929
FXUS63 KLOT 161741
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1141 AM CST MON DEC 16 2024
KEY MESSAGES
- REDUCED VISIBILITIES AND LOCALIZED POCKETS OF FOG INTO THE
EARLY AFTERNOON
- 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 /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...DENSE FOG ADVISORY UNTIL NOON CST TODAY FOR THE IL AND IN
NEARSHORE WATERS.
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.
VISIT US AT WEATHER.GOV/CHICAGO
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