926
FXUS63 KLOT 191208
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
608 AM CST THU DEC 19 2024
KEY MESSAGES
- PERIOD OF SNOW LATE THIS AFTERNOON INTO EARLY THIS EVENING,
ACCUMULATIONS OF UP TO AN INCH OR SO, MAINLY WI BORDER
COUNTIES.
- SECOND SHOT OF SNOW SHOWERS EXPECTED LATE TONIGHT INTO EARLY
FRIDAY MORNING, COATING TO AROUND AN INCH OF ACCUMULATION
EXPECTED, LOCALLY OVER AN INCH CLOSER TO LAKE MICHIGAN IN
NORTHEAST IL.
- LAKE EFFECT SNOW SHOWERS QUICKLY SHIFT FROM IL SHORE INTO
NORTHWEST INDIANA FRIDAY AFTERNOON, LINGERING INTO FRIDAY
EVENING, TAPERING OFF LATE FRIDAY NIGHT. SOME ADDITIONAL
ACCUMULATIONS EXPECTED.
- BELOW NORMAL TEMPERATURES THIS WEEKEND FOLLOWED BY A
PRONOUNCED PATTERN CHANGE TO MILDER CONDITIONS THE WEEK OF
CHRISTMAS ALONG WITH OCCASIONAL OPPORTUNITIES FOR (PRIMARILY)
RAIN.
DISCUSSION
ISSUED AT 325 AM CST THU DEC 19 2024
THROUGH FRIDAY NIGHT:
WATER VAPOR IMAGERY NICELY DEPICTS A LARGE AND VIGOROUS UPPER
TROUGH OVER THE NORTHERN PLAINS WITH JET STREAK SEEN NOSING IN
FROM THE WEST AND BEGINNING TO ROUND THE BASE OF THE TROUGH.
THIS TROUGH IS PROGGED TO DIG EAST-SOUTHEAST INTO THE MID-UPPER
MISSISSIPPI VALLEY/WESTERN GREAT LAKES THIS EVENING, THEN MOVING
EAST TO THE UPPER OHIO VALLEY FRIDAY MORNING.
NO SIGNIFICANT CHANGES TO THE FORECAST THIS MORNING. THE
VARIOUS HI-RES MODELS/CAMS REMAIN LESS CONSISTENT WITH EACH
OTHER AND RUN-TO-RUN, HOWEVER, THE GEFS AND EPS HAVE REMAINED
QUITE CONSISTENT, AS HAVE THEIR RESPECTIVE OPERATIONAL RUNS.
GIVEN THE CONSISTENCY OF THE GEFS/EPS, CONTINUED TO LEAN THE
FORECAST HEAVILY IN THAT DIRECTION, WHICH ALSO IS VERY MUCH IN
LINE WITH THE INHERITED FORECAST THIS MORNING.
AS THE UPPER TROUGH MOVES SOUTHEAST TOWARD THE REGION, THE
ASSOCIATED SURFACE LOW OVER THE DAKOTAS EARLY THIS MORNING WILL
FOLLOW SUIT AND IS PROGGED TO TRACK ACROSS NORTHERN IL THIS
EVENING, THEN EAST INTO NORTHERN IN BY EARLY FRIDAY MORNING.
THIS TRACK SHOULD SAFELY PLACE THE MORE MEANINGFUL SWATH OF
ACCUMULATING SNOW SOLIDLY NORTH OF THE IL/WI BORDER.
HOWEVER, AS LEFT EXIT REGION OF 120KT 300MB JET STREAK
OVERSPREAD THE AREA THIS AFTERNOON, STRONG UPPER LEVEL
DIVERGENCE WILL RESULT IN A PERIOD OF STRONG MID-UPPER LEVEL
ASCENT LATER THIS AFTERNOON INTO EARLY THIS EVENING. THIS MID-
UPPER LEVEL ASCENT COULD BRIEFLY COUPLE WITH MODEST LOW LEVEL
OMEGA ASSOCIATED WITH ISENTROPIC ASCENT CENTERED ON THE 280K SFC
TO PROVIDE FOR A PERIOD OF SNOW LATE THIS AFTERNOON INTO EARLY
THIS EVENING. SOME ASCENT WILL BE USED UP ON TOP-DOWN SATURATION
OF THE COLUMN, WITH THE BEST OVERLAP OF DEEP ASCENT PROGGED TO
LINE UP OVER SOUTHERN WI AND FAR NORTHERN IL. RATHER STEEP MID-
UPPER LEVEL LAPSE RATES AND STRONG ASCENT THROUGH THE DENDRITIC
GROWTH ZONE COULD RESULT IN A BRIEF PERIOD OF BEEFIER SNOWFALL
RATES. GENERALLY EXPECTING AN AROUND AN INCH OR SO ACCUMULATION
WITH THIS FIRST WAVE OF SNOW NEAR THE WI BORDER WITH AMOUNTS
TAPERING SOUTH.
THE FIRST WAVE OF SNOW LOOKS TO BE QUICK HITTING AS STRONGER
ASCENT ASSOCIATED WITH THE LEFT EXIT REGION OF THE JET QUICKLY
MOVES EAST OF THE AREA. MID LEVEL DRY INTRUSION ASSOCIATED WITH
THE MID-UPPER LEVEL JET SHOULD OVERSPREAD THE AREA THIS EVENING,
LIKELY RESULTING IN A LOSS OF ICE NUCLEI NEEDED FOR SNOW.
HOWEVER, MODEST ISENTROPIC ASCENT IS EXPECTED TO CONTINUE, SO
SHOULD SEE SNOW EITHER END OR TAPER OFF TO A FINE SNIZZLE
(SNOW/DRIZZLE MIX). NOT UNCOMMON TO SEE SOME SMALL SHOWERY
CONVECTIVE ELEMENTS TO DEVELOP IN THESE DRY SLOTS WHICH COULD
LEAD TO A COUPLE OF BRIEF SNOW OR SLEET SHOWERS, BUT THE THEME
IN THE WAKE OF WAVE WAVE WILL BE MINIMAL QPF AND TRAVEL IMPACTS
LOOKING UNLIKELY.
SECONDARY VORT DIGGING INTO THE BACK SIDE OF THE TROUGH AMIDST
QUICKLY STEEPENING LOW/MID LEVEL LAPSE RATES AND LINGERING LOW
LEVEL MOISTURE SHOULD LEAD TO FAVORABLE CONDITIONS FOR A WAVE OF
SNOW SHOWERS TO MOVE ACROSS THE AREA LATE TONIGHT INTO EARLY TO
MID MORNING FRIDAY. SOME OF THE MORE ROBUST SNOW SHOWERS COULD
DROP A QUICK COATING TO MAYBE A HALF INCH OR SO OF SNOW, WHICH
GIVEN THE STRONG WINDS AND FALLING TEMPERATURE COULD LEAD TO
TRAVEL IMPACTS. GIVEN THE CONVECTIVE NATURE OF THE SNOW SHOWERS,
SNOWFALL ACCUMULATIONS WILL LIKELY VARY, BUT WOULDN'T BE
SURPRISING TO SEE A COUPLE OF SPOTS PICK UP AN INCH OR SO OF
SNOW BETWEEN 09-15Z TONIGHT, PARTICULARLY ACROSS NORTHEAST IL
WHERE THE LAKE MAY GET INVOLVED.
SPEAKING OF THE LAKE INVOLVEMENT...THE SAME STEEP LOW/MID LEVEL
LAPSE RATES THAT WILL LEAD TO SNOW SHOWER DEVELOPMENT INLAND
WILL ALSO BE PRESENT OVER THE STILL WARM (5-7C) LAKE MICHIGAN
WATERS, WHICH WILL ACCENTUATE INSTABILITY AND LEAD TO SOME LAKE
ENHANCEMENT OF THE SNOW SHOWERS. INITIALLY, THE LAKE ENHANCED
SNOW SHOWERS WILL BE OVER EASTERN WI TONIGHT, BUT AS WINDS BACK,
THAT AREA OF LAKE ENHANCED SNOW SHOWERS IS EXPECTED TO PIVOT
SOUTH DOWN THE WESTERN SHORE OF THE LAKE IN NORTHEAST IL VERY
LATE TONIGHT INTO FRIDAY MORNING. THERMODYNAMICALLY, INSTABILITY
WILL BE PLENTY SUPPORTIVE OF ROBUST LAKE EFFECT SNOW SHOWERS.
HOWEVER, FORECAST SOUNDINGS SUGGEST THAT THE MAJORITY OF THE
CONVECTIVELY DRIVEN STRONG ASCENT LATE TONIGHT INTO FRIDAY
MORNING WILL BE BELOW THE -12C LEVEL LEADING TO UNCERTAINTIES
REGARDING THE QUALITY OF THE LAKE EFFECT SNOW. IF CONVECTION
WERE TO OVERPERFORM (GROW DEEPER THAN FORECAST SOUNDINGS WOULD
SUGGEST) THEN A PERIOD OF MODERATE TO HEAVY SNOW SHOWERS WOULD
BE POSSIBLE OVER NORTHEAST IL AND THE CHICAGO AREA FRIDAY
MORNING WITH HIGHER ACCUMULATIONS. HOWEVER, CONSENSUS OF
FORECAST SOUNDINGS SUGGESTS THAT THE TOP OF THE LAKE ENHANCED
CONVECTION WILL REMAIN LARGELY BELOW THE -12C TO -14C LEVEL, SO
KEPT FORECAST AMOUNTS MODEST FOR NOW, GENERALLY AN INCH OR SO
VERY LATE TONIGHT INTO FRIDAY MORNING OVER THE CHICAGO METRO
AREA.
LAKE EFFECT IS EXPECTED TO CONTINUE ROUNDING THE SOUTHERN RIM OF
THE LAKE INTO NORTHWEST IN FRIDAY AFTERNOON. BY LATER FRIDAY
AFTERNOON, CONTINUED COLD AIR ADVECTION AT THE SURFACE AND
ALOFT SHOULD RESULT IN COLDER CLOUD LAYER TEMPS AND LIKELY
INCREASINGLY EFFICIENT LAKE SNOW PRODUCTION. THIS SHOULD
CONTINUE INTO FRIDAY EVENING BEFORE INVERSION HEIGHTS START
CRASHING, LEADING TO WEAKENING OF LAKE EFFECT SNOW SHOWERS LATER
IN THE EVENING AND MORE SO OVERNIGHT. IT IS GENERALLY LOOKING
LIKELY A 1-3" LAKE EFFECT SNOWFALL OVER LAKE/PORTER COUNTIES
FRIDAY AFTERNOON/EVENING, THOUGH IF A MORE ORGANIZED BAND CAN
GET GOING AND PARK OVER AN AREA FOR A COUPLE OF HOURS, THEN
COULDN'T RULE OUT SOME ISOLATED 4"+ TOTALS SOMEWHERE. THE STRONG
LAKE INDUCED INSTABILITY SHOULD ALLOW FOR STRONG ONSHORE WINDS,
POTENTIALLY GUSTING TO 40 MPH NEAR THE LAKE AT TIMES IN THE
HEAVIER SNOW SHOWERS THAT OCCUR NEAR THE LAND BREEZE CONVERGENCE
ZONE.
NORTHWEST WINDS WILL INCREASE LATE FRIDAY NIGHT AND REMAIN
RATHER STRONG AND GUSTY FRIDAY BEFORE DIMINISHING FRIDAY
EVENING. ANYWHERE THAT HAS SOME LIGHTER, MORE BLOWABLE SNOW ON
THE GROUND WILL LIKELY SEE SOME BLOWING SNOW FRIDAY. DIDN'T ADD
THAT TO THE GRIDS YET AS THERE IS STILL UNCERTAINTY AROUND WHAT
AREAS WILL BE SNOW COVERED AND THE CHARACTER OF SAID SNOW
COVER. WINDS WILL QUICKLY DIMINISH FRIDAY EVENING.
- IZZI
SATURDAY THROUGH WEDNESDAY:
SATURDAY WILL BE CHILLY WITH HIGHS IN THE 20S FOR MOST. IF SKIES
REMAIN CLOUDIER (OR VICE VERSA), TEMPS WOULD LIKELY BE LOWER (OR
HIGHER) THAN IN THE OFFICIAL FORECAST. LIGHT WINDS DUE TO ~1035
MB HIGH PRESSURE NOSING IN FROM THE NORTH WILL THANKFULLY RESULT
IN WIND CHILLS ONLY A BIT LOWER THAN ACTUAL TEMPS. THE LAKE EFFECT
SNOW SETUP IS CURRENTLY FORECAST TO BE A GOOD DEAL LESS FAVORABLE
THAN FRIDAY PM WITH LOWER INVERSION HEIGHTS, DRY AIR ABOVE THE
INVERSION, AND LESS LAKE INDUCED CAPE. THERE'S STILL ENOUGH OF A
SIGNAL FOR THE CONVECTIVE LAYER TO REACH -10 TO -12C AND
SUFFICIENT LIFT THROUGH IT FOR ~15-25% POPS INTO NORTHWEST INDIANA
(MAINLY PORTER COUNTY). IMPACTFUL LAKE EFFECT SNOW APPEARS
UNLIKELY AT THIS JUNCTURE, WITH SMALL FLAKE SIZE AND LIGHTER
INTENSITY CONSIDERING THE MARGINAL PARAMETERS IN PLAY.
SATURDAY NIGHT, HIGH PRESSURE MOVING OVERHEAD WILL LIKELY CUT OFF
ANY SPOTTY LAKE EFFECT SNOW SHOWERS LINGERING INTO THE EVENING.
WHILE CALM WINDS UNDERNEATH THE SURFACE HIGH WOULD SUPPORT GOOD
RADIATIONAL COOLING, WENT A BIT MORE CONSERVATIVE WITH THE LOW
TEMPS (TEENS OUTSIDE CHICAGO AND AROUND 20F/LOW 20S IN THE CITY).
ONSET OF RETURN FLOW AND INITIALLY MODEST WARM ADVECTION ON
SUNDAY SHOULD ENABLE HIGH TEMPS A FEW TO SEVERAL DEGREES ABOVE
SATURDAY'S, BUT STILL A BIT BELOW NORMAL.
A LARGE SCALE PATTERN CHANGE TO MILDER CONDITIONS WILL TAKE PLACE
THE WEEK OF CHRISTMAS. ANOMALOUSLY DEEP TROUGHING IN THE VICINITY
OF THE GULF OF ALASKA WILL INDUCE IMPRESSIVE POSITIVE HEIGHT
ANOMALIES DOWNSTREAM INTO CENTRAL CANADA. MUCH OF NORTH AMERICA
WILL BE EFFECTIVELY FLOODED WITH PACIFIC BORNE AIR MASSES AND
TYPICAL COLD AIR MASSES FOR LATE DECEMBER SHUNTED WELL NORTH-
NORTHEAST.
FAST MOVING CLOSED MID-UPPER LEVEL LOW PRESSURE AND ITS ASSOCIATED
WEAK SURFACE LOW TRAVERSING THE NORTHERN TIER OF THE US WILL
LIKELY BRING A PERIOD OF PRECIPITATION MONDAY-MONDAY EVENING,
MAINLY IN THE FORM OF RAIN. MAIN ELEMENT TO WATCH WOULD BE HOW
QUICKLY PRECIPITATION ONSETS BEFORE LINGERING LOW-LEVEL COOL AND
DRY AIR MASS IS EJECTED, WHICH COULD BRING SOME SNOW MIXED IN, OR
EVEN MIXED PRECIPITATION ISSUES FOR A TIME. ALL IN ALL, THE SETUP
DOES NOT CURRENTLY LOOK FAVORABLE FOR IMPACTFUL WINTER WEATHER IN
OUR AREA.
THE FAIRLY ACTIVE, PROGRESSIVE PATTERN AND STRONG INVERSIONS FROM
THE WELL ABOVE NORMAL WARMTH ALOFT (925-850 MB LAYER) POINTS
TOWARD SOCKED THE AREA BEING IN CLOUDY SKIES, WHICH WILL LIMIT
DIURNAL RANGES. FOR THIS REASON, CHRISTMAS EVE AND CHRISTMAS DAY
ARE NOT LOOKING LIKE A REPEAT OF SOME OF THE EXCEPTIONALLY WARM
ONES WITHIN THE PAST SEVERAL YEARS (IE. ABOVE NORMAL HIGHS BUT MID
30S-LOWER 40S). SOME GUIDANCE SUGGESTS RAIN CHANCES RETURNING
BACK NORTHWARD CHRISTMAS EVE PM INTO CHRISTMAS DAY, THOUGH
CONFIDENCE IS LOW IN SPECIFICS AT THAT RANGE.
CASTRO
AVIATION /12Z TAFS THROUGH 18Z FRIDAY/
ISSUED AT 607 AM CST THU DEC 19 2024
KEY MESSAGES:
- SNOW SHOWERS LATER TODAY AND LATE TONIGHT THROUGH FRIDAY
MORNING LIKELY TO REDUCE VSBY AND MAY CAUSE SOME ACCUM.
- SNOW MAY MIX WITH DZ/FZDZ LATE THIS EVENING OR END FOR A TIME
BEFORE NEXT ROUND LATE TONIGHT/EARLY FRIDAY.
- LOW MVFR TO IFR (POSSIBLY LIFR AT TIMES) CIGS TONIGHT INTO
EARLY FRIDAY.
- GUSTY NORTH-NORTHWEST WINDS FRIDAY MORNING MAY WORSEN VSBY.
WELL ADVERTISED UPPER LEVEL DISTURBANCE WILL AFFECT THE AREA
TODAY INTO FRIDAY. AS THE SYSTEM'S WEAK SURFACE LOW APPROACHES
AND TRACKS ACROSS NORTHERN ILLINOIS, CURRENTLY LIGHT WINDS WILL
BECOME SOUTHEAST AND INCREASE, WITH 15-20 KT GUSTS TOWARDS EVENING.
SCATTERED SNOW SHOWERS MAY THEN MOVE ACROSS THE AREA FROM THE
MID AFTERNOON, IF DRY AIR IS ERODED ENOUGH. A MORE COHERENT
AREA OF SNOW/SNOW SHOWERS IS EXPECTED TO PIVOT IN PRIMARILY THIS
EVENING, FOR WHICH TEMPO IFR VSBY WAS MAINTAINED. A COATING OF
SNOW MAY OCCUR IF THE SNOW COMES DOWN HARD ENOUGH, THOUGH
CONFIDENCE IS ON THE LOWER SIDE REGARDING RUNWAY IMPACTS THIS
EVENING.
DRY AIR ALOFT WILL THEN LIKELY CAUSE THE PRECIPITATION TO TAPER
OFF/MIX WITH DRIZZLE LATE THIS EVENING, OR EVEN TEMPORARILY END
FOR A TIME. THIS IS THE MOST LIKELY TIMEFRAME FOR PREVAILING
IFR (OR LOWER) CIGS. ON THE BACK SIDE OF THE SYSTEM LATE TONIGHT
INTO/THROUGH FRIDAY MORNING, A PERIOD OF MORE ROBUST SNOW
SHOWERS IS EXPECTED, WHICH MAY AGAIN KNOCK VSBY DOWN TO IFR OR
EVEN LIFR AT TIMES IF SNOW RATES ARE HEAVY ENOUGH. GENERALLY
NEAR TO BELOW FREEZING TEMPS GOING INTO THE FRIDAY AM ROUND MAY
GIVE THE BEST CHANCE FOR RUNWAY ACCUMS AND IMPACTS. IN ADDITION,
WINDS WILL SHIFT TO NORTH-NORTHWEST (330-360 DEG) AND QUICKLY
INCREASE WITH GUSTS TO 25 KT OR GREATER 12Z FRIDAY AND ONWARD.
CASTRO
LOT WATCHES/WARNINGS/ADVISORIES
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY FROM 3 AM FRIDAY TO 9 AM CST SATURDAY FOR
THE IL NEARSHORE WATERS.
SMALL CRAFT ADVISORY FROM 6 AM FRIDAY TO 3 PM CST SATURDAY FOR
THE IN NEARSHORE WATERS.
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