721
FXUS65 KMSO 202016
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
116 PM MST WED NOV 20 2024
DISCUSSION
KEY MESSAGES:
- AN ATMOSPHERIC RIVER WILL BRING AMPLE MOISTURE TO THE MOUNTAINS
OF CENTRAL IDAHO AND NORTHWEST MONTANA TODAY THROUGH SATURDAY.
- UNCERTAINTY REGARDING SNOW LEVELS CONTINUES TO PLAGUE THE
FORECAST
- SLIGHT CHANCE FOR FREEZING RAIN IN THE FLATHEAD VALLEY OVERNIGHT
TONIGHT
IF YOU'VE BEEN FOLLOWING ALONG OVER THE LAST FEW DAYS, NOT A LOT
HAS CHANGED IN THE FORECAST GOING INTO THE WEEKEND. A WELL-
DEVELOPED MID LATITUDE CYCLONE, OBVIOUS ON VISIBLE SATELLITE
IMAGERY, CONTINUES TO SPIN OFF THE WEST COAST ABOUT IN LINE WITH
THE CANADIAN BORDER. THE COUNTER CLOCKWISE ROTATION OF THE LOW
WILL SEND MOISTURE ONSHORE SOUTH OF THE LOW PRESSURE CENTER. THIS
PLUME OF MOISTURE, ALSO KNOWN AS AN ATMOSPHERIC RIVER, WILL BRING
PRECIPITATION INTO THE NORTHERN ROCKIES. IN THE NEAR TERM,
PRECIPITATION WILL BE IN THE FORM OF SNOW AS SUB-FREEZING
TEMPERATURES PERSIST. FORECAST PROBLEM #1 IS WHEN, AND TO WHAT
EXTENT, WILL SNOW PERSIST BEFORE TURNING TO RAIN? DEW POINT
TEMPERATURES THIS AFTERNOON WOULD SUGGEST THAT SNOW WILL BE ABLE
TO HOLD ON LONG ENOUGH TO PRODUCE A TRACE TO ONE INCH AS LOW AS
SOME VALLEY FLOORS IN NORTHWEST MT SUCH AS KALISPELL. IF COLD AIR
CAN HOLD ON OVERNIGHT TONIGHT IN THE FLATHEAD VALLEY AND
PRECIPITATION TYPE TRANSITIONS TO RAIN, THERE WILL BE A CHANCE FOR
FREEZING RAIN OVERNIGHT (FORECAST PROBLEM #2). FORECAST
CONFIDENCE IS LOW IN FREEZING RAIN WITH CHANCES FOR FREEZING RAIN
ABOUT 20 PERCENT FOR THE FLATHEAD AND WESTERN MISSION VALLEYS, AS
WELL AS HWY 2 SOUTH OF LIBBY.
IN ADDITION TO THE STRONG CIRCULATION, SATELLITE IMAGERY ALSO
SHOWS THE PLUME OF MOISTURE EXTENDING FROM THE NORTHERN ROCKIES
ALL THE WAY TO THE PACIFIC OCEAN. TOTAL PRECIPITATION AMOUNTS
WITH THIS EVENT WILL BE IMPRESSIVE, ESPECIALLY ACROSS NORTHWEST
MONTANA AND THE CLEARWATER RANGE OF IDAHO. BY SATURDAY MORNING,
MODEL ENSEMBLE MEANS ARE SHOWING 2-4 INCHES OF LIQUID
PRECIPITATION ALONG THE IDAHO-MONTANA BORDER AND MOUNTAINS OF THE
CABINET, CLEARWATER, SWAN, WHITEFISH, FLATHEAD, MISSION, NORTHERN
BITTERROOT RANGES, AS WELL AS THE CONTINENTAL DIVIDE IN GLACIER
PARK. EVEN WITH RISING SNOW LEVELS, THIS AMOUNT OF MOISTURE COULD
TRANSLATE INTO MULTIPLE FEET OF SNOW FOR THE HIGHER TERRAIN ALONG
THE DIVIDE WHERE SNOW LEVELS WILL TOP OUT AT ABOUT 5000 FEET
BEFORE DESCENDING AGAIN. ELSEWHERE, SNOW LEVELS WILL RISE EVEN TO
THE HIGHER PEAKS.
THE UPPER LEVEL FLOW BECOMES WESTERLY TO NORTHWESTERLY BY SUNDAY.
THIS FLOW PATTERN WILL BE CONDUCIVE TO WEAK DISTURBANCES MOVING
THROUGH THE NORTHERN ROCKIES. THE LATEST MODEL RUNS ARE ONLY
SHOWING A 10-15% CHANCE FOR ARCTIC AIR MAKING ITS WAY OVER THE
CONTINENTAL DIVIDE ON SUNDAY. WHILE ENSEMBLE SOLUTIONS LOOKING
INTO THANKSGIVING WEEK ARE NOT EXACTLY IN SYNC, ALL POINT TOWARDS
BETTER CHANCES THAT COOL, WET WEATHER WILL CONTINUE.
AVIATION
THE ATMOSPHERIC RIVER WILL CONTINUE TO SEND MOISTURE
INTO THE NORTHERN ROCKIES TODAY CAUSING WIDESPREAD PRECIPITATION
ALONG WITH LOWERING CEILINGS AND TEMPORARILY DECREASED
VISIBILITY. SNOW LEVELS WILL STAY LOW ENOUGH INTO THIS EVENING FOR
SNOW AT KGPI. THE SNOW LEVELS WILL BE STEADILY RISING OVER THE
NEXT 24-48 HOURS. MOUNTAINS AND PASSES ARE EXPECTED TO BE OBSCURED
THE MAJORITY OF THE TIME.
MSO WATCHES/WARNINGS/ADVISORIES
MT...WINTER WEATHER ADVISORY UNTIL 11 AM MST THURSDAY FOR LOWER CLARK
FORK REGION...WEST GLACIER REGION.
ID...NONE.
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