990
FXUS63 KLOT 160502
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1102 PM CST WED JAN 15 2025
KEY MESSAGES
- SCATTERED GUSTY SNOW SHOWERS MAY LEAD TO LOCAL SLICK SPOTS
MID-EVENING AND OVERNIGHT.
- FRIDAY WILL BE BLUSTERY WITH ABOVE NORMAL TEMPS, THEN THE
COLDEST AIR MASS OF THE SEASON THUS FAR WILL ARRIVE SUNDAY AND
LINGER THROUGH MID NEXT WEEK WITH DANGEROUSLY LOW WIND CHILLS
POSSIBLE AT TIMES.
- RAIN FRIDAY EVENING CHANGING TO WET SNOW BEFORE ENDING EARLY
SATURDAY, PRIMARILY FOR AREAS SOUTHEAST OF I-55.
- DURING THE COLD SNAP, MOST IF NOT ALL OF THE PRECIP WILL BE LAKE
EFFECT SNOW, POTENTIALLY INCLUDING NORTHEAST PORTER CO. AT TIMES.
UPDATE
ISSUED AT 844 PM CST WED JAN 15 2025
OTHER THAN A BIT OF VERY MINOR FINE-TUNING OF THE TIMING, NO
REAL CHANGES MADE TO THE FORECAST FOR THE SNOW SHOWERS TONIGHT.
FIRST BAND SEEN ON RADAR MOVING ACROSS NORTHWESTERN PORTIONS OF
CWA IS ENCOUNTERING DRY LOW LEVELS AND MAINLY PRODUCING VIRGA
TO A FEW FLURRIES. ADDITIONAL ECHOES UPSTREAM ACROSS SOUTHERN WI
WILL LIKELY PRODUCE SCATTERED SNOW SHOWERS ACROSS THE AREA FROM
LATE THIS EVENING INTO THE FIRST HALF OR SO OF THE OVERNIGHT
HOURS. ACCUMULATIONS OF A COATING TO A HALF INCH STILL LOOK
RIGHT ON TARGET.
ONE CHANGE TO THE FORECAST FOR TONIGHT WAS TO REMOVE MENTION OF
FREEZING DRIZZLE AND FLURRIES FOR LATE TONIGHT AND THURSDAY
MORNING. UPSTREAM OBSERVATIONS SHOW VERY LITTLE STRATUS AND WHAT
STRATUS THERE IS REMAINS HIGH BASED AND UNLIKELY TO PRODUCE
DRIZZLE. VAST MAJORITY OF THE CEILING GUIDANCE SUGGESTS THAT
CLOUD BASES WILL REMAIN MOSTLY VFR (>3KFT) LATE TONIGHT INTO
THURSDAY MORNING, WHICH IS WHAT UPSTREAM OBSERVATIONS ALSO SHOW.
FINALLY, THE LATEST FORECAST SOUNDINGS FROM THE HRRR AND RAP
BOTH SHOW MEAN RH VALUES LESS THAN 85% IN THE 0-1KM LAYER WITH
LOW LEVEL SUBSIDENCE. FREEZING DRIZZLE IS VERY CHALLENGING TO
FORECAST, BUT STRUGGLING TO FIND MUCH SUPPORT FOR FZDZ, EXCEPT
IN THE NAM WHICH IS NOTORIOUS FOR BEING TOO MOIST/SATURATED IN
THE BOUNDARY LAYER IN THE WINTER.
DID NUDGE TEMPS UPWARD A BIT OVERNIGHT, ESPECIALLY OVER THE
NORTHWESTERN CWA WHERE LACK OF SNOW COVER AND STRONG SOUTHWEST
WINDS AND RESULTANT WARM AIR ADVECTION LIKELY RESULTS IN SLOWLY
RISING TEMPS TONIGHT. BY SUNRISE, TEMPERATURES SHOULD WARM INTO
THE LOW TO MID 20S, WARMEST OVER THE NORTHWESTERN CWA. UPSTREAM
DEWPOINTS ARE MOSTLY IN THE UPPER SINGLE DIGITS TO MID TEENS,
SO THAT SHOULD RESULT IN T/TD DEPRESSIONS OF AT LEAST 5-8F BY
MORNING, WHICH ALSO SUPPORTS THE NOTION THAT FREEZING DRIZZLE IS
UNLIKELY.
- IZZI
DISCUSSION
ISSUED AT 246 PM CST WED JAN 15 2025
THROUGH THURSDAY NIGHT:
AN AMPLIFYING MID TO UPPER-LEVEL TROUGH ON THE NORTHEAST
PERIPHERY OF A STACKED STRONG JET WILL DIG SOUTHEASTWARD ACROSS
THE WESTERN GREAT LAKES THROUGH TONIGHT. ROBUST DEEP FORCING
WITHIN THE LEFT-EXIT OF THE MID TO UPPER-LEVEL JET AND A RIBBON
OF LOW-LEVEL WAA/ISENTROPIC ASCENT WILL FEED INTO SEASONABLY
STEEP MID-LEVEL LAPSE RATES TO PRODUCE SCATTERED BURSTS OF GUSTY
SNOW SHOWERS ACROSS NORTHERN ILLINOIS AND NORTHWEST INDIANA
MID-EVENING THROUGH THE EARLY OVERNIGHT HOURS. UPSTREAM
OBSERVATIONS SHOW THAT A BROKEN AXIS OF POTENT SNOW SHOWERS IS
ALREADY PRESENT ACROSS CENTRAL MINNESOTA EARLY THIS AFTERNOON,
FURTHER SUPPORTING TONIGHT'S EXPECTATIONS. WHILE THE OVERALL
COVERAGE OF THE SNOW SHOWERS IS LESS CERTAIN, THE STRONGEST SNOW
SHOWERS WILL BE CAPABLE OF PRODUCING QUICK ACCUMULATIONS OF UP
TO A HALF INCH ALONG WITH SW/W GUSTS IN EXCESS OF 25 MPH. BY THE
PRE-DAWN HOURS, ACCUMULATIONS WILL LIKELY RANGE FROM A TRACE TO
A COUPLE LOCALIZED NARROW BANDS UP TO AROUND 1" ACROSS NORTHERN
ILLINOIS (PARTICULARLY NORTHEAST ILLINOIS) AND NORTHWEST
INDIANA.
WITH THE LOSS OF DEEPER MOISTURE BEHIND THE MID/UPPER-LEVEL
TROUGH, CLOUD ICE WILL BECOME LESS PREVALENT IN THE LINGERING
LOW-LEVEL STRATUS. THOUGH LOW-LEVEL ASCENT WILL BE WANING WITH
TIME FROM WEST TO EAST, PATCHY FREEZING DRIZZLE AND/OR LIGHT
POOR-QUALITY SNOW/FLURRIES MAY PROCEED THE SNOW SHOWERS FOR
SEVERAL HOURS. THIS INCLUDES MUCH OF NORTHEAST ILLINOIS THROUGH
AROUND MID-MORNING. OVERALL, SPORADIC IMPACTS ARE POSSIBLE DUE
TO LOCALLY SLIPPERY SPOTS FROM THE SNOW SHOWERS AND FREEZING
DRIZZLE.
TEMPS SHOULD WARM TO AROUND THE FREEZING MARK FOR SEVERAL HOURS
LATE MORNING THROUGH THE AFTERNOON THURSDAY AS STRATUS PERSISTS
ACROSS THE AREA. EVENTUALLY, A GRADUALLY LOWERING INVERSION IN
RESPONSE TO A PASSING MID-LEVEL RIDGE WILL ERODE THE REMAINING
STRATUS FROM SOUTHWEST TO NORTHEAST LATE THURSDAY NIGHT INTO
FRIDAY MORNING.
KLUBER
FRIDAY THROUGH WEDNESDAY:
DEEP 985-990 MB SURFACE LOW PRESSURE WILL PASS NORTH OF LAKE
SUPERIOR ON FRIDAY, WITH THE ASSOCIATED COLD FRONTAL TROUGH
APPROACHING FROM THE MIDWEST. A TIGHT PRESSURE GRADIENT BETWEEN
LOW PRESSURE TO OUR NORTH AND WEST AND EXPANSIVE HIGH PRESSURE TO
THE SOUTHEAST AMIDST A ROBUST LOW-LEVEL JET ALOFT WILL SET THE
STAGE FOR BLUSTERY SOUTH-SOUTHWESTERLY WARM AIR ADVECTION. MOST OF
THE GUIDANCE SCOURS OUT ANY LINGERING LOW CLOUDS EARLY IN THE DAY,
BUT THIS ISN'T A SURE THING. IN ADDITION, THE EXISTING SNOW COVER
OVER THE SOUTHEAST 1/2 TO 2/3 OR SO OF THE CWA (AND WELL SOUTH OF
THERE) CASTS UNCERTAINTY ON HOW WARM TEMPS WILL GET GIVEN THAT
THE BRISK SOUTHERLY FLOW WILL BE OCCURRING OVER EXPANSIVE SNOW COVER.
THERE SHOULD BE A NORTHWEST (MILDER) TO SOUTHEAST HIGH TEMP
GRADIENT OVER THE AREA, POSSIBLY SHARPER IN REALITY THAN WE CAN
DEPICT AT THIS RANGE. OUR AFTERNOON FORECAST OFFICIALLY SHOWS
UPPER 30S TO LOWER 40S NEAR AND NORTHWEST OF I-55 (MILDEST IN
NORTHWEST IL) AND MID 30S SOUTHEAST OF I-55. LOWER MIXING DEPTHS
WHERE TEMPS STAY COOLER SHOULD KEEP WIND GUSTS IN CHECK A BIT,
THOUGH IT'LL BE BLUSTERY AREAWIDE WITH GUSTS UP TO 30-35 MPH.
BANDED PRECIP WILL BLOSSOM IN THE FRONTAL TROUGH FRIDAY EVENING
AS COLD AIR ADVECTION ENSUES ALOFT, PRIMARILY SOUTHEAST OF I-55.
EXPECT A SHARP GRADIENT ON THE NORTHWEST EDGE OF THE PRECIP
SHIELD, WHICH WILL BE FURTHER REFINED IN SUBSEQUENT UPDATES. AN
INITIALLY ABOVE FREEZING PROFILE WILL RESULT IN THE PRECIP
STARTING AS RAIN, THOUGH FOR AREAS THAT DO RECEIVE PRECIP, PTYPE
SHOULD MIX WITH AND THEN CHANGE TO WET SNOW BEFORE ENDING IN THE
PRE-DAWN HOURS OF SATURDAY. A SLUSHY COATING OF SNOW IS POSSIBLE
ON COLDER SURFACES, HOWEVER MOSTLY ABOVE FREEZING AIR TEMPS
WILL LIKELY LIMIT TRAVEL IMPACTS.
TEMPERATURES WILL PRIMARILY FLAT-LINE SATURDAY (LOW 20S NW TO
NEAR 30F SOUTHEAST) AMIDST BLUSTERY NORTH-NORTHWEST WINDS
GUSTING UP TO 30 MPH AND LINGERING POST-FRONTAL STRATUS, WITH
PERHAPS SOME SPOTTY FLURRIES. PM WIND CHILLS WILL BE BACK DOWN
IN THE SINGLE DIGITS AND LOWER TEENS. LAKE EFFECT PARAMETERS
WILL BE SOMEWHAT MARGINAL ON SATURDAY, THOUGH SUFFICIENT TO HOLD
ONTO LOWER CHANCE (~30%) POPS INTO NORTHEAST PORTER COUNTY.
AS CROSS-POLAR FLOW TAKES HOLD SATURDAY NIGHT AND ONWARD, A
TRULY ARCTIC AIR MASS WILL ARRIVE WITH 850 MB TEMPS BOTTOMING
OUT AS LOW AS -25 TO -30C IN THE SUNDAY NIGHT-TUESDAY MORNING
TIMEFRAME. THE LACK OF SNOW COVER IN PARTS OF OUR AREA (WHICH
WILL LIKELY SEE SOME FURTHER EROSION ON FRIDAY) AND UPSTREAM
MAKES IT A BIT UNCERTAIN EXACTLY HOW COLD TEMPS AND WIND CHILLS
MAY GET IN THIS STRETCH. THE LOWEST TEMPS WILL OCCUR SUNDAY
NIGHT-MONDAY AM AND MONDAY NIGHT-TUESDAY AM WHEN SUB-ZERO LOWS
ARE FORECAST AREA WIDE ALONG WITH MINIMUM WIND CHILLS RANGING
FROM -15 TO -25F. DAYTIME HIGH TEMPS WILL LIKELY BE HELD INTO
THE POSITIVE SINGLE DIGITS FOR THE NORTHWEST HALF OF THE AREA ON
SUNDAY AND AREA-WIDE ON MONDAY-TUESDAY. SOME MODERATION BACK TO
MERELY BELOW NORMAL WILL OCCUR BY NEXT WEDNESDAY.
THE NORTHWEST TO WESTERLY FLOW PATTERN DURING THE COLD SNAP WILL
LIKELY RELEGATE PRECIP CHANCES TO PERSISTENT LAKE EFFECT SNOW
FOR THE FAVORED SNOW BELTS, POSSIBLY CLIPPING NORTHEAST PORTER
COUNTY AT TIMES. WHILE WE CERTAINLY CAN'T RULE OUT SUBTLE LOW
AMPLITUDE IMPULSES BEING ABLE TO WRING OUT LIGHT SNOW OR
FLURRIES IN SUCH A COLD PATTERN, THE MAIN MESSAGE IS TO PREPARE
FOR THE AT TIMES DANGEROUSLY COLD CONDITIONS.
CASTRO
AVIATION /06Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 1102 PM CST WED JAN 15 2025
- AT LEAST PERIODIC MVFR CIGS EXPECTED THROUGH TODAY.
- GUSTY NORTHWEST WINDS EXPECTED FROM MID MORNING UNTIL LATER
THIS AFTERNOON.
FIRST BATCH OF FLURRIES AND SCATTERED SNOW SHOWERS WILL BE
SOUTHEAST OF THE TERMINALS BY THE START OF THE 06Z TAFS. ONE
LAST BAND OF SCATTERED SNOW SHOWERS OVER WISCONSIN COULD BRIEFLY
IMPACT THE TERMINALS BETWEEN 0700-0830Z TIME FRAME. AFTER THAT
EXPECT A MIX OF VFR AND MVFR CIGS IS EXPECTED THROUGH MUCH OF
TODAY WITH A TENDENCY TOWARD MORE MVFR LIKELY DURING THE DAY AS
WINDS BECOME NORTHWESTERLY.
GUSTINESS OF THE WESTERLY WINDS ARE EXPECTED TO BECOME LESS
FREQUENT OR ABATE ALTOGETHER DURING THE PREDAWN HOURS BEFORE
BECOMING NORTHWEST AND PICKING UP AGAIN AFTER SUNRISE. SOME
OCCASIONAL PEAK GUSTS AROUND 30KT ARE POSSIBLE LATE THIS MORNING
INTO THE EARLY AFTERNOON. WIND SPEEDS AND GUSTS ARE EXPECTED TO
EASE THIS EVENING.
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 PM CST THURSDAY FOR WINTHROP
HARBOR IL TO GARY IN.
SMALL CRAFT ADVISORY UNTIL 9 PM CST THURSDAY FOR GARY TO BURNS
HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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