152
FXUS63 KLOT 151200
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
600 AM CST SUN DEC 15 2024
KEY MESSAGES
- AREAS OF DENSE FOG THROUGH MIDDAY TODAY, ESPECIALLY FOR
LOCATIONS NEAR AND NORTHWEST OF I-55. LINGERING PATCHY FOG OR
REDEVELOPMENT OF FOG TONIGHT COULD BE DENSE IN SPOTS NEAR AND
NORTH OF I-80.
- TREND TOWARD COLDER, MORE SEASONABLE WEATHER CONDITIONS MID-
WEEK AND BEYOND. LIGHT SNOW ACCUMULATIONS ARE POSSIBLE TUESDAY
NIGHT INTO EARLY WEDNESDAY, MAINLY ACROSS FAR NORTHERN IL.
DISCUSSION
ISSUED AT 324 AM CST SUN DEC 15 2024
THROUGH MONDAY:
OUR MAIN FORECAST CONCERN TODAY PERTAINS TO VISIBILITY TRENDS IN
AREAS OF DENSE FOG. THE NEAR-TERM SIGNAL IN VISIBILITY GUIDANCE
AND WITH UPSTREAM OBSERVATIONAL TRENDS REMAINS STRONG FOR
CURRENT 3/4 TO 3 MILE VISIBILITIES AS OF THIS WRITING IN LIGHT
FOG/DRIZZLE TO BUILD DOWN TO AREAS OF 1/4 MILE OR LESS TOWARD
AND AFTER SUNRISE. THE FILLING WEAK SURFACE LOW RESPONSIBLE FOR
WIDESPREAD PRECIPITATION YESTERDAY AND OVERNIGHT WILL LIFT
NORTH-NORTHEAST AND FADE, LEAVING A LIGHT SOUTHERLY GRADIENT IN
ITS WAKE. WINDS ARE STILL IN THE PROCESS OF COMING DOWN AND DEW
POINT SPREADS OF 1-3F IN SPOTS ARE EXPECTED TO GO DOWN TO NIL IN
MOST LOCATIONS.
GIVEN THE DENSE FOG SIGNAL, AND THE SYNOPTIC SETUP BECOMING
MORE FAVORABLE, OPTED TO "FORECAST" A DENSE FOG ADVISORY FOR
COUNTIES/ZONES NEAR AND NORTHWEST OF THE I-55 CORRIDOR, IN
EFFECT UNTIL NOON CST. SINCE THIS IS A FORECASTED ADVISORY, IT'S
POSSIBLE THAT PORTIONS OF THE ADVISORY ARE SPARED FROM THE WORST
VISIBILITY. THE DAY SHIFT WILL BE ABLE TO ASSESS THE NEED FOR
ANY CHANGES ONCE OBSERVATIONAL TRENDS HAVE BEEN ESTABLISHED.
SOUTH-SOUTHWESTERLY FLOW ABOVE THE INVERSION INCREASING TO
10-15 KT THIS AFTERNOON SHOULD ENABLE VISIBILITY TO IMPROVE.
HOWEVER, WITH VERY MINIMAL DIURNAL CLIMB IN TEMPS AND DEW POINTS
UNDERNEATH SOCKED IN LOW STRATUS, WOULDN'T BE SURPRISED IF
PATCHY DENSE FOG PERSISTS FOR A BIT AFTER THE PLANNED EXPIRATION
OF THE ADVISORY. THIS EVENING INTO THE OVERNIGHT, BOUNDARY LAYER
SOUTHERLY FLOW SHOULD REMAIN LIGHT ENOUGH FOR CONTINUED PATCHY
FOG NEAR/NORTH OF I-80, THOUGH EVEN STRONG SOUTHWESTERLY FLOW
ALOFT IS EXPECTED TO LIMIT THE THREAT FOR DENSE FOG.
STRENGTHENING WARM AIR ADVECTION AND LARGE SCALE ASCENT IN
ADVANCE OF AN APPROACHING UPPER TROUGH SHOULD LEAD TO A
BLOSSOMING AREA OF SHOWERS TO EXPAND/MOVE NORTHEASTWARD ACROSS
THE AREA VERY LATE THIS EVENING AND MORESO OVERNIGHT. TEMPS
TONIGHT SHOULD START OUT NEARLY STEADY, THEN BEGIN RISING
OVERNIGHT AS WAA RAMPS UP AND A WARM FRONT LIFTS NORTHWARD.
SHOWER COVERAGE WILL BE HIGHEST (POPS ~60-80%) NEAR AND
SOUTHEAST OF I-55 IN THE PRE-DAWN HOURS. A RELATIVELY QUICK
PROGRESSION OF THE UPPER TROUGH WILL RESULT IN LINGERING SHOWERS
I-57 AND SOUTHEAST TO END DURING MONDAY MORNING. TEMPERATURES
WILL RISE THROUGH THE COLD FRONTAL PASSAGE AROUND MIDDAY MONDAY.
THE RESULT WILL BE WELL ABOVE NORMAL HIGHS IN THE 50-55F RANGE
NEAR AND SOUTHEAST OF I-55, AND GENERALLY MID TO UPPER 40S
NORTHWEST. BREEZY WESTERLY WINDS WILL GUST UP TO 30 MPH (LOCALLY
TO 35 MPH) MONDAY AFTERNOON BEHIND THE COLD FRONT AS COOLER AIR
FILTERS IN.
CASTRO
MONDAY NIGHT THROUGH SATURDAY:
A PROGRESSIVE, SEASONABLY MILD WEATHER PATTERN IS EXPECTED TO
PERSIST INTO MID-WEEK, BEFORE A MORE AMPLIFIED SHIFT DEVELOPS
LATER IN THE WEEK AND NEXT WEEKEND. THIS WILL LIKELY RESULT IN A
SHIFT TOWARD MORE SEASONABLE TO SLIGHTLY COLDER THAN AVERAGE
TEMPERATURES THURSDAY THROUGH SATURDAY, WITH INTERMITTENT LIGHT
PRECIPITATION CHANCES FROM A SERIES OF MID-LEVEL WAVES.
IN THE WAKE OF MONDAY'S PACIFIC COLD FRONTAL PASSAGE, BREEZY,
DRY AND COOLER BUT STILL SOMEWHAT MILD CONDITIONS ARE EXPECTED
MONDAY NIGHT INTO TUESDAY. HIGHS TUESDAY ARE FORECAST TO REACH
THE UPPER 30S TO LOWER 40S, WITH WINDS GRADUALLY DIMINISHING BY
AFTERNOON. PRECIPITATION CHANCES THEN INCREASE TUESDAY NIGHT, IN
ASSOCIATION WITH A POSITIVE-TILT MID-LEVEL SHORT-WAVE TRACKING
ACROSS THE AREA INTO WEDNESDAY. MOISTURE RETURN LOOKS RATHER
UNIMPRESSIVE WITH THIS SYSTEM, THOUGH FORCING FOR ASCENT AND
SOME INCREASE IN UPPER LEVEL DIVERGENCE BENEATH THE RIGHT
ENTRANCE REGION TO A ~120 KT UPPER LEVEL JET STREAK DOES LEAD TO
GUIDANCE PRODUCING SOME LIGHT (LESS THAN 0.10") QPF TUESDAY
NIGHT INTO WEDNESDAY MORNING AS THE WAVE AND ASSOCIATED COLD
FRONT MOVE THROUGH. THOUGH SOME DIFFERENCES IN PLACEMENT REMAIN
BETWEEN THE GLOBAL MODELS/ENSEMBLES, FORECAST THERMODYNAMIC
PROFILES GENERALLY SUPPORT RAIN CHANGING TO SNOW ACROSS PARTS OF
NORTH CENTRAL IL (WITH SOME LIGHT ACCUMS OF A FEW TENTHS OF AN
INCH) WEDNESDAY NIGHT, WITH MORE OF A RAIN/SNOW MIX FARTHER
SOUTH AND INTO NORTHWEST INDIANA WHERE LOW-LEVEL TEMPS AND WET
BULBS ARE FORECAST TO BE MORE MARGINAL. GREATER PRECIPITATION
POTENTIAL DURING THIS PERIOD APPEARS FAVORED FARTHER SOUTH INTO
THE LOWER OHIO VALLEY, CLOSER TO THE TRACK OF A WEAK SURFACE
WAVE ALONG THE TRAILING PORTION OF THE OLD PACIFIC COLD FRONT.
SOME PHASING OF THIS SHORT WAVE, AND ANOTHER ROTATING ACROSS
THE NORTHERN GREAT LAKES ON WEDNESDAY/WEDNESDAY NIGHT, IS
PROGGED TO LEAD TO AMPLIFICATION OF A LONG-WAVE UPPER TROUGH
OVER THE EASTERN NOAM/CONUS. COMBINED WITH A BUILDING UPPER
RIDGE OVER THE WEST COAST IS EXPECTED TO LEAD TO A COLDER TREND
FOR THE MIDWEST FOR THE REMAINDER OF THE WEEK AND THE WEEKEND.
NBM TEMPS LOCALLY DIP BELOW AVERAGE BY THE WEEKEND, WITH HIGHS
IN THE UPPER 20S FORECAST BY SATURDAY. ECMWF/GFS/CMC ENSEMBLES
ALL DEPICT ANOTHER MID-LEVEL SHORT WAVE TRANSITING THE NORTHERN
PLAINS AND UPPER MIDWEST IN THE THURSDAY THROUGH THURSDAY NIGHT
TIME FRAME WHICH MAY POSE AN ADDITIONAL CHANCE FOR SOME LIGHT
SNOW ESPECIALLY JUST NORTH OF THE CWA ACROSS THE UPPER MIDWEST.
ADDITIONAL VORT LOBES ARE DEPICTED ROTATING ACROSS THE UPPER
MIDWEST/GREAT LAKES REGION INTO THE WEEKEND, AND AGAIN WHILE
BEST LIGHT SNOW CHANCES REMAIN JUST OFF TO THE NORTH/NORTHEAST
OF THE FORECAST AREA, WE'LL CONTINUE TO MONITOR THAT POTENTIAL
OVER THE DOMING DAYS.
RATZER
AVIATION /12Z TAFS THROUGH 18Z MONDAY/
ISSUED AT 600 AM CST SUN DEC 15 2024
MAIN CONCERN:
- DENSE FOG THIS MORNING, WITH LOW CIGS AND REDUCED VSBY
CONTINUING INTO OR THROUGH MONDAY MORNING
LIFR CIGS ARE IN PLACE ACROSS THE AREA, WITH VSBY GENERALLY
DETERIORATING FROM WEST TO EAST, YIELDING FAIRLY WIDESPREAD 1/2
TO 1/4SM VSBY NEAR AND NORTHWEST OF I-55 THROUGH THE LATE MORNING.
THE 1-2SM VSBY AT ORD AND MDW AS OF THIS WRITING IS TREND
DOWNWARD TO 1SM OR LESS, ALONG WITH TEMPORARY 1/4 TO 1/2SM VSBY.
THERE WILL LIKELY BE SOME IMPROVEMENT IN THE EARLY AFTERNOON,
THOUGH CIGS WILL LIKELY REMAIN IN THE 400-700 FT RANGE. WHILE
STRONGER WINDS ALOFT TONIGHT ARE EXPECTED TO LESSEN THE THREAT
FOR DENSE FOG, CAN'T RULE OUT LOCALIZED LIFR VSBY IN THE RFD
AREA. ANOTHER WEATHER SYSTEM WILL BRING A CHANCE FOR SHOWERS AND
DRIZZLE OVERNIGHT INTO MONDAY MORNING, GENERALLY COINCIDING WITH
THE BEST CHANCE FOR LIFR CIGS AND IFR VSBY INTO THE CHICAGO METRO.
LIGHT SOUTH-SOUTHEAST WINDS WILL BECOME SOUTH-SOUTHWEST THIS
AFTERNOON, WITH SPEEDS INCREASING TO NEAR 10 KT OVERNIGHT AND
10-15 KT MONDAY MORNING.
CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...DENSE FOG ADVISORY UNTIL NOON CST TODAY FOR ILZ003-ILZ004-
ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
ILZ020-ILZ021-ILZ032-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CST THIS MORNING FOR THE IL
NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
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