186
FXUS63 KLOT 200014
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
614 PM CST THU DEC 19 2024
KEY MESSAGES
- PERIODS OF SNOW ARE EXPECTED THIS EVENING THROUGH TOMORROW
EVENING. UNTREATED ROADWAYS MAY BECOME SLIPPERY DURING TIMES
WHEN SNOW IS FALLING, INCLUDING DURING THE FRIDAY MORNING
COMMUTE.
- NORTHERLY WINDS WILL GUST 30 TO 40 MPH TOMORROW MORNING AND
AFTERNOON BEHIND A COLD FRONT.
- NEXT RAIN WILL FEATURE OPPORTUNITIES FOR RAINFALL, INCLUDING
DURING THE HOLIDAYS.
DISCUSSION
ISSUED AT 153 PM CST THU DEC 19 2024
THROUGH FRIDAY NIGHT:
REGIONAL WATER VAPOR IMAGERY DEPICTS A WOUND-UP SHORTWAVE IN
SOUTHERN WISCONSIN, A SECONDARY SHORTWAVE WITH AN ASSOCIATED SURFACE
LOW DIVING INTO FAR NORTHWESTERN IOWA, AND A THIRD, BROADER,
SHORTWAVE DROPPING INTO THE PLAINS. SNOW ASSOCIATED WITH THE LEAD
SHORTWAVE HAS LARGELY REMAINED IN SOUTHERN WISCONSIN. IN FACT, PARTS
OF OUR AREA ARE EXPERIENCING PARTLY SUNNY SKIES, WHICH HAS ALLOWED
FOR TEMPERATURES TO RISE TO OR ABOVE FREEZING. REGIONAL RADAR
IMAGERY DEPICTS A SHIELD OF SNOW MAINLY ACROSS SOUTHERN WISCONSIN,
WITH A SECONDARY AREA DEVELOPING FROM SOUTHWESTERN MINNESOTA TO
NORTHEASTERN IOWA. ALL THINGS CONSIDERED, IT'S ACTUALLY A SOMEWHAT
PLEASANT AND QUIET AFTERNOON OUT THERE.
OVER THE NEXT 12 TO 18 HOURS, ALL THREE SHORTWAVES WILL MERGE AND
PHASE WHILE TRAVERSING OVER THE LOWER GREAT LAKES. THE RESULTING
SURFACE LOW WILL PASS DIRECTLY OVER NORTHERN ILLINOIS TONIGHT INTO
TOMORROW MORNING, AND ALLOW FOR SEVERAL PERIODS OF WINTRY
PRECIPITATION IN OUR AREA. THEY ARE AS FOLLOWS:
(1) THIS EVENING THROUGH EARLY FRIDAY MORNING. PRECIPITATION ECHOES
CURRENTLY DEVELOPING FROM SOUTHEASTERN MINNESOTA THROUGH
NORTHEASTERN IOWA ARE EXPECTED EXPAND SOUTHEASTWARD INTO NORTHERN
ILLINOIS AND NORTHEASTERN INDIANA FROM THIS POINT FROWARD.
WHILE PRECIPITATION TYPE SHOULD START AS SNOW, INCREASING
DIFFERENTIAL WARM-AIR ADVECTION MAY CAUSE THERMAL PROFILES TO
WARM SUFFICIENTLY TO SUPPORT A BRIEF PERIOD OF FREEZING RAIN,
PARTICULARLY NEAR THE I- 80 TO I-88 CORRIDORS, AND THEN PERHAPS
EVEN JUST COLD RAIN FOR A TIME MAINLY ALONG AND SOUTH OF I-80.
OWING TO LOW CONFIDENCE ON WHETHER FREEZING RAIN WILL ACTUALLY
MATERIALIZE, AS WELL AS A RELATIVELY WARM GROUND TEMPERATURES
NEAR OR ABOVE FREEZING FROM THIS AFTERNOON, WILL WITHHOLD
INTRODUCING ANY ICE ACCUMULATIONS INTO OUR FORECAST IN SPITE OF
ADVERTISING A STRIPE OF SLIGHT CHANCES (15%) FOR A PERIOD OF
FREEZING RAIN (AGAIN, NEAR THE I-80 AND I-88 CORRIDORS). ALONG
AND NORTHEAST OF I-90, ALL SNOW IS EXPECTED THIS EVENING AND
OVERNIGHT. RELATIVELY WARM THERMAL PROFILES SUPPORT A SOMEWHAT
LOW-RATIO SNOW, WITH ANYWHERE FROM 0.5 TO LOCALLY 2 INCHES
FAVORED ALONG AND NORTHEAST OF I-90. SLICK SPOTS MAY DEVELOP ON
ANY UNTREATED ROADWAY WHERE WINTRY PRECIPITATION (FREEZING RAIN
AND/OR SNOW) FALLS.
(2) DAYBREAK THROUGH MID-FRIDAY MORNING. AS THE PHASING OF
SHORTWAVES COMPLETES, A COLD FRONT ORIGINATING IN MICHIGAN AND
WISCONSIN WILL COLLAPSE SOUTHWESTWARD INTO NORTHERN ILLINOIS AND
INDIANA LEADING TO A PERIOD OF PREDOMINANTLY SNOW. CONTINUED WARM
THERMAL PROFILES SUPPORT A CONTINUED LOW-RATIO/POOR QUALITY SNOW. IN
FACT, SNOW MAY MIX WITH RAIN ACROSS NORTHEASTERN ILLINOIS AND
NORTHWESTERN INDIANA BY MID-FRIDAY MORNING AS LAKE-INDUCED TROUGHING
CAUSES NORTHEASTERLY WINDS TO TRANSPORT ABOVE-FREEZING MARINE
AIRMASS TEMPERATURES INLAND. FOR THESE REASONS, SNOW ACCUMULATIONS
SHOULD RANGE FROM 0.5 TO LOCALLY 2 INCHES, HIGHEST AGAIN GENERALLY
ALONG AND NORTHEAST OF I-90. NORTHERLY WINDS ALONG AND BEHIND THE
COLD FRONT WILL QUICKLY INCREASE WITH FREQUENT GUSTS OF 30 TO 40
MPH. ALL THINGS CONSIDERED, THE FRIDAY MORNING COMMUTE MAY BE SLICK
WITH LOW VISIBILITY DUE TO THE COMBINATION OF FALLING AND BLOWING
SNOW.
(3). FRIDAY AFTERNOON AND EVENING. WHILE THE SNOW (MIXED WITH RAIN)
WILL TAPER ACROSS THE AREA BY EARLY FRIDAY AFTERNOON, A CONVERGENCE
AXIS ALONG THE BACKSIDE OF THE SYSTEM WILL CAUSE A TRANSITORY BAND
OF LAKE EFFECT SNOW TO MOVE DOWN THE LAKE MICHGIAN SHORELINE. AT
THIS POINT, AN ENSEMBLE OF CAM GUIDANCE FAVORS THE BAND SCRAPING THE
ILLINOIS SHORELINE DURING THE AFTERNOON HOURS BEFORE SETTLING IN
NORTHWESTERN INDIANA AFTER SUNSET. SINCE THE UPPER-LEVEL TROUGH AXIS
WILL BE LONG GONE BY THE TIME THE LAKE EFFECT SNOW MAKES IT TO OUR
AREA, STUNTED AND LOWERING INVERSION HEIGHTS (NEAR THE BOTTOM OF IF
NOT ENTIRELY BELOW THE DGZ) SUGGESTS LOW SNOW-RATIO/POOR QUALITY
SNOWFALL WITH ONLY 0.5 TO PERHAPS LOCALLY 3 INCHES OF ADDITIONAL
SNOW, HIGHEST IN NORTHWESTERN INDIANA.
TAKEN ALTOGETHER, THE NEXT 24 TO 36 HOURS WILL BE CHARACTERIZED BY
PERIODS OF WINTRY PRECIPITATION, A PERIOD OF GUSTY NORTHERLY WINDS,
AND A PERIOD OF LAKE EFFECT SNOW. DID GIVE SOME CONSIDERATION TO
WHETHER A WINTER WEATHER ADVISORY WAS WARRANTED FOR PARTS OF
NORTHEASTERN IL WHERE TRAVEL IMPACTS MAY BE MAXIMIZED OVERNIGHT AND
TOMORROW MORNING, BUT OPTED TO WITHHOLD GIVEN SOMEWHAT MARGINAL
SURFACE/GROUND TEMPERATURES, AN EXPECTATION FOR LOW RATIO/ QUALITY
SNOW, AND A SOMEWHAT PROLONGED PERIOD OF TIME DURING WHICH SNOW WILL
BE FALLING. WITH THAT SAID, ALL MOTORIST ARE ENCOURAGED TO LEAVE
EXTRA TRAVEL TIME IF ON THE ROADS TONIGHT AND TOMORROW GIVEN
UNTREATED ROADWAYS WILL STILL BE PRONE TO BECOMING SLIPPERY.
BORCHARDT
SATURDAY THROUGH THURSDAY:
DRIER AIR ADVECTING IN FROM THE NORTHWEST AND LOWERING
INVERSION HEIGHTS SHOULD DIMINISH LES POTENTIAL THROUGH THE DAY
ON SATURDAY FOR PORTIONS OF NORTHWEST INDIANA. SOME SPARSE LIGHT
LES SHOWERS WILL SWING CLOCKWISE NEAR THE ILLINOIS SHORE
SATURDAY NIGHT AS A SURFACE RIDGE PASSES TO THE NORTH, THOUGH A
MODEST LAND BREEZE SHOULD KEEP MUCH OF THE SHOWER ACTIVITY OVER
THE LAKE.
A PROMINENT RIDGE OVER THE WESTERN CONUS EARLY THIS WEEKEND
WILL GRADUALLY DEAMPLIFY WHILE DRIFTING EASTWARD TO THE WESTERN
GREAT LAKES BY SUNDAY NIGHT. BROAD LOW-LEVEL WAA AHEAD OF A
PACIFIC WAVE CROSSING THE NORTHERN GREAT PLAINS COMBINED WITH A
PLUME OF GULF MOISTURE ADVECTING TOWARD THE REGION WILL LIKELY
PROMOTE LIGHT PRECIP OVER THE AREA ON MONDAY BEFORE A COLD FRONT
SHIFTS SOUTH OF THE AREA BY MONDAY EVENING. WITH SURFACE
TEMPERATURES LAGGING STRONGER WAA ALOFT, PRECIP MAY BEGIN AS
FREEZING DRIZZLE/RAIN IN THE MORNING BEFORE LOW-LEVEL THERMO
PROFILES SUFFICIENTLY WARM TO SWITCH OVER TO RAIN BY THE
AFTERNOON.
THE COLD FRONT IS PROGGED TO STALL ACROSS THE OHIO VALLEY
TUESDAY INTO CHRISTMAS DAY, THOUGH THERE ARE VARYING SOLUTIONS
AMONG GUIDANCE WITH THE PHASING AND SUBSEQUENT EVOLUTION OF A
SOUTHERN GREAT PLAINS WAVE DRAWING AN UNSEASONABLY MOIST AIRMASS
NORTHWARD TOWARD THE AREA CHRISTMAS DAY INTO CHRISTMAS NIGHT.
TEMPS SHOULD BE WARM ENOUGH TO SUPPORT ALL RAIN, THOUGH TEMPS
HAVE TRENDED COLDER AND INTO THE 30S FOR CHRISTMAS.
KLUBER
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 153 PM CST THU DEC 19 2024
A PERIOD OF NORTHERLY GALE FORCE WINDS TO 35 KT MAY OCCUR IN
THE ILLINOIS AND INDIANA NEARSHORE WATERS OF LAKE MICHIGAN
TOMORROW MORNING AND 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.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 3 AM FRIDAY TO 9 AM CST SATURDAY FOR
THE IL NEARSHORE WATERS.
SMALL CRAFT ADVISORY FROM 6 AM FRIDAY TO 3 PM CST SATURDAY FOR
THE IN NEARSHORE WATERS.
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