912
FXUS63 KLOT 210501
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1101 PM CST FRI DEC 20 2024
KEY MESSAGES
- LAKE EFFECT SNOW SHOWERS EXPECTED THROUGH TONIGHT AND MAY
GRAZE EASTERN COOK COUNTY EARLY THIS EVENING BEFORE FOCUSING
MAINLY INTO NORTHWEST IN OVERNIGHT. UP TO 1-4" OF SNOW ARE
POSSIBLE, HIGHEST IN LAKE AND PORTER COUNTIES IN INDIANA.
- 20% CHANCE OF LIGHT FREEZING RAIN OR DRIZZLE ACROSS NORTHERN
ILLINOIS MONDAY MORNING.
DISCUSSION
ISSUED AT 229 PM CST FRI DEC 20 2024
THROUGH SATURDAY:
FOCUS IN THE NEAR TERM IS ON THE LAKE EFFECT SNOW POTENTIAL THE
REST OF THIS AFTERNOON THROUGH SATURDAY MORNING.
THE SURFACE LOW RESPONSIBLE FOR THIS MORNING'S SNOWFALL HAS
QUICKLY SHIFTED EAST ACROSS OH/WV ALREADY THIS AFTERNOON. IN ITS
WAKE, EXPANSIVE HIGH PRESSURE HAS SET UP ACROSS THE HIGH PLAINS
RESULTING IN NORTHERLY FETCH ACROSS LAKE MICHIGAN FAVORABLE FOR
LAKE EFFECT SNOW ORIENTED INTO NORTHEAST ILLINOIS AND NORTHWEST
INDIANA. SO FAR THIS AFTERNOON THIS BAND HAS STRUGGLED TO
ORGANIZE INTO A MORE COHESIVE LINE, PARTICULARLY ON THE SOUTHERN
THIRD OF THE LAKE, THOUGH THERE REMAINS A RATHER FORMIDABLE
BAND EAST OF MILWAUKEE/SHEBOYGAN.
AS OF THIS HOUR, THE INITIAL BURSTS OF SNOW HAVE SHIFTED EAST
INTO NORTHWEST INDIANA, WITH VISIBILITY AS LOW AS 1/2 MILE
HAVING OCCURRED AT TIMES IN NORTHERN LAKE COUNTY. NEAR TO ABOVE
FREEZING TEMPERATURES HAVE LIMITED ADDITIONAL IMPACTS SO FAR.
THIS HAS PLACED NORTHEAST ILLINOIS ON THE WEST SIDE OF THE
CONVERGENCE AXIS AND MAY KEEP ILLINOIS SNOW FREE FOR AT LEAST A
COUPLE MORE HOURS. THE EXPECTATION IS FOR THE AFOREMENTIONED
MORE COHESIVE LAKE EFFECT SNOW BAND TO CONTINUE PIVOTING SOUTH
ACROSS THE LAKE THROUGH THE AFTERNOON AND COULD IMPACT AT LEAST
EASTERN PORTIONS COOK COUNTY, INCLUDING THE PORTIONS OF THE CITY
OF CHICAGO EARLY THIS EVENING (ROUGHLY 5-7 PM CST). IF THE CORE
OF THE BAND PUSHES MANAGES TO PUSH ASHORE, A QUICK 1"+ OF SNOW
COULD FALL IN EASTERN PORTIONS OF THE CITY (WORST CASE
SCENARIO). THERE COULD BE A FAIRLY SHARP GRADIENT IN SNOWFALL
ACCUMULATIONS FROM WEST TO EAST ACROSS THE CITY. IN FACT, THE
WESTERN SIDE OF CHICAGO, INCLUDING O'HARE IS EXPECTED TO SEE
LITTLE TO NO ADDITIONAL SNOW.
THIS LAKE EFFECT BAND IS THEN EXPECTED TO SHIFT EAST AND FOCUS
INTO NORTHWEST INDIANA THE REST OF THE EVENING AND OVERNIGHT.
MODELED VERTICAL PROFILES HAVE TRENDED MORE SUPPORTIVE OF HIGHER
SNOWFALL RATES (1" TO BRIEFLY 2"/HR), WITH A COOLER AND DEEPER
CLOUD LAYER COINCIDING WITH STRONGER LIFT WITHIN THE DGZ.
ACCORDINGLY, SNOWFALL AMOUNTS HAVE INCREASED WITH THE AFTERNOON
UPDATE, WITH 2-3", LOCALLY 4" OF SNOW POSSIBLE ACROSS PORTIONS
OF LAKE AND PORTER COUNTIES IN INDIANA THROUGH SATURDAY MORNING.
PLAN TO MESSAGE THIS VIA A SPECIAL WEATHER STATEMENT (SPS) FOR
NOW THOUGH A SHORT FUSED ADVISORY CAN'T BE RULED OUT IF
CONFIDENCE INCREASES IN HIGHER AMOUNTS OCCURRING. THE SNOW BAND
SHOULD LIFT MAINLY OUT OVER THE LAKE ON SATURDAY, THOUGH
OCCASIONAL SNOW SHOWERS MAY DRIFT INLAND ACROSS PORTER COUNTY AT
TIMES.
WHILE THERE MAY BE A PERIOD OF HIGH CLOUDS THAT MOVE OVERHEAD
THIS EVENING, CLEARING SKIES LATE TONIGHT SHOULD ALLOW FOR
TEMPERATURES TO DROP INTO THE SINGLE DIGITS ACROSS INTERIOR
NORTHERN ILLINOIS, WITH TEENS AND LOW 20S EXPECTED GENERALLY
ALONG/EAST OF I-55 AND NEAR THE LAKE. TEMPERATURES REMAIN COOL
ON SATURDAY WITH HIGHS IN THE 20S TO AROUND 30.
PETR
SATURDAY NIGHT THROUGH FRIDAY:
A RIDGE OVER THE WESTERN CONUS EARLY THIS WEEKEND WILL
GRADUALLY DEAMPLIFY WHILE DRIFTING EASTWARD TO THE WESTERN GREAT
LAKES BY SUNDAY NIGHT. A PACIFIC-BASED TROUGH CROSSING THE
WESTERN GREAT LAKES REGION ON MONDAY WILL THEN BRING AN
ASSOCIATED COLD FRONT ACROSS THE FORECAST AREA MONDAY AFTERNOON.
BROAD LOW-LEVEL ISENTROPIC ASCENT WITHIN A RELATIVELY MOIST
PRE-FRONTAL AIRMASS SHOULD SPREAD LIGHT PRECIP ACROSS AT LEAST
NORTHERN PORTIONS OF THE FORECAST AREA LATE SUNDAY NIGHT INTO
MONDAY MORNING. A 1-2C WARM NOSE ADVECTING INTO THE AREA ON
850-900HPA 50 KNOT FLOW WILL YIELD LIQUID PRECIP, THOUGH THERE
IS SOME UNCERTAINTY AS TO WHETHER TEMPS WILL RISE ABOVE FREEZING
BEFORE THE PRECIP BEGINS. LATEST TEMPERATURE GUIDANCE AND AIR
TEMPERATURES EXPECTED TO REMAIN AROUND OR BELOW FREEZING THROUGH
THIS WEEKEND SUGGEST PRECIP MAY BEGIN AS LIGHT FREEZING
RAIN/DRIZZLE THROUGH MID-MORNING, ESPECIALLY CLOSER TO THE
WISCONSIN LINE.
THE COLD FRONT IS PROGGED TO STALL ACROSS THE OHIO VALLEY ON
TUESDAY BEFORE A CONVOLUTED MID-LEVEL PATTERN (RESULTING FROM
FOUR TO FIVE WAVES ROTATING AROUND AND WITHIN A LONGWAVE RIDGE)
LENDS ITSELF TO PLENTY OF UNCERTAINTY FROM CHRISTMAS DAY INTO
NEXT WEEKEND. OVERALL, A SEASONABLY MILD PERIOD WITH PERIODS OF
RAIN ARE IN STORE, THOUGH SPECIFICS WILL NEED TO BE IRONED OUT
IN THE COMING DAYS.
KLUBER
AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
ISSUED AT 1101 PM CST FRI DEC 20 2024
NO SIGNIFICANT AVIATION WEATHER CONCERNS FOR THE 06Z TAFS.
LAKE-EFFECT SNOW BAND HAS PUSHED INLAND EAST/SOUTHEAST OF GYY AS
OF LATE FRIDAY EVENING. WHILE ADDITIONAL SPOTTY LIGHT SNOW
SHOWERS OR FLURRIES CAN'T BE COMPLETELY RULED OUT FOR GYY
OVERNIGHT, THE POTENTIAL FOR ORGANIZED PRECIPITATION APPEARS
TOO LOW FOR TAF MENTION. WILL LIKELY CONTINUE TO SEE PERIODS OF
PATCHY LOWER-VFR CIGS AT TIMES THERE INTO SATURDAY.
FOR THE OTHER TAF SITES, QUIET VFR WEATHER IS EXPECTED THROUGH
THE PERIOD, WITH A MID-DECK DEVELOPING SATURDAY AFTERNOON/EVENING.
SURFACE WINDS WILL REMAIN NORTHWEST TO NORTH-NORTHWEST, WITH
SPEEDS 10-15 KT INITIALLY BUT GRADUALLY DIMINISHING THROUGH THE
PERIOD. SURFACE HIGH PRESSURE RIDGE MOVES INTO THE AREA SATURDAY
EVENING, SETTING UP A PERIOD OF LIGHT AND VARIABLE WINDS WHICH
SHOULD EVENTUALLY SETTLE TO A LIGHT SOUTH-SOUTHEAST TOWARD
SUNRISE SUNDAY MORNING.
RATZER
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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