735
FXUS63 KLOT 191953
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
153 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 /18Z TAFS THROUGH 00Z SATURDAY/
ISSUED AT 1152 AM CST THU DEC 19 2024
A VIGOROUS MID-LEVEL WAVE OVER FAR SOUTHWEST WISCONSIN WILL
TRACK EASTWARD THROUGH THIS EVENING WHILE SPREADING A BROAD
SHIELD OF SNOW AS FAR SOUTH AS THE CHICAGO METRO. DRY LOW-LEVELS
THIS AFTERNOON WILL INITIALLY LIMIT SNOW TO VIRGA ALOFT, THOUGH
SOME SPARSE FLURRIES ARE POSSIBLE. THE SOUTHERN EXTENT OF SNOW
SHIELD IS EXPECTED TO REACH THE CHICAGO TERMINALS BY 23-00Z WITH
PERIODS OF IFR VISIBILITY THROUGH MUCH OF THE EVENING. HAVE
MAINTAINED THE TEMPO IFR VISIBILITY, BUT IT IS POSSIBLE THE
LOWER VISIBILITY MAY PREVAIL FOR SEVERAL HOURS.
A TRAILING SURFACE LOW ASSOCIATED WITH THE PASSING MID-LEVEL
WAVE WILL CROSS THE TERMINALS LATE THIS EVENING THROUGH THE
OVERNIGHT HOURS, PROMOTING A PERIOD OF IFR CEILINGS AND POCKETS
OF SNSH EMBEDDED WITHIN SNDZ. VRB WINDS WILL BEGIN TO FAVOR SW
OR W UNDER 10 KNOTS DURING THIS TIME.
ON THE BACK EDGE OF THE LOW, A MORE FORMIDABLE PERIOD OF SN IS
POSSIBLE FOR SEVERAL HOURS FRIDAY MORNING AMID NNE WINDS GUSTING
TO 25 KNOTS OR HIGHER. THE CURRENT PROB30 FOR IFR VISIBILITY IN
SN MAY NEED TO BE CONVERTED TO A TEMPO, WITH THE POTENTIAL FOR
EVEN SOME BRIEF PERIODS OF LIFR VISIBILITY. BY FRIDAY AFTERNOON,
A LAKE-ENHANCED BAND OF SNOW WILL LIKELY SETTLE NEAR THE LAKE
MICHIGAN SHORE.
KLUBER
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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