916
FXUS63 KLOT 110509
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1109 PM CST MON FEB 10 2025
KEY MESSAGES
- INTERMITTENT FLURRIES WITH A CHANCE FOR A PERIOD OF LIGHT
ACCUMULATING SNOW LATE TONIGHT INTO TUESDAY MORNING, PRIMARILY
NEAR AND NORTH OF I-80. FLURRIES MAY CONTINUE NEAR THE LAKE
THROUGH THE DAY ON TUESDAY.
- WINTER STORM WILL IMPACT THE AREA WEDNESDAY AND WEDNESDAY
NIGHT WITH SNOW, HEAVY AT TIMES, INCLUDING DURING THE
AFTERNOON/EVENING COMMUTE.
- VERY COLD THURSDAY INTO FRIDAY WITH SUBZERO LOWS EXPECTED
THURSDAY NIGHT MANY AREAS.
- ANOTHER WINTER STORM COULD IMPACT THE REGION LATE FRIDAY INTO
THE FIRST HALF OF THE WEEKEND.
UPDATE
ISSUED AT 956 PM CST MON FEB 10 2025
NO CHANGES OF NOTE WERE MADE TO THE GOING FORECAST REGARDING
THE SNOWFALL POTENTIAL FOR TONIGHT INTO TUESDAY.
RECENT HRRR AND RAP RUNS HAVE TRENDED A LITTLE MORE FAVORABLY
TOWARDS LIGHT SNOWFALL OCCURRING OVERNIGHT TONIGHT INTO TUESDAY
MORNING, FAVORING SLIGHTLY STRONGER, THOUGH STILL TRANSIENT,
MID-LEVEL FRONTOGENESIS THAT WOULD YIELD BETTER OMEGA AND MID-
LEVEL SATURATION INTERSECTING THE DENDRITIC GROWTH ZONE. EVEN
WITH THIS THOUGH, FULL SATURATION OF ANY SECTION OF THE
TROPOSPHERIC COLUMN MAY STILL NOT QUITE BE ACHIEVED, AND THE
MODEST DEGREE OF FORCING MAY STILL NOT BE ENOUGH TO FULLY ERODE
THE DRY LOW-LEVEL AIR BENEATH THE CLOUD LAYER AND ALLOW FOR
SNOW TO FALL AT A STEADY ENOUGH CLIP TO ACCUMULATE MUCH, IF AT
ALL.
THUS, CONTINUING WITH AN INTERMITTENT FLURRY MENTION ACROSS
THE ENTIRETY OF OUR FORECAST AREA OVERNIGHT INTO TUESDAY
MORNING AND KEEPING SOME SLIGHT CHANCE AND LOW-END CHANCE POPS
(UP TO 30%) FOR ACCUMULATING SNOW IN OUR FORECAST GRIDS STILL
SEEMS LIKE THE WAY TO GO. OPTED TO KEEP THE MENTIONABLE POPS AS
IS IN THE NORTHERN HALF OF OUR FORECAST AREA AS THIS IS WHERE
THE MID-LEVEL UVV SIGNAL IS MOST PRONOUNCED IN MOST OF THE 00Z
HI-RES GUIDANCE SUITE, THOUGH AN ARGUMENT COULD BE MADE FOR THE
SOUTHWARD EXPANSION OF THESE LOWER-END POPS GIVEN THAT HI-RES
GUIDANCE OFTEN STRUGGLES WITH THE PLACEMENT OF THESE RELATIVELY
SUBTLE FRONTOGENETICAL CIRCULATIONS -- EVEN AT VERY SHORT TIME
RANGES JUST A FEW HOURS OUT.
OGOREK
DISCUSSION
ISSUED AT 236 PM CST MON FEB 10 2025
THROUGH TUESDAY NIGHT:
OUR REGION WILL REMAIN BENEATH FAST WESTERLY FLOW ALOFT
TONIGHT, WITH A STRONG POSITIVE-TILT MID-LEVEL SHORT WAVE
TRACKING NORTH OF THE REGION ACROSS THE UPPER MIDWEST, NORTHERN
GREAT LAKES AND ONTARIO. ADDITIONAL SHEARED MID-LEVEL ENERGY WILL
ZIP FROM THE PLAINS TO THE MID-MISSISSIPPI VALLEY THROUGH EARLY
TUESDAY. THE LOCAL AREA WILL BE SPLIT BY STRONGER FORCING TO THE
NORTH AND BETTER LOW-MID LEVEL BAROCLINICITY TO THE SOUTH.
HOWEVER, WEAK WARM ADVECTION AND ISENTROPIC ASCENT RESPONSE AND
ASSOCIATED TRANSIENT FRONTOGENESIS IS DEPICTED IN MODEL GUIDANCE
TO DEVELOP FROM IA INTO SOUTHERN WI AND THE NORTHERN ILLINOIS
LATE TONIGHT INTO EARLY TUESDAY, WHICH WILL LIKELY INCREASE
SATURATION FOR A TIME.
FORECAST SOUNDINGS DEPICT SOME POTENTIAL FOR LIGHT SNOW AFTER
MIDNIGHT THROUGH MID MORNING PRIMARILY NEAR AND NORTH OF I-80
ACROSS NORTHERN IL. DESPITE MODEST FORCING AND FAIRLY LIMITED
MOISTURE, SUSPECT THAT SOME OF THE DRIER GUIDANCE (INCLUDING
EXTENDED HRRR RUNS IN PARTICULAR) IS NOT ERODING THE LOW-LEVEL
DRY AIR ENOUGH. THE GUIDANCE WITH SUFFICIENT SATURATION (IE. NAMS,
RAP, ECMWF) SUGGESTS TRANSIENT BANDS OF SNOW COMING DOWN AT A
STEADY CLIP. WITH THAT SAID, IT'S STILL UNCERTAIN EXACTLY WHERE
THE DRY AIR WILL BE ERODED ENOUGH TO SUPPORT A PERIOD OF LIGHT
ACCUMULATING SNOW AND REDUCED VISIBILITY (AMIDST OTHERWISE NON-
ACCUMULATING FLURRIES). THUS, KEPT POPS IN THE LOW CHANCE RANGE
(~30%) NEAR/NORTH OF I-80, AND SLIGHT CHANCE (~20%) DOWN TO A BIT
SOUTH OF I-80.
DUE TO THE SHORT DURATION, MODEST FORCING, AND LIMITED MOISTURE
AVAILABILITY, IT STILL LOOKS LIKE SNOW ACCUMULATIONS (IF/WHERE
THEY OCCURS) WOULD BE NO MORE THAN COATING TO A FEW TENTHS/LOCALLY
TO ONE HALF INCH. SURFACE TEMPS WILL BE IN THE UPPER TEENS-MID
20S HOWEVER, WHICH WOULD ALLOW FOR THE POTENTIAL LIGHT
ACCUMULATION TO COAT PAVED SURFACES AND CAUSE SOME TRAVEL
IMPACTS INTO THE AM COMMUTE. FARTHER SOUTH/EAST, FLURRIES ARE
MORE LIKELY WHERE FORCING AND SATURATION IS LESS IMPRESSIVE.
LOW LEVEL WINDS WILL SHIFT NORTHEAST LATER TUESDAY MORNING, WHICH
MAY SUPPORT SOME RELATIVELY WEAK LAKE-EFFECT SNOW SHOWERS AND
FLURRIES INTO PARTS OF NORTHEAST IL/NORTHWEST IN DURING THE DAY.
LAKE-INDUCED THERMODYNAMIC PARAMETERS APPEAR PRETTY MARGINAL, WITH
INVERSION HEIGHTS ONLY AROUND 5 KFT SUGGESTING FAIRLY POOR-
QUALITY SNOW. SEVERAL CAMS CONTINUE TO PRODUCE SOME LIGHT QPF INTO
THE AFTERNOON FROM LAKE AND COOK IL INTO LAKE (IN) AND NORTHWEST
PORTER COUNTIES, AND HAVE MAINTAINED SOME LOW-CHANCE POPS (20%)
FOR LAKE SNOW SHOWERS WITH A DUSTING TO A COUPLE OF TENTHS OF AN
INCH ACCUMULATION. FLURRIES MAY OCCUR DOWNWIND INTO THE
I-65/57/55 CORRIDORS, THOUGH AM NOT EXPECTING ANY ACCUMULATIONS
TOO FAR AWAY FROM THE LAKE, AND WITHHELD ANY FORMAL SNOW ACCUMS
EVEN NEAR THE LAKE DURING THE AFTERNOON HOURS.
AS LOW-LEVEL WINDS BECOME MORE EASTERLY TUESDAY EVENING, LIGHT
LAKE EFFECT SNOW AND FLURRIES SHOULD FOCUS INTO NORTHEAST
ILLINOIS. FINALLY, INITIAL BANDS OF (MAINLY LIGHT) SNOW FROM
OUR IMPENDING WINTER STORM SYSTEM MAY DEVELOP EASTWARD IN THE
PRE-DAWN HOURS. WE INCLUDED A CORRIDOR OF LOW CHANCE (~30%)
POPS PRIMARILY FOR AREAS NORTHWEST OF I-55. BY FAR THE MORE
CONSEQUENTIAL SNOW AND IMPACTS WILL ARRIVE LATER WEDNESDAY
MORNING AND BEYOND, DETAILED IN DEPTH IN THE REST OF THE
DISCUSSION (BELOW).
CASTRO/RATZER
WEDNESDAY THROUGH MONDAY:
VIGOROUS SHORTWAVE TROUGH JUST OFFSHORE BRITISH COLUMBIA IS
PROGGED TO DIG SOUTH INTO THE FOUR CORNERS REGION TOMORROW
(TUESDAY) AFTERNOON BEFORE EJECTING NORTHEAST ACROSS THE PLAINS
AND INTO THE MIDWEST WEDNESDAY. SURFACE CYCLOGENESIS WILL TAKE
PLACE OVER THE LOWER OHIO VALLEY WEDNESDAY IN RESPONSE TO THIS
APPROACHING TROUGH. THIS LOW WILL THEN TRACK NORTHEAST ACROSS
THE OHIO VALLEY. FOR THE FIRST TIME THIS WINTER, THERE WILL BE
AMPLE COLD AIR IN PLACE FOR AN EXPANSIVE SHIELD OF SNOW TO
DEVELOP ON THE COLD SIDE OF THIS SYSTEM.
THERE HAS BEEN A BIT OF A NORTHWEST JOG IN THE TRACK OF THIS
SYSTEM IN THIS MORNING'S GUIDANCE, MOST NOTABLY IN THE NAM.
OFTEN TIMES IN STRONG WARM AIR ADVECTION REGIMES, THE NAM ENDS
UP BEING CLOSEST TO REALITY WITH HOW FAR NORTH THE SNOW/MIX/RAIN
LINE GETS, SO DID LEAN A BIT MORE TOWARD THE NAM SOLUTION
BRINGING SOME FREEZING RAIN AND SLEET NORTH INTO OUR FAR
SOUTHERN AND SOUTHEASTERN CWA.
STRENGTHENING LOW AND PARTICULARLY MID LEVEL WARM AIR
ADVECTION/ISENTROPIC ASCENT WILL COUPLE WITH STRONG UPPER LEVEL
DIVERGENCE, FIRST IN THE ENTRANCE REGION OF DEPARTING 130-140KT
UPPER LEVEL JET STREAM, THEN IN THE LEFT ENTRANCE REGION OF A
SECOND JETLET POKING INTO THE AREA LATE IN THE AFTERNOON AND
INTO THE EVENING. FAIRLY STRONG ISENTROPIC ASCENT COUPLED WITH
THE STRONG UPPER LEVEL DIVERGENCE SHOULD RESULT IN A 6-8 HOUR
PERIOD OF DEEP, STRONG ASCENT, FOCUSED ON WEDNESDAY AFTERNOON
AND EVENING. GUIDANCE SHOWS A FAIRLY IMPRESSIVE NORTHWARD SURGE
OF MOISTURE, WITH PRECIPITABLE WATER VALUES OF AROUND 150% OF
NORMAL WEDNESDAY AFTERNOON, WHICH COUPLED WITH THE DEEP, STRONG
OMEGA SHOULD RESULT IN SOME FAIRLY HEAVY SNOWFALL.
UNFORTUNATELY, AS FAR AS AMOUNTS GO, THERE REMAINS UNCERTAINTY
IN REGARDS TO SPECIFIC TOTALS. THE UNCERTAINTY IS DRIVEN LARGELY
BY THE QUESTIONS IN REGARDS TO THE SNOW:LIQUID RATIOS (SLRS),
WITH CLIMO FOR SOUTHWEST ORIGINATING STORMS AND THE COBB METHOD
BOTH FAVORING SLRS CLOSER TO 10 TO 11:1. NBM GUIDANCE IS
SUBSTANTIALLY HIGHER, CLOSER TO 14-18:1, HIGHEST NORTHWEST CWA.
FOR THIS FORECAST HAVE SPLIT THE DIFFERENCE BETWEEN THE 2
EXTREMES, THOUGH FORECAST SOUNDINGS (PARTICULARLY IN THE WARMER
NAM) SHOW A FAIRLY DEEP ISOTHERMAL LAYER CLOSER TO THE -5C WITH
THE HEAVIER PRECIP, WHICH WOULD TEND TO FAVOR MORE AGGREGATES
AND LOWER SLRS.
REGARDLESS OF THE EXACT SLRS AND SNOWFALL TOTALS, IT LOOKS TO BE
A VERY IMPACTFUL STORM WITH HEAVY SNOWFALL RATES COINCIDING WITH
THE AFTERNOON COMMUTE. THERE IS AN AT LEAST 50% CHANCE OF 6"+
TOTALS, WHICH COUPLED WITH THE EXPECTED IMPACTS, JUSTIFIES A
WINTER STORM WATCH. IF SLRS END UP HIGHER OR IF THERE'S SOME
MODEST LAKE ENHANCEMENT, SOME TOTALS OVER 8" ARE CERTAINLY NOT
OUT OF THE QUESTION. SNOW WILL PROBABLY MIX WITH/CHANGE TO
SLEET/FREEZING RAIN IN THE SOUTHEASTERN CWA LATE WEDNESDAY
AFTERNOON INTO THE EVENING (WITH UNCERTAINTY ON HOW FAR NORTH
THE MIX LINE WILL REACH). THIS WILL LIKELY CUT DOWN ON SNOWFALL
TOTALS, BUT OBVIOUSLY SLEET AND FREEZING RAIN WOULD RESULT IN
TRAVEL ISSUES OF THEIR OWN.
PRECIPITATION RATES SHOULD TAPER LATER WEDNESDAY EVENING INTO
THE OVERNIGHT, SO THE THURSDAY MORNING COMMUTE SHOULD BE MUCH
IMPROVED VS. WEDNESDAY PM. A COUPLE DAYS OF BELOW NORMAL TEMPS
ARE THEN EXPECTED. TEMPS THURSDAY COULD BE STEADY OR EVEN FALL
A DEGREE OR TWO IN THE AFTERNOON, LIKELY MOSTLY IN THE TEENS.
FRESH SNOW COVER AND A 1040MB HIGH IN THE AREA THURSDAY NIGHT
SHOULD SET THE STAGE FOR SUBZERO LOW TEMPS ACROSS MOST OF THE
AREA. THE 12Z GUIDANCE HAS TRENDED COLDER WITH TEMPS FRIDAY,
WHICH IF THIS TREND CONTINUES WITH THE 00Z GUIDANCE, TEMPS FOR
FRIDAY WILL NEED TO BE LOWERED.
ANOTHER STORM SYSTEM COULD BRING MORE WINTRY PRECIPITATION TO
THE REGION LATE FRIDAY INTO AT LEAST THE FIRST HALF OF THE
WEEKEND. THERE'S A GOOD DEAL OF SPREAD STILL WITH THIS SYSTEM
AND THERE ARE LOT OF MOVING PIECES, LEADING TO HIGHER
UNCERTAINTY WITH HOW MUCH/IF THERE IS PHASING. GIVEN THE
UNCERTAINTIES AT THIS DISTANCE, MADE NO CHANGES TO NBM GRIDS.
ANOTHER SHOT OF VERY COLD AIR IS EXPECTED IN THE WAKE OF THAT
SYSTEM LATE IN THE WEEK INTO EARLY NEXT WEEKEND.
- IZZI
AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
ISSUED AT 1100 PM CST MON FEB 10 2025
MAIN CONCERNS:
- INTERMITTENT FLURRIES DEVELOP LATE TONIGHT WITH A 30% CHANCE
FOR A DUSTING OF SNOW THROUGH EARLY TUESDAY AM.
- MVFR CEILINGS AND FLURRIES MAY CONTINUE NEAR THE LAKE THROUGH
THE DAY ON TUESDAY.
- LIGHT SNOW MAY DEVELOP LATE TUESDAY NIGHT AHEAD OF WEDNESDAY'S
WINTER STORM.
LIGHT AND VARIABLE WINDS ARE EXPECTED THIS EVENING, EVENTUALLY
TURNING NORTHERLY AND INCREASING OVERNIGHT. DIRECTIONS TREND
NORTHEASTERLY BY TUESDAY MORNING AROUND 12 KT.
A BAND OF SNOW CURRENTLY OVER NORTHEAST IA AND SOUTHWEST WI
WILL VERY GRADUALLY SHIFT SOUTHEAST TOWARD NORTHERN ILLINOIS
LATE TONIGHT. IT WILL BE BATTLING LOW-LEVEL DRY AIR WITH
EASTWARD EXTENT. WHILE CONFIDENCE REMAINS LOW, IF THE DRY AIR
CAN BE OVERCOME, A LIGHT DUSTING OF SNOW COULD OCCUR BETWEEN 9
AND 15Z ALONG WITH ASSOCIATED MVFR CIGS AND VSBYS (30% CHANCE).
MVFR STRATUS IS THEN EXPECTED TO MOVE IN OFF THE LAKE TUESDAY
MORNING ACROSS THE CHICAGO METRO TERMINALS. EXPECT AT LEAST
INTERMITTENT FLURRIES TO OCCUR BENEATH THIS STRATUS DECK FOR
MUCH OF THE DAY AT ORD/MDW/GYY. WHILE LITTLE TO NO ACCUMULATION
IS CURRENTLY EXPECTED, AN ADDITIONAL LOCALIZED DUSTING OF
0.1-0.2" CAN'T BE RULED OUT.
INTRODUCED ANOTHER PROB30 FOR SNOW TOWARD THE END OF THE
ORD/MDW TAFS TO ACCOUNT FOR THE POTENTIAL FOR LIGHT
ACCUMULATING SNOW DEVELOPING OUT AHEAD OF THE WINTER STORM
EXPECTED TO BRING IMPACTFUL ACCUMULATING SNOW TO THE AREA ON
WEDNESDAY.
PETR
LOT WATCHES/WARNINGS/ADVISORIES
IL...WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH LATE
WEDNESDAY NIGHT FOR ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
ILZ032-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.
IN...WINTER STORM WATCH FROM WEDNESDAY MORNING THROUGH LATE
WEDNESDAY NIGHT FOR INZ001-INZ002.
LM...NONE.
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