521
FXUS63 KLOT 201139
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
539 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 /12Z TAFS THROUGH 18Z SATURDAY/
ISSUED AT 539 AM CST FRI DEC 20 2024
AVIATION FORECAST CONCERNS FOR 12Z TAFS:
- PERIOD OF SNOW WITH IFR (BRIEFLY LIFR) EARLY THIS MORNING,
GRADUALLY TAPERING OFF AFTER 13-14Z.
- WINDS TURN NORTH-NORTHEAST AND BECOME GUSTY AT 25 TO 30 KT
TODAY.
- LAKE EFFECT SNOW SHOWERS LIKELY TO PERSIST THIS AFTERNOON INTO
THIS EVENING, ESPECIALLY AT KGYY.
AN AREA OF ACCUMULATING SNOW CONTINUES TO SHIFT ACROSS THE
TERMINALS EARLY THIS MORNING ALONG THE NORTHWESTERN PERIPHERY OF
AN EXITING AREA OF LOW PRESSURE OVER IN. VSBYS AND CIGS WILL
REMAIN IFR, WITH A BRIEF PERIOD OF LIFR POSSIBLE, FOR THE NEXT
HOUR OR TWO (THROUGH 14Z), BEFORE THE SNOW BEGINS TO TAPPER
OFF. EYES WILL THEN BE ON THE LIKELY DEVELOPMENT OF A BAND OF
LAKE EFFECT SNOW SHOWERS LATER THIS MORNING INTO THE AFTERNOON.
PRESENTLY, IT APPEARS THE HIGHEST THREAT FOR THESE SNOW SHOWERS
WILL REMAIN EAST OF ORD. HOWEVER, WITH CONFIDENCE REMAINING LOW
WITH HOW FAR INLAND ACROSS NORTHEASTERN IL THESE SHOWERS WILL
GET, WE HAVE OPTED TO HOLD ONTO THE GOING PROB30 MENTION FOR
THEM AT ORD AND MDW. OVERALL, THE HIGHEST CHANCES FOR THESE LAKE
EFFECT SNOW WILL BE AT GYY THIS AFTERNOON INTO THIS EVENING.
WINDS ON THE BACK SIDE OF THIS EXITING AREA OF LOW PRESSURE WILL
TURN GUSTY FROM THE NORTH-NORTHEAST TODAY. GUSTS UP TO 30 KT
ARE EXPECTED FROM MID TO LATE MORNING INTO THIS AFTERNOON. WINDS
WILL THEN ABATE THIS EVENING, WHILE SETTLING INTO A NORTH-
NORTHWESTERLY DIRECTION.
KJB
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 UNTIL 3 PM CST SATURDAY FOR THE IN
NEARSHORE WATERS.
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