047
FXUS63 KLOT 221137
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
537 AM CST MON DEC 22 2025
KEY MESSAGES
- TEMPERATURE MODERATION WILL BEGIN TODAY WITH ABOVE NORMAL
TEMPS EXPECTED THROUGH SATURDAY.
- THERE IS A LOW (LESS THAN 20%) CHANCE FOR LIGHT RAIN/SPRINKLES
ACROSS FAR NORTHERN IL THIS AFTERNOON/EVENING.
- A PERIOD OF SOMEWHAT DREARY CONDITIONS IS SHAPING UP WEDNESDAY
AND THURSDAY WITH PERIODS OF DRIZZLE AND SHOWERS, LOW CLOUDS,
FOG, AND STEADY TEMPERATURES GENERALLY IN THE 40S.
DISCUSSION
ISSUED AT 244 AM CST MON DEC 22 2025
THROUGH TUESDAY:
THE SURFACE HIGH THAT BROUGHT US THE SUNNY WEATHER YESTERDAY
CONTINUES ITS EASTWARD PROGRESSION ACROSS THE APPALACHIANS THIS
MORNING WHILE A SUBTLE SHORTWAVE IMPULSE SWINGS ACROSS THE
DAKOTAS. AHEAD OF THE SHORTWAVE, A PLUME OF MID-LEVEL CLOUD
COVER HAS OVERSPREAD NORTHERN IL AND NORTHWEST IN AND LOOKS TO
REMAIN OVERHEAD THROUGH AT LEAST THIS EVENING. WHILE RADAR HAS
CONTINUED TO SHOW SOME REFLECTIVITY ECHOES OVERHEAD, THE VERY
DRY SUB-CLOUD LAYER (AS NOTED IN RECENT AIRCRAFT SOUNDING OUT OF
ORD) HAS BEEN PREVENTING ANY PRECIPITATION FROM REACHING THE
SURFACE. WITH THIS LOW-LEVEL DRY AIR EXPECTED TO REMAIN IN PLACE
THROUGH MUCH OF THE DAY, SUSPECT DRY CONDITIONS WILL PREVAIL
FOR MANY. HOWEVER, THERE IS A PLUME OF MOISTURE (CENTERED AROUND
2000 TO 3500FT) THAT IS ADVECTING NORTHWARD OUT OF THE SOUTHERN
PLAINS AND GULF COAST. THIS MOISTURE IS PROGGED TO ARRIVE INTO
NORTHERN IL TOWARDS MID-AFTERNOON AND COULD SUPPORT A PERIOD OF
LIGHT RAIN AND/OR SPRINKLES/DRIZZLE LATE THIS AFTERNOON AND
EVENING. GIVEN THAT FORECAST SOUNDINGS SHOW THE MOISTURE DEPTHS
AROUND 1500 FT AND THE RELATIVELY WEAK FORCING, CONFIDENCE ON
ANY PRECIPITATION OCCURRING REMAINS LOW (<15-20%). THAT SAID,
DID OPT TO INCLUDE A FORMAL SPRINKLE MENTION IN THE FORECAST FOR
PORTIONS OF FAR NORTHEAST IL AND NORTHWEST IN AS A PRECAUTION.
OUTSIDE OF THE CLOUDS AND NON-ZERO PRECIPITATION CHANCES,
TEMPERATURES TODAY WILL BEGIN TO MODERATE AS SOUTH-SOUTHWEST
WINDS PUMP IN WARMER AIR TO THE REGION. THEREFORE, EXPECT HIGHS
TO TOP OUT IN THE LOW TO MID-40S TODAY (UPPER 30S NEAR IL-WI
LINE) DESPITE THE CLOUD COVER. ADDITIONALLY, WINDS THIS
AFTERNOON COULD ALSO BECOME BREEZY IF SUFFICIENT MIXING OCCURS
WITH GUSTS AROUND 20-25 MPH.
HEADING INTO TONIGHT, ANY RAIN/DRIZZLE/SPRINKLES THAT
MATERIALIZE WILL GRADUALLY SHIFT EAST BY MIDNIGHT WITH GUSTS
ALSO DIMINISHING DURING THE EARLY EVENING HOURS. SO CONDITIONS
TONIGHT WILL BE TRANQUIL WITH MOSTLY CLOUDY SKIES AND
TEMPERATURES COOLING SLIGHTLY INTO THE MID TO UPPER 30S
AREAWIDE.
WHILE A WEAK COLD FRONT IS EXPECTED TO MOVE THROUGH THE AREA ON
TUESDAY, TEMPERATURES BEHIND THE FRONT LOOK TO STILL BE MILD
WITH HIGHS IN THE MID TO UPPER 40S DUE TO PLUME OF WARMER AIR
ALOFT (+8 TO +12 C TEMPERATURES AT 850 MB). SKIES ON TUESDAY
ALSO LOOK TO SCATTER OUT BEHIND THE FRONT WITH PARTLY CLOUDY
SKIES CURRENTLY FORECAST, BUT THERE IS A CHANCE THAT SOME
LINGERING LOW-LEVEL MOISTURE KEEPS A STRATUS DECK AROUND. IF
THIS STRATUS DECK DOES MATERIALIZE, SLIGHTLY COOLER
TEMPERATURES COULD OCCUR TUESDAY AFTERNOON.
YACK
TUESDAY NIGHT THROUGH NEXT MONDAY:
ENSEMBLE MODEL GUIDANCE REMAINS IN STEADFAST AGREEMENT THAT
BROAD UPPER-LEVEL QUASI-ZONAL FLOW WILL BE ESTABLISHED ACROSS
THE NORTHERN UNITED STATES BY THE MIDDLE OF THE WEEK, LARGELY
OWING TO ANOMALOUSLY STRONG TROUGHING ANCHORED ALONG THE PACIFIC
COAST. THE LOW-LEVEL BAROCLINIC ZONE WILL BE SOMEWHAT OFFSET
FROM THE UPPER-LEVEL JET AND EVEN SOUTH OF OUR AREA BY TUESDAY
NIGHT AFTER A REINFORCING PERIOD OF NORTH TO NORTHEASTERLY WINDS
IN THE WAKE OF A SURFACE HIGH PRESSURE SYSTEM MOVING INTO
SOUTHERN ONTARIO. TUESDAY NIGHT IS HENCE SHAPING UP TO BE FAIRLY
QUIET WITH PARTLY CLOUDY SKIES AND OVERNIGHT LOWS IN THE UPPER
20S TO LOWER 30S.
ON WEDNESDAY, NEUTRAL ADVECTIVE AND HEIGHT TENDENCIES AS WELL
AS INCREASING UPPER-LEVEL CLOUD COVER SUGGEST A SLOW START TO
DAY WITH TEMPERATURES ONLY GRADUALLY CLIMBING THROUGH THE 30S
AND INTO THE LOW 40S BY EARLY AFTERNOON. ENSEMBLE GUIDANCE HAS
BEEN REMARKABLY CONSISTENT IN SHOWING A LOW-AMPLITUDE SHORTWAVE
SHEDDING FROM THE AFOREMENTIONED AGGREGATE PACIFIC TROUGHING AND
"RIDING" THE PREVAILING ROSSBY WAVE INTO THE LOWER GREAT LAKES
WEDNESDAY EVENING. INCREASING WARM-AIR ADVECTION AHEAD OF THE
SHORTWAVE AND ATOP THE BAROCLINIC ZONE WILL LEAD TO A BLOSSOMING
AREA OF DRIZZLE AND LOW-INTENSITY RAIN SHOWERS, WITH BOTH EPS
AND GEFS PROBABILITIES FOR >0.01" OF QPF EXCEEDING 80% ACROSS
PARTS OF OUR AREA. OWING TO A FAULTY QMD BIAS CORRECTION METHOD
THAT ERRONEOUSLY IGNORES LOW QPF AMOUNTS (TYPICAL OF DRIZZLE),
AUTOMATED NBM POPS WEDNESDAY EVENING THROUGH EARLY THURSDAY
MORNING WERE WOEFULLY TOO LOW. SO, WENT AHEAD AND RE-INTRODUCED
LIKELY (>55%) CHANCES FOR SHOWERS AND DRIZZLE INTO OUR FORECAST,
FOCUSED AFTER SUNSET WEDNESDAY EVENING. WITH LITTLE MOVEMENT IN
THE PLACEMENT OF THE BAROCLINIC ZONE, TEMPERATURES SHOULD STAY
STEADY IN THE UPPER 30S TO LOWER 40S OVERNIGHT.
ON THURSDAY/CHRISTMAS DAY, ENSEMBLE MODEL GUIDANCE HAS
CONTINUED A TREND OF THE QUASI-ZONAL UPPER-LEVEL FLOW BRIEFLY
TRANSITIONING TO RIDGING ACROSS THE UPPER MISSISSIPPI RIVER
VALLEY. STRENGTHENING MID-LEVEL SUBSIDENCE WILL SIMILARLY
STRENGTHEN THE LOW-LEVEL INVERSION, SETTING THE STAGE FOR
TRAPPED MOISTURE TO TRANSITION INTO FOG. ANY SUBTLE WAVE
TRAVERSING THROUGH THE RIDGE WOULD BE PRONE TO EXCITING PERIODS
OF DRIZZLE, THOUGH HAVE CAUTIOUSLY INTRODUCED IN A RELATIVE
MINIMA IN POPS TO THE 10 TO 20% THROUGH EARLY AFTERNOON.
THURSDAY EVENING, A MORE FORMIDABLE UPPER-LEVEL SHORTWAVE WILL
PROPAGATE THROUGH THE JET TO OUR NORTH LEADING TO THE
DEVELOPMENT OF A LOW PRESSURE SYSTEM SOMEWHERE IN THE GREAT
LAKES. WITH MEAN LOW POSITIONING IN ENSEMBLE GUIDANCE NORTH OF
OUR AREA, CURRENTLY FAVOR ANOTHER PERIOD OF WARM-AIR ADVECTION
THAT SHOULD SUPPORT ANOTHER PERIOD OF DRIZZLE AND LOW-INTENSITY
RAIN SHOWERS ACROSS OUR AREA. WITH BOTH EPS AND GEFS GUIDANCE
DEPICTING >60% CHANCES FOR >0.01" OF QPF ESPECIALLY WITH
NORTHWARD EXTENT, HAVE AGAIN BOOSTED POPS BEYOND THE PALTRY NBM,
ESPECIALLY AFTER SUNSET.
IN TERMS OF TEMPERATURES ON CHRISTMAS DAY... THE AUTOMATED NBM
IS EGREGIOUSLY OUT OF PHASE WITH INPUT ENSEMBLE DATA IN THE
FORECAST PLACEMENT AND WIDTH OF THE LOW-LEVEL BAROCLINIC ZONE.
FOR EXAMPLE, THE MEAN HIGH TEMPERATURE FROM THE 00Z EPS, GEFS,
AND CMCE AT CHICAGO O'HARE IS 44F AMIDST A STEADY NORTHEAST WIND
OFF LAKE MICHIGAN. MEANWHILE, THE HIGH TEMPERATURE FORECAST
FROM THE NBM AT CHICAGO O'HARE IS 57F WITH A STOUT SOUTHWESTERLY
BREEZE. WITH THIS IN MIND, WILL CONTINUE TO ESSENTIALLY IGNORE
THE NBM AND INSTEAD RELY ON OTHER, MORE TRUSTWORTHY, BLENDED
GUIDANCE TO FEATURE HIGH TEMPERATURES GENERALLY IN THE 40S
(NORTH) TO LOWER 50S (SOUTH). A FEW UPPER 50 TO LOWER 60 DEGREE
READINGS MAY STILL BE IN PLAY SOMEWHERE NEAR THE IL-9 CORRIDOR
DEPENDING ON THE EXACT PLACEMENT OF THE WARM FRONT, THOUGH AREAS
NORTH OF I-80 WILL BE NOWHERE CLOSE TO THAT WARM. AND,
CONSIDERING DETERMINISTIC GUIDANCE IS TRENDING EVEN COOLER THAN
ENSEMBLE COUNTERPARTS (MEAN HIGH FROM THE 00Z ECMWF, GFS, AND
CMC AT O'HARE IS 41F), IT'S POSSIBLE OUR FORECAST HIGH
TEMPERATURES CHRISTMAS DAY REMAIN SOME 3 TO 5 DEGREES TOO WARM
IN SOME LOCATIONS.
FORECAST CONFIDENCE LOWERS FROM FRIDAY ONWARD OWING TO GROWING
UNCERTAINTIES IN THE LOCATION OF THE LOW-LEVEL BAROCLINIC ZONE.
SUSPECT THE NBM-DELIVERED HIGHS ON FRIDAY AND SATURDAY IN THE
UPPER 50S TO AROUND 60 MAY STILL BE RUNNING TOO WARM GIVEN INPUT
ENSEMBLE DATA ARE MORESO IN THE 40S TO AROUND 50, BUT WILL
FORGO ANY CHANGES FOR NOW IN FAVOR OF SEEING WHERE THINGS LIE AS
WE GET CLOSER. BY EARLY NEXT WEEK, ENSEMBLE MODEL GUIDANCE
CONTINUES TO SUGGEST THE PACIFIC TROUGH WILL SHIFT EASTWARD
TOWARD THE GREAT LAKES AND POTENTIALLY PHASE WITH NORTHERN WAVES
EMANATING OUT OF CANADA. WILL NOTE A GROWING SIGNAL FOR A
PERIOD OF COLD/BELOW AVERAGE TEMPERATURES BEHIND WHATEVER
COMES OF THAT SYSTEM TO CLOSE OUT 2025.
BORCHARDT
AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
ISSUED AT 537 AM CST MON DEC 22 2025
AVIATION KEY MESSAGES:
- MARGINAL LLWS CONDITIONS MAY OCCUR BETWEEN 12-18Z THIS
MORNING, MAINLY FOR SMALL AIRCRAFT.
- EXPANSIVE MVFR STRATUS LIFTING ACROSS THE MID-MISSISSIPPI
RIVER WILL SPREAD ACROSS THE TERMINALS BY EARLY AFTERNOON.
- INTERMITTENT RAINDROPS MAY ACCOMPANY THE STRATUS THIS
AFTERNOON. NO DROPS IN VISIBILITY ARE EXPECTED.
- INVERSION HEIGHTS ABOVE THE STRATUS LAYER WILL LOWER THROUGH
THE NIGHT, LEADING TO CORRESPONDING LOWERING CLOUD BASES. THE
EXPECTATION IS FOR MVFR TO TRANSITION TO IFR OVERNIGHT, AND
POSSIBLY FLIRT WITH LIFR TOWARD DAYBREAK.
- WINDS WILL STAY SOUTHERLY THROUGH THE DAY BEFORE VEERING
SOUTHWESTERLY AND EVENTUALLY WESTERLY THROUGH THE LATTER HALF
OF THE TAF PERIOD.
DISCUSSION:
A PASSING UPPER-LEVEL WAVE RIDING ATOP A RESIDUALLY DRY LOW-
LEVEL AIRMASS CONTINUES TO GENERATE EXPANSIVE MID-LEVEL CLOUDS
(BASED NEAR 10000FT) AND VIRGA ACROSS THE AIRSPACE THIS
MORNING. AS THE SYSTEM SLIDES EASTWARD THROUGH THE NEXT 3 TO 6
HOURS, A MODEST LOW-LEVEL JET CURRENTLY POSITIONED WEST OF THE
MISSISSIPPI RIVER TO SLIDE EASTWARD OVER THE TERMINALS. WITH
FORECAST SOUNDINGS DEPICTING AROUND 40 TO 45 KT OF FLOW LARGELY
CENTERED AT 2500FT, CONTINUE TO SUSPECT THAT FORMAL LLWS
THRESHOLDS WILL NOT BE MET. WITH THAT SAID, CANNOT RULE OUT A
PIREP OR TWO FROM LLWS FROM SMALL AIRCRAFT MAINLY BETWEEN 12
TO 18Z THIS MORNING.
ATTENTION THEN TURNS TOWARD AN EXPANSIVE REGION OF STRATUS EXTENDING
FROM THE GULF ALL THE WAY UP THROUGH THE MIDDLE MISSISSIPPI
RIVER VALLEY. SOUTHWESTERLY STEERING FLOW WILL GUIDE THE STRATUS
TO THE TERMINALS BY EARLY AFTERNOON. USING RAP 925/850MB RH
FIELDS AS A PROXY FOR THE LEADING EDGE OF THE STRATUS, WILL GO
AHEAD AND NUDGE UP THE ARRIVAL TIME AT ALL TERMINALS TO AS EARLY
AS 19Z AT RFD AND 20Z AT ORD/MDW. CLOUD BASES WILL GRADUALLY
LOWER WITH TIME AS THE STRATUS SPREADS OVER THE TERMINALS,
STARTING AT 3000 TO 3500 FT AND LOWERING TO 2500 FT BY MID
AFTERNOON. CONFIDENCE IN STRATUS ARRIVAL TIME IS MEDIUM TO
HIGH.
INTERESTINGLY, RAP/HRRR FORECAST SOUNDINGS DEPICT THE STRATUS
DEPTH GROWING TO NEARLY 5KFT AS IT APPROACHES THE TERMINALS.
WHEN COMBINED WITH CONTINUED WARM-AIR ADVECTION (E.G., LIFT) ON
THE SOUTHERN SIDE OF THE AFOREMENTIONED UPPER-LEVEL WAVE
TRAVERSING THE GREAT LAKES, DO THINK THERE MAY BE INTERMITTENT
RAIN DROPS THAT FALL ACROSS THE AREA. SO, WILL INTRODUCE PROB30
GROUPS FOR -SHRA AT ALL TERMINALS, FOCUSED ON THE AFTERNOON
HOURS. AT THIS POINT, DO NOT FORESEE ANY REAL VISIBILITY
REDUCTIONS OWING TO CLOUD BASES REMAINING ABOVE 2000 FT.
CONFIDENCE IN PRECIPITATION OCCURRING IS LOW (30% CHANCE).
AFTER SUNSET, BUILDING MID-LEVEL HEIGHTS IN THE WAKE OF THE
UPPER-LEVEL WAVE AND ASSOCIATED SUBSIDENCE WILL CAUSE THE
INVERSION AT THE TOP OF THE STRATUS LAYER TO LOWER. TIME/HEIGHT
PROFILES OF MOISTURE FROM THE HRRR/RAP SHOW A CLASSIC LOWERING
STRATUS SIGNAL, WITH CLOUD BASES FALLING THROUGH MVFR AND INTO
IFR THROUGH THE OVERNIGHT HOURS. BY THEN, THE DEPTH OF THE
STRATUS SHOULD BE THIN ENOUGH TO LIMIT ADDITIONAL PRECIPITATION
GENERATION. DO HAVE TO NOTE A SIGNAL IN CAM GUIDANCE FOR
ADVECTION FOG TO DEVELOP AND RACE EASTWARD ACROSS THE AIRSPACE
TOWARD DAYBREAK PRESUMABLY AS THE INVERSION BUILDS CLOSER TO THE
SURFACE. HOWEVER, GIVEN A SOMEWHAT TIGHT LOW-LEVEL PRESSURE
GRADIENT MAINTAINING SURFACE LOW, SIMPLY INTERPRET SUCH A SIGNAL
AS A CONTINUED LOWERING STRATUS DECK. CONFIDENCE IN CEILING
HEIGHT TRENDS OVERNIGHT IS MEDIUM.
TOWARD OR JUST AFTER THE END OF THE 24 TO 30 HOUR TAF PERIOD,
SUSPECT HOLES IN THE STRATUS WILL BECOME MORE PREVALENT AS
CONTINUED 25 TO 30 KT OF FLOW ATOP THE LOWERING INVERSION LEADS
TO ENTRAINMENT OF DRY AIR. SO, WILL CAUTIOUSLY ADVERTISE A
TRANSITION FROM SCT TO BKN TOWARD THE END OF THE 30-HOUR TAF
WINDOWS AT ORD/MDW AS CLOUD BASES APPROACH 500FT. WITH THAT
SAID, CONFIDENCE IN HOW FAST STRATUS ERODES IS LOW.
FINALLY, THE NORTH-TO-SOUTH ORIENTATION OF THE PRESSURE
GRADIENT BETWEEN A DEPARTING HIGH PRESSURE SYSTEM OVER THE
EASTERN SEABOARD AND AGGREGATE SURFACE TROUGHING IN THE PLAINS
SHOULD MAINTAIN GENERALLY SOUTHERLY WINDS (DIRECTION 170 TO 190)
THROUGH DAYLIGHT HOURS. AFTER SUNSET, THE EXPECTATION IS FOR
WINDS TO GRADUALLY VEER SOUTHWESTERLY AND THEN WESTERLY AS A
SURFACE LOW EMERGES FROM THE AGGREGATE TROUGHING OUT WEST AND
TRACKS EASTWARD ACROSS THE UPPER GREAT LAKES. CONFIDENCE IN THE
WIND FORECAST IS MEDIUM TO HIGH.
BORCHARDT
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...NONE.
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