762
FXUS63 KLOT 150924
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
324 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 /06Z TAFS THROUGH 12Z MONDAY/
ISSUED AT 1051 PM CST SAT DEC 14 2024
PRIMARY FORECAST CONCERNS INCLUDE...
LIFR/VLIFR CIGS/VIS WITH DRIZZLE/POSSIBLE DENSE FOG.
LIGHT RAIN WILL TRANSITION TO DRIZZLE OVER THE NEXT FEW HOURS AS
CIGS QUICKLY LOWER INTO IFR/LIFR AND EVENTUALLY INTO VLIFR FOR
MUCH OF THE AREA OVERNIGHT. DENSE FOG IS LIKELY ACROSS NORTHWEST
IL AND INTO THE NORTHWEST SUBURBS OF CHICAGO INCLUDING AT DPA
AND HAVE INCLUDED TEMPO 1/4SM AT RFD/DPA. DENSE FOG IS POSSIBLE
AT ORD AS WELL, BUT CONFIDENCE REMAINS LOW BUT DO EXPECT SEVERAL
HOURS OF VISIBILITY UNDER 1SM AT ORD AND HAVE INCLUDED
PREVAILING 3/4SM. FOR MDW/GYY, MUCH OF THE GUIDANCE KEEPS VIS
ABOVE 1/2SM AND GIVEN THE SOMEWHAT CONSISTENT FORECASTS OVER THE
PAST SEVERAL HOURS FOR THE AXIS OF POTENTIAL DENSE FOG, WILL
CONTINUE WITH PREVAILING 1SM, THOUGH EVEN HERE ONLY MEDIUM
CONFIDENCE. ONCE THE FOG DEVELOPS IT MAY CONTINUE THROUGH LATE
SUNDAY MORNING WITH THE LOWEST VISIBILITIES POSSIBLY AFTER
SUNRISE. FOG LOOKS TO LIFT BY EARLY SUNDAY AFTERNOON WITH CIGS
LIFTING TO AT LEAST IFR. THERE IS SOME POTENTIAL FOR MVFR CIGS
BY SUNDAY EVENING OR EVENING SCATTERING, BUT CONFIDENCE IS TOO
LOW TO INCLUDE WITH THIS FORECAST. THERE ALSO APPEARS TO BE
ANOTHER CHANCE FOR FOG AND LOWER CIGS LATE SUNDAY NIGHT INTO
EARLY MONDAY MORNING.
SOUTHEAST WINDS MAY STILL GUST INTO THE 15-20KT RANGE FOR A FEW
MORE HOURS THEN SPEEDS ARE EXPECTED TO DIMINISH UNDER 10KTS
OVERNIGHT. DIRECTIONS WILL TURN MORE SOUTH/SOUTHEAST LATER
SUNDAY MORNING AND EVENTUALLY TO THE SOUTH/SOUTHWEST SUNDAY
AFTERNOON. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CST THIS MORNING FOR THE IL
NEARSHORE WATERS.
SMALL CRAFT ADVISORY UNTIL 4 AM CST EARLY THIS MORNING FOR THE
IN NEARSHORE WATERS.
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