903
FXUS65 KMSO 280818
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
218 AM MDT SUN JUN 28 2026
DISCUSSION
KEY MESSAGES:
- PERIODS OF HEAVY SNOW AND DANGEROUS BACKCOUNTRY CONDITIONS WILL
CONTINUE ACROSS SOUTHWEST MONTANA, GENERALLY ABOVE 6500 FEET.
RISKS INCLUDE SEVERE HYPOTHERMIA, TREACHEROUS HIGH-COUNTRY
TRAVEL, AND POSSIBLE FALLING TREES DUE TO HEAVY SNOW.
- A MASSIVE INFLUX OF MOISTURE WILL BRING HEAVY RAINFALL TO
NORTHWEST MONTANA TODAY THROUGH MONDAY. EXPECT RAPID STREAM
RISES AND POTENTIAL ROCKFALLS FROM THE GLACIER NATIONAL PARK
REGION DOWN TO THE MISSION VALLEY. CAMPERS NEAR STREAMS AND
RESERVOIRS SHOULD CHOOSE A DIFFERENT LOCATION.
- FOG AND LOW CLOUDS MAY IMPACT AVIATION AND REGIONAL TRAVEL TUESDAY
MORNING. BEYOND TUESDAY, AN UNSTABLE WEATHER PATTERN WILL
PERSIST, BRINGING DAILY SHOWER AND THUNDERSTORM CHANCES WHILE
KEEPING TEMPERATURES NEAR OR BELOW AVERAGE INTO EARLY JULY.
STEADY LIGHT TO MODERATE RAINFALL HAS RETURNED OVERNIGHT AND THIS
MORNING TO LOCATIONS ALONG AND SOUTH OF THE I-90 CORRIDOR IN
WESTERN MONTANA. SNOW IS CURRENTLY FALLING AT ELEVATIONS ABOVE
7000 FEET.
TRAVEL OVER HIGH MOUNTAIN PASSES ALONG FOREST SERVICE ROADS IN
SOUTHWEST MONTANA WILL REMAIN HAZARDOUS IF NOT DANGEROUS DUE TO
HEAVY RAIN AND SNOW THROUGH THE DAY AND INTO MONDAY. THE RISK OF
HYPOTHERMIA WILL CONTINUE TO BE A CONCERN FOR THOSE TRAVELING IN
THE BACKCOUNTRY, DUE TO THE SNOW, COLD TEMPERATURES, AND BREEZY
WINDS. A STRONG LOW PRESSURE SYSTEM WILL CONTINUE TO STRENGTHEN
TODAY, WHICH WILL INTENSIFY PRECIPITATION AND CAUSE SNOW LEVELS TO
FALL TO AROUND 6500 FEET OR LOWER. HIGHLY TRAVELED MOUNTAIN
PASSES, SUCH AS MACDONALD, HOMESTAKE AND LOST TRAIL PASSES SHOULD
STAY MOSTLY RAIN, THROUGH THERE IS A 40 TO 50 PERCENT PROBABILITY
RAIN COULD SWITCH TO HEAVY WET SNOW FOR A SHORT PERIOD OF TIME
AROUND SUNRISE, THAT WILL SIGNIFICANTLY REDUCE VISIBILITY. THE
LOCATIONS WITH THE HIGHEST CONCERN FOR WINTER-LIKE CONDITIONS
REMAINS THE HIGH MOUNTAIN ROADS, OFF OF THE MAIN ROADWAYS.
MOUNTAIN SNOWFALL IS NOT THE ONLY CONCERN, THE MODERATE TO HEAVY
RAINFALL THAT WILL DEVELOP AS THE LOW STRENGTHENS, WILL BRING IT’S
OWN HAZARDS AS WELL. FOR RESIDENTS AND VISITORS IN THE MOUNTAINS
ALONG THE CONTINENTAL DIVIDE, INCLUDING GLACIER NATIONAL PARK
SHOULD KEEP AN EYE OUT FOR RAPID RISES IN SMALL CREEKS AND STREAMS
AND ALSO THE THREAT FOR ROCK AND MUDSLIDES ALONG STEEP TERRAIN.
THE MOST LIKELY RAINFALL AMOUNTS WILL RANGE BETWEEN 2.50” TO 4.50”
OF RAIN TODAY THROUGH THE DAY ON MONDAY.
BY TUESDAY, WINDS WILL RELAX AND WE WILL TRANSITION INTO A WEAK
RIDGE OF HIGH PRESSURE WITH SLIGHTLY UNSTABLE AIR. GIVEN THE
WIDESPREAD PRECIPITATION, FOG AND LOW CLOUDS COULD CAUSE TRAVEL
ISSUES ACROSS THE REGION TUESDAY MORNING. THE LINGERING MOISTURE
WILL ALSO SUPPORT SCATTERED SHOWERS LATER IN THE DAY.
AFTER TUESDAY, LONG-RANGE MODELS INDICATE AN UNSTABLE WEATHER
PATTERN REMAINING OVER THE REGION. A BROAD RIDGE OF HIGH PRESSURE
OVER THE EASTERN PACIFIC WILL FORCE STORM SYSTEMS DOWN TOWARD THE
PACIFIC NORTHWEST, LEADING TO DAILY CHANCES FOR SHOWERS AND
THUNDERSTORMS THROUGH THE REST OF THE WEEK. HIGHS SHOULD RETURN TO
AVERAGE FOR EARLY JULY. LOOKING AHEAD TO THE 4TH OF JULY
(SATURDAY), ABOUT 40% OF MODELS DEPICT AN UNSTABLE PATTERN
CONDUCIVE TO DAILY SHOWERS AND THUNDERSTORM ACTIVITY, WHILE 60%
SUGGEST A QUIETER WEATHER PATTERN, WHICH COULD PAVE THE WAY FOR A
DRIER EVENING FOR FIREWORKS.
AVIATION
STEADY LIGHT TO MODERATE RAINFALL HAS RETURNED OVERNIGHT
AND THIS MORNING TO LOCATIONS ALONG AND SOUTH OF THE I-90
CORRIDOR IN WESTERN MONTANA. SNOW IS CURRENTLY FALLING AT
ELEVATIONS ABOVE 7000 FEET. CEILINGS WILL CONTINUE TO LOWER AS THE
DAY PROGRESSES AS MOISTURE STREAMS AROUND A STRENGTHENING LOW
PRESSURE SYSTEM IN CENTRAL MONTANA. THE HEAVIEST RAINFALL AND
LOWEST CEILINGS ARE EXPECTED TO BE THIS EVENING FOR ALL AVIATION
SITES, STARTING AROUND 29/0000Z, AND REMAINING THAT WAY THROUGH
THE DAY ON MONDAY. CONDITIONS ARE NOT EXPECTED TO DRY OUT AND
IMPROVE UNTIL MID-DAY TUESDAY. RAIN SHOULD TAPPER OFF BY THEN, BUT
DUE TO ALL OF THE RECENT MOISTURE, WIDESPREAD FOG AND LOW STRATUS
IS EXPECTED TO BE A CONCERN FOR TRAVEL TUESDAY MORNING.
HYDROLOGY
AS MENTIONED IN THE KEY MESSAGES, ANOMALOUSLY HIGH
MOISTURE CONTENT (200% OF NORMAL) WILL FUEL HEAVY PRECIPITATION
INTO MONDAY ACROSS WESTERN MONTANA, ESPECIALLY NORTHWEST MONTANA
ON TONIGHT THROUGH MONDAY. SNOW LEVELS RISING TOWARD 8000 FEET
WILL SUPPORT EFFICIENT, HEAVY RAINFALL. WHEN COMBINED WITH SOUTH-
TO-WESTERLY WINDS PICKING UP EXTRA MOISTURE OFF FLATHEAD LAKE,
RAIN INTENSITIES WILL INCREASE. HIGH-RESOLUTION WEATHER MODELS
SHOW POSSIBLE RAIN RATES OVER 0.30 INCHES PER HOUR IN THE LEWIS,
NORTHERN SWAN, AND MISSION RANGES TONIGHT INTO MONDAY MORNING.
FORTUNATELY, EXISTING SNOWPACK IS MINIMAL, SO THIS WILL TRANSLATE
PURELY TO INCREASED STREAM RUNOFF. AREAS PRONE TO ROCKFALLS OR
MUDSLIDES—INCLUDING UPPER ELEVATIONS OF THE GOING-TO-THE-SUN
ROAD—COULD BE VULNERABLE DEPENDING ON EXACTLY WHERE THE HEAVIEST
RAIN SETS UP. CAMPERS AND RVS PARKED CLOSE TO STREAMS OR THE
HUNGRY HORSE RESERVOIR SHOULD CONSIDER REPOSITIONING TO HIGHER
GROUND TODAY.
MSO WATCHES/WARNINGS/ADVISORIES
MT...WINTER STORM WARNING UNTIL 9 AM MDT MONDAY FOR
BITTERROOT/SAPPHIRE MOUNTAINS...BUTTE/BLACKFOOT REGION.
ID...WINTER STORM WARNING UNTIL 9 AM MDT MONDAY FOR EASTERN LEMHI
COUNTY...WESTERN LEMHI COUNTY.
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