203
FXUS63 KLOT 200917
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
317 AM CST FRI DEC 20 2024
KEY MESSAGES
- PERIODS OF SNOW EARLY THIS MORNING, WITH ACCUMULATIONS UP TO 1
INCH POSSIBLE. UNTREATED SURFACES EXPECTED TO BECOME SNOW
COVERED AND SLICK.
- LAKE EFFECT SNOW SHOWERS LATE MORNING/EARLY AFTERNOON FOR
NORTHEAST IL, THEN MOVING INTO NORTHWEST IN BY MID/LATE
AFTERNOON. SNOWFALL TOTALS 1-3 INCHES POSSIBLE.
- NORTHERLY WIND GUSTS TO 35 MPH THROUGH THIS AFTERNOON.
- WARMER TEMPS WITH PERIODIC LOW CHANCES FOR RAIN NEXT WEEK.
DISCUSSION
ISSUED AT 317 AM CST FRI DEC 20 2024
THROUGH SATURDAY:
LOW PRESSURE WILL BE MOVING SOUTHEAST OF THE AREA EARLY THIS
MORNING AND AS IT DOES, COOLER AIR WILL SETTLE ACROSS THE AREA
ENDING ANY MIXED PRECIP TYPE CONCERNS. AS A WAVE ROTATES AROUND
THE DEPARTING LOW, SNOW IS EXPECTED TO INCREASE IN COVERAGE
ACROSS NORTHERN IL IN THE PREDAWN HOURS AND CONTINUE ACROSS MOST
OF THE AREA DURING THE MORNING COMMUTE, ENDING FROM NORTHWEST
TO SOUTHEAST THROUGH THE MID/LATE MORNING. SOME PERIODS OF
MODERATE SNOW WILL BE POSSIBLE DURING THIS TIME PERIOD. WHILE
NEW SNOWFALL AMOUNTS WILL LIKELY BE AN INCH OR LESS, AS TEMPS
SLOWLY DROP BELOW FREEZING, UNTREATED SURFACES ARE EXPECTED TO
BECOME SNOW COVERED AND SLICK.
A BAND OF LAKE EFFECT SNOW IS EXPECTED TO DEVELOP LATE THIS
MORNING AND STEADILY MOVE EAST/SOUTHEAST ACROSS NORTHEAST IL
FROM LATE MORNING THROUGH EARLY AFTERNOON AND THEN SHIFT INTO
NORTHWEST IN. THERE IS STILL SOME UNCERTAINTY REGARDING HOW FAR
INLAND THIS BAND MAY REACH ACROSS NORTHEAST IL. WHEREVER THE
BAND DOES SET-UP, A SHORT PERIOD OF MODERATE TO PERHAPS HEAVY
SNOW WILL BE POSSIBLE WITH A QUICK INCH OR TWO OF ACCUMULATION.
ONCE IT SHIFTS INTO NORTHWEST IN, A BIT LONGER RESIDENCE TIME
MAY LEAD TO HIGHER SNOW TOTALS THOUGH IT WILL STILL BE SLOWLY
MOVING EAST. SNOWFALL TOTALS IN THE 1-3 INCH RANGE SEEM
REASONABLE BUT THERE COULD BE LOCALLY HIGHER AMOUNTS. THE LAKE
EFFECT SNOW SHOWERS MAY CONTINUE INTO SATURDAY ACROSS NORTHEAST
PORTER COUNTY BUT CONFIDENCE IS LOW.
AS THE SURFACE LOW DEPARTS AND THE GRADIENT TIGHTENS, WINDS WILL
INCREASE AROUND/JUST AFTER DAYBREAK WITH A PERIOD OF GUSTS INTO
THE 35 MPH RANGE EXPECTED. THESE NORTHERLY WINDS WILL REMAIN
GUSTY INTO THE AFTERNOON THOUGH SOME DECREASE IS EXPECTED BY
MID/LATE AFTERNOON AND THEN WIND SPEEDS AND GUSTS SHOULD
DIMINISH QUICKLY THIS EVENING.
TEMPS TODAY ARE A BIT TRICKY, MUCH OF THE GUIDANCE IS TRYING TO
WARM TEMPS THROUGH MIDDAY BEFORE FALLING LATER THIS AFTERNOON
AND HAVE TEMPS WARMING A DEGREE OR TWO FROM THIS MORNING'S LOWS
BUT TEMPS MAY BE STEADY OR EVEN BEGIN TO SLOWLY FALL THIS
AFTERNOON. LOWS TONIGHT EXPECTED TO DIP INTO THE LOW TEENS
ACROSS NORTHWEST IL, PERHAPS A FEW SINGLE DIGITS IN THE USUAL
COOL SPOTS AND THEN HIGHS ONLY REBOUND INTO THE 20S FOR HIGHS ON
SATURDAY. CMS
SATURDAY NIGHT THROUGH THURSDAY:
A LARGE SCALE PATTERN CHANGE TO MILD WEATHER IS EXPECTED ACROSS
THE CONUS FOR THE HOLIDAY WEEK. THIS WILL TAKE PLACE BY EARLY NEXT
WEEK AS A STRONG AND PERSISTENT ZONALLY ORIENTED MID-LATITUDE
PACIFIC JET DEVELOPS ALONG THE SOUTHERN PERIPHERY OF ANOMALOUSLY
DEEP TROUGHING ACROSS THE GULF OF ALASKA. THIS PARTICULAR PACIFIC
PATTERN WILL FAVOR PROGRESSIVE AND MILD WEST-TO-EAST MOVING
WEATHER SYSTEMS ACROSS THE CONUS THROUGH THE HOLIDAY WEEK. WITH
COLD AIRMASS EXPECTED TO BE SHUNTED WELL NORTH OF THE REGION, THE
PROSPECTS FOR A WHITE CHRISTMAS THIS YEAR ARE LOOKING RATHER
BLEAK.
AS THIS WEATHER PATTERN SHIFT TAKES PLACE, THE FIRST IN A SERIES
OF QUICK MOVING PACIFIC IMPULSES IS EXPECTED TO SLIDE EASTWARD
INTO THE WESTERN GREAT LAKES ON MONDAY. MOST OF THE PRECIPITATION
WITH THIS IMPULSE LOOKS TO LARGELY REMAIN NORTH AND EAST OF OUR
AREA. NEVERTHELESS, THE PASSAGE OF A WEAK COLD FRONT ON MONDAY
SUPPORTS THE MENTION FOR SOME LOW (20-40%) CHANCES FOR RAIN MONDAY
INTO MONDAY EVENING. THE FRONT, AND THE MAIN THREAT OF RAIN,
SHOULD SETTLE TO OUR SOUTH ACROSS PARTS OF DOWNSTATE IL AND IN
FOR TUESDAY. LOW CHANCES (20% OR LESS) FOR RAIN WILL CONTINUE
LOCALLY INTO CHRISTMAS DAY. HOWEVER, WITH THE NEXT PACIFIC WAVE
EXPECTED TO SLIDE ACROSS THE OZARKS, OUR AREA MAY LARGELY REMAIN
DRY FOR CHRISTMAS. THIS NEARLY CONTINUOUS PATTERN OF PACIFIC
IMPULSES IS LIKELY TO PERSIST THROUGH THE END OF PERIOD.
THE MILD WEATHER PATTERN NEXT WEEK WILL SUPPORT WELL ABOVE NORMAL
TEMPERATURES IN THE 40S FOR DAYTIME HIGHS. WELL ABOVE NORMAL
OVERNIGHT LOW TEMPERATURES ARE ALSO LIKELY IN THIS PATTERN.
KJB
AVIATION /00Z TAFS THROUGH 06Z SATURDAY/
ISSUED AT 615 PM CST THU DEC 19 2024
AVIATION FORECAST CONCERNS FOR 00Z TAFS:
- PERIODS OF SNOW SHOWERS TONIGHT INTO FRIDAY MORNING/MIDDAY.
PERIODS OF IFR/BRIEF LIFR CONDITIONS LIKELY WITH HEAVIEST
SNOW SHOWERS.
- SURFACE WINDS BACK FROM SE TO NE OVERNIGHT, BECOMING GUSTY
FROM THE NORTH NEAR 30 KTS FRIDAY.
- LAKE EFFECT SNOW SHOWERS MAY PERSIST INTO FRIDAY EVENING FOR
KGYY.
A STRONG MID-LEVEL DISTURBANCE WAS MOVING INTO THE AREA LATE
THIS AFTERNOON, AND WILL TRACK ACROSS THE TERMINALS OVERNIGHT
AND INTO FRIDAY BEFORE SHIFTING EAST OF THE REGION FRIDAY
AFTERNOON. IN THE IMMEDIATE NEAR-TERM, A NORTH-SOUTH BAND OF
MODERATE/HEAVY SNOW WAS MOVING INTO THE CHICAGO METRO
TERMINALS, AND IS EXPECTED TO BRING A BRIEF PERIOD OF IFR/LIFR
CONDITIONS AND ACCUMULATIONS BETWEEN 00-01Z. LIGHTER SNOW
SHOWERS ARE THEN EXPECTED TO PERSIST INTO TONIGHT (AND MAY
BECOME MIXED WITH DRIZZLE AT TIMES BETWEEN HEAVIER SNOW
SHOWERS), WITH SOME INCREASE IN COVERAGE AND INTENSITY EXPECTED
AGAIN AFTER MIDNIGHT BENEATH THE MAIN UPPER TROUGH AXIS. STRONG
NORTHERLY WINDS AND COLDER TEMPERATURES FRIDAY MORNING WILL
LIKELY SUPPORT AN ADDITIONAL PERIOD OF LAKE-ENHANCED/LAKE-
EFFECT SNOW SHOWERS AND IFR/BRIEF LIFR CONDITIONS FOR THE
CHICAGO TERMINALS. LAKE-EFFECT WILL SLOWLY SHIFT SOUTHEAST
FRIDAY AFTERNOON, BUT MAY LINGER ACROSS KMDW THROUGH 19-20Z AND
KGYY POTENTIALLY INTO EARLY EVENING.
THE SURFACE LOW PRESSURE CENTER ASSOCIATED WITH THIS SYSTEM WAS
OVER EASTERN IA EARLY THIS EVENING, AND WILL TRACK ACROSS THE
FORECAST AREA TONIGHT. CURRENT HIGH-RES MODEL GUIDANCE TAKES THE
CENTER OF THE LOW JUST SOUTH OF THE TERMINALS, RESULTING IN SURFACE
WINDS BACKING FROM CURRENT SOUTHEAST DIRECTIONS TO NORTHEAST AS
THE LOW PASSES LATE TONIGHT. WINDS WILL BECOME NORTHERLY FRIDAY
MORNING ON THE BACK SIDE OF THE LOW, LIKELY GUSTING NEAR 30 KTS
AT TIMES INTO EARLY AFTERNOON BEFORE GRADUALLY DIMINISHING.
RATZER
MARINE
ISSUED AT 245 AM CST FRI DEC 20 2024
A PERIOD OF NORTHERLY GALE FORCE WINDS TO 35 KT MAY OCCUR IN
THE ILLINOIS AND INDIANA NEARSHORE WATERS OF LAKE MICHIGAN
THIS MORNING INTO EARLY THIS AFTERNOON AS A LOW PRESSURE SYSTEM
TRACKS ACROSS THE LOWER GREAT LAKES. OWING TO LOW CONFIDENCE ON
THE FREQUENCY OF GALE FORCE GUSTS, WILL WITHHOLD ISSUING A GALE
WARNING IN THIS FORECAST PACKAGE. HOWEVER, WILL ADVERTISE A FEW
GALE FORCE GUSTS IN THE NEARSHORE AND SMALL CRAFT ADVISORY
PRODUCTS.
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CST SATURDAY FOR THE IL
NEARSHORE WATERS.
SMALL CRAFT ADVISORY FROM 6 AM THIS MORNING TO 3 PM CST
SATURDAY FOR THE IN NEARSHORE WATERS.
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