806
FXUS63 KLOT 202336
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
536 PM CST TUE JAN 20 2026
KEY MESSAGES
- ACCUMULATING SNOW AND ASSOCIATED TRAVEL IMPACTS WILL DEVELOP
LATE THIS EVENING ACROSS FAR NORTHERN IL AND GRADUALLY SHIFT
SOUTH THROUGH EARLY WEDNESDAY MORNING, ENDING BY MID MORNING.
THE HIGHEST AMOUNTS OF 3-5" ARE FORECAST NEAR THE WI/IL STATE
LINE, TAPERING TO LESS THAN 1" SOUTH OF I-80. THE WINTER WEATHER
ADVISORY WAS EXPANDED SLIGHTLY SOUTH TO THE I-88/290 CORRIDOR.
- A STRONG ARCTIC FRONT MAY BRING A PERIOD OF GUSTY SNOW SHOWERS
WEDNESDAY EVENING FOLLOWED BY PATCHY BLOWING SNOW OVERNIGHT.
- A SECOND ARCTIC FRONT WILL USHER IN AN EXTENDED PERIOD OF
DANGEROUS COLD THURSDAY NIGHT THROUGH EARLY NEXT WEEK.
DISCUSSION
ISSUED AT 305 PM CST TUE JAN 20 2026
THROUGH WEDNESDAY:
YET ANOTHER QUICK HIT OF SNOW IS ON THE WAY TONIGHT INTO EARLY
WEDNESDAY MORNING, WITH THE ACCUMULATING SNOW SHUTTING OFF BY 8-9
AM WEDNESDAY. THERE SHOULD BE TWO RELATIVE "PHASES" OF THE SNOW
TONIGHT, DRIVEN BY A WEAK CLIPPER TYPE LOW PRESSURE SYSTEM
TRACKING OVER THE REGION.
THE FIRST PHASE OF THE EVENT WILL OCCUR BETWEEN ABOUT 8 PM AND 1
AM CST. STRENGTHENING WARM ADVECTION ALOFT WILL TIGHTEN UP THE
LOWER TO MID-LEVEL BAROCLINIC ZONE ACROSS THE REGION, RESULTING IN
AN AT LEAST TRANSIENT ROBUST FRONTOGENETICAL CIRCULATION AMIDST
GRADUALLY INCREASING LARGE SCALE ASCENT. BANDED SNOW WILL TAKE
SHAPE IN THIS TIMEFRAME, THOUGH TIME WILL BE NEEDED TO COMPLETE
TOP-DOWN SATURATION BEFORE ACCUMULATIONS COMMENCE. IN THIS
REGARD, THE HIGHER RESOLUTION (HIRES) GUIDANCE SHOULD HAVE A
BETTER HANDLE ON THE HIGHER QPF COMMON IN THESE MESOSCALE BANDS.
CONCEPTUALLY, WITH THE SURFACE LOW TRACK FROM SOUTHEAST IOWA TO
ACROSS NORTHERN/FAR NORTHERN ILLINOIS, THE NARROW MESOSCALE
BANDING SHOULD SET UP SOMEWHERE NEAR/NORTH OF I-88.
MUCH OF THE HIRES GUIDANCE CONTINUED TO FAVOR CLOSER TO THE
WISCONSIN STATE LINE AND UP INTO SOUTHERN WI WITH THIS INITIAL
BANDING. HOWEVER, SOME VARIANCE WAS UNSURPRISINGLY NOTED WITH THE
POSITION OF THE BANDING. EVEN AT A NEAR-TERM LEAD TIME, IT'S
COMMON FOR THE F-GEN DRIVEN BANDING TO END UP SHIFTED NORTH OR
SOUTH OF A CONSENSUS. FURTHERMORE, DRY AIR AND/OR SUBSIDENCE SOUTH
OF THE BAND OFTEN RESULTS IN A SHARPER GRADIENT BETWEEN
ACCUMULATING SNOW AND NO SNOW/NON-ACCUMULATING FLURRIES THAN CAN
BE REALISTICALLY DEPICTED IN THE GRIDDED DATABASE. WITH STEEP MID
TO UPPER LEVEL LAPSE RATES AND ROBUST LIFT MODELED THROUGH A
FAIRLY DEEP DGZ, RATIOS COULD BE AS HIGH AS 15 TO 20:1. THIS COULD
YIELD HOURLY SNOWFALL RATES UP TO ~1"/HOUR OR SO, EVEN BRIEFLY
HIGHER.
IN THE SECOND PHASE OF THE EVENT, SNOW WILL LIKELY EXPAND FAIRLY
FAR SOUTH WITH TIME, THOUGH WITH BETTER COLUMN MOISTURE STILL
FARTHER NORTH. THIS WILL OCCUR AS GOOD HEIGHT FALLS FROM A
NEUTRALLY TILTED BUT MODERATELY STRONG MID-LEVEL SHORT-WAVE
OVERSPREAD THE REGION IN THE 1-3 AM TIMEFRAME. DESPITE THE
TYPICALLY SOMEWHAT UNFAVORABLE SURFACE LOW TRACK, A WEST-SOUTHWEST
TO EAST-NORTHEAST ORIENTED BAND OF SNOW WILL SPREAD SOUTHEAST. THE
SNOW WILL BE MORE PROGRESSIVE DURING THIS TIMEFRAME, WITH THE
SHORTEST RESIDENCE TIME (UP TO 1-2 HOURS) SOUTH OF THE IL AND
KANKAKEE RIVER VALLEYS. CONFIDENCE INCREASED SUFFICIENTLY IN THIS
PORTION OF THE EVENT TO SUPPORT INCREASING POPS TO CATEGORICAL
(75-100%) FARTHER SOUTH. STEEP MID AND UPPER LEVEL LAPSE RATES ARE
MODELED DURING THE PRE-DAWN/EARLY WEDNESDAY TIMEFRAME, SO CAN
ENVISION RATIOS TO OR ABOVE 15:1 AND RATES AS HIGH AS 1/2" TO 1"
PER HOUR.
ALL IN ALL, TAKING THE VARIOUS COMPETING FACTORS INTO ACCOUNT FOR
THIS EVENT, OUR OFFICIAL FORECAST CONTINUES TO PEG THE WI STATE
LINE COUNTIES FOR THE HIGHEST POTENTIAL FOR 3-5" (LOCALLY
HIGHER/TO 6" IF BANDING OVERPERFORMS). WITH INCREASED CONFIDENCE
IN THE BROADER AREA OF SNOW EXTENDING INTO THE WEDNESDAY MORNING
COMMUTE AND CAUSING TRAVEL IMPACTS, AMOUNTS NEAR/NORTH OF I-88
CAME UP TO ROUGHLY 2-4". GIVEN THE LIKELIHOOD OF ACCUMULATING SNOW
AND TEMPS SOLIDLY BELOW FREEZING AFFECTING THE 5-8 AM PORTION OF
THE COMMUTE, OPTED TO EXPAND THE WINTER WEATHER ADVISORY SOUTH TO
INCLUDE THE I-88/I-290 CORRIDOR COUNTIES AS WELL AS CHICAGO. THE
EVENING SHIFT WILL TAKE ANOTHER LOOK AT OBSERVATIONAL AND GUIDANCE
TRENDS FOR ANY POSSIBLE ADDITIONAL CHANGES TO THE ADVISORY.
THE GOOD NEWS, SO TO SPEAK FOR THIS EVENT, IS THAT STRONGER
WEST-SOUTHWEST WINDS WILL NOT DEVELOP UNTIL AFTER THE SNOW ENDS.
BY THAT TIME, TEMPERATURES WILL BE WELL INTO THE 20S AND EVEN LOW-
MID 30S IN SPOTS FARTHER SOUTH, SO NOT CURRENTLY ANTICIPATING MUCH
IN THE WAY OF BLOWING SNOW ISSUES MID-LATE WEDNESDAY MORNING INTO
THE AFTERNOON. WEDNESDAY WILL BE THE LAST DAY BEFORE WE ENTER A
MULTI-DAY DEEP FREEZE, USHERED IN BY A SERIES OF ARCTIC FRONTS.
THIS FIRST FRONT WILL RESULT IN A PERIOD OF SNOW SHOWERS INTO
WEDNESDAY EVENING, DETAILED IN THE NEXT SECTION BELOW.
CASTRO
WEDNESDAY NIGHT THROUGH TUESDAY:
HIGH LATITUDE BLOCKING ACROSS ALASKA AND GREENLAND (ASSOCIATED
WITH THE NEGATIVE PHASE OF THE ARCTIC OSCILLATION) WILL CONTINUE
TO FOSTER ANOMALOUS TROUGHING ACROSS THE CENTRAL AND EASTERN
CONUS THROUGH THE FORECAST PERIOD. THIS LARGE SCALE HEMISPHERIC
PATTERN IS STRONGLY CORRELATED WITH BELOW TO MUCH BELOW NORMAL
TEMPERATURES, WITH INCREASED ARCTIC INTRUSIONS INTO OUR AREA.
ACCORDINGLY, OUR PRIMARY FORECAST CONCERN THROUGH THE PERIOD
CONTINUES TO FOCUS ON WHAT SHOULD BE OUR NEXT PERIOD OF
DANGEROUSLY COLD WEATHER, PARTICULARLY THURSDAY NIGHT THROUGH
SATURDAY.
THE FIRST OF TWO ARCTIC FRONTS IS SLATED TO SHIFT ACROSS OUR
AREA IN CONJUNCTION WITH THE NEXT LOW AMPLITUDE MID-LEVEL
IMPULSE DIVING SOUTHEASTWARD FROM THE UPPER MIDWEST WEDNESDAY
EVENING. ANOTHER QUICK (1 TO 3 HOUR) "THUMP" OF SNOW SHOWERS IS
LIKELY (60%+) TO ACCOMPANY THIS FRONT WEDNESDAY EVENING.
STEEPENING LOW- LEVEL LAPSE RATES AHEAD OF THE FRONT INTO EARLY
EVENING DOES ADD SOME CONCERN FOR POSSIBLE SNOW SQUALLS WITH
THIS FRONT, PARTICULARLY AS AN NOTABLE PRESSURE RISE/FALL
COUPLET LOOKS TO ACCOMPANY THE FRONT. HOWEVER, THERE IS SOME
SIGNAL THAT THE STRONGER WIND GUSTS MAY LAG A BIT BEYOND THE
MOST FAVORED PERIOD OF SNOW SHOWERS WEDNESDAY EVENING, THUS
POTENTIALLY LIMITING THE THREAT FOR TRUE DANGEROUS SNOW SQUALLS
TO SOME DEGREE. NEVERTHELESS, A QUICK HITTING PERIOD OF
ACCUMULATING SNOW IS EXPECTED IN THE 6-10 PM WINDOW WEDNESDAY
EVENING, AND THIS IS LIKELY TO RESULT IN SOME ADVERSE IMPACTS TO
TRAVEL ACROSS NORTHERN IL DUE TO REDUCED VISIBILITIES (BRIEFLY
UNDER A 1 MILE) AND SNOW COVERED ROADS.
ANY SNOW THAT ACCUMULATES, IN ADDITION TO THE SNOW THAT FALLS
TONIGHT NORTH OF I-80 COULD RESULT IN SOME BLOWING AND DRIFTING
SNOW LATER WEDNESDAY EVENING AND NIGHT, PARTICULARLY AS WEST-
NORTHWESTERLY WINDS INCREASE FOLLOWING THE COLD FRONTAL PASSAGE.
THIS COULD PARTICULARLY BECOME PROBLEMATIC IF THE WINDS IN THE
WAKE OF THE COLD FRONT END UP GUSTING IN EXCESS OF 40 MPH ACROSS
NORTHERN IL, AS SOME GUIDANCE DOES SUGGEST. HOWEVER, FORECAST
GUIDANCE DOES CONTINUE TO DISPLAY SOME RATHER LARGE SPREAD WITH
WIND MAGNITUDES ACROSS NORTHERN IL WEDNESDAY EVENING. THIS
APPEARS TO BE TIED TO SOME NOTED DIFFERENCES IN THE FORECAST
TRACK OF THE SURFACE PRESSURE TROUGH WEDNESDAY EVENING, AND THE
ENSUING STRENGTH OF THE COLD AIR ADVECTION EXPECTED IN ITS WAKE
LATER WEDNESDAY EVENING. NEVERTHELESS, WITH BLUSTERY WEST-
NORTHWEST WINDS LIKELY TO BECOME GUSTY UP TO AT LEAST 30 MPH
LATER WEDNESDAY EVENING AND NIGHT, I HAVE MAINTAINED THE MENTION
FOR PATCHY BLOWING SNOW NORTH FOR AREAS NORTH OF I-80.
BLUSTERY AND COLDER CONDITIONS ARE EXPECTED ON THURSDAY, THOUGH
THE REAL DANGEROUS COLD IS NOT EXPECTED UNTIL LATER THURSDAY
NIGHT INTO FRIDAY FOLLOWING THE SECOND AND STRONGER ARCTIC
FRONT. THIS PERIOD OF DANGEROUS COLD WILL BE THE MAIN WEATHER
STORY (PARTICULARLY THURSDAY NIGHT THROUGH SATURDAY). DURING
THIS PERIOD, ENSEMBLE GUIDANCE FAVORS THE HIGH LATITUDE BLOCKING
TO FORCE A SIGNIFICANT SOUTHWARD SHIFT OF A PIECE OF THE POLAR
VORTEX INTO SOUTHERN CANADA AND THE UPPER GREAT LAKES REGION. AT
THE SURFACE, THIS WOULD BE LIKELY TO DRIVE A 1050+ MB ARCTIC
SURFACE HIGH AND ACCOMPANYING AIRMASS SOUTHEASTWARD ACROSS
SASKATCHEWAN AND THE DAKOTAS INTO THE WESTERN GREAT LAKES FRIDAY
INTO SATURDAY. THE MAGNITUDE OF THIS ARCTIC AIRMASS LOOKS TO
KEEP MOST AREAS WITH ACTUAL AIR TEMPERATURES REMAINING BELOW
ZERO FROM EARLY FRIDAY MORNING THROUGH SATURDAY MORNING. WIND
CHILLS OF -15 TO -30 ARE ALSO EXPECTED, WITH THE LOWEST VALUES
EXPECTED FRIDAY MORNING.
WE WILL ALSO HAVE TO KEEP AN EYE ON THE EVOLUTION OF WHAT COULD
BE A MAJOR WINTER STORM ACROSS MUCH OF THE SOUTHERN CONUS
FRIDAY INTO SATURDAY. AT THIS TIME, IT APPEARS WE MAY BE LOCKED
DEEP ENOUGH IN THE POLAR AIRMASS TO KEEP ALL OF THE WINTRY
PRECIPITATION SAFELY TO OUR SOUTH, BUT WE WILL CONTINUE TO
MONITOR THIS.
KJB
AVIATION /00Z TAFS THROUGH 06Z THURSDAY/
ISSUED AT 536 PM CST TUE JAN 20 2026
SNOW IS EXPECTED TO SPREAD ACROSS THE REGION LATE THIS EVENING
AND OVERNIGHT. THE PRESENCE OF DRY LOW-LEVEL AIR, COMBINED WITH
WOBBLES IN THE MODELED LOW TRACK ARE DRIVING SOME UNUSUALLY
LARGE UNCERTAINTY IN THE FORECAST AT THIS RELATIVELY SHORT TIME
RANGE. RECENT NEAR-TERM GUIDANCE HAS TRENDED NORTH, BUT IS NOT
HANDLING ONGOING ACTIVITY ACROSS CENTRAL AND NORTHERN IOWA WELL.
AS A RESULT, DID NOT MAKE SIGNIFICANT CHANGES TO THE PREVIOUS
TAFS WHICH HANDLE THE TIMING AND UNCERTAINTY ASPECTS WELL.
EXPECTATION IS FOR IFR VSBYS TO DEVELOP AT ORD/MDW AFTER ABOUT
06-07Z AS ASCENT INCREASES ALOFT. ALSO INTRODUCED SOME TEMPO
GROUPS TO HIGHLIGHT THE POTENTIAL FOR MVFR VSBYS JUST PRIOR TO
06Z WITH INITIAL TOP-DOWN SATURATION. THERE IS A SIGNIFICANT
AMOUNT OF UNCERTAINTY REGARDING THE PROSPECTS FOR LOWER VSBYS
(3/4 TO 1/2 SM) LATE TONIGHT AND INTO EARLY WEDNESDAY MORNING.
AS SUCH, ELECTED TO MAINTAIN PROB30 GROUPS AT THIS TIME. AT GYY,
GIVEN THE MODELED FORCING, DID ELECT TO ADD TWO TEMPO GROUPS TO
HIGHLIGHT THE SNOW POTENTIAL. SOME GUIDANCE SUGGESTS PERIODS OF
HEAVY SNOW CAN'T BE RULED OUT, AND WILL KEEP A CLOSE EYE ON
TRENDS ON THE SOUTHERN GRADIENT TONIGHT. AT RFD, CONFIDENCE IS
HIGHEST IN A PERIOD OF LOWER CIGS/VSBYS TONIGHT, AND HAVE
MANTAINED THE TEMPO GROUP FOR 1/2 SM SN LATE TONIGHT.
LIGHT SOUTHEASTERLY WINDS AT THE CHICAGO-AREA TERMINALS THIS
EVENING WILL TURN SSW LATER TONIGHT, WITH LIGHT/VARIABLE WINDS
AT RFD. WINDS WILL THEN TURN W/WNW WEDNESDAY MORNING WITH
INTERMITTENT GUSTS AROUND 20 KTS INTO THE AFTERNOON.
A ROBUST DISTURBANCE WILL MOVE THROUGH THE REGION WEDNESDAY
EVENING. SIGNIFICANT SPREAD IN THE HANDLING OF THIS FEATURE
RESULTS IN CONTINUED HIGH UNCERTAINTY IN THE FORECAST EVOLUTION,
BUT A POTENTIAL FOR STRONG AND GUSTY WESTERLY WINDS ALONG WITH
SNOW SHOWERS IS EVIDENT. DUE TO UNCERTAINTY, INTRODUCED PROB30
GROUPS IN THE EXTENDED ORD/MDW TAFS.
CARLAW
HYDROLOGY
ISSUED AT 258 PM CST MON JAN 19 2026
AREA HYDROGRAPHS AND ICE SPOTTER REPORTS CONFIRM THAT ICE
CONTINUES TO DEVELOP AND SPREAD ACROSS AREAL RIVERS. LOCALIZED ICE
JAMS ARE ALREADY PRESENT ALONG THE ROCK, DES PLAINES, AND FOX
RIVERS. CONTINUED COLD TEMPERATURES TONIGHT AND TUESDAY WILL
CAUSE ICE TO CONTINUE EXPANDING AND THICKENING.
AFTER A BRIEF WARM-UP IN TEMPERATURES TO ABOVE 20 DEGREES ON
TUESDAY-WEDNESDAY, ANOTHER PUSH OF BITTERLY COLD TEMPERATURES
WILL ARRIVE THURSDAY NIGHT. AS A RESULT, THE THREAT FOR ICE
JAMS INCLUDING LOCALIZED FLOODING WILL INCREASE FURTHER TOWARD
THE END OF THE WEEK AND THROUGH THE WEEKEND.
- NWS CHICAGO
LOT WATCHES/WARNINGS/ADVISORIES
IL...WINTER WEATHER ADVISORY UNTIL 8 AM CST WEDNESDAY FOR ILZ003-
ILZ004-ILZ005-ILZ006.
WINTER WEATHER ADVISORY FROM 10 PM THIS EVENING TO 8 AM CST
WEDNESDAY FOR ILZ008-ILZ011-ILZ012-ILZ013-ILZ103-ILZ104.
IN...NONE.
LM...NONE.
VISIT US AT WEATHER.GOV/CHICAGO
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