350
FXUS63 KLOT 171746
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1146 AM CST FRI JAN 17 2025
KEY MESSAGES
- ABOVE NORMAL TEMPERATURES AND BLUSTERY CONDITIONS EXPECTED
TODAY
- RAIN MOVES IN FROM THE SOUTH TONIGHT CHANGING OVER TO A
RAIN/SNOW MIX. ANY SLUSHY ACCUMS ARE EXPECTED ON COLD SURFACES
AND MAINLY SOUTH AND EAST OF I-55
- COLDEST AIRMASS OF THE SEASON ARRIVES SATURDAY NIGHT AND
LINGERS THROUGH TUESDAY NIGHT.
- DANGEROUSLY COLD WIND CHILLS (FROSTBITE POTENTIAL IN ~30
MINUTES) PROBABLE SUNDAY AND MONDAY NIGHTS.
- MOST OF THE PRECIP DURING THE COLD SNAP WILL BE LAKE EFFECT
SNOW EAST OF OUR NORTHWEST INDIANA COUNTIES, THOUGH FLURRIES
ARE POSSIBLE ON MONDAY.
DISCUSSION
ISSUED AT 259 AM CST FRI JAN 17 2025
THROUGH SATURDAY:
NIGHTTIME SATELLITE IMAGERY SHOWS CLOUDS ARE DRAPED OVER AN
UPPER LEVEL RIDGE FROM MINNESOTA TO NORTHEASTERN INDIANA.
HOWEVER, THERE IS LESS CLOUD COVER IN THE SOUTHERN PORTION OF
THE FORECAST AREA. SURFACE OBS JUST SOUTH OF THE FORECAST DOMAIN
HAVE SHOWN ISOLATED LOCATIONS WITH LOWER VIS LIKELY DUE TO THE
COMBINATION OF BETTER RADIATIONAL COOLING UNDER THE CLEARER
SKIES AS WELL AS THE THICKER SNOWPACK. AS A RESULT, THERE IS A
CHANCE FOR SOME PATCHY FOG THIS MORNING, MAINLY AROUND AND SOUTH
OF US-24, BUT CONFIDENCE IS LOW FOR DENSE FOG DEVELOPMENT.
AS THE UPPER LEVEL RIDGE MOVES EAST THROUGH FRIDAY, SKIES WILL
CLEAR. TEMPERATURES ARE EXPECTED TO RISE ABOVE FREEZING TO AT
OR JUST ABOVE 40 DEGREES. AS THE SURFACE PRESSURE GRADIENT
STRENGTHENS THIS MORNING, SOUTHERLY WINDS WILL INCREASE CREATING
BLUSTERY CONDITIONS THROUGH THE DAY. WIND GUSTS CAN BE EXPECTED
BETWEEN 30 TO 35 MPH (THOUGH A LOCALIZED PEAK TO 40 MPH IS
CERTAINLY POSSIBLE).
THE NEXT UPPER LEVEL WAVE OVER THE SOUTHERN PLAINS WILL
TRAVERSE NORTHEASTWARD THROUGH THE DAY AND OVER CENTRAL
ILLINOIS TONIGHT. TEMPS SHOULD BE WARM ENOUGH AS THE SYSTEM
ARRIVES FOR THE INITIAL PRECIPITATION TO BEGIN AS RAIN. BUT
WITH REINFORCING COLD AIR ADVECTION MOVING IN SWIFTLY BEHIND IT,
THERE IS AN EXPECTED TRANSITION TO A RAIN/SNOW MIX (OR EVEN
BRIEFLY ALL SNOW) ALONG THE BACKSIDE AS TEMPERATURES DROP
OVERNIGHT. WITH DRY AIR MOVING INTO NORTHWEST ILLINOIS THROUGH
THE EVENING, THE MAIN PRECIPITATION POTENTIAL WILL BE SOUTH AND
EAST OF A PERU TO EVANSTON LINE, AND ANY MINOR/SLUSHY SNOW
ACCUMS ARE EXPECTED TO BE RELEGATED EAST OF I-55.
THE SYSTEM IS EXPECTED TO MOVE EAST AFTER DAYBREAK MAKING FOR
QUIETER CONDITIONS ON SATURDAY. WINDS WILL TURN OUT OF THE
NORTHWEST BEHIND THE FRONT. THERE IS A CHANCE FOR SOME
LINGERING LAKE EFFECT SNOW AROUND PORTER COUNTY INDIANA THROUGH
SATURDAY MORNING, BUT GIVEN THE MORE MARGINAL SET-UP, THE
FORECAST WAS LEFT AT JUST A "SLIGHT CHANCE" MENTION FOR
ADDITIONAL SNOW. HOWEVER, THOSE NORTHWEST WINDS WILL INCREASE
THROUGH THE DAY CREATING BLUSTERY CONDITIONS ONCE AGAIN, WITH
GUSTS UP TO 30 MPH.
DK
SATURDAY NIGHT THROUGH THURSDAY:
AN UPSTREAM ARCTIC AIRMASS WILL MAKE AN INITIAL CONCERTED PUSH
TOWARDS THE REGION SATURDAY NIGHT AND INTO SUNDAY. WHILE THE
CORE OF COLDEST AIR WILL REMAIN TO OUR NORTHWEST THROUGH THIS
PERIOD, A FAIRLY STOUT PRESSURE GRADIENT WILL BE IN PLACE AS AN
IMPRESSIVE MID 1040 MB HIGH BEGINS TO BUILD INTO THE WESTERN
DAKOTAS. THIS WILL HOLD WINDS UP AROUND 15 MPH THROUGH SATURDAY
NIGHT WHICH, WHEN COMBINED WITH AIR TEMPERATURES MEANDERING DOWN
THROUGH THE SINGLE DIGITS WILL LIKELY YIELD WIND CHILL VALUES
IN THE -10 TO -20 DEGREE RANGE BY DAYBREAK SUNDAY.
AS THIS RESERVOIR OF COLD AIR SPILLS INTO THE REGION, A FAIRLY
ROBUST SHORTWAVE WILL PUSH OVERHEAD INTO SUNDAY MORNING. AN
ATTENDANT BRIEF F-GEN CIRCULATION WILL DEVELOP, DRIVING AN
INCREASE IN MID AND UPPER LEVEL SATURATION, MAINLY ABOVE 750 MB.
GUIDANCE IS QUIET ON THE QPF FRONT, AND WITH FORECAST SOUNDINGS
REVEALING FAIRLY DRY CONDITIONS BELOW 800 MB, HAVE LEFT THINGS
DRY IN THE GRIDDED FORECASTS, ALTHOUGH WILL CONTINUE TO KEEP AN
EYE ON THIS TIME PERIOD.
STEEP LOW-LEVEL LAPSE RATES ON SUNDAY ALONG WITH THE CONTINUED
TIGHT SURFACE PRESSURE GRADIENT LOOKS TO SUPPORT GUSTS
INCREASING THROUGH ABOUT 25 MPH DURING THE LATE MORNING AND
AFTERNOON. WITH AIR TEMPERATURES MOVING LITTLE THROUGH THE DAY,
WIND CHILL VALUES WILL NOT "WARM" MEANINGFULLY EVEN IN THE
AFTERNOON.
THE CORE OF THE ARCTIC AIRMASS WILL BUILD INTO THE REGION
DURING THE SUNDAY NIGHT THROUGH MONDAY NIGHT TIME FRAME. SUB-
ZERO AIR TEMPERATURES ARE FORECAST ACROSS THE ENTIRE FORECAST
AREA BOTH SUNDAY AND MONDAY NIGHTS, WITH MONDAY NIGHT LOOKING
LIKE THE COLDEST OF THE STRETCH AS AN IMPRESSIVE CORE OF
APPROXIMATELY -30 C AIR AT 850 MB ARRIVES ACROSS NORTHERN
ILLINOIS. MINIMUM WIND CHILLS DURING THIS PERIOD LOOK TO FALL
SOLIDLY INTO THE -15TO -25 F RANGE, WITH SOME LOCALIZED
OCCURRENCES OF -30 F VALUES CERTAINLY IN PLAY DEPENDING ON WIND
TRENDS. GIVEN CURRENT MODEL OUTPUT AND ONGOING TRENDS (OR LACK
THEREOF), IT DOES LOOK LIKE WE'LL BE HEADED FOR COLD WEATHER
ADVISORIES FOR THE SUNDAY EVENING THROUGH TUESDAY MORNING TIME
FRAME. TUESDAY NIGHT WILL ALSO BE A COLD ONE WITH THE CENTER OF
THE 1040+ ARCTIC HIGH SLIDING OVERHEAD, BUT WINDS MAY END UP A
SMIDGE LIGHTER THAN PREVIOUS NIGHTS. TEMPERATURES WILL THEN
MODERATE INTO THE END OF THE WEEK, PERHAPS BACK UP TOWARDS
FREEZING.
WHILE GENERALLY PRECIPITATION-FREE CONDITIONS ARE EXPECTED
DURING THE LONG TERM, THE BACK EDGE OF LAKE EFFECT SNOW SHOWERS
MAY INTERMITTENTLY BUILD ACROSS PARTS OF PORTER COUNTY
THROUGHOUT THIS ARCTIC INTRUSION. THIS IS PARTICULARLY TRUE
SATURDAY NIGHT INTO SUNDAY NIGHTS BEFORE SURFACE WINDS BACK MORE
WESTERLY AND SOUTHWESTERLY. HAVE HELD ONTO SOME LOW-END POPS
INTO TUESDAY ACROSS PORTER CO. AS A RESULT. IN ADDITION, AS
HIGHLIGHTED BY THE PREVIOUS SHIFT, A VORT MAX EMBEDDED WITHIN
THE FAST CYCLONIC FLOW IS FORECAST TO SLICE THROUGH THE REGION
MONDAY AFTERNOON AND EVENING. WHILE FORECAST SOUNDINGS DON'T
DEPICT PARTICULARLY THICK CLOUD LAYERS, JUST A BIT OF ASCENT
THROUGH A NEAR-SATURATED -30 C LAYER MAY BE ENOUGH TO SQUEEZE
OUT SOME VERY FINE SNOW.
CARLAW
AVIATION /18Z TAFS THROUGH 00Z SUNDAY/
ISSUED AT 1146 AM CST FRI JAN 17 2025
FORECAST CONCERNS INCLUDE...
STRONG/GUSTY SOUTH/SOUTHWEST WINDS THROUGH EARLY THIS EVENING.
LOW LEVEL WIND SHEAR THROUGH THIS EVENING.
CHANCE OF RAIN, PERHAPS MIXING WITH SNOW LATE THIS EVENING.
COLD FRONT/WIND SHIFT TO STRONG/GUSTY NORTHWEST WINDS TONIGHT.
SOUTH/SOUTHWEST WINDS WILL GUST INTO THE 25-30KT RANGE THROUGH
SUNSET AND THEN STEADILY DIMINISH THROUGH THE EARLY EVENING,
THOUGH PERIODIC GUSTS ARE STILL EXPECTED PAST SUNSET. LOW LEVEL
WINDS WILL REMAIN STRONG THROUGH THE EARLY EVENING AND EVEN WITH
THE PREVAILING SURFACE GUSTS, ENOUGH DIRECTIONAL DIFFERENCE TO
CONTINUE LOW LEVEL WIND SHEAR MENTION THIS AFTERNOON AND ADDED
IT FOR A FEW HOURS THIS EVENING UNTIL WINDS FINALLY DIMINISH.
SURFACE WINDS MAY DIMINISH TO 10KTS OR LESS LATE THIS EVENING
AS THEY TURN MORE WESTERLY AND THEN A COLD FRONT WILL MOVE
ACROSS THE AREA EARLY SATURDAY MORNING, SHIFTING WINDS TO THE
NORTHWEST, POSSIBLY NORTH/NORTHWEST WITH GUSTS INCREASING INTO
THE MID 20KT RANGE, WHICH WILL LIKELY CONTINUE THROUGH MIDDAY
SATURDAY, WITH GUSTS POSSIBLY DIMINISHING SOME SATURDAY
AFTERNOON.
AN AREA OF LIGHT RAIN IS EXPECTED TO DEVELOP MID/LATE EVENING,
RIGHT ACROSS THE CHICAGO TERMINALS WITH A SHARP CUTOFF EXPECTED
ON THE NORTHWEST SIDE OF THIS PRECIPITATION. WHERE EXACTLY THIS
CUTOFF WILL OCCUR MAY MEAN ORD/DPA STAYS DRY WHILE MDW SEES SOME
LIGHT RAIN. OPTED TO INCLUDE PROB MENTION FOR ORD/DPA FOR A FEW
HOURS, AND TRENDS WILL NEED TO BE MONITORED. AS THIS LIGHT RAIN
ENDS AHEAD OF THE COLD FRONT, IT MAY MIX WITH SOME WET SNOW
FLAKES AND ADDED A MIX FOR ALL THE CHICAGO TERMINALS.
LOW VFR CIGS, 3-4KFT ARE EXPECTED TO DEVELOP THIS EVENING AND
CONTINUE THROUGH SATURDAY MORNING, POSSIBLY SCATTERING OUT
SATURDAY AFTERNOON. MVFR CIGS ARE POSSIBLE LATE THIS EVENING AND
OVERNIGHT, ESPECIALLY AT GYY BUT OVERALL CONFIDENCE IS LOW. CMS
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 3 PM CST SATURDAY FOR WINTHROP
HARBOR IL TO BURNS HARBOR IN.
SMALL CRAFT ADVISORY UNTIL 3 PM CST SUNDAY FOR BURNS HARBOR TO
MICHIGAN CITY IN.
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