802
FXUS63 KLOT 182324
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
524 PM CST TUE FEB 18 2025
KEY MESSAGES
- BITTERLY COLD CONDITIONS ARE EXPECTED THROUGH THURSDAY MORNING.
- INTERMITTENT FLURRIES ARE EXPECTED LATE WEDNESDAY AFTERNOON
THROUGH WEDNESDAY NIGHT WITH A CHANCE (20-40%) OF LIGHT
MEASURABLE SNOW ACCUMULATIONS AND SLIPPERY TRAVEL.
- LIGHT LAKE EFFECT SNOW SHOWERS WILL CLIP NORTHEAST PORTER
COUNTY AT TIMES THROUGH WEDNESDAY, WITH A BETTER CHANCE (50%+)
OF LIGHT ACCUMULATIONS AND SLIPPERY TRAVEL WEDNESDAY NIGHT.
- TEMPERATURES ARE EXPECTED TO GRADUALLY WARM BACK TOWARD AND
EVEN ABOVE NORMAL LATE THIS WEEK INTO EARLY NEXT WEEK.
DISCUSSION
ISSUED AT 300 PM CST TUE FEB 18 2025
THROUGH WEDNESDAY NIGHT:
OUR WEATHER WILL CONTINUE TO BE DOMINATED BY THE STRONG,
EXPANSIVE, AND COLD 1050S MB SURFACE HIGH PRESSURE OVER THE
NORTHERN PLAINS THIS (TUESDAY) AFTERNOON. THE SURFACE HIGH
CENTER WILL MOVE TO THE CENTRAL AND SOUTHERN PLAINS BY EARLY
THURSDAY WHILE WEAKENING SLIGHTLY TO ABOUT 1045 MB.
TONIGHT (TUESDAY NIGHT) INTO WEDNESDAY MORNING WILL NOT BE QUITE
AS COLD AS THIS MORNING, BUT STILL VERY COLD. 850 MB TEMPS 2-5
DEGREES CELSIUS ABOVE THOSE OF THIS MORNING AND HIGH CLOUDS
STREAMING OVERHEAD WILL SERVE AS A LIMITING FACTOR ON HOW COLD
TEMPS WILL GET (RELATIVELY SPEAKING). THE HIGH CLOUD COVER IS
EXPECTED TO BE MOST TRANSLUCENT OVER FAR NORTHERN ILLINOIS. THIS
FACTOR PLUS A BIT DEEPER SNOW COVER, EXTENSIVE UPSTREAM SNOW
COVER, AND DEW POINTS RUNNING BELOW MUCH OF THE GUIDANCE, SHOULD
ENABLE TEMPS TO FALL TO AS LOW AS THE LOW-MID SINGLE DIGITS
BELOW ZERO. SUB-ZERO LOWS SHOULD GENERALLY BE CONFINED TO
NEAR/NORTH OF I-80 AND AWAY FROM CHICAGO, WITH THE COLDEST TEMPS
(LOCALLY -7 TO -8F) AND WIND CHILLS AS LOW AS 20 BELOW ZERO
NEAR AND WEST OF THE FOX VALLEY IN FAR NORTHERN ILLINOIS. OPTED
TO ADD IN KANE AND LAKE (IL) COUNTIES INTO THE COLD WEATHER
ADVISORY IN EFFECT FROM MIDNIGHT TO 10 AM WEDNESDAY.
A SOUTHERN STREAM WAVE PASSING TO OUR SOUTH ON WEDNESDAY WILL
RESULT IN THICKENING MID-LEVEL CLOUDS, LIKELY RESULTING IN HIGH
TEMPS SIMILAR TO A BIT BELOW THOSE OF TODAY. WIND CHILLS MAY
CLIMB A FEW DEGREES ABOVE ZERO IN THE SOUTHEAST HALF OF THE AREA
DURING THE AFTERNOON, BUT THEN DIP BACK BELOW ZERO INTO THE
EVENING. A CLOSED 500 MB SHORT-WAVE WILL PIVOT EAST-SOUTHEAST
ACROSS THE AREA IN THE AFTERNOON AND EVENING. FORECAST SOUNDINGS
FOR MUCH OF THE AREA INDICATE A DEEP LAYER OF RH INCREASING TO
80%+ AND ASCENT COLDER THAN -20C (COLDER THAN FAVORED -12 TO
-18C DGZ). DESPITE THE INHERENTLY DRY ARCTIC AIR MASS IN PLACE,
CONCERN REMAINS FOR VERY SMALL SNOWFLAKES ("ARCTIC DUST"),
CAPABLE OF CAUSING SLIPPERY TRAVEL CONDITIONS AND REDUCED
VISIBILITY.
WITH THIS ISSUANCE, INCLUDED INTERMITTENT FLURRIES MENTION
AREAWIDE IN THE LATE AFTERNOON THROUGH MOST OF THE NIGHT, AND
EXPANDED THE AREA WITH MEASURABLE POPS. THE BEST CHANCE FOR MORE
WIDESPREAD LIGHT SNOW ACCUMULATIONS (ASSOCIATED WITH 30-40%
POPS) IS PEGGED FOR AREAS NEAR AND SOUTHEAST OF I-55 IN THE
EVENING. EVEN FOR LOCATIONS FARTHER NORTHWEST, AT LEAST
LOCALIZED COATINGS OF SNOW WOULDN'T BE SURPRISING. WITH ROAD
TREATMENTS LESS EFFECTIVE AT VERY COLD TEMPS AND MOST OF THE
MORE STEADILY FALLING SNOW AFTER SUNSET, SOME TRAVEL IMPACTS ARE
PROBABLE IF THIS SCENARIO COMES TO FRUITION.
LAKE EFFECT THERMODYNAMICS WILL IMPROVE WEDNESDAY NIGHT WITH 850
MB TEMPS COOLING A FEW DEGREES C. NORTHWESTERLY BOUNDARY LAYER
FLOW WILL CONTINUE TO FAVOR LOCATIONS EAST OF PORTER COUNTY FOR
MORE APPRECIABLE ACCUMULATIONS AND ASSOCIATED IMPACTS, THOUGH
THE SYNOPTIC PLUS LAKE EFFECT/LAKE ENHANCEMENT COMBO COULD
RESULT IN LOCALIZED ACCUMS NEAR 1" IN NORTHEAST PORTER. PRIOR TO
TOMORROW EVENING, VERY LIGHT, GENERALLY NON-ACCUMULATING, LAKE
EFFECT SNOW/FLURRIES MAY CONTINUE TO AFFECT NORTHEAST PORTER CO.
AT TIMES.
FINALLY, WEDNESDAY NIGHT INTO THURSDAY MORNING WILL BE THE FINAL
NIGHT OF FLIRTING WITH -20F COLD WEATHER ADVISORY CRITERIA IN
PARTS OF NORTHERN AND PERHAPS CENTRAL IL. THE COLDEST CONDITIONS
WILL OCCUR PRIMARILY TOWARD DAYBREAK THURSDAY AS CLOUDS CLEAR
AND TEMPS DROP BACK TO NEAR 0F TO A BIT BELOW ZERO (LOWEST
WESTERN HALF OF THE CWA).
CASTRO
THURSDAY THROUGH TUESDAY:
LAKE EFFECT SNOW SHOWERS MAY CONTINUE ACROSS NORTHEAST PORTER
COUNTY ON THURSDAY. ANY ADDITIONAL SNOW ACCUMULATION WOULD BE
LIGHT.
COLD TEMPERATURES WILL CONTINUE THURSDAY NIGHT INTO FRIDAY
MORNING WITH LOWS AGAIN IN THE LOW/MID SINGLE DIGITS, POSSIBLY
AROUND ZERO ACROSS THE NORTHWEST CWA. WINDS ARE EXPECTED TO BE
LIGHT FRIDAY MORNING AS HIGH PRESSURE APPROACHES THE AREA, THUS
WIND CHILLS MAY ONLY BE IN THE ZERO TO -10 RANGE THURSDAY NIGHT
INTO FRIDAY MORNING. FROM THERE, A GRADUAL WARM UP IS EXPECTED
WITH HIGHS IN THE 20S FRIDAY AND POSSIBLY LOWER 30S SATURDAY.
THE MODELS ARE HINTING AT A WEAK WAVE MOVING ACROSS THE AREA
SATURDAY AFTERNOON INTO SATURDAY EVENING. BLENDED GUIDANCE HAS
REMAINED DRY DURING THIS TIME PERIOD AND THERE IS NOT MUCH
SUPPORT IN THE ENSEMBLES FOR PRECIPITATION DURING THIS TIME
PERIOD AND THUS MAINTAINED THE GOING DRY FORECAST. BUT THIS TIME
PERIOD WILL NEED TO BE MONITORED FOR POSSIBLE LIGHT PRECIP,
PERHAPS MAINLY LIGHT SNOW IF ANYTHING MATERIALIZED.
TEMPS THEN LOOK TO RISE ABOVE NORMAL AND POSSIBLY INTO THE 40S
EARLY NEXT WEEK. WILL NEED TO MELT THE EXISTING SNOW PACK SO
THESE HIGHS COULD BE A LITTLE OPTIMISTIC UNTIL HIGHER DEWPOINTS
MAKE IT INTO THE AREA. ANOTHER WEAK SYSTEM MAY BRING THE CHANCE
OF RAIN TO THE AREA MONDAY AFTERNOON INTO MONDAY NIGHT. CMS
AVIATION /00Z TAFS THROUGH 06Z THURSDAY/
ISSUED AT 524 PM CST TUE FEB 18 2025
THE ONLY AVIATION WEATHER CONCERN IS A POTENTIAL FOR
FLURRIES/LIGHT SNOW WEDNESDAY AFTERNOON AND EVENING.
HIGH-LEVEL CLOUD COVER WILL INCREASE AND GRADUALLY BUILD
DOWNWARDS INTO WEDNESDAY MORNING, BUT VFR CONDITIONS ARE
EXPECTED WITH NW WINDS AROUND 10 KTS. FROM MID-LATE WEDNESDAY
AFTERNOON THROUGH THE EVENING, INTERMITTENT FLURRIES/LIGHT SNOW
WILL BE POSSIBLE, MAINLY AT THE CHICAGO-AREA SITES. IT'S
POSSIBLE THAT SNOW FALLS STEADILY ENOUGH TO DROP VSBYS BRIEFLY
TO MVFR AT TIMES, ALTHOUGH THE CHANCES FOR THIS REMAIN UNDER 30
PERCENT RIGHT NOW, AND HAVE LEFT A MENTION OUT OF THE LATEST
TAFS. EVEN THIS VERY LIGHT SNOW MAY RESULT IN A DUSTING WITH
VERY COLD TEMPERATURES IN THE SINGLE DIGITS/LOWER TEENS.
CARLAW
LOT WATCHES/WARNINGS/ADVISORIES
IL...COLD WEATHER ADVISORY FROM MIDNIGHT TONIGHT TO 10 AM CST
WEDNESDAY FOR ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012.
IN...NONE.
LM...NONE.
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