459
FXUS65 KMSO 181936
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
136 PM MDT MON MAY 18 2026
DISCUSSION
KEY MESSAGES:
- SHOWERS WITH A FEW THUNDERSTORMS(20-50% CHANCE) TODAY THROUGH
WEDNESDAY, ESPECIALLY ACROSS NORTHWEST MONTANA AND OVER THE
CONTINENTAL DIVIDE.
- FROST/FREEZE CONCERNS REMAIN FOR TUESDAY MORNING.
- MOUNTAIN SNOW ABOVE 6000 FEET WITH RAW/COLD BACKCOUNTRY
CONDITIONS LATER WEDNESDAY INTO THURSDAY MORNING OVER THE
DIVIDE, GLACIER NATIONAL PARK, EXTENDING WEST TOWARDS THE IDAHO
BORDER.
- RETURN TO SUMMER-LIKE CONDITIONS AS MODERATING TEMPERATURES
ARE EXPECTED LATE WEEK INTO NEXT WEEKEND.
TEMPERATURES THIS MORNING WERE QUITE COLD ACROSS THE REGION.
WHENEVER YOU GET SUCH A COLD TROUGH ORIGINATING FROM THE NORTHERN
LATITUDES LIKE ALASKA, YOU ALMOST HAVE TO EXPECT POSSIBLE FROSTY
MORNINGS. THE RAVALLI COUNTY AIRPORT LOCATED NORTHEAST OF HAMILTON
SAW A LOW OF 21 DEGREES! LOOKING BACK AT CLIMATOLOGY, ITS REALLY
NOT THAT UNCOMMON TO GET THESE TYPES OF SYSTEMS IN MAY WHERE HIGHS
STRUGGLE TO GET OUT OF THE 30S AND 40S. LOOKING AT THE
CLIMATOLOGY FOR THE HAMILTON AREA FOR THE SECOND HALF OF MAY THE
COLDEST LOW TEMPERATURE RECORDED WAS 24 DEGREES SET ON MAY 22,
2010. THE STEVENSVILLE COOP RECORDED A 15 DEGREES ON MAY 30, 2001.
EVEN OTHER LOCATIONS, LIKE THE HIGHER 3000 FEET VALLEYS IN IDAHO
WERE IN THE UPPER 20S THIS MORNING. WITH THE CHANCE FOR FROST
CONTINUING INTO TUESDAY MORNING, WE ISSUED MORE FROST ADVISORIES.
WE LISTED THE PERCENTILE RANGES OF LOW TEMPERATURES FOR SELECT
LOCATIONS IN THE "ADDITIONAL DETAILS" SECTION TO HELP PEOPLE MAKE
DECISIONS ON IF THEY NEED TO COVER THINGS OR NOT. FOR THOSE WHO
ARE UNAWARE, WE TYPICALLY WILL NOT ISSUE FROST/FREEZE ADVISORIES
FOR THE HIGH VALLEYS(I.E. SOUTHWEST MONTANA/DIVIDE) THAT WILL
CONTINUE TO HAVE SUB- FREEZING NIGHTS FOR THE REMAINDER OF MAY
UNTIL THE CLIMATOLOGICAL LAST FROST/FREEZE DAY. IF WE GET A COLD
SYSTEM THEREAFTER, WE MAY CHOOSE TO ISSUES ADVISORIES THEN.
FOR THE NEXT TWO DAYS, WE'LL BE UNDER CYCLONIC FLOW WITH
DISTURBANCES DROPPING OUT OF SOUTHERN BRITISH COLUMBIA. THERE IS
GENERALLY A 20 TO 50 PERCENT CHANCE FOR LIGHTNING OVER THE
MOUNTAINS OF NORTHWEST MONTANA THIS AFTERNOON AND EVENING. OTHER
IMPACTS FROM THE SHOWERS WILL BE GUSTY WINDS TO 35 MPH IN THE
STRONGER CELLS.
THEN SHORTWAVE ENERGY FROM THE GULF OF ALASKA WILL CARVE A LARGER
UPPER LEVEL TROUGH WHICH DIGS OVER MONTANA LATER WEDNESDAY INTO
THURSDAY. WITH THE COLDER TEMPERATURES ALOFT, THIS WILL DRIVE THE
SNOW LEVELS DOWN TO AROUND 6000 FEET. THIS COULD BRING ADDITIONAL
SNOWFALL TO THE MOUNTAINS OVER THE DIVIDE, INCLUDING GLACIER
NATIONAL PARK. ALSO THE ASSOCIATED COLD FRONT WILL SHIFT SOUTH AND
WEST WITH TIME JUST AFTER PEAK HEATING WHICH WILL ALLOW FOR
POSSIBLE SHOWERS AND THUNDERSTORMS. THE LATEST PROBABILITIES SHOW
THE BEST INSTABILITY PRESENT ACROSS NORTHWEST MONTANA. THIS
SYSTEM WILL BASICALLY MOVE OFF INTO THE NORTHERN PLAINS BY
THURSDAY INTO FRIDAY WHILE WE WILL HAVE RISING HEIGHTS (MODERATING
TEMPERATURES).
A GENERAL RIDGE WILL BUILD BY NEXT WEEKEND AND TEMPERATURES WILL
RISE BACK INTO THE 70S AND 80S ACROSS THE REGION. THERE IS A HINT
THAT A TROUGH MAY EITHER BRING MILD AND WINDY CONDITIONS ON
SUNDAY, OR DIG ACROSS THE REGION BY EARLY NEXT WEEK.
AVIATION
SCATTERED SHOWERS AND THUNDERSTORMS WILL BE POSSIBLE
TODAY ACROSS THE REGION. THE MORE INTENSE CELLS WILL BE FOUND
ACROSS NORTHWEST MONTANA, ESPECIALLY INITIATED OVER THE MOUNTAINS.
THEY WILL HAVE A GENERAL STORM MOTION FROM NORTHWEST TO SOUTHEAST
AROUND 23 KNOTS. MAIN IMPACTS WILL BE ISOLATED LIGHTNING, BRIEF
VISIBILITY REDUCTION AND GUSTY WINDS TO 30 KNOTS. A FEW LINGERING
SHOWERS ARE POSSIBLE TONIGHT ACROSS NORTHWEST MONTANA THANKS TO A
SHORTWAVE DROPPING OUT OF BRITISH COLUMBIA. THERE COULD BE PATCHY
FOG ALONG RIVER BOTTOMS ELSEWHERE TUESDAY MORNING. ANOTHER ROUND
OF SHOWERS WITH A FEW THUNDERSTORMS ARE EXPECTED TUESDAY AFTERNOON
AND EVENING WITH THE FOCUS OVER THE DIVIDE AND POINTS WEST.
MSO WATCHES/WARNINGS/ADVISORIES
MT...FROST ADVISORY FROM MIDNIGHT TONIGHT TO 9 AM MDT TUESDAY FOR
BITTERROOT/SAPPHIRE MOUNTAINS...FLATHEAD/MISSION VALLEYS...
LOWER CLARK FORK REGION...MISSOULA/BITTERROOT VALLEYS.
ID...FROST ADVISORY FROM MIDNIGHT MDT /11 PM PDT/ TONIGHT TO 9 AM MDT
/8 AM PDT/ TUESDAY FOR EASTERN LEMHI COUNTY...NORTHERN
CLEARWATER MOUNTAINS...SOUTHERN CLEARWATER MOUNTAINS.
FROST ADVISORY FROM 11 PM THIS EVENING TO 9 AM PDT TUESDAY FOR
OROFINO/GRANGEVILLE REGION.
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