602
FXUS65 KMSO 240430
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
930 PM MST TUE DEC 23 2025
UPDATE
DENSE FOG IS VISIBLE IN CAMERAS IN BOTH THE CAMAS PRAIRIE AND
LEMHI COUNTY, ID. FOR THIS REASON A DENSE FOG ADVISORIES HAVE
BEEN ISSUED FOR THOSE LOCATIONS. IN LEMHI COUNTY, FOG IS EXPECTED
TO REMAIN BELOW 4300 FEET. TIMING OF FOG BURNING OFF WILL BE
BETWEEN 3 AND 5 AM, AS SOUTHERLY WINDS ARE EXPECTED TO DEVELOP AND
ERODE THE SURFACE STABILITY.
AVIATION
A PLUME OF MOISTURE IS CURRENTLY STREAMING INTO NORTH
CENTRAL IDAHO AND WEST CENTRAL MONTANA AND WILL BE SHIFTING TO THE
NORTH DURING THE AFTERNOON HOURS. THIS MOISTURE IS CAUSING A
WINTRY MIX OF RAIN, SNOW AND FREEZING RAIN AT KHRF AND KMSO. ONCE
THE PRECIPITATION REACHES KGPI BY EARLY AFTERNOON, THE TERMINAL
COULD EXPERIENCE A BRIEF ONSET OF THIS WINTRY MIX. SNOW LEVELS
WILL BE GRADUALLY RISING TODAY CAUSING THE PRECIPITATION TYPE TO
BECOME MOSTLY RAIN FOR THE TERMINAL AFFECTED BY THIS PULSE OF
MOISTURE. THE PRECIPITATION WILL BE DIMINISHING FROM SOUTH TO
NORTH THROUGHOUT THE AFTERNOON AND EVENING.
PREV DISCUSSION
/ISSUED 1218 PM MST TUE DEC 23 2025/
DISCUSSION...
KEY MESSAGES:
- RISK FOR AREAS OF LIGHT SNOW/FREEZING RAIN ACROSS NORTHWEST
MONTANA THIS EVENING AND TONIGHT.
- MILD TEMPERATURES, INCREASING SOUTHERLY WINDS FOR IDAHO AND THE
HIGHER TERRAIN WEDNESDAY.
- COLD FRONT FRIDAY: A ROBUST COLD FRONT WILL BRING A DEFINITIVE
CHANGEOVER FROM RAIN TO SNOW FOR ALL VALLEY FLOORS AND GUSTY
WINDS.
TODAY THROUGH TONIGHT: SATELLITE AND REGIONAL RADAR OBSERVATIONS
THIS AFTERNOON HIGHLIGHT A PLUME OF PACIFIC MOISTURE EXTENDING
FROM NORTHERN CALIFORNIA INTO THE NORTHERN ROCKIES. THIS MOISTURE
HAS OVERRUN EXISTING COLD AIR TRAPPED IN AREA MOUNTAIN VALLEYS,
LEADING TO A WINTRY MIX OF SNOW AND FREEZING RAIN ACROSS THE
US-93, I-90, AND HIGHWAY 200 CORRIDORS OF WEST-CENTRAL MONTANA.
TEMPERATURES WILL SLOWLY MODERATE THROUGH THE LATE AFTERNOON AND
EARLY EVENING AS THIS AREA OF PRECIPITATION MOVES NORTHWARD INTO
NORTHWEST MONTANA. THE RISK FOR SNOW AND FREEZING RAIN WILL BE
MONITORED CLOSELY THIS EVENING FOR FLATHEAD AND LINCOLN COUNTIES,
WHERE TEMPERATURES ARE CURRENTLY IN THE UPPER 20S AND LOW 30S.
WHILE THESE AREAS SHOULD MODERATE INTO THE MID TO UPPER 30S BY
THIS EVENING, IF THIS WARMING DOES NOT OCCUR, ANOTHER ROUND OF
WINTER WEATHER ADVISORIES MAY BE NEEDED FOR NORTHWEST MONTANA.
WEDNESDAY THROUGH THURSDAY: A LONGWAVE TROUGH WILL REMAIN OFF THE
WESTERN U.S. COAST FROM WEDNESDAY THROUGH THURSDAY, WITH A STRONG
ATMOSPHERIC RIVER MOVING INLAND ACROSS SOUTHERN CALIFORNIA AND
THROUGH THE GREAT BASIN INTO CENTRAL IDAHO. MODELS SUGGEST A
COMPACT SURFACE LOW WILL MOVE QUICKLY NORTHWARD NEAR THE OREGON
AND WASHINGTON COASTS WEDNESDAY MORNING, PUSHING A WARM FRONT
THROUGH THE NORTHERN ROCKIES. SOUTHERLY WINDS WILL INCREASE ACROSS
SOUTHERN IDAHO COUNTY AND THE HIGHER TERRAIN, WITH GUSTS OF 30 TO
50 MPH. THE US-95 CORRIDOR STANDS OUT IN PARTICULAR, WITH A 30%
CHANCE FOR GUSTS TO REACH 50 MPH LATE WEDNESDAY MORNING INTO THE
EARLY AFTERNOON FOR GRANGEVILLE AND THE VICINITY. SNOW LEVELS WILL
QUICKLY RISE, REACHING ABOVE 6,500 FEET AS THE NEXT WAVE OF
PRECIPITATION ARRIVES WEDNESDAY AFTERNOON INTO THURSDAY MORNING. A
MODEST SOUTHERLY GRADIENT WILL MODERATE VALLEY TEMPERATURES, WITH
HIGHS REACHING INTO THE 40S. THE AIRMASS WILL BECOME VERY WARM
FOR THIS TIME OF YEAR, WITH 700MB TEMPERATURES REACHING NEAR 0C,
WHICH IS NEAR THE 95TH PERCENTILE. WITH GROUND TEMPERATURES
HOVERING NEAR FREEZING, BLACK ICE IS A CONCERN WHERE RAIN FALLS
OVERNIGHT INTO THURSDAY MORNING. UNFORTUNATELY, THIS MILD AIR MASS
WILL LIKELY DASH HOPES FOR A WHITE CHRISTMAS FOR MANY IN THE
LOWER ELEVATIONS.
FRIDAY THROUGH SATURDAY: A QUICK-MOVING TROUGH IS PROJECTED TO
DIVE SOUTHWARD FROM BRITISH COLUMBIA AND ALBERTA FRIDAY INTO
SATURDAY. SNOW LEVELS WILL BEGIN FALLING FRIDAY MORNING, WITH WET
SNOW ACCUMULATING ACROSS AREA PASSES SUCH AS LOOKOUT, LOLO, AND
MARIAS AS THE NEXT WAVE OF PRECIPITATION ENTERS THE REGION. SNOW
LEVELS WILL QUICKLY FALL TO VALLEY FLOORS FRIDAY NIGHT INTO
SATURDAY MORNING AS A COLD FRONT ARRIVES. GUSTY WINDS WILL
ACCOMPANY THE FRONT, WITH PEAK GUSTS RANGING FROM 25 TO 50 MPH.
CONSIDERABLE UNCERTAINTY EXISTS FOR VALLEY WIND GUSTS AS THE FRONT
ARRIVES, AS MODELS CURRENTLY DISAGREE ON THE PRESSURE RISES ALONG
THE FRONT. THIS WILL BE CLOSELY MONITORED, AS THE NATIONAL BLEND
OF MODELS (NBM) CURRENTLY SHOWS A 20% TO 40% CHANCE FOR GUSTS OF
40 MPH OR GREATER FOR WESTERN MONTANA VALLEYS. FURTHERMORE, THIS
WILL BE THE PRIMARY PERIOD TO WATCH FOR VALLEY SNOWFALL; AREAS
ALONG THE IDAHO-MONTANA BORDER, SUCH AS LINCOLN COUNTY, HAVE A
GREATER THAN 60% CHANCE FOR ONE INCH OR MORE OF SNOWFALL. THE
PATTERN SUGGESTS A FAVORABLE ENVIRONMENT FOR CONVECTIVE SNOW
SHOWERS OR BANDS TO DEVELOP BEHIND THE FRONT, FAVORING BRIEF
PERIODS OF MODERATE TO HEAVY SNOW.
SUNDAY ONWARDS: ENSEMBLE GUIDANCE IS IN STRONG AGREEMENT
REGARDING A PERIOD OF HIGH PRESSURE SUNDAY INTO EARLY NEXT WEEK.
THE PATTERN BECOMES UNSETTLED ONCE AGAIN AROUND THE JANUARY 1ST TO
3RD TIMEFRAME, WITH A FAVORABLE SETUP FOR THE RETURN OF PACIFIC
MOISTURE AND WIDESPREAD PRECIPITATION, INCLUDING POTENTIAL
ATMOSPHERIC RIVERS. DUE TO UNCERTAINTY IN THE TIMING OF THE RIDGE
BREAKDOWN, PLEASE STAY TUNED FOR FURTHER UPDATES.
AVIATION...A PLUME OF MOISTURE IS CURRENTLY STREAMING INTO NORTH
CENTRAL IDAHO AND WEST CENTRAL MONTANA AND WILL BE SHIFTING TO THE
NORTH DURING THE AFTERNOON HOURS. THIS MOISTURE IS CAUSING A
WINTRY MIX OF RAIN, SNOW AND FREEZING RAIN AT KHRF AND KMSO. ONCE
THE PRECIPITATION REACHES KGPI BY EARLY AFTERNOON, THE TERMINAL
COULD EXPERIENCE A BRIEF ONSET OF THIS WINTRY MIX. SNOW LEVELS
WILL BE GRADUALLY RISING TODAY CAUSING THE PRECIPITATION TYPE TO
BECOME MOSTLY RAIN FOR THE TERMINAL AFFECTED BY THIS PULSE OF
MOISTURE. THE PRECIPITATION WILL BE DIMINISHING FROM SOUTH TO
NORTH THROUGHOUT THE AFTERNOON AND EVENING.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...DENSE FOG ADVISORY UNTIL 5 AM MST /4 AM PST/ WEDNESDAY FOR
EASTERN LEMHI COUNTY...OROFINO/GRANGEVILLE REGION.
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