747
FXUS63 KLOT 212339
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
539 PM CST TUE JAN 21 2025
KEY MESSAGES
- WIND CHILLS OF -15F TO -20F WILL PERSIST THROUGH THIS EVENING.
THE COLD WEATHER ADVISORY REMAINS IN EFFECT UNTIL MIDNIGHT.
- ACCUMULATING SNOW (LOCALLY UP TO 1") IS EXPECTED WEDNESDAY
THROUGH EARLY THURSDAY MORNING LEADING TO SLIPPERY TRAVEL.
- LAKE EFFECT SNOW MAY LINGER ACROSS NORTHEAST PORTER COUNTY
DURING THE DAY ON THURSDAY.
DISCUSSION
ISSUED AT 304 PM CST TUE JAN 21 2025
THROUGH WEDNESDAY NIGHT:
AN ARCTIC HIGH CONTINUES TO SLIDE ACROSS THE MIDWEST AND GREAT
LAKES THIS AFTERNOON WHICH HAS RESULTED IN BITTERLY COLD
TEMPERATURES DESPITE THE NEARLY UNABATED SUNSHINE. HIGHS TODAY
HAVE LEVELED OFF AROUND 0 DEGREES AND LOOK TO REMAIN NEARLY
STATIONARY THROUGH SUNSET. WITH WINDS EXPECTED TO REMAIN BREEZY
THROUGH THIS EVENING, WIND CHILLS IN THE -15F TO -20F RANGE ARE
EXPECTED TO PREVAIL THROUGH MIDNIGHT. THEREFORE, THE COLD
WEATHER ADVISORY WAS EXTENDED UNTIL MIDNIGHT TO HIGHLIGHT THIS
THREAT.
AFTER MIDNIGHT, TEMPERATURES ARE EXPECTED TO GRADUALLY WARM AS
STRONG SOUTH-SOUTHWEST WINDS DEVELOP AND ADVECT IN "WARMER" AIR.
HOWEVER, TEMPERATURES ARE STILL EXPECTED TO BE IN THE SINGLE
DIGITS THROUGH THE NIGHT WITH WIND CHILLS HOVERING IN THE -10F
TO -15F RANGE. SINCE THESE WIND CHILL MAGNITUDES ARE ABOVE THE -20F
THRESHOLD USED FOR ADVISORIES, THE EXPECTATION IS FOR NO
ADDITIONAL COLD WEATHER HEADLINES TO BE ISSUED ONCE THE
AFOREMENTIONED ONE EXPIRES. THOUGH WIND GUSTS WILL BE INCREASING
INTO THE 20-25 MPH RANGE OVERNIGHT WHICH WILL KEEP A NOTABLE
CHILL IN THE AIR TO START OUR WEDNESDAY.
THE BROAD SHORTWAVE TROUGH CURRENTLY IN ALBERTA WILL BE
PIVOTING INTO THE NORTHERN PLAINS AND EVENTUALLY THE GREAT LAKES
THROUGH THE DAY ON WEDNESDAY. AS THIS OCCURS, THE SURFACE LOW
AHEAD OF THE SYSTEM WILL BE PIVOTING INTO THE NORTHERN GREAT
LAKES WHICH WILL FORCE A COLD FRONT TO ADVANCE ACROSS THE UPPER
MIDWEST WEDNESDAY MORNING. SNOW IS EXPECTED TO DEVELOP BENEATH
THIS WAVE AND ALONG THE COLD FRONT AND BEGIN TO MOVE INTO FAR
NORTHERN IL TOWARDS THE MID-MORNING AND EARLY AFTERNOON HOURS.
HOWEVER, THE FRONT IS FORECAST TO GET HUNG UP NEAR THE IL-WI
LINE WHICH SHOULD KEEP THE BULK OF THE SNOW ALONG THE STATE LINE
AND POINTS NORTH THROUGH MUCH OF THE DAY. THOUGH, THE FRONT IS
EXPECTED TO PIVOT SOUTHWARD THROUGH THE REST OF THE AREA LATE
WEDNESDAY AFTERNOON AND EVENING AS THE SHORTWAVE DIVES INTO THE
OHIO RIVER VALLEY. SINCE THE FRONT SHOULD BE FAIRLY PROGRESSIVE
AS IT MOVES ACROSS NORTHERN IL AND NORTHWEST IN, THE DURATION OF
SNOW AT ANY ONE LOCATION SHOULD BE ONLY 3-4 HOURS AT MOST AND
THUS KEEP ACCUMULATIONS AROUND HALF AN INCH. HOWEVER, LOCALLY
HIGHER TOTALS UP TO AN INCH ARE POSSIBLE ESPECIALLY NEAR THE IL-
WI LINE WHERE LONGEVITY IS FORECAST TO BE MAXIMIZED.
REGARDLESS, THE COLD SURFACE TEMPERATURES WILL ALLOW FOR SOME
SNOW COVERED ROADS AND SLIPPERY TRAVEL SO BE SURE TO USE CAUTION
IF TRAVELING TOMORROW.
SNOW IS EXPECTED TO GRADUALLY TAPER AFTER MIDNIGHT WEDNESDAY
NIGHT, THOUGH SOME LINGERING LIGHT SNOW AND FLURRIES MAY PERSIST
INTO THURSDAY MORNING FOR AREAS EAST OF I-57. OTHERWISE, EXPECT
TEMPERATURES TO WARM INTO THE 20S WEDNESDAY AFTERNOON BEFORE
COOLING INTO THE TEENS WEDNESDAY NIGHT. WINDS WILL ALSO BE GUSTY
ON WEDNESDAY AS THE SYSTEM MOVES THROUGH WITH GUSTS IN THE
25-30 MPH RANGE LIKELY AND LOCALLY HIGHER GUSTS POSSIBLE. THOUGH
WINDS WILL SUBSIDE WEDNESDAY EVENING AS THE SYSTEM BEGINS TO
EXIT TO OUR EAST.
YACK
THURSDAY THROUGH TUESDAY:
SNOW WILL BE WINDING DOWN TOWARD DAYBREAK ON THURSDAY THOUGH A
FEW FLURRIES MAY LINGER NEAR/SOUTHEAST OF I-55 THROUGH MID
MORNING. CONDITIONS BECOME FAVORABLE FOR LAKE EFFECT SNOW
ORIENTED INTO NORTHEAST PORTER COUNTY DURING THE DAY ON THURSDAY
(NORTHWESTERLY FETCH, INCREASING INVERSION HEIGHTS, STEEP LAPSE
RATES AND SATURATION THROUGH THE DGZ) WHICH COULD RESULT IN
ADDITIONAL ACCUMULATIONS (LOCALLY UP TO 0.5-1") AND CONTINUED
SLICK TRAVEL. CLOUD DEPTHS BEGIN TO THIN TOWARD THURSDAY EVENING
WITH THE MAIN AREA OF SNOW SHIFTING FURTHER EAST.
WINDS WILL BE BREEZY OUT OF THE NORTHWEST IN THE WAKE OF A COLD
FRONTAL PASSAGE THURSDAY MORNING WHICH COMBINED WITH
TEMPERATURES ONLY WARMING INTO THE UPPER TEENS TO LOWER 20S WILL
RESULT IN WIND CHILLS IN THE SINGLE DIGITS PERSISTING DURING
THE DAY. LOW TEMPERATURES THURSDAY NIGHT DROP INTO THE SINGLE
DIGITS ONCE AGAIN AREAWIDE WITH WIND CHILLS IN THE 0 TO -10
DEGREE RANGE. MORE OF THE SAME IS EXPECTED ON FRIDAY WITH HIGHS
IN THE LOWER 20S. A RETURN TO SOUTHWESTERLY FLOW FRIDAY NIGHT
SHOULD ALLOW TEMPERATURES TO SLOWLY RISE THROUGH THE NIGHT WITH
TEMPERATURES ON SATURDAY CURRENTLY FORECAST TO BRIEFLY WARM
ABOVE FREEZING (33-36F) IN THE AFTERNOON.
WHILE THERE REMAINS VARIABILITY IN THE HANDLING OF THE UPPER
LEVEL PATTERN INTO EARLY NEXT WEEK, THE GENERAL EXPECTATION IS
FOR AN ELONGATED POSITIVELY TILTED UPPER LEVEL TROUGH
(POTENTIALLY CUT-OFF LOW) TO DEVELOP ACROSS THE WESTERN CONUS
WITH QUASI-ZONAL FLOW EXTENDING EAST. THIS PATTERN LARGELY
FAVORS KEEPING MORE MOISTURE LADEN WEATHER DISTURBANCES LARGELY
SOUTH OF THE AREA AND IS SUPPORTED BY THE LATEST ENSEMBLE
GUIDANCE KEEPING PROBS FOR >0.1" OF PRECIPITATION GENERALLY
UNDER 20% THROUGH EARLY NEXT WEEK.
PETR
AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/
ISSUED AT 539 PM CST TUE JAN 21 2025
AVIATION FORECAST CONCERNS FOR 00Z TAFS:
- BRIEF LULL IN WINDS THIS EVENING, THEN BECOMING BREEZY AGAIN
FROM THE SOUTHWEST LATE TONIGHT AND WEDNESDAY WITH GUSTS ~25
KTS.
- PERIOD OF LIGHT SNOW POSSIBLE WEDNESDAY MORNING ACROSS FAR
NORTHERN IL/SOUTHERN WI, LOWER PROBABILITY FOR CHICAGO
TERMINALS, GREATER FOR KRFD. MVFR/BRIEF IFR POSSIBLE.
- ANOTHER MORE LIKELY POTENTIAL FOR A PERIOD OF LIGHT SNOW
WEDNESDAY EVENING. IFR/LOW-MVFR POSSIBLE.
WEAK SURFACE HIGH PRESSURE RIDGE WAS PASSING SOUTH OF THE
TERMINALS EARLY THIS EVENING, WITH DECREASING WINDS BACKING TO
THE SOUTHWEST. THESE SOUTHWEST WINDS ARE EXPECTED TO INCREASE
AGAIN AFTER MIDNIGHT TONIGHT, WITH GUSTS AROUND 25 KT DEVELOPING
AND PERSISTING INTO THE DAY WEDNESDAY. WINDS WILL THEN EASE
GRADUALLY WEDNESDAY EVENING.
CLEAR SKIES ACROSS THE REGION THIS EVENING WILL EVENTUALLY GIVE
WAY TO INCREASING MID-LEVEL VFR CLOUD COVER OVERNIGHT, IN
RESPONSE TO A STRENGTHENING LOW LEVEL JET AND ASSOCIATED WARM
ADVECTION (ESPECIALLY ALOFT) AHEAD OF AN APPROACHING UPPER LEVEL
TROUGH. SATURATION IN RESPONSE TO THIS FORCING WILL RESULT IN
VFR CLOUD BASES CONTINUING TO LOWER WEDNESDAY, WITH SOME
POTENTIAL FOR AN AREA OF LIGHT SNOW TO DEVELOP FROM IA INTO WI
AND POTENTIALLY FAR NORTHERN/NORTHWEST IL DURING THE MORNING.
CONFIDENCE IS SOMEWHAT LOW IN THE SOUTH/SOUTHEASTWARD EXTENT OF
THE SNOW POTENTIAL, AS VARIOUS MODEL GUIDANCE SHOWS CONSIDERABLE
SPREAD IN THE DEGREE OF SATURATION IN LOWER LEVELS. KRFD HAS
THE HIGHEST PROBABILITY OF SEEING SNOW DURING THE MORNING, WITH
DECREASING LIKELIHOOD FARTHER SOUTHEAST INTO THE CHICAGO AREA. A
LULL IS THEN EXPECTED BY MIDDAY ESPECIALLY FOR THE CHICAGO
TERMINALS.
A BETTER POTENTIAL FOR LIGHT SNOW LOOKS TO COME LATER IN THE
DAY/EVENING ACROSS THE AREA, AS THE MAIN UPPER LEVEL TROUGH
MOVES THROUGH. HAVE INDICATED A PERIOD OF PREVAILING LIGHT SNOW
AFTER 22Z AT KRFD AND AFTER 00Z IN THE KORD/KMDW 30-HOUR TAFS,
ALONG WITH A PROB MENTION OF IFR CONDITIONS IN THE 02Z-06Z
PERIOD. GIVEN RELATIVELY COLD TEMPERATURES, SOME ACCUMULATION
IS LIKELY (GENERALLY LESS THAN AN INCH) WITH BOTH THE POTENTIAL
MORNING AND EVENING PERIODS OF LIGHT SNOW.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...COLD WEATHER ADVISORY UNTIL MIDNIGHT CST TONIGHT FOR ILZ003-
ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...COLD WEATHER ADVISORY UNTIL MIDNIGHT CST /1 AM EST/ TONIGHT
FOR INZ001-INZ002-INZ010-INZ011-INZ019.
LM...SMALL CRAFT ADVISORY FROM 10 PM THIS EVENING TO 6 PM CST
WEDNESDAY FOR THE IL AND IN NEARSHORE WATERS.
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