286
FXUS63 KARX 202110
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
ISSUED BY NATIONAL WEATHER SERVICE DES MOINES IA
310 PM CST WED NOV 20 2024
KEY MESSAGES
- SNOW SHOWERS WILL PERSIST THIS EVENING. MINOR SNOW
ACCUMULATIONS AND SOME SLICK AREAS MAY DEVELOP.
- STRONG NORTHWEST WINDS POSSIBLE THURSDAY WITH GUSTS
APPROACHING 45 MPH IN SOME AREAS. SOME SNOW ACCUMULATIONS MAY
OCCUR FAR EAST DURING THE MORNING.
DISCUSSION
ISSUED AT 303 PM CST WED NOV 20 2024
A BROAD AREA OF SNOW SHOWERS ARE ONGOING THIS AFTERNOON.
VISIBILITIES HAVE BEEN REDUCED TO 3-5SM AT TIMES THOUGH THERE HAVE
BEEN A FEW BRIEF REDUCTIONS LOWER. CHARLES CITY DID HAVE 3/4SM AND
SNOW BRIEFLY THIS AFTERNOON. ROAD TEMPERATURES HAVE BEEN NEAR TO
SLIGHTLY ABOVE AIR TEMPERATURES THIS AFTERNOON AND SOME OF THAT HAVE
BEEN DRIVEN BY SOME SOLAR RADIATION STILL PASSING THROUGH THE
CLOUDS. THUS ROAD CONDITIONS REMAIN WET. THE ROAD TEMPERATURES WILL
DROP WITH THE LOSS OF SOLAR RADIATION TOWARDS SUNSET SO THERE IS A
POTENTIAL FOR A FEW SLICK SPOTS TO DEVELOP. IN ADDITION THE SNOW
SHOWERS WILL PERSIST INTO THE EVENING AS DEEPER SATURATION OCCURS.
AS THE SATURATION OCCURS, THE DENDRITIC GROWTH ZONE (DGZ) WILL
INCREASE IN DEPTH TO NEAR 10 KFT. THOUGH THE VERTICAL ASCENT WITHIN
THIS DGZ IS MODEST, THAT TRANSITION OF THE SNOW CRYSTAL MORPHOLOGY
WILL FAVOR LARGER FLAKES THAN WHAT HAVE OCCURRED TODAY AND MAY
RESULT IN LIGHT SNOW ACCUMULATIONS. THIS LIGHT SNOW MAY FATHER
SUPPORT A FEW SLICK PATCHES THIS EVENING, ESPECIALLY IN SHELTERED
AREAS WHERE THE SNOW IS NOT REMOVED BY THE WIND. THIS MODEST FORCING
WILL DIMINISH AFTER MIDNIGHT. THE BREEZY TO GUSTY CONDITIONS WILL
PERSIST OVERNIGHT AS THE BOUNDARY LAYER IS EXPECTED TO REMAIN MIXED
MUCH OF THIS PERIOD. THE WIND THIS EVENING WILL BE SIMILAR TO WHAT
IS CURRENTLY ONGOING BUT EXPECT AN UPTICK IN THE WIND BY LATE
TONIGHT AS SURFACE LOW PRESSURE INTENSIFIES OVER NORTHERN LAKE
MICHIGAN.
THE INTENSIFYING SURFACE LOW PRESSURE IS IN RESPONSE TO A STRONG
VORTICITY LOBE ROTATING AROUND THE PARENT UPPER LOW PRESSURE THAT IS
MOVING FROM CENTRAL MINNESOTA TO TO LOWER MICHIGAN BY THURSDAY
MORNING. THE VORTICITY LOBE/PV ANOMALY HAS AN IMPRESSIVE PV 1.5
PRESSURE SURFACE DOWN TO NEAR 600 MB. THE SURFACE AND UPPER LEVEL
FEATURES WILL DIVE STRAIGHT DOWN LAKE MICHIGAN. THIS TRAJECTORY WILL
PULL LOW LEVEL MOISTURE INTO WISCONSIN FROM LAKE MICHIGAN. IN
ADDITION, THE LARGE UPPER LEVEL LOW ORIENTATION WILL RESULT IN A NON-
TYPICAL MID-LEVEL MOISTURE ADVECTION FROM THE NORTH ATLANTIC AND THE
COMBINATION OF MOISTURE ADVECTION WILL RESULT IN PWAT VALUES OVER
ONE HALF INCH INTO CENTRAL WISCONSIN. A NARROW ROBUST BAND OF
PRECIPITATION IS EXPECTED ALONG THE WESTERN PERIPHERY OF THE SURFACE
LOW WHICH SHOULD REMAIN MOSTLY EAST OF THE FORECAST AREA. JUNEAU AND
ADAMS COUNTY WILL HAVE THE GREATEST CHANCE FOR ACCUMULATING
PRECIPITATION. THERE IS SOME UNCERTAINTY ON PRECIPITATION TYPE BUT
THE INITIAL PRECIPITATION SHOULD INITIALIZE AS SNOW. SURFACE DEW
POINTS SHOULD INCREASE SOME DURING THE DAY THOUGH TIMING OF THE
INCREASING DEW POINTS AND THE IMPACT ON WET BULB TEMPERATURES WILL
DICTATE THE PRECIPITATION TYPE. FORECAST TEMPERATURES ARE LIKELY
STILL ON THE HIGH SIDE WHILE THE PRECIPITATION IS OCCURRING SO ANY
SNOW POTENTIAL MAY LINGER THROUGH MUCH OF THE MORNING BEFORE A
POTENTIAL TRANSITION TO LIGHT RAIN AS SURFACE WET BULB
TEMPERATURE BEGIN TO RISE ABOVE 34-35 DEGREES. THERE ARE A FEW
SIGNALS THAT THE ACCUMULATING SNOW OF AN INCH OR MORE COULD
REACH JUNEAU AND ADAMS COUNTIES.
THE OTHER CHALLENGE IS THE WIND POTENTIAL ON THURSDAY. THE PRESSURE
GRADIENT INCREASES QUICKLY LATE TONIGHT OVER NORTHERN WISCONSIN LATE
TONIGHT AND WILL EXPAND ACROSS MUCH OF THE AREA ON THURSDAY. THE
PRESSURE GRADIENT VALUES OF 45-55 UBARS/KM IS IMPRESSIVE AND WILL BE
MAXIMIZED FROM 12-21Z BEFORE THE FILLING LOW LEADS TO A GRADUAL
RELAXING OF THE GRADIENT. MIXING DEPTHS WILL BE RELATIVELY SHALLOW
AND BELOW 2 KFT BUT PEAK MIXED LAYER WINDS RANGING FROM THE UPPER
30S TO MID 40S KT RANGE WILL BE PRESENT. THE LACK OF STRONG COLD
ADVECTION AND ACTUALLY IN THIS CASE, IT WILL BE WARM ADVECTION, WILL
LIMIT THE MIXING EFFICIENCY OF THE PEAK MIXED LAYER WINDS BUT STILL
WILL SUPPORT THE ISALLOBARIC COMPONENT. OVERALL, MODEL GUIDANCE,
BLENDS AND PROBS ARE NOT HANDLING THIS WIND VERY WELL AND HAVE
TRENDED TO NEAR THE NBM 90TH PERCENTILE AND THERE IS AN INCREASING
CHANCE THAT WIND HEADLINES COULD BE REQUIRED. ANY SNOW ACCUMULATIONS
OVER THE EAST COULD COMPLICATE THESE HEADLINE DECISIONS.
LITTLE CHANGE TO THE FORECAST BEYOND THURSDAY. THE UPPER LOW WILL
CONTINUE TO DEPART TO THE EAST WITH THE UPPER LEVEL FLOW
TRANSITIONING TO A MORE WESTERLY/ZONAL PATTERN. OVER THE WEEKEND.
ANOTHER UPPER LEVEL SYSTEM WILL DESCEND INTO THE NORTHERN PLAINS
EARLY NEXT WEEK AND WILL BRING ANOTHER SURGE OF NEAR TO BELOW NORMAL
TEMPERATURES WITH HIGH TEMPERATURES RETREATING BACK INTO THE 30S.
THIS SYSTEM WILL ALSO BRING LIGHT SNOW POTENTIAL BACK TO THE REGION.
AVIATION /18Z TAFS THROUGH 18Z THURSDAY/
ISSUED AT 1130 AM CST WED NOV 20 2024
MVFR CIGS ARE EXPECTED MUCH OF THE PERIOD THOUGH CIGS WILL
BOUNCE ABOVE AND BELOW 2 KFT AT TIMES. SNOW SHOWERS REMAIN IN
THE FORECAST INTO THIS EVENING. PERIODS WITH THE HIGHEST CHANCE
WITH LOWEST VISIBILITIES ARE HIGHLIGHTED. GUSTY WEST/NORTHWEST
WINDS TODAY WILL DIMINISH SLIGHTLY OVERNIGHT BUT NEAR SURFACE
MIXING WILL HELP GUSTS PERSIST. STRONGER WINDS ARE EXPECTED BY
THURSDAY MORNING WITH SOME GUSTS OVER 35 KTS POSSIBLE. VERY
LIGHT SNOW MAY ALSO PASS THROUGH THE AREA THU MORNING.
ARX WATCHES/WARNINGS/ADVISORIES
WI...NONE.
MN...NONE.
IA...NONE.
DISCUSSION...DONAVON/DMX
AVIATION...DONAVON/DMX
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