719
FXUS63 KLOT 171128
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
528 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 /12Z TAFS THROUGH 18Z SATURDAY/
ISSUED AT 528 AM CST FRI JAN 17 2025
THE MAIN AVIATION WEATHER CONCERNS ARE:
- DEVELOPING STRONG AND GUSTY SSW WINDS THIS MORNING AND
AFTERNOON. A FEW GUSTS TO 35 KNOTS POSSIBLE.
- MARGINAL LLWS TODAY.
- PERIOD OF RAIN THIS EVENING, MAINLY SOUTH AND EAST OF ORD.
PRECIPITATION LARGELY LOOKS TO END PRIOR TO A CHANGEOVER TO
SNOW.
- FROPA WITH A NORTHWEST WIND SHIFT TONIGHT. GUSTS AROUND 25
KNOTS INTO SATURDAY. POTENTIAL FOR MVFR CIGS.
VFR WITH PASSING CIRRUS WILL PREVAIL THIS MORNING. PATCHES OF
FZFG WELL TO THE SOUTH OF C90 SHOULD REMAIN AWAY FROM THE
CHICAGO-AREA TERMINALS.
WINDS EARLY THIS MORNING WILL REMAIN LIGHT, UNDER ABOUT 10
KNOTS, AND MAY OCCASIONALLY BECOME SSE AT ORD/MDW. THROUGH MID
TO LATE MORNING, SOUTHWESTERLY WIND GUSTS WILL DEVELOP AND WILL
INCREASE INTO THE AFTERNOON. A FEW GUSTS NEAR 35 KNOTS APPEAR
POSSIBLE, ALTHOUGH CURRENTLY DON'T LOOK PERSISTENT ENOUGH TO
JUSTIFY INCREASING GUSTS IN THE TAF THAT HIGH AT THIS TIME. A
50-55 KNOT LLJ IS ALSO INTENSIFYING OVERHEAD AND WILL LEAD TO A
PERIOD OF MARGINAL LLWS TODAY.
RAIN WILL DEVELOP AND SPREAD NORTHEASTWARD THIS EVENING AND
OVERNIGHT. THE HIGHEST CHANCES ARE AT GYY AND THEN DIMINISH WITH
NORTHWEST EXTENT. HAVE LEFT ORD DRY FOR NOW GIVEN THE LATEST
INDICATIONS OF A SIGNIFICANT PRECIPITATION GRADIENT. AT THIS
TIME, IT LARGELY LOOKS LIKE PRECIPITATION WILL END RIGHT BEFORE
SURFACE TEMPERATURES COOL ENOUGH TO SUPPORT A CHANGEOVER. THAT
SAID, CAN'T RULE OUT A BRIEF PERIOD OF -RASN/-SNRA AT GYY IF
PRECIPITATION LINGERS BEYOND 07/08Z.
FINALLY, A COLD FRONT WILL DELIVER A NORTHWEST WIND SHIFT TO THE
TERMINALS OVERNIGHT. WINDS LOOK TO GUST TO ABOUT 25 KNOTS
FOLLOWING FROPA AND THROUGH SATURDAY MORNING. WHILE POST-FRONTAL
MVFR CIGS ARE POSSIBLE, CHANCES REMAIN JUST A BIT TOO LOW FOR A
FORMAL INTRODUCTION IN THE TAFS AT THIS TIME.
CARLAW
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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