926
FXUS63 KLOT 220858
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
258 AM CST WED JAN 22 2025
KEY MESSAGES
- ACCUMULATING SNOW (LOCALLY UP TO 1") TODAY THROUGH EARLY
THURSDAY MORNING LEADING TO SLIPPERY TRAVEL AT TIMES.
- LAKE EFFECT SNOW MAY LINGER ACROSS NORTHEAST PORTER COUNTY
DURING THE DAY ON THURSDAY.
DISCUSSION
ISSUED AT 258 AM CST WED JAN 22 2025
THROUGH THURSDAY:
PRETTY TRICKY FORECAST AHEAD THROUGH THE NEXT 18-24 HOURS OR SO
AS SEVERAL FAIRLY WEAK/LOW-AMPLITUDE DISTURBANCES SHUTTLE ACROSS
THE REGION. EVERYTHING FROM A SYNOPTIC PERSPECTIVE LOOKS A BIT
"DISJOINTED", WITH DCVA, JET FORCING, WAA, AND TRANSIENT F-GEN
CIRCULATIONS ALL PRESENT, YET NOT REALLY CO-LOCATED OR
PARTICULARLY ROBUST AT ANY GIVEN TIME. COMBINE THIS WITH
LINGERING DRY AIR IN THE LOW-LEVELS AS THE RECENT ARCTIC AIRMASS
DEPARTS, FORECAST CONFIDENCE REGARDING SPECIFIC SNOW TIMING AND
CHANCES REMAINS A BIT LOWER THAN NORMAL FOR THIS TIME RANGE.
THAT SAID, LIGHT SNOW DOES LOOK LIKE A GOOD BET THROUGH TONIGHT
(AMOUNTS LOCALLY UP TO ABOUT AN INCH), AND WITH VERY COLD GROUND
TEMPERATURES, SLIPPERY TRAVEL CONDITIONS APPEAR LIKELY.
THE FIRST DISTURBANCE IN THE INCOMING WAVE TRAIN IS CURRENTLY
MEANDERING ACROSS SOUTHERN IOWA EARLY THIS MORNING.
STRENGTHENING SOUTHWESTERLY/WESTERLY FLOW IS FACILITATING A
CONSIDERABLE INCREASE IN WARM ADVECTION/ISENTROPIC UPGLIDE WHICH
SEEMS TO BE RESPONSIBLE FOR THE CURRENT EXPANSION OF 15-25 DBZ
RETURNS ACROSS CENTRAL IOWA. GIVEN THE RECENT INCREASE OF LIGHT
SNOW OBSERVATIONS, IT'S CLEAR THAT AT LEAST THE SHORT LEAD TIME
HRRR FORECASTS ARE TOO DRY. WHETHER OR NOT THIS EXTENDS TO LATER
THIS MORNING REMAINS TO BE SEEN, BUT GIVEN RECENT MODEL TRENDS
NOTED IN THE RAP, ECMWF, AND HRDPS, COUPLED WITH THE UPSTREAM
RADAR PRESENTATION, COORDINATED WITH NEIGHBORING OFFICES TO
BOOST SNOW CHANCES THIS MORNING ACROSS NORTHERN ILLINOIS.
SPECIFICALLY, INCREASED POPS INTO THE LIKELY CATEGORY ROUGHLY
ALONG AND NORTH OF I-88, AND THE EXPANDED CHANCE POPS SOUTH OF
I-80 IN THE 9 AM TO 1 PM TIME FRAME. NOT EXPECTING ANYTHING MORE
THAN PERHAPS A FEW FLURRIES AS YOU HEAD SOUTH ABOUT THE
KANKAKEE RIVER.
THANKFULLY, FORCING WITH THIS FIRST WAVE STILL DOESN'T LOOK TOO
SUBSTANTIAL, WITH UVVS GENERALLY PEAKING IN THE -2 TO -4
UBAR/SEC RANGE, INCREASING A BIT AS YOU STEP NORTH OF THE
WISCONSIN STATE LINE. THE MAJORITY OF THIS ASCENT IS NOW MODELED
TO REMAIN ABOVE THE DENDRITIC GROWTH ZONE, AND THIS MAY END UP
CUTTING SLRS DOWN A BIT, EVEN WITH THE DGZ AN IMPRESSIVE 3-4 KM
DEEP AT TIMES THIS MORNING.
FORCING FROM THIS FIRST WAVE WILL BEGIN TO MOVE NORTH OF THE
REGION EARLY THIS AFTERNOON. THIS SHOULD TEMPORARILY REDUCE
PRECIPITATION COVERAGE, ALTHOUGH SINCE WE NEVER FULLY LOSE
ASCENT ACROSS NORTHERN ILLINOIS, AT LEAST INTERMITTENT VERY
LIGHT SNOW MAY PERSIST THROUGH THE AFTERNOON AS TEMPERATURES
ATTEMPT TO CLIMB INTO THE MID 20S.
THE NEXT WAVES APPROACH THIS EVENING AND OVERNIGHT AHEAD OF A
COLD FRONT. THIS WILL DRIVE AN EXPANSION OF LIGHT SNOW ACROSS
MUCH OF THE REGION, INITIALLY NORTH AND WEST OF ABOUT I-55, AND
THEN PRESSING SOUTH AND EAST FROM THERE. HAVE NOTED A DEVELOPING
SIGNAL IN SOME GUIDANCE, INCLUDING THE RAP, HRDPS, AND ECMWF,
INDICATING SOME POTENTIAL FOR SOME SE-NE ORIENTED BANDED
SNOWFALL DEVELOPING LATE THIS EVENING IN THE VICINITY OF A
PONTIAC TO VALPARAISO LINE, COINCIDENT WITH SOME MID-LEVEL
DEFORMATION AND RIGHT ON THE CYCLONIC SHEAR SIDE OF A DEPARTING
UPPER JET STREAK. THIS SIGNAL IS NOT UBIQUITOUS ACROSS THE MODEL
SUITE HOWEVER, BUT CONCEPTUALLY MAKES SENSE. HAVE INCREASED
POPS AND SLIGHTLY BOOSTED SNOW AMOUNTS IN THE VICINITY TO
ACCOUNT FOR THIS POTENTIAL.
A COLD FRONT WILL ARRIVE LATE TONIGHT (AFTER MIDNIGHT IN OUR
NORTHWEST) AND WILL BEGIN TO SHUT SNOW CHANCES OFF FROM
NORTHWEST TO SOUTHEAST THROUGH EARLY THURSDAY MORNING. THE
LATEST FORECAST WILL SHOW SNOWFALL TOTALS FOR THIS EVENT FROM A
DUSTING TO LOCALLY AROUND AN INCH.
LAKE EFFECT SNOW SHOWERS MAY LINGER ACROSS PARTS OF FAR NE LAKE
AND PORTER COUNTIES THROUGH THURSDAY. THERMODYNAMIC PARAMETERS
AREN'T VERY SIGNIFICANT, SUGGESTING ANY ADDITIONAL ACCUMULATIONS
SHOULD BE LIMITED TO AN INCH OR LESS.
CARLAW
THURSDAY NIGHT THROUGH TUESDAY:
THE UPPER LEVEL TROUGH WILL EXIT TO THE EAST ON THURSDAY NIGHT.
BRIEF RIDGING AND HEIGHT RISES ON FRIDAY WILL CREATE DRY
CONDITIONS WITH LIGHT SOUTHWEST WINDS AIDING IN WARM AIR
ADVECTION DRAWING HIGH TEMPERATURES BACK UP TO 20S AREAWIDE.
MEDIUM RANGE GUIDANCE IS PROJECTING SUBTLE WAVES TO TRAVERSE
AROUND THE BROADER LOW OVER THE HUDSON BAY FRIDAY NIGHT INTO
SATURDAY. MOST MODELS KEEP THESE WAVES MOVING OVER MINNESOTA
INTO WISCONSIN (IF NOT KEEPING THEM ENTIRELY IN CANADA).
HOWEVER, THERE ARE A FEW ENSEMBLE MEMBERS THAT HAVE THE WAVES
DROPPING FAR ENOUGH SOUTH TO TOUCH THE WISCONSIN/ILLINOIS
STATELINE. THIS WAVE WILL BE ACCOMPANIED BY AN APPROXIMATELY 100
KNOT JET AT 300 MB, DCVA AHEAD OF IT, A PLUME OF HIGHER 700 MB
RHS AND STRONG WARM AIR ADVECTION. LOOKING AT MODEL SOUNDINGS,
SOME GUIDANCE IS SUGGESTING SOME SATURATION IN THE MID LEVELS.
HOWEVER, THE BETTER LAPSE RATES REMAIN IN WISCONSIN, WITH ONLY
ONLY WEAK FORCING NOTED ACROSS OUR REGION. THERE WAS
CONSIDERATION FOR ADDING SOME NON-ACCUMULATING FLURRIES TO THE
FORECAST, BUT THERE WAS JUST NOT ENOUGH CONFIDENCE TO ADD IT AT
THE PRESENT TIME. WINDS COULD BE GUSTING IN THE AFTERNOON
BETWEEN 20 TO 25 MPH, BUT OTHERWISE THE ONLY OTHER THING TO NOTE
FOR SATURDAY IS THAT THE WARM AIR ADVECTION COULD PROVIDE THE
AREA WITH THE FIRST OPPORTUNITY FOR HIGH TEMPERATURES TO TOUCH
JUST ABOVE THE FREEZING MARK SINCE LAST SATURDAY.
AS AN UPPER TROUGH DROPS DOWN THE PACIFIC COAST OVER THE
WEEKEND WITH A CLOSED LOW SETTING UP OVER SOCAL, BROAD ZONAL
FLOW ALOFT IS EXPECTED TO SET UP OVER THE CONUS EAST OF THE
ROCKIES. COLD AIR ADVECTION BEHIND THE PREVIOUSLY MENTIONED
SATURDAY WAVE WILL LIMIT HIGH TEMPERATURES BACK DOWN IN THE 20S.
HOWEVER, WINDS TURN BACK TO THE SOUTHWEST EARLY NEXT WEEK FOR
HIGH TEMPERATURES TO REBOUND ONCE AGAIN ABOVE FREEZING INTO THE
MIDDLE OF NEXT WEEK. ENSEMBLES ARE SUGGESTING THAT AN EMBEDDED
WAVE IN THE FLOW COULD PASS OVER THE AREA EARLY NEXT WEEK,
THOUGH THERE IS STILL VARIABILITY IN THE MAGNITUDE AND PLACEMENT
(TIMING AND LOCATION) OF THE WAVE. WITH HIGHS ABOVE FREEZING
AND LOWS BELOW 32F, P-TYPE WOULD BE DEPENDENT ON WHEN IT CAME
THROUGH. HOWEVER, MOST ENSEMBLE MEMBERS ARE KEEPING THE BETTER
MOISTURE AWAY FROM THE FORECAST AREA EITHER TO THE NORTH OF
SOUTH AND THUS KEEPING THE RISK FOR ANY PRECIPITATION OUT OF THE
CURRENT FORECAST.
ALL THAT WRITING REALLY JUST TO SAY THAT WE HAVE DRY FORECAST
THROUGH THE LONG TERM WITH MODULATING TEMPERATURES AROUND
SEASONAL NORMS, WITH A CHANCE FOR ABOVE NORMAL TEMPERATURES
EARLY NEXT WEEK.
DK
AVIATION /06Z TAFS THROUGH 12Z THURSDAY/
ISSUED AT 1106 PM CST TUE JAN 21 2025
AVIATION FORECAST CONCERNS FOR 06Z TAFS:
- BLUSTERY SOUTH-SOUTHWEST WINDS GUSTING AROUND 25 KTS OVERNIGHT
AND WEDNESDAY.
- TWO POTENTIAL PERIODS OF LIGHT SNOW. FIRST WEDNESDAY
MORNING/MIDDAY MAINLY TOWARD THE IL/WI BORDER, THE SECOND
HIGHER PROBABILITY PERIOD FOR ALL SITES WEDNESDAY
EVENING/OVERNIGHT. MVFR/IFR POSSIBLE.
VFR MID-DECK WILL DEVELOP AND THICKEN ACROSS THE AREA OVERNIGHT,
WITHING A WARM ADVECTION REGIME ASSOCIATED WITH A 50 KT LOW
LEVEL JET AHEAD OF AN APPROACHING MID-LEVEL WAVE AND WARM FRONT.
PATCHY LIGHT SNOW IS EXPECTED TO DEVELOP ON THE NOSE OF THE LOW
LEVEL JET FROM IA INTO WI, AND ACROSS PORTIONS OF FAR
NORTH/NORTHWEST IL WEDNESDAY MORNING. KRFD APPEARS TO HAVE THE
GREATEST POTENTIAL FOR LIGHT SNOW DURING THIS PERIOD, THOUGH
HAVE MAINTAINED A PROB30 MENTION FOR CHICAGO TERMINALS (EXCEPT
KGYY). FORECAST MODELS VARY WITH THE DEGREE OF SATURATION IN
LOWER LEVELS, WITH LESS LIKELIHOOD OF SNOW THE FARTHER SOUTH AND
SOUTHEAST OF THE IL/WI BORDER. PATCHY FLURRIES OR VERY LIGHT
SNOW COULD LINGER AROUND KRFD INTO THE AFTERNOON, THOUGH A LULL
IS GENERALLY EXPECTED UNTIL THE SECOND (MORE LIKELY) PERIOD OF
LIGHT SNOW DEVELOPS LATE IN THE DAY/EVENING INTO THE OVERNIGHT
HOURS WITH THE APPROACH OF THE MAIN MID-LEVEL WAVE AND A SURFACE
COLD FRONT. THIS IS MORE LIKELY TO PRODUCE A PERIOD OF MVFR
CEILINGS AND LOW-MVFR/IFR VSBYS ACROSS MUCH OF THE AREA
BY LATER WEDNESDAY EVENING/EARLY OVERNIGHT. SNOW SHOULD END WITH
THE COLD FRONTAL PASSAGE, PREDAWN THURSDAY MORNING.
SURFACE WINDS HAVE BACKED SOUTHWEST AHEAD OF THE APPROACHING
SYSTEM, AND HAVE BEGUN TO INCREASE LATE THIS EVENING AS THE 50
KT LOW LEVEL JET SPREADS OVERHEAD. SOUTHWEST WINDS WILL LIKELY
CONTINUE TO GUST AROUND 25 KTS (WITH A FEW HIGHER GUSTS
POSSIBLE) THROUGH EARLY/MID AFTERNOON BEFORE EASING. WINDS WILL
SHIFT NORTHWEST BEHIND THE COLD FRONT, PREDAWN THURSDAY MORNING.
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 6 PM CST THIS EVENING FOR THE IL
AND IN NEARSHORE WATERS.
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