080
FXUS63 KLOT 171047
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
447 AM CST TUE DEC 17 2024
KEY MESSAGES
- BRIEF PERIOD OF SNOW EXPECTED LATE TONIGHT INTO WEDNESDAY
MORNING.
- ANOTHER ROUND OF SNOW AND TRAVEL IMPACTS POSSIBLE THURSDAY
INTO FRIDAY MORNING, PARTICULARLY FOR AREAS NORTH OF I-80.
- PERIODS OF LAKE EFFECT SNOW POSSIBLE FRIDAY AFTERNOON THROUGH
SATURDAY NIGHT MAY INCLUDE PARTS OF NORTHEAST IL AT TIMES.
DISCUSSION
ISSUED AT 237 AM CST TUE DEC 17 2024
THROUGH WEDNESDAY:
TRICKY PART OF THE FORECAST TODAY IS WITH CLOUD COVER AND THE
STRATUS DECK SEEPING SOUTH INTO NORTHERN IL EARLY THIS MORNING.
GENERALLY SPEAKING, GUIDANCE HASN'T BEEN HANDLING THIS ALL THAT
WELL, THOUGH RAP 925MB RH LOOKS THE MOST REASONABLE AND WOULD
SUGGEST THE STRATUS COULD HANG AROUND TODAY OVER NORTHERN
PORTIONS OF THE CWA. HAVE BEEFED UP SKY COVER IN THE GRIDS TO
ACCOUNT FOR THIS. ELSEWHERE, HIGH AND EVENTUALLY MID LEVEL
CLOUDINESS WILL OVERSPREAD THE AREA TODAY IN ADVANCE POSITIVELY
TILTED, FAIRLY QUICK MOVING SHORTWAVE TROUGH PROGGED TO MOVE
ACROSS THE CWA TONIGHT.
WATER VAPOR IMAGERY EARLY THIS MORNING SHOWS THE AFOREMENTIONED
SHORTWAVE TROUGH MOVING TOWARD THE NORTHERN AND CENTRAL HIGH
PLAINS. THIS FEATURE IS PROGGED TO CONTINUE MOVING QUICKLY
EASTWARD WHILE AMPLIFYING A BIT AS IT MOVES INTO THE MID-UPPER
MISSISSIPPI VALLEY TONIGHT, THEN EAST ACROSS THE GREAT LAKES
AND INTO THE LOWER GREAT LAKES WEDNESDAY.
THE ABSENCE OF ANY RETURN FLOW IN ADVANCE OF THE TROUGH WILL
LEAVE IT RATHER MOISTURE STARVED. HOWEVER, AS THE TROUGH
AMPLIFIES THIS AFTERNOON AND TONIGHT, GUIDANCE IS IN GOOD
AGREEMENT IN DEVELOPING A LOW-MID LEVEL F-GEN ZONE ASSOCIATED
WITH THE THERMALLY DIRECT CIRCULATION IN THE RIGHT ENTRANCE
REGION OF 100KT+ UPPER LEVEL JET STREAK. THERE IS VARIABILITY
AMONG THE VARIOUS MODELS ON THE DEPTH/STRENGTH OF THIS F-GEN
CIRCULATION AS IT MOVES ACROSS OUR CWA TONIGHT, WHICH GIVEN THE
LIMITED AVAILABLE MOISTURE, HAS SOME PRETTY BIG IMPLICATIONS IN
HOW MUCH QPF THIS SYSTEM WILL BE ABLE TO SQUEEZE OUT AS IT
MOVES ACROSS THE AREA LATE TONIGHT INTO EARLY WED AM.
INITIALLY, A PERIOD OF STRONGER FORCING WILL GO TOWARD SATURATING
THE LOW-MID LEVELS, HOWEVER THE MAJORITY OF GUIDANCE SUGGESTS THAT
THE TOP-DOWN SATURATION PROCESS WILL TAKE PLACE QUICKLY ENOUGH FOR
THERE TO STILL BE A BAND OF SNOW TO ACCOMPANY THE LOW-MID LEVEL
F-GEN CIRCULATION AS IT MOVES ACROSS THE AREA. ONE NOTABLE
EXCEPTION IS THE GFS, WHICH IS WEAKER/MORE SHALLOW WITH THE F-GEN
CIRCULATION AND LARGELY BRINGS THIS TROUGH THROUGH WITH VERY
LITTLE PRECIPITATION. CERTAINLY CAN'T TOTALLY RULE THIS SCENARIO,
HOWEVER, GIVEN THE MAJORITY OF GUIDANCE WOULD SUPPORT AT LEAST A
BRIEF PERIOD OF SOME SNOW LATE TONIGHT INTO TOMORROW MORNING, FELT
LIKELY POPS WERE JUSTIFIED.
MOST LIKELY SCENARIO IS A BAND OF TWO OF SNOW RESULTS IN A 1-3
HOUR PERIOD OF SOME SNOW LATE TONIGHT INTO EARLY WED MORNING, WITH
HIGHEST SNOW CHANCES AND MOST LIKELY HIGHEST (ALBEIT STILL PRETTY
SMALL) ACCUMULATIONS NORTH OF I-80 AND ESPECIALLY NORTH OF
I-88/I-290. SHOULD GENERALLY SEE A COATING TO PERHAPS LOCALLY AN
INCH OR SO OF SNOW, DEPENDING ON THE SPEED OF TOP-DOWN SATURATION
AND INTENSITY OF THE F-GEN BAND. NAM/WRF ARW ARE AMONG THE MORE
MODELS WITH THE STRONGER F-GEN CIRCULATION AND MORE IMPRESSIVE
SATURATED NEGATIVE EPV, INDICATIVE OF LESS STABILITY/MORE
INSTABILITY WHICH COULD SUPPORT A STRONGER BAND OF SNOW. THESE
MORE WORST CASE SCENARIOS COULD SUPPORT A SWATH OF 1-2" OF SNOW
WITH MORE SIGNIFICANT TRAVEL IMPACTS. WHILE THIS ISN'T THE MOST
LIKELY SCENARIO, GUIDANCE OFTEN STRUGGLES WITH F-GEN DRIVEN
CIRCULATIONS AND THESE TENDS TO BE THE TYPE OF SCENARIOS THAT CAN
OVERPERFORM.
EVEN IF IT DOESN'T OVERPERFORM, THE TIMING IS LESS THAN IDEAL,
LEADING UP TO AND DURING THE BEGINNING PART OF THE MORNING COMMUTE
WEDNESDAY. TEMPERATURES SHOULD BE AT OR BELOW FREEZING AND ALLOW
FOR SOME ACCUMULATION ON UNTREATED ROADWAYS, PARTICULARLY AWAY
FROM THE URBAN HEAT ISLAND OF CHICAGO. IF THE SYSTEM WERE TO
OVERPERFORM, THEN TRAVEL IMPACTS COULD BE MORE SIGNIFICANT GIVEN
THE INOPPORTUNE TIMING, SO THIS WILL BE SOMETHING TO KEEP A CLOSE
EYE ON. COULD BE A SHORT PERIOD OF SOME DRIZZLE/SNIZZLE
IMMEDIATELY IN THE WAKE OF THE BAND AS CLOUD DECK LOSES ICE NUCLEI
PRIOR TO THE LOSS OF LOW LEVEL FORCING. PRECIP SHOULD EXIT OUR NW
IN COUNTIES BY MID MORNING WED WITH INCREASING WESTERLY AND
SEASONABLY CHILLY AIR ADVECTING INTO THE AREA IN THE WAKE OF THE
SYSTEM.
- IZZI
WEDNESDAY NIGHT THROUGH MONDAY:
THE LONG TERM WILL START OUT CALM AND RELATIVELY CHILLY
UNDERNEATH ~1030 MB SURFACE HIGH PRESSURE TRANSITING THE REGION.
INCREASING HIGH CLOUDS LOOK TO LIMIT HOW COLD TEMPS MAY GET IN
AN OTHERWISE FAVORABLE RADIATIONAL COOLING SETUP.
FOR THURSDAY INTO FRIDAY, WHILE KEY DIFFERENCES EXIST IN THE
GUIDANCE, THERE'S GOOD OVERALL AGREEMENT IN A FAIRLY VIGOROUS
CLIPPER TYPE WAVE MOVING ACROSS THE UPPER MS VALLEY AND WESTERN
GREAT LAKES. A DISTINCT MAJORITY OF MOST RECENT OPERATIONAL
MODELS AND ENSEMBLE MEMBERS GENERALLY FAVOR NORTH OF OUR AREA
FOR THE HIGHEST POTENTIAL FOR A FEW TO SEVERAL INCHES OF SNOW
ACCUMULATION. HOWEVER, GIVEN THE STRONGEST 500 MB FORCING WITH
THE PARENT SHORT-WAVE PUSHING ACROSS ILLINOIS, THERE STILL
APPEARS TO BE MEANINGFUL WIGGLE ROOM IN THE TRACK OF THE
CLIPPER. EVEN IN THE CURRENT GENERAL CONSENSUS, NORTH-SOUTH
DIFFERENCES IN THE EXACT TRACK OF THE WEAK SURFACE LOW ARE
IMPORTANT.
POPS IN THE 20-40% RANGE ARE INDICATED IN THE GRIDDED FORECAST
INTO FAR NORTHERN ILLINOIS AS EARLY AS THURSDAY MORNING, THOUGH
THE COLUMN MAY STILL BE TOO DRY FOR WARM ADVECTION SNOW IN THE
MORNING. RECENT NAM RUNS ARE AN OUTLIER IN THIS REGARD, THOUGH
WE'LL CERTAINLY BE KEEPING A CLOSE EYE ON TRENDS GIVEN THE
POTENTIAL FOR IMPACTS TO PART OF THE MORNING COMMUTE IN A NAM-
LIKE SOLUTION.
IF THE CLIPPER SURFACE LOW TRACKS FAR ENOUGH SOUTH, THE
ACCUMULATING SNOW BAND(S) IN WISCONSIN COULD CONCEIVABLY PIVOT
INTO AREAS NORTH OF I-88 LATE THURSDAY AFTERNOON AND THURSDAY
EVENING, WHERE OUR GRIDS HAVE SOME POPS UP TO LIKELY/60%
INDICATED. CONCEPTUALLY, SCATTERED SNOW SHOWERS MAY OCCUR
FARTHER SOUTH NEAR THE CORE OF THE MID-LEVEL PVA AS LAPSE RATES
STEEPEN FROM COLD (NEAR -30C AT 500 MB) TEMPS ALOFT. LATER
THURSDAY NIGHT INTO FRIDAY MORNING, IN A STRENGTHENING NORTHERLY
COLD ADVECTION REGIME ON THE BACK SIDE OF THE CLIPPER, AREAS OF
SNOW/SNOW SHOWERS SHOULD EXTEND FARTHER SOUTH. WE'LL ALSO HAVE
TO MONITOR FOR LAKE ENHANCEMENT FROM EASTERN WI SOUTHWARD INTO
NORTHEAST ILLINOIS. MAIN TAKEAWAY IS THAT THE CURRENT BEST
CHANCE FOR TRAVEL IMPACTS IN ANY PORTION OF OUR AREA APPEARS TO
BE THE THURSDAY EVENING COMMUTE THROUGH THE FRIDAY MORNING COMMUTE.
EXPANSIVE NEAR 1040 MB HIGH PRESSURE WILL SLOWLY BUILD IN FROM
THE NORTHWEST FRIDAY THROUGH SATURDAY, TYPICALLY A FAVORABLE
SETUP FOR LAKE EFFECT SNOW ON SOUTHERN LAKE MICHIGAN. 850 MB
TEMPS FORECAST TO GET DOWN INTO THE -12 TO -15 CELSIUS RANGE
SUGGEST FAVORABLE THERMODYNAMICS AND LAKE INDUCED INSTABILITY
GIVEN LAKE TEMPS STILL IN THE 40S. MODEL FORECAST SOUNDINGS ALSO
INDICATE SUFFICIENT MOISTURE AND INVERSION HEIGHTS TO SUPPORT
HIGHER SNOWFALL RATES. THE BIGGEST UNANSWERABLE QUESTION AT THIS
POINT IS EXACTLY HOW BOUNDARY LAYER CONVERGENCE SETS UP,
DICTATING THE TRAJECTORY OF LAKE EFFECT BAND(S). BASED ON OUR
FORECAST WIND DIRECTIONS, NORTHWEST INDIANA MAY BE MORE FAVORED
FRIDAY AFTERNOON THROUGH EARLY SATURDAY, POSSIBLY MIGRATING
WESTWARD INTO NORTHEAST IL AND FAR NW INDIANA ON SATURDAY-
SATURDAY NIGHT. ANOTHER WILDCARD FOR THE LAKE EFFECT IS THE
INFLUENCE OF MESOLOW DEVELOPMENT, IF THAT OCCURS.
ASIDE FROM THE LAKE EFFECT POTENTIAL, CAN'T COMPLETELY RULE OUT
A WEAKENING BAND OF SNOW APPROACHING OUR WESTERN CWA SATURDAY
MORNING. OTHERWISE, THE BIGGER STORY WILL BE A STRETCH OF BELOW
NORMAL TEMPERATURES THROUGH THE WEEKEND. THIS LOOKS TO BE
SHORT-LIVED, WITH VERY STRONG MEDIUM-RANGE GUIDANCE AGREEMENT IN
A PATTERN CHANGE TO MILDER CONDITIONS FOR THE WEEK OF CHRISTMAS.
CASTRO
AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/
ISSUED AT 447 AM CST TUE DEC 17 2024
- A 1-3 HOUR PERIOD OF IFR IN SNOW EXPECTED LATE TONIGHT/VERY
EARLY WED MORNING
AN EXTENSIVE SHIELD OF MVFR CIGS HAS BEEN OOZING SOUTHWARD
OVERNIGHT AND HAS OVERSPREAD MOST OF THE TERMINALS. MVFR CIGS
ARE LIKELY TO START OUT THE 12Z TAFS THIS MORNING, BUT
CONFIDENCE IS EXTREMELY LOW WHEN, AND EVEN IF, THE MVFR CIGS
WILL CLEAR OUT TODAY. SOME HOLES HAVE OPENED IN THE PAST COUPLE
OF HOURS OVER WEST CENTRAL WI/NE IA, WHICH IF THAT TREND WERE TO
CONTINUE THIS MORNING THEN VFR CONDITIONS COULD DEVELOP MUCH
EARLIER THAN CURRENTLY FORECAST. ALSO POSSIBLE THAT THOSE HOLES
FILL IN AND MVFR CIGS PERSIST THROUGH THE DAY AND INTO TONIGHT.
A BAND OR TWO OF SNOW IS EXPECTED TO MOVE SOUTHEASTWARD ACROSS
THE TERMINALS OVERNIGHT TONIGHT, POSSIBLY LINGERING INTO THE
VERY EARLY MORNING HOURS WEDNESDAY. WHILE SNOW ISN'T EXPECTED TO
LAST LONG AT ANY ONE GIVEN LOCATION, A SHORT PERIOD OR TWO OF
IFR VSBY IN LIGHT SNOW IS LIKELY (AROUND 60% CHANCE). A COATING
TO LOCALLY AN INCH OF SNOW COULD ACCUMULATE, PARTICULARLY AT RFD
AND DPA.
SOME DRIZZLE OR VERY FINE SNOW/SNIZZLE COULD LINGER AN HOUR OR
TWO AFTER THE SNOW BANDS MOVE THROUGH. WHILE ANY DRIZZLE/SNIZZLE
WILL END, MVFR CIGS WILL PROBABLY LINGER WEDNESDAY MORNING. WEST
WINDS WILL BECOME LIGHT/VARIABLE THIS EVENING, THEN LIGHT
NORTHWEST OVERNIGHT, THEN BEGIN INCREASING FROM THE NORTHWEST
DURING THE FINAL FEW HOURS OF THE ORD AND MDW 30 HOUR TAFS.
- IZZI
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY UNTIL 9 AM CST THIS MORNING FOR GARY TO
BURNS HARBOR IN-BURNS HARBOR TO MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page