Issued by NWS Missoula, MT
000 FXUS65 KMSO 221558 AAA AFDMSO AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE MISSOULA MT 858 AM MST SUN NOV 22 2009 .DISCUSSION...DID AN UPDATE TO INCREASE CHANCE OF SNOW ACROSS THE MISSOULA/BITTERROOT VALLEYS THIS MORNING. THE LOW LEVELS OF THE ATMOSPHERE HAVE MOISTENED ENOUGH TO ALLOW PRECIPITATION TO FALL EARLIER THAN PREVIOUSLY EXPECTED. LOCATIONS DOWN THE MISSOULA/BITTERROOT VALLEY CAN EXPECT UP TO AN INCH OF SNOW ACCUMULATION BY THIS AFTERNOON...LOCALLY UP TO 3 INCHES OVER THE HIGHER BENCHES SURROUNDING THE VALLEY FLOOR. OTHERWISE FORECAST LOOKS ON TRACK WITH SNOWY FORECAST...ESPECIALLY OVER THE HIGHER TERRAIN OF NORTH CENTRAL IDAHO. WINTER WEATHER ADVISORIES REMAIN IN PLACE...PLEASE SEE WWW.WEATHER.GOV/MISSOULA FOR FURTHER DETAILS. && .AVIATION...ANOTHER ACTIVE WEATHER DAY IS EXPECTED WITH AN UPPER TROUGH AND COLD FRONTAL PASSAGE. THE FRONT WILL MOVE THROUGH NORTHERN IDAHO BY 22/1800Z THEN THROUGH WESTERN MONTANA BY 22/2200Z. WIDESPREAD SNOW SHOWERS WILL RESULT...SOME HEAVY AT TIMES. INCREASED WEST WINDS WILL ALSO BE OF CONCERN AS BLOWING SNOW MAY LIMIT VISIBILITY. LOW CEILINGS WILL OBSCURE HIGHER TERRAIN ACROSS THE ENTIRE REGION. ACCUMULATING SNOW BETWEEN 1 TO 2 INCHES IS POSSIBLE ON KMSO AND KGPI RUNWAYS BETWEEN 22/1800Z AND 22/2100Z. SHOWERS WILL BEGIN TO DECREASE BY LATE AFTERNOON...BUT LINGER WELL INTO THE EVENING MAINLY OVER NORTHERN IDAHO AND THE BITTERROOT MOUNTAINS. && .PREV DISCUSSION... /ISSUED 319 AM MST SUN NOV 22 2009/ DISCUSSION...A POTENT WEATHER SYSTEM IS MOVING THROUGH THE PACIFIC NORTHWEST EARLY THIS MORNING...AND WILL PRODUCE WIDESPREAD SNOW SHOWERS ACROSS THE NORTHERN ROCKIES TODAY. THE HEAVIEST SNOW WILL DEVELOP OVER NORTHERN IDAHO AND THE BITTERROOTS FROM MID MORNING THROUGH EARLY AFTERNOON AHEAD OF A SURFACE COLD FRONT. HOWEVER...GOOD OROGRAPHIC LIFT AND INSTABILITY WILL CONTINUE THE SHOWERS IN THOSE LOCATIONS THROUGH THE DAY AND INTO THE EVENING. LIGHTER SNOW ACCUMULATIONS ARE EXPECTED THROUGHOUT THE REMAINDER OF WESTERN MONTANA. WINDS WILL INCREASE AS THE FRONT APPROACHES AND REMAIN UP INTO THE EVENING. WINDS IN THE GRANGEVILLE AREA ARE EXPECTED TO REACH ADVISORY CRITERIA (IN EXCESS OF 30 MPH WITH GUSTS INTO THE 40S) EARLY THIS MORNING THROUGH MIDDAY AS THE WINDS IN THE LOWER ATMOSPHERE AND THE SURFACE PRESSURE GRADIENT INCREASE. WINDS IN THE MOUNTAINS NEAR GRANGEVILLE HAVE BEEN GUSTING TO WELL ABOVE 40 MPH ALREADY. THE SOUTHERLY WINDS GENERALLY INHIBIT SHOWER ACTIVITY OVER THE CAMAS PRAIRIE...BUT THE FRONT MAY BE STRONG ENOUGH TO OVERCOME THE DOWNSLOPING. THEREFORE...EXPECT SOME BLOWING SNOW IN THAT AREA AS WELL AS IN THE CLEARWATER AND BITTERROOT MOUNTAINS WITH THE PASSAGE OF THE FRONT. HIGH PRESSURE WILL BUILD OVER THE AREA MONDAY AND MONDAY NIGHT. HOWEVER...THE FLOW WILL STILL CONTAIN SUBSTANTIAL MOISTURE. THEREFORE...LIGHT SNOW SHOWERS ARE POSSIBLE ACROSS NORTHWEST MONTANA...AND SOME CLOUD COVER IS EXPECTED OVER WEST CENTRAL MONTANA AND NORTH CENTRAL IDAHO. TUESDAY THROUGH THE WEEK...HIGH PRESSURE RIDGING IS STILL THE MAIN FEATURE FOR THE EXTENDED FORECAST PERIOD. WHILE THIS PATTERN ALLOWS FOR A DRIER AND MUCH TAMER TREND IN WEATHER...ONE IMPACT THAT WILL CONTINUE TO BE MONITORED WILL BE FOG DEVELOPMENT UNDER THE STRENGTHENING RIDGE. AS THE NORTHERN ROCKIES FALLS UNDER INCREASINGLY STABLE AIR AND RESULTING INVERSIONS DEVELOP BY MIDWEEK...FOG FORMATION IN VALLEYS OF WESTERN MONTANA AND CENTRAL IDAHO WILL BE KEY...PARTICULARLY FOR ANY HOLIDAY TRAVEL. AS OF RIGHT NOW...THE BEST DAYS FOR FOG FORMATION WILL BE WEDNESDAY MORNING AND AGAIN ON THURSDAY MORNING. ANY INVERSIONS THAT FORM AND HELP KEEP FOG TRAPPED IN VALLEYS WILL DEPEND ON THE STRENGTH AND POSITION OF THE RIDGE...WHICH WILL ALL NEED TO BE MONITORED AS TIME PASSES. && .MSO WATCHES/WARNINGS/ADVISORIES... MT... WINTER WEATHER ADVISORY UNTIL 11 PM MST THIS EVENING FOR BITTERROOT/SAPPHIRE MOUNTAINS. WINTER WEATHER ADVISORY UNTIL 11 PM MST THIS EVENING FOR LOWER CLARK FORK REGION. ID...WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR OROFINO/GRANGEVILLE REGION. WIND ADVISORY UNTIL NOON PST TODAY FOR LOWER HELLS CANYON/SALMON RIVER REGION-OROFINO/GRANGEVILLE REGION. WINTER WEATHER ADVISORY UNTIL 10 PM PST THIS EVENING FOR NORTHERN CLEARWATER MOUNTAINS-SOUTHERN CLEARWATER MOUNTAINS. && $$ SHORT TERM...PALLISTER LONG TERM....WHITMORE AVIATION...PALLISTER
