIDANCE.
THERE REMAINS FAIRLY GOOD AGREEMENT IN A VIGOROUS CLIPPER TYPE
WAVE MOVING ACROSS THE UPPER MS VALLEY AND WESTERN GREAT
LAKES. A DISTINCT MAJORITY OF MOST RECENT OPERATIONAL MODELS AND
ENSEMBLE MEMBERS GENERALLY FAVOR NORTH OF OUR AREA WHERE THE
BETTER MID LEVEL MOISTURE IS AVAILABLE FOR THE HIGHEST POTENTIAL
FOR A FEW TO SEVERAL INCHES OF SNOW ACCUMULATION. HOWEVER,
GIVEN THE STRONGEST 500 MB FORCING WITH THE PARENT SHORT-WAVE
PUSHING ACROSS ILLINOIS, THERE STILL APPEARS TO BE MEANINGFUL
WIGGLE ROOM IN THE TRACK OF THE CLIPPER. EVEN IN THE CURRENT
GENERAL CONSENSUS, NORTH-SOUTH DIFFERENCES IN THE EXACT TRACK OF
THE WEAK SURFACE LOW ARE IMPORTANT.
POPS ARE INTRODUCED IN THE FORECAST IN THE 20-30% RANGE INTO
FAR NORTHERN ILLINOIS AS EARLY AS THURSDAY MORNING AS THE SYSTEM
STARTS TO MOVE OVER THE STATE. HOWEVER, THERE IS LOWER
CONFIDENCE IN ONSET TIME IF THE COLUMN IS STILL BE TOO DRY FOR
WARM ADVECTION SNOW. IF THE CLIPPER SURFACE LOW TRACKS FAR
ENOUGH SOUTH, THE ACCUMULATING SNOW BAND(S) IN WISCONSIN COULD
CONCEIVABLY PIVOT INTO AREAS NORTH OF I-88 LATE THURSDAY
AFTERNOON AND THURSDAY EVENING, WHERE OUR GRIDS HAVE SOME POPS
UP TO LIKELY/60% AND THESE HAVE BEEN MAINTAINED IN THE AFTERNOON
FORECAST PACKAGE. CONCEPTUALLY, SCATTERED SNOW SHOWERS MAY
OCCUR FARTHER SOUTH NEAR THE CORE OF THE MID- LEVEL PVA AS LAPSE
RATES STEEPEN FROM COLD (NEAR -30C AT 500 MB) TEMPS ALOFT.
LATEST 12Z GUIDANCE KEPT THE TRACK SIMILAR TO PREVIOUS RUNS, BUT
SHOWS A SLIGHTLY WEAKER LOW.
LATER THURSDAY NIGHT INTO FRIDAY MORNING, IN A STRENGTHENING
NORTHERLY COLD ADVECTION REGIME ON THE BACK SIDE OF THE CLIPPER,
AREAS OF SNOW/SNOW SHOWERS SHOULD EXTEND FARTHER SOUTH. WE'LL
ALSO HAVE TO MONITOR FOR LAKE ENHANCEMENT FROM EASTERN WI
SOUTHWARD INTO NORTHEAST ILLINOIS. MAIN TAKEAWAY IS THAT THE
CURRENT BEST CHANCE FOR TRAVEL IMPACTS IN ANY PORTION OF OUR
AREA APPEARS TO BE THE THURSDAY EVENING COMMUTE THROUGH THE
FRIDAY MORNING COMMUTE.
EXPANSIVE NEAR 1040 MB HIGH PRESSURE WILL SLOWLY BUILD IN FROM
THE NORTHWEST FRIDAY THROUGH SATURDAY, TYPICALLY A FAVORABLE
SETUP FOR LAKE EFFECT SNOW ON SOUTHERN LAKE MICHIGAN. 850 MB
TEMPS FORECAST TO GET DOWN INTO THE -12 TO -15 CELSIUS RANGE
SUGGEST FAVORABLE THERMODYNAMICS AND LAKE INDUCED INSTABILITY
GIVEN LAKE TEMPS STILL IN THE 40S. MODEL FORECAST SOUNDINGS ALSO
INDICATE SUFFICIENT MOISTURE AND INVERSION HEIGHTS TO SUPPORT
HIGHER SNOWFALL RATES. THE BIGGEST UNANSWERABLE QUESTION AT THIS
POINT IS EXACTLY HOW BOUNDARY LAYER CONVERGENCE SETS UP,
DICTATING THE TRAJECTORY OF LAKE EFFECT BAND(S). BASED ON OUR
FORECAST WIND DIRECTIONS, NORTHWEST INDIANA MAY BE MORE FAVORED
FRIDAY AFTERNOON THROUGH EARLY SATURDAY, POSSIBLY MIGRATING
WESTWARD INTO NORTHEAST IL AND FAR NW INDIANA ON SATURDAY-
SATURDAY NIGHT. ANOTHER WILDCARD FOR THE LAKE EFFECT IS THE
INFLUENCE OF MESOLOW DEVELOPMENT, IF THAT OCCURS.
ASIDE FROM THE LAKE EFFECT POTENTIAL, CAN'T COMPLETELY RULE OUT
A WEAKENING BAND OF SNOW APPROACHING OUR WESTERN CWA SATURDAY
MORNING. OTHERWISE, THE BIGGER STORY WILL BE A STRETCH OF BELOW
NORMAL TEMPERATURES THROUGH THE WEEKEND. THIS LOOKS TO BE SHORT-
LIVED, WITH VERY STRONG MEDIUM-RANGE GUIDANCE AGREEMENT IN A
PATTERN CHANGE TO MILDER CONDITIONS FOR THE WEEK OF CHRISTMAS.
ENSEMBLE GUIDANCE IS SUGGESTING THAT AT SOME POINT ANOTHER
DISTURBANCE WILL MOVE WEST TO EAST ON OR AROUND CHRISTMAS EVE.
BUT WITH SEVERAL MEMBERS DISAGREEING ON TIME AND LOCATION, IT
IS HARD TO HAVE CONFIDENCE TO SAY HOW WET THINGS WOULD BE
CASTRO/DK
AVIATION /06Z TAFS THROUGH 12Z THURSDAY/
ISSUED AT 1150 PM CST TUE DEC 17 2024
FORECAST CONCERNS INCLUDE:
- PERIOD OF SNOW WITH IFR TO LIFR VSBY, POSSIBLE IFR CIGS
OVERNIGHT.
- MVFR CIGS WEDNESDAY.
- GUSTY NORTHWEST WINDS WEDNESDAY AFTERNOON.
A NARROW BAND OF SNOW, MODERATE TO HEAVY AT TIMES, EXTENDS FROM
EASTERN IA INTO NORTHWEST IL AND WILL CONTINUE MOVING EAST-
SOUTHEAST ACROSS THE AREA INTO EARLY WEDNESDAY. AT RFD,
EXPECTING 1-2 HOURS OF TEMPORARY LIFR VSBY AND IFR CIGS, WITH A
BIT LOWER CONFIDENCE DOWNSTREAM. LIGHT SNOW/FLURRIES WILL SOON
START AHEAD OF THE STEADY LIGHT SNOW AT DPA AND ORD. SKEPTICAL
OF THE NEAR TERM MODEL GUIDANCE THAT SHOWS A STEADY WEAKENING
TREND IN THE BAND AS IT MOVES ACROSS NORTHEAST IL IN THE PREDAWN
HOURS. GIVEN THE CURRENT RADAR PRESENTATION, MADE NO SIGNIFICANT
CHANGES FOR THE CHICAGO METRO SITES, THOUGH DID INDICATE A BIT
LOWER VSBY AT DPA AND ORD.
THE SOUTHERN EDGE OF THE PRECIPITATION, ALONG A VYS/LOT/MDW
LINE, MAY MIX WITH SOME LIGHT RAIN, WHICH MAY KEEP VISIBILITIES
A BIT HIGHER. THE PRECIPITATION IS EXPECTED TO END ACROSS THE
CHICAGO AREA SHORTLY AFTER DAYBREAK. PREVAILING CIGS ARE
EXPECTED TO REMAIN MVFR THOUGH BRIEF IFR CIGS WOULD BE POSSIBLE
IF HEAVIER SNOW PERSISTS INTO THE CHICAGO METRO. MVFR CIGS ARE
THEN EXPECTED TO PERSIST INTO WEDNESDAY AFTERNOON, POSSIBLY
SCATTERING OUT WEDNESDAY EVENING.
LIGHT WINDS OVERNIGHT, FAVORING NORTHWEST TO WEST, WILL BECOME
PREVAILING NORTHWEST WEDNESDAY MORNING AT 10-15 KT, WITH GUSTS
INTO THE LOWER 20KT RANGE WEDNESDAY AFTERNOON. GUSTS ARE
EXPECTED TO DIMINISH WITH SUNSET WEDNESDAY EVENING.
CMS/CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 3 PM TO 10 PM CST WEDNESDAY FOR GARY
TO BURNS HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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