615
FXUS63 KLOT 151757
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1157 AM CST WED JAN 15 2025
KEY MESSAGES
- SNOW SHOWERS MAY LEAD TO LOCAL SLICK SPOTS TONIGHT INTO
THURSDAY MORNING.
- A STORM SYSTEM LOOKS BRING A MIX OF RAIN AND SNOW TO THE
GENERAL REGION IN THE LATE FRIDAY THROUGH EARLY SUNDAY
TIMEFRAME.
- AFTER A BRIEF LATE WEEK WARM-UP, ANOTHER PUSH OF ARCTIC AIR
WILL ARRIVE OVER THE WEEKEND. TEMPERATURES EARLY NEXT WEEK
MAY BE THE COLDEST THUS FAR OF THE WINTER SEASON.
DISCUSSION
ISSUED AT 205 AM CST WED JAN 15 2025
THROUGH THURSDAY:
A RECENT HAND SURFACE ANALYSIS REVEALED A BROAD SURFACE HIGH
PRESSURE SYSTEM CENTERED OVER THE MID-MISSISSIPPI RIVER VALLEY.
CLEAR SKIES, LIGHT WINDS, A FRESH SNOWPACK, AND A RESIDENT DRY
AIRMASS HAS ALLOWED FOR TEMPERATURES TO DROP TOWARD 0F THIS MORNING.
THROUGHOUT THE DAY, THE SURFACE HIGH PRESSURE SYSTEM WILL SETTLE
SOUTHWARD. THE INCREASING PRESSURE GRADIENT ALONG THE NORTHERN FLANK
OF THE SURFACE HIGH WILL ACCORDINGLY ALLOW FOR LOW-LEVEL
SOUTHWESTERLY FLOW AND WARM AIR ADVECTION TO GRADUALLY INCREASE
THROUGHOUT THE DAY. ASSUMING SKIES STAY CLEAR (WHICH ASSUMES THE
MASS OF CLOUDS IN IOWA DOESN'T MAKE IT HERE), HIGHS THIS
AFTERNOON SHOULD TOP OFF IN THE UPPER TEENS TO LOWER 20S. IN
ALL, IT'S LOOKING LIKE A QUIET DAY.
TONIGHT, AN UPPER-LEVEL SHORTWAVE (CURRENTLY RACING FROM
SASKATCHEWAN INTO SOUTHERN MANITOBA) WILL DIVE INTO THE GREAT LAKES.
INCREASING UPPER-LEVEL DIFFLUENCE IN THE LEFT EXIT REGION OF AN
ASSOCIATED JET STREAK, MODEST DCVA AHEAD OF THE SHORTWAVE, AND
RELATIVELY STEEP MID-LEVEL LAPSE RATES (INCLUDING THROUGH THE DEPTH
OF THE DGZ) WILL PROVIDE SUFFICIENT FORCING FOR ASCENT TO SUPPORT
WIDESPREAD SNOW SHOWERS AS THE WAVE PASSES OVERHEAD. A DPROG/DT ON
500MB HEIGHTS FROM A VARIETY OF DETERMINISTIC AND ENSEMBLES VALID AT
DAYBREAK THURSDAY REVEALS A SUBTLE TREND SOUTHWESTWARD IN THE
POSITION OF THE UPPER-LEVEL WAVE, SUPPORTING INCREASING POPS
INTO THE LOW TO MID-RANGE (20 TO 50%) FOR SHOW SHOWERS ACROSS
OUR ENTIRE AREA, WITH THE HIGHEST CHANCES ACROSS NORTHEASTERN IL
AND NORTHWESTERN IN. COVERAGE OF SNOW SHOWERS SHOULD BE HIGHEST
GENERALLY BETWEEN MIDNIGHT AND DAYBREAK. COMPARED TO THE PAST
FEW ROUNDS OF SNOW, COVERAGE OF ACCUMULATIONS TONIGHT INTO
TOMORROW MORNING SHOULD BE LESS WIDESPREAD AND "STREAKY," WITH
SOME AREAS PICKING UP A QUICK FEW TENTHS OF AN INCH RIGHT NEXT
TO OTHERS THAT ONLY SEE A DUSTING. INCREASINGLY BREEZY NORTHWEST
WINDS MAY ALSO LEAD TO POCKETS OF LOW VISIBILITY OVERNIGHT, AS
WELL.
THURSDAY WILL START RELATIVELY CHILLY WITH WIND CHILLS IN THE SINGLE
DIGITS. FLURRIES AND ISOLATED SNOW SHOWERS MAY VERY WELL PREVAIL
THROUGH EARLY THURSDAY AFTERNOON UNTIL THE BACK EDGE OF THE UPPER-
LEVEL CYCLONIC SHEAR AXIS SLIDES OVERHEAD. MOSTLY CLOUDY SKIES AND
HIGHS IN THE LOWER 30S WILL MARK THE AFTERNOON.
BORCHARDT
THURSDAY NIGHT THROUGH TUESDAY:
MID-LEVEL RIDGING IS EXPECTED TO BE BUILDING INTO THE CENTRAL
CONUS AND GREAT LAKES REGIONS THURSDAY NIGHT INTO THE DAY ON
FRIDAY. THIS WILL RESULT IN STOUT WARM ADVECTION ACROSS NORTHERN
IL AND NORTHWEST IN WHICH WILL ALLOW TEMPERATURES TO WARM
NICELY INTO THE UPPER 30S TO AROUND 40 BY FRIDAY AFTERNOON.
HOWEVER, THERE HAS BEEN SOME GUIDANCE THAT IS SHOWING MORE CLOUD
COVER ADVANCING INTO THE AREA ON FRIDAY AHEAD OF A DEVELOPING
STORM SYSTEM (MORE ON THIS BELOW) WHICH WOULD MEAN COOLER
TEMPERATURES IF IT VERIFIES. AT THIS TIME, THE GUIDANCE MEMBERS
WITH THE HIGHER CLOUD COVERAGE AND COOLER TEMPERATURES APPEAR TO
BE OUTLIERS AND THUS HAVE MAINTAINED THE TREND FOR FRIDAY TO BE
THE WARMEST DAY OF THE WEEK. THOUGH, WILL KEEP AN EYE ON TRENDS
IN CASE THINGS CHANGE.
WHILE OUR AREA ENJOYS A BREAK FROM THE COLD AND ACTIVE WEATHER,
AN UPPER LOW IS EXPECTED TO MOVE ONSHORE IN SOUTHERN CALIFORNIA
AND GRADUALLY BROADEN INTO A SHORTWAVE TROUGH AS IT TRAVERSES
ACROSS THE SOUTHWEST CONUS ON FRIDAY. AT THE SAME TIME, A
BROADER TROUGH ASSOCIATED WITH THE POLAR JET IS EXPECTED TO BE
DIVING INTO THE NORTHERN CONUS AND EVENTUALLY PHASING WITH THE
SOUTHERN SHORTWAVE FRIDAY NIGHT INTO SATURDAY. GUIDANCE (BOTH
ENSEMBLE AND DETERMINISTIC) REMAINS IN GOOD AGREEMENT ON THE
SYSTEM EVOLUTION AND A THE DEVELOPMENT OF A SURFACE LOW NEAR THE
LOWER MISSISSIPPI VALLEY THAT SHOULD LIFT INTO THE OHIO RIVER
VALLEY FRIDAY NIGHT THROUGH THE DAY ON SATURDAY. AS THIS OCCURS,
CONFIDENCE IS GROWING THAT OUR AREA SHOULD SEE AT LEAST SOME
PRECIPITATION FROM THIS SYSTEM FRIDAY NIGHT THROUGH EARLY
SATURDAY, ESPECIALLY THOSE SOUTHEAST OF I-55. HOWEVER, THERE IS
STILL SOME LINGERING UNCERTAINTY ON WHERE EXACTLY THE SURFACE
LOW AND NORTHERN EDGE OF THE PRECIPITATION SHIELD WILL TRACK SO
BE MINDFUL THAT SUBTLE SHIFTS MAY STILL OCCUR.
THAT SAID, PRECIPITATION LOOKS TO START OUT FRIDAY NIGHT IN THE
FORM OF RAIN AS SURFACE TEMPERATURES AND THE LOWER-ATMOSPHERE
WILL BE ABOVE FREEZING. THOUGH, THERE HAS BEEN A NOTABLE TREND
IN GUIDANCE FOR THE MID-LEVELS TO COOL MUCH QUICKER FRIDAY NIGHT
AS A COLD FRONT SWINGS THROUGH THE AREA. THEREFORE, IT APPEARS
THAT THE TRANSITION FROM RAIN TO EITHER ALL SNOW OR A RAIN-SNOW
MIX (PENDING SURFACE TEMPERATURE RESPONSE) MAY BE QUICKER THAN
ORIGINALLY THOUGHT. WITH THE DURATION OF SNOW/WINTRY MIX
POSSIBLY LASTING LONGER, THERE IS A GROWING SIGNAL THAT SOME
SNOW ACCUMULATIONS COULD OCCUR BEFORE PRECIPITATION ENDS
SATURDAY MORNING, PARTICULARLY IN THE SOUTHEASTERN PORTIONS OF
THE CWA. RIGHT NOW IT IS STILL FAR TOO EARLY TO DISCUSS SPECIFIC
SNOW AMOUNTS AND ASSOCIATED IMPACTS, BUT THIS IS SOMETHING WE
WILL CONTINUE TO MONITOR GOING FORWARD.
FURTHERMORE, THERE ALSO REMAINS THE POTENTIAL FOR SOME LAKE
EFFECT SNOW SHOWERS TO DEVELOP ON THE BACKSIDE OF THE SYSTEM
LATE SATURDAY INTO THE DAY ON SUNDAY. WHILE INITIALLY IT LOOKS
AS IF THE LAKE EFFECT SHOULD BEGIN IN PORTIONS OF NORTHWEST IN
DUE TO NORTHERLY WINDS, IT SHOULD QUICKLY PIVOT INTO NORTHERN IN
AND LOWER MI AS WINDS TURN WEST-NORTHWESTERLY ON SUNDAY. THAT
SAID, LAKE EFFECT PARAMETERS DO NOT LOOK OVERLY IMPRESSIVE
DESPITE THE STRONG SIGNAL IN GUIDANCE SO THERE IS THE CHANCE THE
LAKE EFFECT COULD FAIL TO MATERIALIZE ALL TOGETHER OR BE MUCH
WEAKER THAN SOME GUIDANCE DEPICTS. FOR NOW HAVE MAINTAINED SOME
15-20% POPS IN NORTHWEST IN AND POINTS EAST TO ACCOUNT FOR THIS
POSSIBILITY AND WILL CONTINUE TO WATCH TRENDS AS WE GET CLOSER.
FINALLY, THE PHASING OF THE TWO AFOREMENTIONED FEATURES WILL
RESULT IN BROAD TROUGHING ESTABLISHING OVER THE CONTINENTAL
UNITED STATES FROM SUNDAY THROUGH AT LEAST THE END OF NEXT WEEK.
THIS TROUGHING WILL ALLOW LOBES OF ARCTIC AIR TO DIVE INTO THE
REGION RESULTING IN A RETURN TO BELOW AVERAGE TEMPERATURES AND
BITTERLY COLD WIND CHILLS HEADING INTO NEXT WEEK. GUIDANCE
REMAINS IN GREAT AGREEMENT THAT TEMPERATURES DURING THE EARLY
PART OF NEXT WEEK MAY STRUGGLE TO GET OUT OF THE SINGLE DIGITS
DURING THE SUNDAY TO TUESDAY TIMEFRAME. IN FACT, FORECAST HIGHS
ON MONDAY ARE FORECAST TO BE SOLIDLY IN THE SINGLE DIGITS WITH
OVERNIGHT LOWS SUNDAY NIGHT AND MONDAY NIGHT WELL BELOW ZERO.
COUPLE THESE TEMPERATURES WITH THE MODEST NORTHWEST WINDS
FORECASTED AND CONDITIONS LOOK TO BE IN PLACE TO GET WIND CHILLS
IN THE -15F TO -25F RANGE. ADDITIONALLY, ANY SHORTWAVES THAT
DIVE THROUGH THE PATTERN COULD BRING THE CHANCES FOR SNOW
SHOWERS. WHILE THERE IS NOT ANY NOTABLE SIGNALS FOR SUCH
ACTIVITY THROUGH AT LEAST THE MIDDLE OF NEXT WEEK, THE PATTERN
DOES LOOK TO BECOME MORE FAVORABLE FOR SHORTWAVES TO PIVOT
THROUGH OUR AREA SO STAY TUNED.
YACK
AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 1157 AM CST WED JAN 15 2025
AVIATION FORECAST CONCERNS FOR THE 18Z TAFS:
- PERIOD OF SNOW SHOWERS LIKELY TOWARD MIDNIGHT INTO THE PRE-
DAWN HOURS. PATCHY IFR VIS AND ACCUMS TO A HALF INCH POSSIBLE.
- PATCHY -FZDZ POSSIBLE AFTER 08-09Z INTO MID-THURSDAY MORNING.
- BREEZY SW WINDS INCREASING WITH GUSTS 20-25 KT BY THIS EVENING,
EVENTUALLY BECOMING NW GUSTS 25 KT THURSDAY MORNING BEHIND A
COLD FRONT.
QUIET VFR AVIATION WEATHER TO PERSIST FOR THE REMAINDER OF THE
AFTERNOON AND EARLY EVENING HOURS, WITH SOUTHWEST WINDS
GRADUALLY INCREASING AND GUSTING 20-25 KT. SHOULD SEE A GRADUAL
INCREASE IN MID-LEVEL VFR CLOUD COVER BY EARLY THIS EVENING AS
WELL.
ATTENTION THEN TURNS TO A MID-LEVEL DISTURBANCE CURRENTLY
DIGGING SOUTHEAST ACROSS THE UPPER MIDWEST, WHICH WILL APPROACH
THE LOCAL AREA LATER THIS EVENING AND TRANSIT THE AREA OVERNIGHT
INTO THURSDAY MORNING. THIS IS EXPECTED TO PRODUCE A PERIOD OF
SNOW SHOWERS AND IFR VSBYS AND MVFR CEILINGS ACROSS THE
TERMINALS FROM LATE EVENING/MIDNIGHT INTO THE PRE-DAWN HOURS.
THESE MAY PRODUCE UP TO A HALF INCH OF DRY/FLUFFY ACCUMULATION
(HIGHEST PROBABILITY OVER THE CHICAGO TERMINALS - LIKELY LOWER
FARTHER WEST AT KRFD). AFTER 08-09Z, MODEL FORECAST SOUNDINGS
INDICATE A DECREASE IN SATURATION AND LOSS OF CLOUD-ICE, WHICH
SUGGESTS THE POTENTIAL FOR PATCHY FREEZING DRIZZLE WHICH MAY
PERSIST INTO MID-THURSDAY MORNING BEFORE PRECIP ENDS. CONFIDENCE
DECREASES IN THE DEPTH OF THE LOW-LEVEL MOIST LAYER AND STRATUS
DECK DURING THIS TIME, THUS HAVE INCLUDED THE FZDZ AS A PROB30
AS OPPOSED TO THE PREVAILING/TEMPO MENTION OF SHSN PRIOR TO
08-09Z.
A COLD FRONT WILL MOVE ACROSS THE AREA IN THE 15-16Z THURSDAY
TIME FRAME, WHICH WILL SHIFT WINDS TO THE NORTHWEST WITH A
RENEWED ROUND OF GUSTS AROUND 25 KTS. PRECIPITATION POTENTIAL
SHOULD END QUICKLY WITH THE PASSAGE OF THE FRONT, WITH LINGERING
MVFR CEILINGS GRADUALLY RISING (AND POTENTIALLY BECOMING VFR IN
THE AFTERNOON).
RATZER
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 3 PM CST
THURSDAY FOR THE IL AND IN NEARSHORE WATERS.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page