509
FXUS63 KLOT 161157
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
557 AM CST MON DEC 16 2024
KEY MESSAGES
- DENSE FOG ADVISORY UNTIL 11 AM CST, WHILE THERE WILL BE SOME
VARIABILITY IN VISIBILITY, AREAS OF DENSE FOG (VSBY NEAR 0SM
AT TIMES) ARE EXPECTED THROUGH THIS MORNING, LIKELY GROWING
MORE WIDESPREAD TOWARD AND AFTER SUNRISE
- POTENTIAL FOR LIGHT SNOW ACCUMULATIONS AND TRAVEL IMPACTS
TUESDAY NIGHT INTO WEDNESDAY MORNING, MAINLY NEAR AND NORTH OF
I-80 IN ILLINOIS.
- ANOTHER ROUND OF SNOW AND TRAVEL IMPACTS POSSIBLE THURSDAY INTO
EARLY FRIDAY.
DISCUSSION
ISSUED AT 253 AM CST MON DEC 16 2024
THROUGH TUESDAY:
IT IS A FOGGY AND VERY MILD (BY MID-DECEMBER STANDARDS)
MORNING. DENSE FOG IS CURRENTLY PATCHY IN NATURE AND WE WILL
LIKELY SEE CONTINUED FLUCTUATIONS IN VISIBILITY THE NEXT FEW
HOURS, PARTICULARLY AS A BAND OF SHOWERS MOVES ACROSS EASTERN
CWA. HOWEVER, PRESSURE GRADIENT ACROSS THE AREA IS EXPECTED TO
GROW BAGGIER AND LIGHTER WINDS BY AND AFTER 12Z, WHICH WILL
LIKELY LEAD TO DENSE FOG BECOMING MORE WIDESPREAD ACROSS THE
AREA. HAVE EXPANDED THE DENSE FOG ADVISORY TO COVER THE ENTIRE
CWA AND EXTENDED EXPIRATION TIME TO 17Z/11 AM CST. THIS MAY BE A
BIT ON THE LONG SIDE, BUT WITH SOME GUIDANCE HANGING ONTO
PATCHY DENSE FOG UNTIL 18-19Z, FELT IT WOULD BE EASIER TO SHOOT
LONG AND LET DAYSHIFT CANCEL EARLY IF FOG DISSIPATES EARLIER
THAN EXPECTED. FOG IS EXPECTED TO DISSIPATE AS A COLD FRONT
MOVES ACROSS THE CWA LATE THIS MORNING THROUGH EARLY-MID
AFTERNOON.
THE CURRENT BAND OF SHOWERS SHOULD BE EAST OF THE AREA BY 12Z,
HOWEVER, THERE COULD BE SOME REGENERATION OF SHOWERS,
PARTICULARLY EAST OF I-57 AFTER 12Z AND THROUGH THE MID-LATE
MORNING HOURS.
A SOMEWHAT COLDER (CLOSER TO MORE SEASONABLE) AND DRIER AIR
MASS WILL FILTER INTO THE AREA BEHIND THE COLD FRONT LATER THIS
AFTERNOON AND TONIGHT. WHILE THERE WILL LIKELY BE SOME CLEARING
TONIGHT, ESPECIALLY SOUTHERN CWA, MID-HIGH LEVEL CLOUDINESS
WILL ALREADY BEGIN OVERSPREADING THE AREA IN ADVANCE OF THE NEXT
WAVE TUESDAY AFTERNOON, ESPECIALLY NORTH OF I-80.
- IZZI
TUESDAY NIGHT THROUGH SUNDAY:
MODEST NORTHWESTERLY COLD AIR ADVECTION TUESDAY NIGHT WILL USHER
IN A COLD ENOUGH FOR SNOW THERMAL PROFILE INTO MOST OF THE AREA,
WITH THE POSSIBLE EXCEPTION OF THE FAR SOUTHERN CWA. MEANWHILE, A
RELATIVELY ROBUST POSITIVELY TILTED SHORT-WAVE TROUGH CROSSING THE
MID-MS VALLEY AND UPPER LEVEL JET SUPPORT WILL PROVIDE LARGE SCALE
FORCING FOR ASCENT. THE AFOREMENTIONED CAA WILL HELP TIGHTEN THE
LOWER AND MID-LEVEL THERMAL GRADIENT, IMPLYING AT LEAST TRANSIENT
850-500 MB FRONTOGENESIS AND SOME POTENTIAL FOR MESOSCALE BANDING
ENHANCEMENT OF PRECIPITATION RATES.
THE ECMWF/EPS SUITE HAS EXHIBITED GOOD RUN TO RUN CONSISTENCY IN
THIS SETUP OVER THE PAST SEVERAL CYCLES, WITH THE OTHER GUIDANCE
GENERALLY IN SUPPORT. IN COLLABORATION WITH NEIGHBORING WFOS,
BUMPED UP POPS INTO THE LIKELY (~60%) RANGE NEAR AND NORTH OF I-80
IN ILLINOIS. 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.
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 30-40% RANGE, HIGHEST NORTHWEST 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.
LAKE EFFECT SNOW MAY OCCASIONALLY TAKE AIM AT PORTIONS OF OUR
NORTHWEST INDIANA COUNTIES.
CASTRO
AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
ISSUED AT 557 AM CST MON DEC 16 2024
MAIN CONCERN:
- LIFR TO VLIFR CONDITIONS THIS MORNING IMPROVING THIS
AFTERNOON.
FOG, DENSE AT TIMES, AND/OR VERY LOW CIGS WILL PERSIST UNTIL A
FEW HOURS AFTER A COLD FRONT PASSAGE LATER THIS MORNING. PUSHED
BACK NOTABLE IMPROVEMENT IN THE TAFS GIVEN THE EXPANSE OF LOW
CONDITIONS UPSTREAM AND EXPECTATION OF WESTERLY WINDS TAKING
LONGER TO RAMP UP. ONCE THE WESTERLY WIND GUSTS TO AROUND 25 KT
ARRIVE, ANY REMAINING LOW VSBY WILL ERODE AND CIGS WILL
GRADUALLY RISE. THERE'S SOME UNCERTAINTY ON THE TIMING OF RETURN
TO VFR CIGS THIS EVENING AND OVERNIGHT, AS AT LEAST PATCHY MVFR
CIGS COULD LINGER OVER THE AREA.
CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...DENSE FOG ADVISORY UNTIL 11 AM CST THIS MORNING FOR ILZ003-
ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...DENSE FOG ADVISORY UNTIL 11 AM CST /NOON EST/ THIS MORNING FOR
INZ001-INZ002-INZ010-INZ011-INZ019.
LM...DENSE FOG ADVISORY UNTIL NOON CST TODAY FOR THE IL AND IN
NEARSHORE WATERS.
SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 3 AM CST
TUESDAY FOR WINTHROP HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY FROM 2 PM THIS AFTERNOON TO 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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