529
FXUS65 KMSO 170930
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
330 AM MDT TUE MAR 17 2026
DISCUSSION
KEY MESSAGES:
- A RIDGE BUILDING OVER THE WESTERN U.S. WILL CAUSE ABOVE AVERAGE
TEMPERATURES THIS WEEK, REACHING MORE THAN 20 DEGREES ABOVE
AVERAGE IN SOME PLACES LATE THIS WEEK.
- AN ATMOSPHERIC RIVER WILL CONTINUE FLOWING AROUND THE PERIPHERY
OF THE RIDGE INTO NEXT WEEKEND CAUSING LIGHT TO MODERATE
PRECIPITATION IN NORTHWEST MONTANA INTO THE WEEKEND.
THE RIDGE BUILDING OVER THE WESTERN U.S. WILL CAUSE CAUSE ABOVE
AVERAGE TEMPERATURES THIS WEEK. THE WARMEST TEMPERATURES WILL BE
ALONG INTERSTATE-90 AND SOUTH ON THURSDAY AND FRIDAY, WHEN BUTTE
IS FORECAST TO BE MORE THAN 20 DEGREES ABOVE AVERAGE. NORTH OF
INTERSTATE-90 WILL ALSO BE ABOVE AVERAGE, BUT LESS SO DUE TO THE
IMPACTS OF AN ATMOSPHERIC RIVER.
USUALLY WHEN WE DISCUSS ATMOSPHERIC RIVERS, THE MAIN IMPACT IS
HEAVY PRECIPITATION, BUT THAT IS NOT THE CASE THIS TIME. WHILE
THIS EVENT DOES TECHNICALLY MEET THE CRITERIA OF AN ATMOSPHERIC
RIVER BASED ON THE AMOUNT OF WATER VAPOR IN THE ATMOSPHERE AND HOW
FAST IT IS MOVING, OTHER FACTORS ARE PREVENTING IT FROM PRODUCING
HEAVY RAIN. THE BIGGEST FACTOR IS THAT IT WILL BE EMBEDDED IN
ANTICYCLONIC FLOW, WHICH HELPS TO SUPPRESS LIFT AND RAIN
PRODUCTION. STILL, IT WILL BE ABLE TO PRODUCE LIGHT TO MODERATE
RAIN AT TIMES, ESPECIALLY IN THE MOUNTAINS. IT WILL ALSO PRODUCE
CONSISTENT CLOUD COVER OVER NORTHWEST MONTANA, PREVENTING IT FROM
WARMING UP AS MUCH AS OTHER PARTS OF THE STATE. THE MOST NOTABLE
ASPECT OF THIS ATMOSPHERIC RIVER IS IT'S DURATION. IT WILL LAST
ALL WEEK, AND CURRENT FORECASTS HAVE IT DYING OUT EARLY SATURDAY
MORNING. THE MAIN CONCERN FOR IMPACTS WILL BE WARM TEMPERATURES IN
THE MOUNTAINS COMBINED WITH FREQUENT PRECIPITATION CAUSING
WORSENING AVALANCHE CONDITIONS. FOR THOSE PLANNING TO GO INTO THE
BACK COUNTRY, PLEASE MONITOR THE AVALANCHE FORECAST CENTERS FOR
THE LATEST CONDITIONS THIS WEEK.
AVIATION
WITH ALL OF THE RECENT PRECIPITATION AND LOW
ELEVATION SNOW MELT UNDER A BUILDING RIDGE, FOG IS A CONCERN AT
TERMINALS KGPI AND KMSO. CURRENT FORECASTS DO NOT HAVE ANY FOG IN
THEM THOUGH. CLOUD COVER AND STRONG WINDS AT THE RIDGE TOP LEVEL
CAUSING SOME WEAK MIXING ARE KEEPING THE FOG FROM FORMING. AS
THESE FACTORS ARE EXPECTED TO CONTINUE THROUGH THE MORNING, WE
HAVE NOT PUT ANY FOG IN THE FORECAST, BUT WE ARE AWARE THAT WITH
ALL THE BOUNDARY LAYER MOISTURE, WE ARE ON A KNIFES EDGE, AND IF
THE BOUNDARY LAYER DECOUPLES AND WINDS DIE DOWN BEFORE SUN RISE,
FOG COULD FORM QUICKLY THIS MORNING.
MSO WATCHES/WARNINGS/ADVISORIES
MT...NONE.
ID...NONE.
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