212
FXUS63 KLOT 202029
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
229 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 /18Z TAFS THROUGH 00Z SUNDAY/
ISSUED AT 1215 AM CST FRI DEC 20 2024
FORECAST CONCERNS FOR THE TERMINALS...
- LAKE EFFECT SNOW RESULTING IN REDUCED VISIBILITIES AT GYY AND
POSSIBLY ORD AND MDW
- MVFR CEILINGS NEAR THE LAKE THIS AFTERNOON BECOMING VFR
OVERNIGHT INTO SATURDAY MORNING
- BREEZY NORTH-NORTHWEST WINDS GUSTING 25-30 KTS THIS AFTERNOON
AN UPPER DISTURBANCE CONTINUES TO PIVOT OVER THE GREAT LAKES
WHICH HAS ALLOWED FOR THE DEVELOPMENT OF A BAND OF LAKE EFFECT
SNOW ACROSS CENTRAL LAKE MICHIGAN. THIS BAND IS EXPECTED TO
PIVOT SOUTHWARD THIS AFTERNOON AND BEGIN TO IMPACT PORTIONS OF
NORTHEAST IL AND EVENTUALLY NORTHWEST IN THIS EVENING THROUGH
THE EARLY OVERNIGHT HOURS. WHILE CONFIDENCE REMAINS HIGH ON THE
GENERAL TIMING AND LAKE EFFECT SNOW OCCURRING, THE INLAND EXTENT
OF THE BAND, ESPECIALLY NEAR ORD AND MDW, REMAINS UNCERTAIN.
THAT SAID, HAVE DECIDED TO MAINTAIN THE PROB30 FOR SNOW SHOWERS
AT ORD AND MDW BETWEEN 18Z AND 21Z WHICH SEEMS REASONABLE AT
THIS TIME. REGARDLESS, AREAS NEAR THE LAKE (INCLUDING GYY) WILL
SEE IFR VISIBILITIES (AROUND 1SM) ASSOCIATED WITH THE LAKE
EFFECT SNOW AND ACCUMULATIONS IN THE 1-2 INCH RANGE THOUGH
LOCALLY HIGHER AMOUNTS UP TO 3 INCHES ARE POSSIBLE IN NORTHWEST
IN. SINCE ORD AND MDW MAY REMAIN JUST OUTSIDE OF THE MAIN BAND,
IMPACTS AT THESE TERMINALS ARE EXPECTED TO BE LESS WITH 2-3 MILE
VISIBILITIES AND MAYBE A COUPLE TENTHS TO AROUND HALF AN INCH
OF ACCUMULATION.
AWAY FROM THE LAKE AND LAKE EFFECT SNOW, DRY CONDITIONS ARE
EXPECTED TO PREVAIL THROUGH THE FORECAST PERIOD. WHILE MOST
AREAS WILL SEE SKIES CONTINUING TO SCATTER OUT THIS AFTERNOON,
THOSE NEAR THE LAKE WILL CONTINUE TO SEE MVFR CEILINGS PREVAIL
THROUGH THIS EVENING BEFORE THE LAKE EFFECT CLOUDS SHIFT EAST
INTO NORTHWEST AND NORTHERN IN ALLOWING VFR CONDITIONS TO RETURN
LATE TONIGHT INTO SATURDAY MORNING. THE EXCEPTION HOWEVER, WILL
BE AT GYY WERE MVFR CEILINGS LOOK TO PREVAIL THROUGH THE END OF
THE FORECAST PERIOD.
FINALLY, WINDS TODAY WILL REMAIN BREEZY OUT OF THE NORTH-
NORTHWEST WITH GUSTS IN THE 25-30 KT RANGE THROUGH EARLY
EVENING. HOWEVER, WINDS COULD VERY BETWEEN A 350-010 DIRECTION
AT ORD, MDW, AND GYY WITH THE LAKE EFFECT SNOW THIS AFTERNOON.
GUSTS ARE EXPECTED TO DIMINISH AFTER SUNSET WITH 5-10 KT WINDS
REMAINING THROUGH THE CONCLUSION OF THE PERIOD.
YACK
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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