674
FXUS63 KARX 041706
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1105 AM CST SUN DEC 4 2022
DISCUSSION
(TODAY THROUGH SATURDAY)
ISSUED AT 400 AM CST SUN DEC 4 2022
KEY MESSAGES:
- NARROW (40-50 MILES WIDE) BAND OF LIGHT SNOW FALLS MONDAY
MORNING. STILL ASSESSING THE EXACT BAND LOCATION, BUT AMOUNTS
ARE EXPECTED TO BE LESS THAN AN INCH WITH MINIMAL TRAVEL
IMPACTS.
- LOW (20%) RISK OF LIGHT SNOW ON TUESDAY, WITH A STRONGER SYSTEM
POSSIBLY IMPACTING THE REGION ON THURSDAY.
- MODERATING TEMPERATURES FOR THE UPCOMING WEEK.
UPCOMING WEEK: MODERATING TEMPERATURES
SOUTHWESTERLY FLOW BECAME ESTABLISHED OVERNIGHT AND PERSISTS
THROUGH MIDDAY MONDAY. AFTER OUR BRIEF BOUT OF ARCTIC AIR
YESTERDAY, HIGHS TODAY CLIMB BACK INTO THE 30S. WINDS REMAIN
ELEVATED TONIGHT AND PREVENT LOWS FROM FALLING TOO FAR, ENABLING
HIGHS ON MONDAY TO STILL REACH THE 30S BEFORE A COLD FRONT PASSES
THROUGH. THE FLOW WEAKENS FOR TUESDAY AND WEDNESDAY UNDER A PAIR
OF RIDGES. TUESDAY SHOULD BE THE COOLEST DAY OF THE PERIOD IN THE
WAKE IF THE FRONT WITH HIGHS ON WEDNESDAY HINGING ON HOW QUICKLY
RETURN FLOW ARRIVES. BY THURSDAY WE SHOULD SEE AN INCREASING FETCH
OF SOUTHERLY FLOW AHEAD OF THE NEXT SYSTEM LIFTING FROM THE
SOUTHWEST. EVEN IN THE WAKE OF THIS LOW THERE IS LITTLE IN THE WAY
OF UPSTREAM POLAR AIR, THUS TEMPERATURES SHOULD REMAIN SEASONAL
RIGHT INTO NEXT WEEKEND. THIS IS REFLECTED WELL IN THE NBM
PERCENTILE PROGS WITH MINIMAL SPREAD NOTED IN THE SEVEN DAY PERIOD
ASIDE FROM WEDNESDAY.
MONDAY: NARROW BAND OF LIGHT SNOW
DESPITE BEING ONLY ABOUT 24 HOURS OUT, A NUMBER OF SMALL BUT
MEANINGFUL ISSUES CHIP AWAY AT THE CONFIDENCE FOR THE MONDAY
MORNING SNOW BAND. CONFIDENCE IS HIGH THAT WE WILL SEE A SNOW
BAND, BUT IT SHOULD BE VERY NARROW IN NATURE--POSSIBLY ONLY A
COUNTY OR TWO WIDE--AS IT SLIDES WEST TO EAST THROUGH THE
FORECAST AREA. THE FORCING FOR ASCENT WILL BE MESOSCALE, MODEST,
AND BRIEF (2-4 HOURS) IN NATURE AT ANY GIVEN LOCATION. LASTLY, A
DRY LOWER TROPOSPHERE WILL SUBLIMATE AND FURTHER CONSTRICT THE
AREAS IMPACTED BY THE BAND.
ALL COMBINED, IT IS EASY TO SEE HOW NORMAL VARIABILITY IN THE
GUIDANCE AT THIS TIME RANGE FROM AN EVENT COULD MEAN THE
DIFFERENCE BETWEEN NO SNOW AND A QUICK 0.5-1.0". DID TREND POPS
HIGHER (AND BROADER) SINCE THE QUESTION HAS SHIFTED TO WHERE WILL
THE BAND SET UP VERSUS WHETHER A BAND WILL FORM, BUT THE
AFOREMENTIONED ISSUES PREVENT HONING IN ON A SPECIFIC CORRIDOR
FOR HIGHER POPS. THAT MAY NOT BE FEASIBLE UNTIL WE ARE 6-12 HOURS
AWAY FROM THE ONSET OF SNOW.
BOTTOM LINE, IMPACTS FROM THIS SNOW BAND ARE LOOKING TO BE LOW
AND CONFINED IN SPACE AND TIME. VISIBILITIES MAY BRIEFLY FALL TO
1/2 A MILE IN THE HEAVIEST SNOW FOR AN HOUR OR TWO. WHEN DEALING
WITH MESOSCALE SNOW BANDS, THERE IS ALWAYS A RISK THAT IF THE
FORCING IS STRONGER OR MOISTURE DEEPER THEN EXPECTED, SNOW AMOUNTS
COULD QUICKLY INCREASE. INDEED, A FEW EXPLICIT MEMBERS DO
HIGHLIGHT THIS ALBEIT LOW (< 5-10%) POTENTIAL. THE SHORT-LIVED
NATURE OF THE BAND LIMITS THE PLAUSIBLE RANGE OF SNOW AMOUNTS, BUT
THE FAR OUTLIER SOLUTIONS ARE NEARING 2".
TUESDAY: SECOND ROUND OF LIGHT SNOW
WE GO THROUGH THIS SAME GAME WITH SNOW POTENTIAL FOR TUESDAY AS A
WEAK PERTURBATION IN THE CYCLONIC FLOW SPAWNS ANOTHER THIN
CORRIDOR OF MID-LEVEL ISENTROPIC ASCENT/FGEN FORCING THAT BATTLES
AGAINST A DRIER NEAR-SURFACE AIRMASS. QPF PROGS DEPICT A NARROW
REGION OF LIGHT SNOW, BUT CONFIDENCE IN PLACEMENT OF THE BAND IS
EVEN LESS THAN FOR MONDAY'S EVENT AND HAVE LEFT THE 20% NBM POPS
UNCHANGED. AS WITH TOMORROW'S BAND, SNOW AMOUNTS SHOULD BE LIGHT
AND IMPACTS LOW.
THURSDAY: BETTER POTENTIAL FOR WIDESPREAD PRECIPITATION
AS THE LONGWAVE OVER THE WESTERN CONUS PROGRESSES EASTWARD
MIDWEEK, MEDIUM RANGE SOLUTIONS ARE CONVERGING ON A LEE CYCLONE
EJECTING FROM THE FRONT RANGE AND TRACKING THROUGH THE MID-
MISSISSIPPI RIVER VALLEY THURSDAY/THURSDAY NIGHT. TIMING
DIFFERENCES ARISE AT THIS POINT WITH ABOUT 12 HOURS OF SPREAD
BETWEEN THE VARIOUS CLUSTERS. THE INTRA-MODEL VARIABILITY IS ALSO
SOMEWHAT FLUID AND STILL HONING IN ON THE TRACK OF THE LOW.
COMPARED TO YESTERDAY, THERE HAS BEEN A NORTHWARD SHIFT IN THE
SYSTEM AND ITS ATTENDANT DEFORMATION BAND--FURTHER INTO THE
FORECAST AREA. THIS SYSTEM PRESENTS THE NEXT OPPORTUNITY FOR
WIDESPREAD PRECIPITATION. LOWER TROPOSPHERIC THERMAL PROFILES WILL
PLAY A KEY ROLE IN PRECIPITATION TYPE AND ONLY A COUPLE DEGREE
SHIFT COULD QUICKLY CHANGE THE FORECAST. WITH THE GUIDANCE NOW
LOCKING IN ON THE CYCLONE STRUCTURE, THERE HAS BEEN A NOTABLE
UPTICK IN THE >3" SNOW PROBABILITIES (30-40%) OVER IOWA WITH THE
00Z EPS. THIS IS CERTAINLY A SYSTEM TO WATCH OVER THE COMING DAYS.
AVIATION
(FOR THE 18Z TAFS THROUGH 18Z MONDAY AFTERNOON)
ISSUED AT 1105 AM CST SUN DEC 4 2022
PERIODS OF LIGHT SNOW MONDAY WITH POTENTIAL FOR VERY NARROW BAND OF
MVFR/IFR SNOW. POSITIONING UNCLEAR. AMOUNTS WILL BE RELATIVELY
MINOR, GENERALLY UNDER 1".
CIGS: INCREASING CLOUDS OVERNIGHT WITH A LOWERING TOWARD 12Z.
POTENTIAL TO DROP INTO MVFR WITH RELATED BANDS OF SNOW, BUT LOCATION
OF THIS RELATIVELY NARROW BAND OF LOWER CIGS IS UNCERTAIN...AND WILL
HOLD VFR CIGS FOR THE MOMENT. EXPECT SOME IMPROVEMENT LATER IN THE
MORNING AS FIRST SNOW BAND EXITS, BUT CHANCE FOR MORE LATER IN THE
AFTERNOON, AND POTENTIALLY ANOTHER LOWERING IN CIGS.
WX/VSBY: LIGHT SNOW MOVES IN TOWARD 12Z MONDAY. GENERALLY MVFR
VSBYS, BUT A NARROW BAND OF IFR LOOKS LIKELY. WHETHER THAT WOULD
IMPACT THE TAF SITES IS UNCLEAR. EXPECT SOME TAPERING OFF IN THE
LATE MORNING WITH MORE LIGHT SNOW BY MID AFTERNOON. THIS AREA COULD
SHIFT JUST NORTH OF KRST/KLSE. OVERALL ACCUMULATIONS APPEAR MINOR AT
THIS TIME.
WINDS: SOUTHERLY INTO TONIGHT, SWINGING NORTHWEST BY MID/LATE
MORNING MONDAY AS A COLD FRONT PUSHES THROUGH. SOME LLWS CONCERNS AT
KLSE FROM MID TO LATE EVENING TONIGHT WITH 1.5-2KFT WINDS NEARING
40KTS.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
DISCUSSION...SKOW
AVIATION.....RIECK
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