854
FXUS63 KLOT 181739
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
1139 AM CST WED DEC 18 2024
KEY MESSAGES
- SOME SNOW IS PROBABLE AS EARLY AS THURSDAY, BUT MORE SO
THURSDAY NIGHT INTO FRIDAY MORNING. BEST CHANCES FOR
ACCUMULATION ARE NORTH OF I-80 AND PARTICULARLY CLOSER TO THE
WI/IL BORDER.
- LAKE EFFECT SNOW SHOWER THREAT RAMPS UP LATE THURSDAY NIGHT
AND POTENTIALLY CONTINUES THROUGH FRIDAY EVENING. TOO EARLY
FOR SPECIFIC, BUT SOME MEANINGFUL LAKE EFFECT SNOW
ACCUMULATIONS ARE POSSIBLE SOMEWHERE IN OUR CWA.
DISCUSSION
ISSUED AT 329 AM CST WED DEC 18 2024
THROUGH TONIGHT:
MAIN CONCERN OVERNIGHT HAD BEEN THE NARROW F-GEN DRIVEN SNOW
BAND THAT PRODUCED MODERATE TO HEAVY SNOW RATES PRIMARILY NORTH
OF I-88 AND WEST OF I-355 AND I-94 (IN LAKE CO) OVERNIGHT. THE
BAND HAS INDEED WEAKENED STEADILY WITHIN THE PAST HOUR (AS OF
THIS WRITING), MUCH SLOWER THAN DEPICTED BY MOST NEAR-TERM
GUIDANCE. IF THIS WEAKENING TREND CONTINUES AND NO
REINTENSIFICATION IS NOTED, THE CHANCES FOR WINTRY TRAVEL
IMPACTS FARTHER SOUTH ARE MUCH LOWER THAN FEARED GIVEN TEMPS
STILL ABOVE FREEZING AT THIS TIME. CHANCES ARE INCREASING THAT
THE CENTRAL AND SOUTHERN CHICAGO METRO, AND REALLY ANYWHERE NEAR
AND SOUTH OF I-80 VERY WELL COULD BE SPARED. THIS WOULD BE
ASIDE FROM TEMPORARY VISIBILITY REDUCTIONS FROM SLIGHTLY HIGHER
SNOWFALL RATES AT TIMES AS THE BROKEN BAND OF SNOW PROGRESSES
SOUTHEAST. THERE ALSO COULD BE RAIN MIXED IN AT TIMES NEAR AND
SOUTHEAST OF I-55.
ONCE THE LIGHT SNOW AND ANY RAIN COMES TO AN END BY THE MID TO
LATE MORNING, THE REST OF THE DAY WILL BE QUIET WITH BREEZY
NORTHWESTERLY WINDS. CONFIDENCE IS LOW IN SKY COVER TRENDS, AS A
LARGE EXPANSE OF STRATUS (WITH SOME HOLES IN IT) EXTENDS WELL
UPSTREAM. ANY MEANINGFUL CLEARING WOULD LIKELY BE TOWARD OR AFTER
SUNSET AND BE SHORT-LIVED AS SKIES ONCE AGAIN BECOME MOSTLY CLOUDY
OVERNIGHT. THIS WILL RESULT IN A SMALLER DIURNAL TEMPERATURE
RANGE WITH HIGHS ONLY IN THE 32-37F RANGE, MILDEST SOUTHEAST, AND
LOWS OF 22-28F TONIGHT. IF SKIES REMAIN CLEAR ENOUGH FOR LONGER
THAN EXPECTED THROUGH LATE EVENING PRIOR TO MID-HIGH CLOUDS
ARRIVING IN IN ADVANCE OF THURSDAY'S CLIPPER SYSTEM, THEN LOWS
MAY BE A BIT COOLER IN SPOTS.
CASTRO
THURSDAY THROUGH TUESDAY:
THURSDAY THROUGH FRIDAY MORNING
ATTENTION WILL QUICKLY TURN TO THE NEXT SYSTEM THAT WILL BE HOT
ON THE HEELS OF THIS MORNING'S. WATER VAPOR IMAGERY EARLY THIS
MORNING SHOWS AN MID-UPPER LEVEL TROUGH OFF THE COASTS OF
BRITISH COLUMBIA AND THE PACIFIC NORTHWEST, WITH A COUPLE OF
EMBEDDED CLOSED LOWS. THE SOUTHERN CLOSED CIRCULATION IS PROGGED
TO BREAK AWAY FROM THE ONE OFF THE BC COAST AND MOVE QUICKLY
EAST ACROSS THE NORTHERN ROCKIES TODAY, THEN DIG SOUTHEAST FROM
THE NORTHERN PLAINS TONIGHT TO THE MID-MISSISSIPPI VALLEY AND
WESTERN GREAT LAKES.
SOME PRETTY SIZABLE DIFFERENCES EXIST BETWEEN THE GLOBAL MODELS
AND THE CAMS WITH RESPECT TO THE STRUCTURE AND TRACK OF WHAT
LOOKS TO BE A SOMEWHAT COMPLEX TROUGH. GFS/ECMWF/GEM SHOW TWO
DISTINCT VORTS EMBEDDED WITHIN THIS TROUGH AS IT DIGS SOUTHEAST
INTO REGION THURSDAY, WITH LEAD VORT AFFECTING OUR AREA DURING
THE DAY THURSDAY WITH POTENTIAL FOR WARM AIR ADVECTION DRIVEN
PRECIPITATION DEVELOPING DURING THE DAY. STRONGEST FORCING AND
BEST CHANCE OF SNOW DURING THE DAY WILL BE ACROSS OUR NORTHERN
CWA, ESPECIALLY NEAR THE IL/WI BORDER.
SECONDARY VORT EMBEDDED WITHIN THIS BROADER TROUGH IS PROGGED TO
STRENGTHEN AS IT DIGS SOUTHEAST TO DOWNSTATE IL THURSDAY NIGHT.
ASSOCIATED SFC LOW IS PROGGED TO MOVE INTO NORTHERN IL THURSDAY
EVENING. WHILE THERMAL PROFILES AND EXPLICIT MODEL OUTPUT
SUGGEST PRECIP WILL BE ALL SNOW, SFC LOW TRACK THIS FAR NORTH
SUGGESTS THAT PRECIP SHOULD DECREASE IN INTENSITY OR END FOR A
TIME. WE COULD EVEN SEE A BIT OF LIGHT RAIN, DRIZZLE, OR SLEET
MIXING IN FOR A TIME THURSDAY EVENING, ESPECIALLY SOUTH OF
I-88/I-290.
THEN AS THE SFC LOW AND ASSOCIATED MID-UPPER LEVEL TROUGH
CONTINUE EASTWARD, A COLD FRONT EXTENDING WEST FROM THIS LOW IS
SHOULD MOVE SOUTH ACROSS THE AREA SHIFTING WINDS TO NORTH AND
ALLOWING COLDER AIR TO SPILL INTO THE AREA. STRONG COLD LOW/MID
LEVEL COLD AIR ADVECTION SHOULD SIGNIFICANTLY STEEPEN UP LAPSE
RATES, WITH FORECAST SOUNDINGS FROM THE GFS AND PARTICULARLY
ECMWF LOOKING QUITE SUPPORTIVE OF SOME HEALTHY SNOW SHOWERS
LATE THURSDAY NIGHT INTO FRIDAY MORNING, ESPECIALLY FOR THE
EASTERN HALF OR TWO THIRDS OF THE CWA. IF THE CURRENT FORECAST
SOUNDINGS END UP CLOSE TO VERIFYING, WE'LL NEED TO WATCH FOR THE
POTENTIAL FOR SOME IMPACTFUL/BRIEFLY HEAVY SNOW SHOWERS
ASSOCIATED WITH THE COLD FRONT IN THE 3-9 AM CST TIME RANGE
FRIDAY. SYSTEM SNOWS SHOULD EXIT STAGE RIGHT BY MID-LATE MORNING
FRIDAY, THOUGH CERTAINLY CAN'T RULE OUT SOME LINGERING FLURRIES
INTO THE AFTERNOON.
IT'S IMPORTANT TO STRESS A COUPLE OF THINGS ABOUT THE ABOVE
DISCUSSION. FIRST, THAT IS BASED ON THE ECMWF/GFS/GEM SOLUTIONS
WHICH HAVE REASONABLY STRONG SUPPORT FROM THEIR RESPECTIVE
ENSEMBLES. IF THEY WERE TREND FARTHER NORTH (WHICH IS CERTAINLY
POSSIBLE), MORE IN LINE WITH THE NAM/CAM GUIDANCE, THEN WINTRY
WX IMPACTS HERE WOULD BE SIGNIFICANTLY LESS. ALSO, WORTH NOTING
THAT THE DUAL VORT STRUCTURE OF THE TROUGH LIKELY LENDS TO SOME
MORE ROOM FOR ERROR IN THE MODELS' HANDLING OF THE INTERACTION
OF THE TO VORTS, WHICH ULTIMATELY COULD IMPACT THE TRACK OF THIS
SYSTEM AND THE LOCATION OF BETTER FORCING. DUE TO THIS,
FORECAST CONFIDENCE IS ON THE LOWER RANGE OF THE SPECTRUM.
LAKE EFFECT LATE THURSDAY NIGHT THROUGH FRIDAY EVENING:
ATTENTION WILL TURN TO LAKE EFFECT SNOW THREAT ON THE BACK SIDE
OF THIS SYSTEM. AS MENTIONED ABOVE, VERY STEEP LOW/MID LEVEL
LAPSE RATES IN THE COLD AIR ADVECTION REGIME ON THE BACK SIDE OF
THIS SYSTEM WITH FURTHER ACCENTUATE THE LAKE INDUCED
INSTABILITY, LIKELY RESULTING IN A WINDOW OF SOME VERY FAVORABLE
LAKE EFFECT THERMODYNAMICS.
BACKING LOW LEVEL FLOW IN THE WAKE OF THE DEPARTING LOW SHOULD
RESULT IN LAKE EFFECT SNOW INITIALLY OVER EASTERN WI THURSDAY
NIGHT, GRADUALLY SHIFTING SOUTHWARD AND POTENTIALLY INTO NE IL
AND THE CHICAGO AREA LATE THURSDAY NIGHT/EARLY FRIDAY MORNING,
BEFORE ULTIMATELY SHIFTING INTO NORTHWEST INDIANA LATER FRIDAY
MORNING AND FRIDAY AFTERNOON. THE LOW-LEVEL FLOW IN THE GFS
WOULD KEEP THE LAKE EFFECT THREAT INTO NORTHEAST ILLINOIS MUCH
LONGER THAN THE ECMWF, BUT LOOKS PRETTY WONKY AND REALLY DOESN'T
FIT WELL WITH CONCEPTUAL MODEL AND CLIMO, SO SIDED WITH THE
ECMWF AND KEEP THE LES MORE PROGRESSIVELY MOVING INTO NORTHWEST
INDIANA.
GIVEN THE IMPRESSIVE THERMODYNAMICS THAT ARE CURRENTLY BEING
ADVERTISED, THERE COULD BE A QUICK, BUT MEANINGFUL THUMPING OF
LAKE EFFECT SNOW SOMEWHERE IN OUR CWA IN THE LATE THURSDAY NIGHT
THROUGH FRIDAY EVENING TIME FRAME. FAR TOO EARLY TO GET INTO
SPECIFICS REGARDING AMOUNTS OF LOCATION. ALSO, SIMILARLY TO WHAT
WAS MENTIONED IN THE SYNOPTIC DISCUSSION OF THIS SYSTEM, THIS IS
LAKE EFFECT FORECAST IS BASED ON THE GLOBAL MODEL CONSENSUS
TRACK OF THE SFC LOW ACROSS NORTHERN IL. IF THE TRACK WERE TO
END UP FARTHER NORTH, MORE IN LINE WITH THE NAM/CAM GUIDANCE,
THEN THE LAKE EFFECT THREAT INTO NE IL WOULD BE MUCH LOWER AND
THE BRUNT OF THE LAKE EFFECT COULD END UP AFFECTING NORTHEASTERN
PORTER COUNTY AND POINTS EASTWARD. STAY TUNED...
FRIDAY NIGHT ONWARD:
AFTER INVERSION HEIGHTS LOWER AND WINDS BACK PUSHING LAKE EFFECT
THREAT EAST OF OUR AREA LATER FRIDAY NIGHT, THE GFS AND ECMWF
SUGGEST THERE COULD BE A BRIEF RETURN OF SOME LAKE EFFECT SNOW
SHOWERS INTO OUR NW IN LAKE SNOW BELT COUNTIES LATER SATURDAY
INTO SATURDAY EVENING WITH A WEAK SECONDARY PUSH OF COLD AIR.
THAT LAST GASP OF COLD AIR CURRENTLY LOOKS SHORT LIVED WITH THE
UPPER TROUGH MOVING EAST AND WARM AIR ADVECTION BEGINNING BY
LATE SUNDAY AND IN EARNEST BY EARLY NEXT WEEK. TEMPERATURES
SHOULD REBOUND BACK TO ABOVE AVERAGE WITH SOME POTENTIAL FOR
SOME RAIN WITH THE NEXT SYSTEM EARLY NEXT WEEK.
- IZZI
AVIATION /18Z TAFS THROUGH 00Z FRIDAY/
ISSUED AT 1139 AM CST WED DEC 18 2024
FORECAST CONCERNS FOR THE TERMINALS...
- PERIODS OF VFR/MVFR CEILINGS THIS AFTERNOON BECOMING VFR
OVERNIGHT
- BREEZY SOUTH-SOUTHEAST WINDS THURSDAY AFTERNOON
- LOW CHANCE (LESS THAN 15%) FOR FLURRIES THURSDAY
AFTERNOON/EVENING
A BROAD AREA OF 2500-3500 FT CLOUDS CONTINUES TO ADVANCE OUT OF
WI TOWARDS NORTHERN IL WHICH LOOKS TO PERSIST THROUGH EARLY
EVENING. WHILE MOST UPSTREAM OBSERVATION ARE SOLIDLY IN THE
MVFR CATEGORY, THERE CONTINUES TO BE SEVERAL VFR OBSERVATIONS AS
WELL SO EXPECT THE CEILINGS TO LIKELY FLUCTUATE BETWEEN MVFR
AND VFR THIS AFTERNOON. REGARDLESS, CEILINGS SHOULD LIFT ABOVE
3500 FT AFTER MIDNIGHT AS A WEAK SURFACE HIGH PIVOTS THROUGH AND
WORKS TO SCATTER OUT THE LOWER CLOUDS.
OTHERWISE, WINDS TODAY WILL REMAIN A TAD BREEZY OUT OF THE
NORTHWEST WITH OCCASIONAL GUSTS IN THE 15-20 KT RANGE THIS
AFTERNOON. THOUGH, THE AFOREMENTIONED HIGH WILL ALLOW WINDS TO
EASE THIS EVENING AND BECOME LIGHT AND VARIABLE OVERNIGHT. WINDS
WILL BECOME SOUTHERLY THURSDAY MORNING AND THEN SOUTH-
SOUTHEASTERLY BY 18Z THURSDAY WITH GUSTS IN THE UPPER TEEN TO
LOWER 20 KT RANGE POSSIBLE THURSDAY AFTERNOON.
FINALLY, THERE IS ALSO A GROWING POTENTIAL FOR A SOME FLURRIES
TO DEVELOP THURSDAY AFTERNOON AHEAD OF AN UPPER-LEVEL
DISTURBANCE. SINCE THERE IS A GREAT DEAL OF UNCERTAINTY AS TO
HOW MUCH MID-LEVEL MOISTURE WILL BE PRESENT, CONFIDENCE ON THE
FLURRIES IS TOO LOW TO WARRANT A FORMAL MENTION IN THE TAFS AT
THIS TIME. THAT SAID, IF FLURRIES DO MATERIALIZE ANY ASSOCIATED
ACCUMULATIONS SHOULD BE VIRTUALLY NON-EXISTENT DUE TO POOR SNOW
QUALITY.
YACK
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 10 PM CST
THIS EVENING FOR GARY TO BURNS HARBOR IN-BURNS HARBOR TO
MICHIGAN CITY IN.
VISIT US AT WEATHER.GOV/CHICAGO
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