893
FXUS65 KMSO 212005
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
205 PM MDT TUE APR 21 2026
DISCUSSION
KEY MESSAGES:
- HIGH-BASED THUNDERSTORMS WILL DEVELOP LATE THIS AFTERNOON AND
EVENING ACROSS NORTH-CENTRAL IDAHO AND NORTHWEST MONTANA,
BRINGING A PRIMARY THREAT OF ERRATIC, GUSTY OUTFLOW WINDS.
- WIDESPREAD RAIN DEVELOPS ON WEDNESDAY, ALONG WITH MUCH COOLER
TEMPERATURES, AND A RETURN TO WINTER DRIVING CONDITIONS OVER
MOUNTAIN PASSES BY THURSDAY.
- A HIGHLY UNCERTAIN BUT POTENTIALLY IMPACTFUL BAND OF HEAVY,
WET SNOW COULD DISRUPT THE THURSDAY AND FRIDAY MORNING COMMUTES
ACROSS WESTERN MONTANA VALLEYS.
THE UPPER-LEVEL RIDGE THAT BROUGHT US RECORD-CHALLENGING WARMTH
WILL BEGIN TO BREAK DOWN THIS EVENING. TODAY WILL BE THE WARMEST
DAY OF THE WEEK, WITH VALLEY HIGHS SOARING INTO THE UPPER 70S AND
MID 80S. THE COMBINATION OF DRY FUELS, LOW RELATIVE HUMIDITY, AND
INCREASING SOUTHWEST BREEZES WILL MAINTAIN ELEVATED FIRE WEATHER
CONDITIONS ACROSS SOUTHWEST MONTANA THROUGH THE EARLY EVENING.
AS WE HEAD INTO LATE AFTERNOON AND EVENING, INCREASING PACIFIC
MOISTURE AND THERMODYNAMIC INSTABILITY WILL INTERACT WITH THE
DEPARTING RIDGE. THIS WILL TRIGGER ISOLATED TO SCATTERED
THUNDERSTORMS, INITIALLY FOCUSING OVER NORTH-CENTRAL IDAHO BEFORE
SPREADING INTO NORTHWEST MONTANA. BECAUSE THE LOWER LEVELS OF OUR
ATMOSPHERE REMAIN QUITE DRY, THESE STORMS WILL BE HIGH-BASED WITH
THE MAIN THREAT BEING LOCALIZED WIND GUSTS OVER 40 MPH.
A DRASTIC, LARGE-SCALE PATTERN SHIFT BEGINS WEDNESDAY MORNING AS
A STRONG PACIFIC COLD FRONT SWEEPS ACROSS THE NORTHERN ROCKIES.
WIDESPREAD PRECIPITATION IS EXPECTED THROUGH THURSDAY NIGHT. THE
HIGHEST LIQUID PRECIPITATION TOTALS, LOCALLY EXCEEDING ONE INCH,
ARE EXPECTED ALONG THE CLEARWATER MOUNTAINS AND THE CONTINENTAL
DIVIDE. BEHIND THE FRONT, TEMPERATURES WILL PLUMMET 30 TO 40
DEGREES FROM TODAY'S HIGHS.
AS THIS COLD AIR MASS SETTLES OVER THE REGION, SNOW LEVELS WILL
DROP RAPIDLY WEDNESDAY NIGHT INTO THURSDAY. BY THURSDAY MORNING,
SNOW LEVELS WILL FALL TO BETWEEN 3,000-4,000 FEET. THIS WILL BRING
WET, ACCUMULATING SNOW TO HIGHER VALLEYS AND MOUNTAIN PASSES,
INCLUDING LOOKOUT, LOLO, HOMESTAKE, AND MACDONALD PASSES.
TRAVELERS SHOULD PREPARE FOR SLUSHY ACCUMULATIONS AND WINTER
DRIVING CONDITIONS ALONG THE DIVIDE.
WE ARE ALSO TRACKING A NARROW, HIGHLY UNCERTAIN BAND OF HEAVY
PRECIPITATION THAT COULD IMPACT THE THURSDAY MORNING COMMUTE
ACROSS NORTHWEST MONTANA. IF THIS BAND SETS UP OVER THE VALLEYS,
HEAVY, WET SNOW WILL CREATE PERIODS OF LOW VISIBILITY, SLUSHY
ROADS, SNAPPED TREE BRANCHES, AND ISOLATED POWER OUTAGES.
FURTHERMORE, A SECOND BAND OF SNOW COULD DEVELOP OVER WEST-CENTRAL
MONTANA THURSDAY NIGHT, TIMED PERFECTLY TO DISRUPT THE FRIDAY
MORNING COMMUTE.
THE COOL AND SHOWERY PATTERN WILL PERSIST THROUGH THE UPCOMING
WEEKEND AS A BROAD UPPER-LEVEL TROUGH STALLS OVER THE WESTERN
UNITED STATES.
AVIATION
VFR CONDITIONS WILL HOLD ACROSS THE AIRSPACE THROUGH
THE AFTERNOON, WITH INCREASING SOUTHWEST WINDS ALOFT MIXING DOWN
TO TERMINAL SITES (KBTM, KSMN). EXPECT AFTERNOON WIND GUSTS OF 20
TO 25 KNOTS. BY LATE AFTERNOON (AROUND 22/00Z), INCREASING
INSTABILITY WILL TRIGGER HIGH-BASED THUNDERSTORMS, PRIMARILY
IMPACTING NORTH-CENTRAL IDAHO INITIALLY BEFORE SPREADING INTO
NORTHWEST MONTANA. BECAUSE OF THE DRY LOWER ATMOSPHERE, THESE
CELLS WILL BE CAPABLE OF PRODUCING ERRATIC OUTFLOW GUSTS IN EXCESS
OF 35 KNOTS. A STRONG COLD FRONT SWEEPS FROM WEST TO EAST
BEGINNING EARLY WEDNESDAY MORNING. ALL TERMINALS WILL EXPERIENCE
RAPIDLY LOWERING CEILINGS, SHIFTING WINDS, AND WIDESPREAD MVFR TO
LOCALIZED IFR CONDITIONS IN MODERATE RAIN BY THE AFTERNOON.
EXTENSIVE MOUNTAIN OBSCURATIONS ARE EXPECTED IN THE HIGH TERRAIN,
ALONG WITH DROPPING SNOW LEVELS WEDNESDAY AND WEDNESDAY NIGHT.
HYDROLOGY
RAPID SNOWMELT DRIVEN BY TODAY'S TEMPERATURES IN THE 70S AND 80S
WILL COMBINE WITH WIDESPREAD RAINFALL WEDNESDAY AND THURSDAY.
THIS WILL RESULT IN NOTABLE RISES ON LOCAL RIVERS AND STREAMS.
WHILE MAJOR RIVER FLOODING IS NOT EXPECTED, WATER PONDING IN
POORLY DRAINED AREAS IS LIKELY. MODERATE RAINFALL OVER STEEP
TERRAIN WILL ALSO INCREASE THE RISK OF SMALL ROCKSLIDES.
MSO WATCHES/WARNINGS/ADVISORIES
MT...WINTER WEATHER ADVISORY FROM 6 PM WEDNESDAY TO NOON MDT FRIDAY
FOR BITTERROOT/SAPPHIRE MOUNTAINS...BUTTE/BLACKFOOT
REGION...POTOMAC/SEELEY LAKE REGION.
WINTER WEATHER ADVISORY FROM 6 PM WEDNESDAY TO 6 PM MDT THURSDAY
FOR WEST GLACIER REGION.
ID...WINTER WEATHER ADVISORY FROM 6 PM WEDNESDAY TO NOON MDT FRIDAY
FOR EASTERN LEMHI COUNTY...WESTERN LEMHI COUNTY.
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page