FXUS65 KMSO 160312
AFDMSO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
812 PM MST TUE DEC 15 2009

.DISCUSSION...SNOW RATIOS ARE A BIT MORE ROBUST ACROSS NORTHWEST
MONTANA THAN EARLIER THOUGHT...CAUSING SLIGHTLY BETTER
ACCUMULATIONS THIS EVENING. THEREFORE...HAVE ISSUED A WINTER
WEATHER ADVISORY FOR SNOW THROUGHOUT NORTHWEST MONTANA. BEST
ACCUMULATIONS WILL LIKELY BE NORTH OF A LINE FROM LOOKOUT PASS TO
THE NORTH END OF FLATHEAD LAKE AND THROUGH GLACIER NATIONAL PARK.

&&

.AVIATION...A WARM FRONT THAT MOVED THROUGH IDAHO AND NORTHWEST
MONTANA HAS BROUGHT ENOUGH MOISTURE THAT PATCHY FOG WILL LIKELY
DEVELOP TONIGHT. SNOW WILL MOVE BACK INTO THE AREA AS WELL LATE
TONIGHT AFFECTING THE GPI AND MSO TERMINALS.

&&

PREV DISCUSSION... /ISSUED 312 PM MST TUE DEC 15 2009/

DISCUSSION...PRECIPITATION AMOUNTS HAVE BECOME LIGHT THIS
AFTERNOON AS THE WARM FRONT THAT PRODUCED RAIN AND SNOW THIS
MORNING MOVES FARTHER NORTH. WITH NOT MUCH MORE PRECIPITATION
EXPECTED UNTIL LATER TONIGHT HAVE CANCELED THE REMAINING HIGHLIGHTS
THAT WERE IN EFFECT FOR NORTHWEST MONTANA. MOISTURE CONTINUES TO
STREAM INTO IDAHO AND MONTANA FROM WASHINGTON AND OREGON BUT
WITHOUT ANY FORCING MAINLY TERRAIN DRIVEN SNOW AND RAIN CAN BE
EXPECTED OVER HIGHER ELEVATIONS. THE EXCESS MOISTURE BROUGHT ON BY
THE RAIN AND MELTING SNOW WILL CONTRIBUTE TO PATCHY FOG FORMATION
TONIGHT IN NORTHWEST MONTANA AND CENTRAL IDAHO. TEMPERATURES WILL
NOT COOL OFF MUCH TONIGHT WITH THE ABUNDANT MOISTURE AND CLOUD
COVER IN PLACE. THE WARM FRONT WILL MOVE BACK SOUTH LATE TONIGHT
BRINGING THE POTENTIAL FOR A MIX OF RAIN AND SNOW. WITH THE AIR SO
WARM THERE IS NOT MUCH POTENTIAL FOR ICE GROWTH WHICH WILL LIMIT
SNOW RATIOS TO LESS THAN 10 TO 1. THIS MEANS NOT A LOT OF
ACCUMULATION AND ALSO INCREASES THE POTENTIAL FOR POCKETS OF
FREEZING RAIN IN NORTHWEST MONTANA SIMILAR TO WHAT WAS SEEN THIS
MORNING. RAIN AND SNOW WILL CONTINUE TO FALL OVER CENTRAL IDAHO
THROUGH THIS TIME FRAME WHICH COULD CREATE PROBLEMS WITH MELTING
SNOW...SEE HYDROLOGY SECTION BELOW.

WEDNESDAY NIGHT THROUGH TUESDAY...PROGRESSIVE WESTERLY FLOW 
ALOFT IS ANTICIPATED TO BE PRESENT OVER WESTERN MONTANA AND NORTH 
CENTRAL IDAHO DURING THIS TIME. COLD AIR REMAINING WITHIN THE 
VALLEYS OF NORTHWEST MONTANA COUPLED WITH MOIST WESTERLY FLOW WILL 
MAINTAIN A THREAT OF LIGHT FREEZING RAIN THROUGH EARLY THURSDAY. 
HAZARDOUS TRAVEL CONDITIONS MAY EXIST DURING THIS TIME IN NORTHWEST 
MONTANA. A WEAK PACIFIC STORM SYSTEM MOVING WITHIN THE WESTERLY FLOW 
WILL MIX NORTHWEST MONTANA MORE EFFECTIVELY THURSDAY 
MORNING...ENDING THE FREEZING RAIN THREAT IMMEDIATELY AHEAD OF A 
WEAK SURFACE COLD FRONT. COLD FRONT IS A SLIGHT MISNOMER 
HOWEVER...AS THE AIR MASS BEHIND THE SURFACE FRONT WILL NOT BE MUCH 
COOLER THAN AHEAD OF THE FRONT. MUCH OF THE SNOW THAT IS EXPECTED TO 
ACCOMPANY THIS SYSTEM WILL OCCUR OVER HIGH TERRAIN AND BE LIGHT IN 
INTENSITY...LASTING FOR AN EXTENDED PERIOD. BY FRIDAY...BRIEF HIGH 
PRESSURE IS EXPECTED TO DEVELOP OVER THE REGION...WITH RESIDUAL LOW 
LEVEL MOISTURE AND MOSTLY CLEAR SKIES ALLOWING FOR FREEZING FOG 
DEVELOPMENT. AFTER FRIDAY...MODEL UNCERTAINTY GROWS 
INCREDIBLY...WITH A VERY ACTIVE PATTERN DEVELOPING THROUGHOUT THE 
NORTHERN HEMISPHERE. GIVEN MODEL DISAGREEMENT AND UNCERTAINTY... 
PRECIPITATION EXPECTATIONS NEAR CLIMATOLOGY HAVE BEEN MAINTAINED. 
THERE ARE HINTS OF ARCTIC AIR GRAZING JUST WEST OF THE CONTINENTAL 
DIVIDE DURING SUNDAY...HOWEVER TEMPERATURES ARE EXPECTED TO BE NEAR 
OR SLIGHTLY ABOVE THE SEASONAL AVERAGE THROUGH SUNDAY.

&&

.HYDROLOGY...WARM TEMPERATURES AND RAIN WILL LEAD TO MELTING SNOW
RESULTING IN PONDING WATER IN THE OROFINO...PIERCE AREA AND OTHER
LOCATIONS IN CLEARWATER COUNTY IDAHO. SNOWMELT AND RAINFALL RUNOFF
ENTERING THE OROFINO CREEK COULD CAUSE ICE JAM MOVEMENT WEDNESDAY
AND THURSDAY.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY UNTIL 8 AM MST WEDNESDAY FOR LOWER CLARK 
     FORK REGION.

     WINTER WEATHER ADVISORY UNTIL 8 AM MST WEDNESDAY FOR 
     FLATHEAD/MISSION VALLEYS-KOOTENAI/CABINET REGION-WEST 
     GLACIER REGION.

ID...NONE.
&&

$$

UPDATE...WHITMORE/ALLEGRETTO
PREVIOUS...LOEFFELBEIN/ZUMPFE/DEROSA