573
FXUS63 KARX 011748
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
1148 AM CST SAT FEB 1 2025
KEY MESSAGES
- SNOW EXPECTED NORTH OF I-94 LATE THIS AFTERNOON THROUGH SUNDAY
MORNING. AMOUNTS HAVE TRENDED LOWER WITH AN INCH OR LESS
EXPECTED. WINTRY MIX, INCLUDING FREEZING DRIZZLE, IN WESTERN
WI.
- MILD THIS WEEKEND WITH HIGHS IN THE 40S AND 50S ON SUNDAY.
SOME DAILY RECORD HIGHS MAY BE CHALLENGED. TEMPERATURES COOL
DOWN TO START NEXT WEEK WITH HIGHS RETURNING TO NEAR AVERAGE.
- ADDITIONAL LIGHT SNOW POSSIBLE (30-50%) SUNDAY NIGHT AND
MONDAY WITH MINOR ACCUMULATIONS.
DISCUSSION
ISSUED AT 319 AM CST SAT FEB 1 2025
TODAY - SUNDAY: COMPLEX SNOW & DRIZZLE FORECAST
A VERY TRICKY LOW QPF PRECIPITATION SCENARIO ONCE AGAIN PLAGUES OUR
REGION LATER TODAY AND INTO TONIGHT AS AN UPPER-LEVEL SHORTWAVE
APPROACHES THE REGION WITH PIECES OF VORTICITY EMBEDDED WITHIN IT.
SNOW VERSUS DRIZZLE CONTINUES TO BE THE PRIMARY FORECAST
CONSIDERATION AS DETERMINISTIC MODEL SOUNDINGS CONTINUE TO GIVE
LITTLE CONFIDENCE IN HOW SATURATION PROFILES WILL MANIFEST. WILL
START WITH THE HIGHEST CONFIDENCE ITEMS FIRST, WITH THE FACT THAT
ACCUMULATING SNOW REMAINS LIKELY NORTH OF I-94 THROUGH TONIGHT.
HOWEVER, GIVEN SOME OF THE RECENT CAMS HAVE REALLY SHIFTED THINGS
FURTHER NORTH, HAVE TRENDED ACCUMULATIONS LOWER IN THE MOST RECENT
FORECAST WITH THE 01.00Z HREF HAVING FAIRLY LOW (20-60%)
PROBABILITIES FOR EVEN 0.5" NORTH OF I-94.
OTHERWISE, AS STATED EARLIER THE FORECAST TRENDS ALONG AND SOUTH OF
I-94 ARE VERY MESSY WITH QUESTIONS REGARDING SNOW AND DRIZZLE.
OVERALL, THE KEY FACTORS IN PLAY WILL BE HOW DOES OUR SATURATION
MANIFEST THROUGHOUT THE PERIOD, ARE SURFACE TEMPERATURES ABLE TO
COOL TO FREEZING, AND WILL WE HAVE LIFT TO GENERATE PRECIPITATION.
AS FAR AS FORCING IS CONCERNED, PIECES OF VORTICITY ADVECTION WITHIN
THE SHORTWAVE AND WARM ADVECTION WILL BE THE PRIMARY SOURCE OF LIFT
THROUGHOUT THE EVENING AND INTO THE OVERNIGHT. AS A RESULT,
EXPECTING LIFT TO BE SOMEWHAT TRANSIENT ASIDE FROM THE WARM
ADVECTION LATE EVENING AND OVERNIGHT WHICH SOME OF THE CAMS DEVELOP
PRECIPITATION ON NEAR AND SOUTH OF I-90. SATURATION PROFILES IN
RECENT RUNS OF THE CAMS HAVE DEVIATED SOME FROM PREVIOUS RUNS FOR
THE MORNING AND AFTERNON HOURS, WITH A FAIRLY PRONOUNCED DRY LAYER
THAT SLOWLY SATURATES THROUGH THE AFTERNOON AS SHOWN IN THE 01.03Z
RAP AT LA CROSSE. THIS DRAWS INTO QUESTION THE EARLIER THINKING
WHERE WE MAY START OFF AS SNOW INITIALLY.
THE QUESTION THEN BECOMES AS THIS DRY LAYER NARROWS, IF SEEDER-
FEEDER CAN TAKE OVER AND PRODUCE SNOW BY LATE EVENING, BASED ON IF
THERE IS ENOUGH LIFT IN THE LAYER TO PRODUCE SNOW ALOFT AT A HIGH
ENOUGH RATE TO EVAPORATE THE DRY LAYER BENEATH. CONSEQUENTLY, THIS
LATE EVENING THROUGH EARLY MORNING SUNDAY PERIOD IS THE MOST
CHALLENGING PART OF THE FORECAST AS VERY SUBTLE SHIFTS IN THE
SATURATION WILL HAVE SUDDEN DIFFERENCES IN P-TYPE. FOR EXAMPLE, IF
THIS PROGGED DRY LAYER IS STEEPER WITH LOW-LEVEL SATURATION IN
PLACE, THE ICE PRODUCING CLOUD LAYER ALOFT MAY NOT BE ABLE TO FEED
TO LOW-LEVEL LAYER AND WOULD RESULT IN DRIZZLE AS OPPOSED TO SNOW
AND VICE VERSA IF IT CAN. AN ADDITIONAL WRINKLE AS WELL WILL BE
SURFACE TEMPERATURES, WITH TEMPERATURES HOVERING JUST ABOVE
FREEZING. HOWEVER, WITH DEWPOINTS BELOW FREEZING, CANNOT RULE OUT
SOME WET-BULBING THAT WOULD RESULT IN SURFACE FREEZING TEMPERATURES
WITH DRIZZLE. SO THIS MAY BE A CASE WHERE WE WILL HAVE TO MONITOR
SHORT-TERM TRENDS CLOSELY AS SNOW, FREEZING DRIZZLE AND DRIZZLE WILL
LIKELY ALL BE IN PLAY DEPENDING ON SOME OF THE AFOREMENTIONED
FACTORS. OVERALL THOUGH, QPF IS GENERALLY EXPECTED TO BE VERY LIGHT
WITH THIS (UNDER 40% OF SEEING OVER 0.05" OF QPF OUTSIDE OF TAYLOR
COUNTY IN THE 01.00Z HREF), SO FREEZING DRIZZLE WILL BE THE PRIMARY
IMPACTFUL PRECIPITATION MODE WHERE IT MANIFESTS BEFORE PRECIPITATION
EXITS ON SUNDAY MORNING. OTHERWISE, FAIRLY MILD TEMPERATURES ARE ON
TAP FOR SUNDAY WITH HIGHS IN THE 40S TO PERHAPS INTO THE 50S IN
SOME LOCATIONS. AS A RESULT, SOME DAILY RECORD HIGHS MAY BE
CHALLENGED LOCATIONS.
SUNDAY NIGHT - MONDAY: COOLING DOWN, OVERNIGHT/MORNING SNOW CHANCES
AS WE HEAD INTO SUNDAY EVENING, STRONGER MID-LEVEL FRONTOGENESIS
WILL SLOWLY MEANDER ITS WAY INTO THE LOCAL AREA. BASED ON THE NAM
AND RAP SOUNDINGS, SOME PROFOUND DIFFERENCES CAN BE NOTED AS THIS
FORCING MAKES IT WAY IN. THE GENERAL IDEA THAT CAN BE NOTED IS A
PRONOUNCED DRY LAYER BETWEEN 700MB AND 900MB THAT SATURATES FROM TOP-
DOWN. THE KEY DIFFERENCES BETWEEN THE NAM AND RAP ALTHOUGH IS THE
RAP LACKS NEAR-SURFACE SATURATION THAT THE NAM HAS WHICH WOULD
SUGGEST SOME POSSIBILITY OF FREEZING DRIZZLE. HOWEVER, GIVEN THAT
THE NAM TENDS TO BIAS TOWARDS SATURATION, THINKING THIS IS LESS
LIKELY. SO GENERALLY WILL SATURATE THIS COLUMN AND EVENTUALLY
SEEDER-FEEDER WILL TAKE OVER AND ENABLE SNOW PRODUCTION.
AS WE HEAD INTO EARLY MONDAY MORNING, SATURATION IN THE RECENT
NAM/GFS SOUNDINGS SUGGEST BY 09Z OR SO WE SHOULD BE UNIFORMLY SNOW
WITH SATURATION WELL INTO THE DENDRITIC GROWTH ZONE. AS A RESULT,
WITH SOME MID-LEVEL FRONTOGENETIC FORCING WITHIN THE NAM/GFS, FAIRLY
DECENT SHOT THAT A BAND OF LIGHT SNOW PERSISTS THROUGH THE MORNING
HOURS ON MONDAY. OVERALL, THE GRAND ENSEMBLE (GEFS/EC/CANADIAN
ENSEMBLE) HAS HIGH PROBABILITIES (70-90%) FOR MEASURABLE SNOW NORTH
OF I-90 WHICH MAKES SENSE GIVEN THE FRONTOGENETIC SETUP.
ADDITIONALLY, THE GRAND ENSEMBLE HAS SOME PROBABILITIES (20-40%) FOR
ACCUMULATIONS OF AN INCH OR MORE. ALTHOUGH THIS IS ASSUMING A 10:1
SNOW TO LIQUID RATIO AND BASED ON DETERMINISTIC SOUNDINGS, A
CLIMATOLOGICAL SLR OF 13:1 APPEARS MORE LIKELY SUGGESTING THESE
PROBABILITIES ARE LIKELY SLIGHTLY HIGHER. OTHERWISE, AS THIS SNOW
EXITS THE REGION, ZONAL FLOW ALOFT AND SOME NORTHWESTERLY SURFACE
FLOW WILL COOL TEMPERATURES FOR MONDAY WITH AFTERNOON TEMPERATURES
GENERALLY IN THE 20S TO LOWER 30S AREAWIDE. SO WHILE SEASONABLE FOR
THIS TIME OF YEAR, CERTAINLY COOLER THAN WHAT WE HAVE EXPERIENCED
OVER THE PAST WEEK.
TUESDAY - FRIDAY: SEASONABLE TEMPERATURES PERSIST, UNCERTAIN
PRECIPITATION CHANCES
AS WE HEAD PAST MONDAY, ZONAL FLOW BECOMING THE DRIVING FORCE INTO
THE MIDDLE OF THE WEEK WITH DETERMINISTIC GUIDANCE BEING PRETTY
UNIFORM ON AGREEING THAT TEMPERATURES WILL BE NEAR SEASONABLE WITH
THE EXCEPTION OF TUESDAY WHERE SOME NORTHWESTERLY TO NORTHERLY FLOW
MAY KEEP TEMPERATURES SOMEWHAT COOLER. THIS IS SHOWN IN THE MEDIAN
HIGH TEMPERATURES IN THE RECENT GRAND ENSEMBLE (GEFS/EC/CANADIAN
ENSEMBLE) WITH VALUES IN THE TEENS TO MIDDLE 20S AREAWIDE. AS WE
HEAD INTO THE MIDDLE OF THE WEEK, DETERMINISTIC GUIDANCE HAS A
FAIRLY CONSISTENT SIGNAL OF AN UPPER-LEVEL TROUGH PASSING NORTH OF
THE LOCAL AREA. HOWEVER, VAST DISAGREEMENT ON HOW ANY DEVELOPING
SURFACE LOW MAY MANIFEST AS A RESULT. THE DETERMINISTIC EC IS MORE
ROBUST THAN THE GFS WITH A RAPIDLY DEVELOPING SURFACE LOW THAT
PUSHES SOUTHEAST OF OUR REGION, SUBJECTING TO THE DEFORMATION ZONE.
PROBABILISTIC GUIDANCE IN THE GRAND ENSEMBLE (GEFS/EC/CANADIAN
ENSEMBLE) HAS SOME HIGH PROBABILITIES (60-80%) DURING THIS PERIOD
FOR MEASURABLE PRECIPITATION, SO WOULD LIKELY BE OUR NEXT POTENTIAL
WINDOW FOR MEANINGFUL PRECIPITATION AFTER MONDAY.
AVIATION /18Z TAFS THROUGH 18Z SUNDAY/
ISSUED AT 1145 AM CST SAT FEB 1 2025
GUSTY SOUTHEAST WINDS AND SOME TRANSIENT MVFR CIGS WILL
CONTINUE THIS AFTERNOON AHEAD OF THE NEXT APPROACHING SYSTEM.
INCREASING LOW LEVEL MOISTURE WILL CAUSE CIGS TO DETERIORATE TO
IFR BY LATE EVENING AND LIKELY TO LIFR AT RST IN THE EARLY
MORNING HOURS. THIS SIGNAL FOR LOW CIGS HAS BEEN CONSISTENT IN
RECENT MODEL RUNS, LENDING HIGHER CONFIDENCE IN IFR TO LIFR
LIKELIHOOD (>60%). THERE IS LOWER CONFIDENCE IN PRECIPITATION
POTENTIAL AS A SYSTEM QUICKLY SWINGS THROUGH THE REGION
TONIGHT. LIFT IS FAVORABLE, BUT THERE ARE QUESTIONS ON HOW
SATURATED THE LOWER ATMOSPHERE WILL BECOME, IMPACTING
PRECIPITATION LIKELIHOOD AT THE TAF SITES AS WELL AS PRECIP
TYPES/AMOUNTS. HIGHEST PRECIP CHANCES (40 TO 70%) LOOK TO
DEVELOP ALONG/EAST OF THE MISSISSIPPI RIVER, SO HAVE INCLUDED
SMALL CHANCES AT LSE. MAY START OUT AS A BRIEF PERIOD OF LIGHT
SNOW BEFORE TRANSITIONING TO DRIZZLE OR FREEZING DRIZZLE,
DEPENDING ON WARMING SURFACE TEMPERATURES. REGARDLESS, PRECIP
AMOUNTS WILL BE VERY MINOR. RST STANDS A RATHER LOW CHANCE OF
ANY PRECIP (<20%) AND DUE TO LOW CONFIDENCE HAVE LEFT MENTION
OUT OF THIS TAF ISSUANCE BUT WILL MONITOR UPSTREAM CONDITIONS
THIS AFTERNOON.
AS THE SYSTEM EXITS EARLY SUNDAY MORNING, WINDS WILL BECOME
LIGHTER OUT OF THE SOUTH TO SOUTHWEST. RESIDUAL LOW LEVEL
MOISTURE MAY ALLOW FOR AREAS OF FOG TO DEVELOP, ESPECIALLY WEST
OF THE MISSISSIPPI RIVER. QUESTIONS ON HOW THICK THIS FOG MAY
DEVELOP OR IF IT MAY JUST MANIFEST AS VERY LOW STRATUS, BUT
HAVE INTRODUCED IFR VSBY AT RST. CIGS AND VSBY SHOULD BEGIN TO
IMPROVE ACROSS THE AREA BY LATE MORNING.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
DISCUSSION...NAYLOR
AVIATION...KURZ
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