858
FXUS63 KARX 261151
AFDARX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LA CROSSE WI
551 AM CST THU DEC 26 2024
KEY MESSAGES
- FREEZING DRIZZLE THREAT HAS STARTED TO MANIFEST ITSELF THIS
MORNING AND A WINTER WEATHER ADVISORY HAS BEEN ISSUED WITH
SLICK ROADS AND SLIDEOFFS BEING REPORTED.
- PERIODS OF DRIZZLE/SHOWERS TODAY INTO MIDDAY FRIDAY WITH MORE
WIDESPREAD RAIN FRIDAY AFTERNOON AND NIGHT. STORM TOTAL
RAINFALL AMOUNTS COULD EXCEED 1/2 OF AN INCH. WIDESPREAD FOG,
LIKELY DENSE, DURING THIS PERIOD.
- WARMER WEATHER FOR THE END OF THE WEEK AND THE WEEKEND WITH
HIGHS IN THE LOW TO MID 40S, COOLING OFF STEADY BACK TO
SEASONAL VALUES BY THE MIDDLE OF NEXT WEEK.
- CANNOT RULE OUT SNOW/RAIN MONDAY INTO EARLY TUESDAY.
UPDATE
ISSUED AT 405 AM CST THU DEC 26 2024
NO SOONER HAD THIS MORNING'S DISCUSSION GONE OUT THE DOOR, THE
FREEZING DRIZZLE PICKED UP AGAIN ON RADAR AND AT THE OFFICE,
PRODUCING A GLAZING OF ICE ON THE PARKING LOT AND CARS. A FEW
TRAFFIC SLOWDOWNS HAVE SHOWN UP AND AT LEAST ONE SLIDEOFFS
REPORTED ON INTERSTATE 39. GIVEN THAT THE MAIN IMPACTS ARE
EXPECTED OVER THE NEXT 6-8 HOURS, DID GO AHEAD AND ISSUE A
WINTER WEATHER ADVISORY FOR FAR SOUTHEAST MN/NORTHEAST IOWA AND
MUCH OF SOUTHWESTERN WISCONSIN THROUGH NOON.
DISCUSSION
ISSUED AT 325 AM CST THU DEC 26 2024
THIS MORNING: FREEZING DRIZZLE POTENTIAL
THE CHRISTMAS MIRACLE THAT GAVE US THE UNEXPECTED BREAK IN THE
CLOUDS YESTERDAY AFTERNOON MAY HAVE BEEN A GRINCH IN DISGUISE
AS AIR AND ROAD TEMPERATURES IN THE CLEARING FELL 5-10 DEGREES
BEFORE STRATUS MOVED BACK IN LATER IN THE EVENING. THESE
TEMPERATURES HAVE BEEN SLOW TO RECOVER OVERNIGHT WITH READINGS
IN THE MID TO UPPER 20S AS OF 3 AM, WHEREAS AREAS UNDER THE
CLOUDS FURTHER SOUTH AND WEST STAYED IN THE LOW TO MID-30S.
DEEPER LOW-LEVEL MOISTURE HAS BEEN SLOWLY SPREADING NORTHWARD
OVERNIGHT WITHIN A REGION OF WEAK ISENTROPIC ASCENT FROM
280-290K. THE HREF MEMBERS HAVE BEEN OVERESTIMATING THE AERIAL
EXTENT OF THE DRIZZLE COVERAGE SO FAR TONIGHT, WITH THE HOURLY
RUNS OF THE RAP/HRRR PULLING THE THREAT AREA FOR DRIZZLE
SLIGHTLY FARTHER EAST WITH EACH SUCCESSIVE RUN.
THIS DOES NOT INSTILL CONFIDENCE IN THE COVERAGE OR IMPACTS OF
ANY FREEZING DRIZZLE THROUGH THE MID-MORNING HOURS. ONE BAND OF
FREEZING DRIZZLE DID LIFT THROUGH SOUTHWESTERN WISCONSIN BETWEEN
MIDNIGHT AND 2AM, BUT IMPACTS WERE NEGLIGIBLE AND NO GLAZING
WAS OBSERVED AT THE OFFICE HERE IN LA CROSSE. THERE IS A RIBBON
OF STRONGER ASCENT THAT LIFTS NORTHWARD ALONG THE 290K SURFACE
THIS MORNING, FOCUSED MAINLY EAST OF THE MISSISSIPPI RIVER
BETWEEN 6 AM AND NOON. THE MANIFESTATION OF THIS LIFT MAY BE
STARTING TO APPEAR ON THE DMX/DVN RADAR SCOPES IN CENTRAL AND
EASTERN IOWA, THOUGH THERE HAS BEEN LITTLE FOR DRIZZLE REPORTED
WITH IT.
THE MRMS SUB-FREEZING PAVEMENT TEMPERATURE PROBABILITIES HAVE
BEEN SLOWLY ERODING NORTHWARD AND THIS MIRRORS THE OBSERVATIONS
FROM THE SOUTHERN FORECAST AREA WHERE WET BULB AND ROAD
TEMPERATURES HAVE RISEN ABOVE FREEZING. BY MIDDAY, THE THREAT
FOR FREEZING DRIZZLE SHOULD BE OVER WITH THE WARMER AIR HAVING
SETTLED IN PLACE.
THIS AFTERNOON - SAT MORNING: STRETCH OF WET WEATHER AND FOG
A WEAKENING MERIDIONAL MID TO UPPER LEVEL TROPOSPHERIC TROUGH
SLIDES OUT OF THE GREAT PLAINS AND INTO THE MID-MISSISSIPPI
RIVER VALLEY BY FRIDAY MORNING. GIVEN ITS FILLING NATURE,
SYNOPTIC LIFT WILL BE LIMITED AND THE THREAT FOR ANY SHOWERS
DRIVEN BY TRANSIENT MESOSCALE FORCING. THE LOW LEVEL MOISTURE
WILL BE IN PLACE FOR CONTINUED DRIZZLE OR LIGHT SHOWERS AND THE
CONVECTIVE MODELS ARE EMPHASIZING THE SCATTERED NATURE OF ANY
PRECIPITATION IN THEIR POP/COMPOSITE REFLECTIVITY PROGS. DIDN'T
LOWER POPS TOO MUCH GIVEN THAT FACT THAT MOST FOLKS SHOULD SEE
PRECIPITATION, BUT DID LEAN INTO THE SCATTERED/SHOWER WORDING IN
THE WORDED FORECAST TO HIGHLIGHT THIS POINT.
A DEEPER, NEGATIVELY-TILTED WAVE DRIVES INTO THE BACKSIDE OF
THIS WEAKENING TROUGH BY MIDDAY FRIDAY AND EJECTS OVER THE
REGION FRIDAY NIGHT INTO EARLY SATURDAY MORNING. THE ARRIVAL OF
THIS WAVE HELPS REINVIGORATE THE MODEST SURFACE LOW THAT FORMED
WITH THE LEAD WAVE OVER TEXAS, PULLING THE LOW NORTHWARD AND
DEEPENING IT SLIGHTLY IN THE PROCESS. THIS BROAD LOW TRACKS OVER
THE REGION FRIDAY AFTERNOON AND NIGHT WITH THE ACCOMPANYING
DEEPER MOISTURE AND ISENTROPIC/ KINEMATIC LIFT DRIVING MODERATE
RAINFALL WITH ITS PASSAGE. ROUGHLY 40 TO 60 PERCENT OF THE LREF
MEMBERS HAVE A STORM TOTAL QPF OF OVER 1/2 OF AN INCH BY
SATURDAY MORNING, AN INCREASE FROM WHAT WE WERE SEEING
YESTERDAY.
THE STEADY FETCH OF LOW-LEVEL MOISTURE AHEAD OF THIS SYSTEM HAS
RESULTED IN THE FORMATION OF DENSE FOG ACROSS MUCH OF CENTRAL
AND EASTERN IOWA EARLY THIS MORNING. THIS DEEPER MOISTURE
ADVANCES OVER THE AREA TODAY AND SHOULD ONLY BE AMPLIFIED BY THE
MELTING OF OUR CURRENT SNOWPACK. DID ISSUE A DENSE FOG ADVISORY
FOR PARTS OF NORTHEAST IOWA AND FAR SOUTHWESTERN WISCONSIN FOR
THIS ROUND OF FOG THIS MORNING, BUT IN ALL LIKELIHOOD WE WILL
NEED A LARGER FOG ADVISORY LATER TODAY WITH THE 26.00Z HREF
HAVING PROBABILITIES OF 80- 100 PERCENT OF VISIBILITIES OF LESS
THAN A QUARTER OF A MILE AFTER 2 TO 4 PM TODAY. THIS FOG MAY
LINGER WELL INTO FRIDAY AND ONLY LIFT WHEN THE HEAVIER RAINFALL
ARRIVES IN THE AFTERNOON.
WARM WEEKEND AHEAD, COOLING OFF NEXT WEEK
THE INCREASE IN LOW-LEVEL MOISTURE WILL BRING WITH IT WARMER
TEMPERATURES. DESPITE THE CLOUDS AND FOG, TEMPERATURES ARE
EXPECTED TO TOP 40 DEGREES FOR MOST LOCALES FOR FRIDAY AND
SATURDAY WITH LOWS ONLY FALLING A FEW DEGREES EACH NIGHT UNDER
THE DENSE CLOUD COVER. THERE A SIGNAL IN THE EXTREME FORECAST
INDEX THAT WE COULD SEE RECORD WARM LOWS WITH ROUGHLY 60 TO 70
PERCENT OF THE EPS MEMBERS EXCEEDING THE MODEL CLIMATOLOGY AND
THE CURRENT NBM FORECAST FOR ROCHESTER AND LA CROSSE MEETING OR
EXCEEDING THE DAILY RECORDS ON SATURDAY. EVEN WITH THE DEPARTURE
OF THE SHORTWAVE ON SATURDAY, FLOW REMAINS FROM THE SOUTHWEST
AHEAD OF A SECONDARY SHORTWAVE EMBEDDED WITHIN THE MEAN LONGWAVE
TROUGH, WHICH DOES NOT CLEAR THE AREA UNTIL MONDAY. THEREFORE,
TEMPERATURES FOR SUNDAY AND MONDAY WILL BE SIMILAR TO FRIDAY AND
SATURDAY. THE FLOWS TURNS NORTHWESTERLY AND COOLER AIR SLIDES
SOUTHEASTWARD LATE MONDAY THROUGH THE REMAINDER OF THE WEEK,
WITH LOWER TROPOSPHERIC TEMPERATURES SLOWLY FALLING THROUGH THE
PERIOD FROM +1 C ON MONDAY TO -15 C BY THURSDAY. AS A RESULT,
SURFACE TEMPERATURES SHOULD FALL TO OR BELOW AVERAGE BY MIDWEEK.
MONDAY - EARLY TUESDAY: RAIN/SNOW CLIPPING THE REGION
A 150-KT JET STREAK USHERS A COMPACT VORT LOBE ACROSS THE
ROCKIES AND INTO THE CENTRAL CONUS SUNDAY INTO EARLY MONDAY.
GIVEN THAT THE ENERGY LOBE IS STILL OUT OVER THE ALEUTIANS AND
LINGERS OVER THE PACIFIC UNTIL MIDDAY SUNDAY, IT IS NOT
SURPRISING THAT THERE IS MODEST SPREAD IN THE TRACK OF THE
SURFACE LOW RESPONSE ACROSS THE MID-MISSOURI RIVER VALLEY. THERE
HAS BEEN A SLIGHT NORTHWARD SHIFT IN THE TRACK IN THE LREF
PROGS OVER THE LAST 24 HOURS, DRIVEN HEAVILY BY THE GEFS MEMBERS
THAT ARE STARTING TO SHOW THE SURFACE LOW RECURVING TO THE
NORTHEAST SOONER THAN THE EPS SOLUTIONS. THIS BEARS WATCHING AS
SUCH A SHIFT DOES BRING PRECIPITATION FARTHER NORTHWARD INTO THE
FORECAST AREA, LIKELY IN THE FORM OF SNOW BY MONDAY EVENING
WITH NEARLY ALL LREF/NBM MEMBERS DEPICTING SURFACE TEMPERATURES
BELOW THE MID-30S BY SUNSET. IT REMAINS TOO EARLY TO SPECULATE
ON AMOUNTS, BUT SUCH SYSTEMS CAN OFTEN DELIVER A MODEST AMOUNT
OF SNOW IN A HURRY IF CONDITIONS LINE UP.
AVIATION /12Z TAFS THROUGH 12Z FRIDAY/
ISSUED AT 551 AM CST THU DEC 26 2024
IFR/LIFR CONDITIONS ARE EXPECTED THROUGHOUT THE TAF PERIOD WITH LOW-
STRATUS OVERHEAD. ONGOING FREEZING DRIZZLE THIS MORNING WILL
CONTINUE UNTIL AROUND 15Z-18Z WHEN SURFACE TEMPERATURES WILL WARM
ABOVE FREEZING AND DRIZZLE WILL NO LONGER POSE ANY AVIATION IMPACTS.
ADDITIONALLY, ALREADY NOTING FOG OVERSPREADING MUCH OF THE LOCAL
AREA THIS MORNING WITH KRST REPORTING IFR VSBYS AS OF RECENT
OBSERVATIONS WITH WARMER, MOISTURE RICH AIR MOVING OVER THE
PRESENT SNOWPACK. AS WE CONTINUE THROUGHOUT THE MORNING,
EXPECTING VISIBILITIES TO CONTINUE TO DROP TO 1/2SM WITH SOME
SPOTS FALLING EVEN LOWER. OVERALL, GUIDANCE HAS BEEN ON THE
SLOWER SIDE WITH THIS FOG BASED ON OBSERVATIONAL TRENDS SO HAVE
INTRODUCED 1/2SM FOG AS EARLY AS 14Z AT KRST AND THIS COULD BE
ON THE LATER SIDE. PROBABILITIES FOR 1/4SM ARE VERY HIGH
(70-100%) PAST SUNSET FOR MUCH OF THE AREA OUTSIDE OF RIVER
VALLEYS IN THE 26.00Z HREF SO HAVE OPTED TO INTRODUCE 1/4SM AT
KRST BY 23Z. HOWEVER, CANNOT RULE OUT SOME PERIODS OF 1/4SM
EARLIER THAN THIS. FOG WILL LIKELY LINGER THROUGH MUCH OF THE
OVERNIGHT WITH SHOWERS BEGINNING TO INCREASE IN COVERAGE TOWARDS
FRIDAY MORNING AHEAD OF AN INCOMING WEATHER SYSTEM. THE FOG
WILL LIKELY DISPERSE ONCE MORE WIDESPREAD RAINFALL MOVES INTO
THE REGION ON FRIDAY.
ARX WATCHES/WARNINGS/ADVISORIES
WI...DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR WIZ043-044-
053>055-061.
WINTER WEATHER ADVISORY UNTIL NOON CST TODAY FOR WIZ017-029-
033-034-041>044-053>055.
MN...DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR MNZ094>096.
WINTER WEATHER ADVISORY UNTIL NOON CST TODAY FOR MNZ088-096.
IA...DENSE FOG ADVISORY UNTIL 9 AM CST THIS MORNING FOR IAZ008>011-
018-019-029-030.
WINTER WEATHER ADVISORY UNTIL NOON CST TODAY FOR IAZ010-011.
UPDATE...SKOW
DISCUSSION...SKOW
AVIATION...NAYLOR
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