000 FXUS65 KMSO 231528 AFDMSO AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE MISSOULA MT 828 AM MST MON NOV 23 2009 .UPDATE...INCREASED CLOUDS AND ADJUSTED A FEW TEMPERATURES FOR TODAY... && .DISCUSSION...A MOIST AND UNSTABLE NORTHWEST UPPER FLOW WILL KEEP A THREAT OF SNOW SHOWERS OVER THE INTERMOUNTAIN WEST TODAY...MOST PREVALENT THIS MORNING. IN ADDITION...TEMPERATURES DID NOT GET AS COLD LAST NIGHT AS PREVIOUSLY FORECAST...WHICH WILL HAVE AN IMPACT ON TODAYS HIGHS. ADJUSTED A FEW LOCATIONS HIGHER TODAY IN THE SOUTH AS EXPECTING DECENT MIXING DESPITE THE CLOUDS. && .AVIATION...PERIODICALLY LOWERED CEILINGS AND VISIBILITIES WILL EXIST ACROSS WEST CENTRAL MONTANA AND CENTRAL IDAHO THROUGH MID MORNING AS LINGERING SNOW SHOWERS CONTINUE. THESE SHOWERS ARE EXPECTED TO WEAKEN THROUGHOUT THE MORNING AND DIMINISH BY THIS AFTERNOON WITH CONDITIONS IMPROVING AS A RESULT...THOUGH MOUNTAINS WILL REMAIN OBSCURED FOR MOST OF TODAY. ANOTHER WEATHER SYSTEM WILL MOVE ACROSS NORTHWEST MONTANA TONIGHT...MAINLY AFTER MIDNIGHT. THIS WAVE IS ONCE AGAIN EXPECTED TO BRING LOWERED CEILINGS AND OBSCURED TERRAIN FROM SHOWERS. KGPI STANDS THE BEST CHANCE OF BEING IMPACTED MAINLY AFTER 24/0600Z. && .PREV DISCUSSION... /ISSUED 617 AM MST MON NOV 23 2009/ ..FOG TO POTENTIALLY IMPACT HOLIDAY TRAVEL ON WEDNESDAY AND THANKSGIVING DAY... DISCUSSION...SNOW SHOWERS ARE EXPECTED TO CONTINUE ACROSS WEST CENTRAL MONTANA AND THE HIGHER ELEVATIONS OF CENTRAL IDAHO THIS MORNING. THESE SHOWERS WILL BEGIN TO TAPER OFF GRADUALLY BY MID MORNING AS THIS SYSTEM WEAKENS FURTHER. UNSTABLE NORTHWEST FLOW WILL HELP MOUNTAIN SHOWERS TO LAST THROUGH THE AFTERNOON BUT ONLY LIGHT ACCUMULATIONS ARE ANTICIPATED. A DEVELOPING RIDGE OF HIGH PRESSURE WILL BE THE NEXT FEATURE TO SET UP ACROSS THE NORTHERN ROCKIES...BUT THIS RIDGE WILL BE INTERRUPTED BRIEFLY BY A QUICK MOVING SYSTEM THAT WILL GRAZE NORTHWEST MONTANA TUESDAY. LATEST FORECAST MODELS HAVE BEEN GOING BOLDER WITH PRECIPITATION EXPECTED OUT OF THIS WAVE OF ENERGY. THE BULK OF MOISTURE IS STILL EXPECTED TO REMAIN FAR ENOUGH NORTH OF THE CANADIAN BORDER...THOUGH QUITE A BIT OF LIFT ASSOCIATED WITH THIS DISTURBANCE WILL HELP GENERATE SNOW...PARTICULARLY IN NORTHWEST AND WEST CENTRAL MONTANA THROUGH TUESDAY AFTERNOON. CONDITIONS WILL QUICKLY IMPROVE AS THE HIGH PRESSURE RIDGE REBUILDS AND STRENGTHENS TUESDAY EVENING INTO WEDNESDAY MORNING. DEPENDING ON HOW MUCH MOISTURE RESULTS FROM TUESDAYS ROUND OF PRECIP...THIS COULD CREATE EVEN MORE AVAILABLE LOW LEVEL MOISTURE WHICH WOULD FURTHER AID IN FOG DEVELOPMENT EXPECTED FOR MID WEEK. THE BIGGEST STORY FOR THE EXTENDED FORECAST PERIOD FROM WEDNESDAY THROUGH THE WEEKEND WILL BE THE POTENTIAL FOR FREEZING FOG BOTH WEDNESDAY AND THANKSGIVING DAY. MODEL MOISTURE FIELDS IN THE UPPER ATMOSPHERE BEGIN TO DRY WEDNESDAY AND ATMOSPHERIC SOUNDINGS SHOW MODEST STABILITY DEVELOPING NEAR THE SURFACE. THIS SHOULD EQUATE TO AT LEAST SOME PATCHY FOG DEVELOPING WEDNESDAY MORNING...DEPENDING UPON LOCAL FACTORS SUCH AS CHANNELING...DRAINAGE WINDS...CLOUD COVER AND THE AMOUNT OF LOW LEVEL MOISTURE AVAILABLE. THE LONGEVITY OF THE POTENTIAL FOG WEDNESDAY IS STILL SOMEWHAT IN QUESTION. BUT AT THIS TIME IT DOES APPEAR TO DISSIPATE SOMEWHAT LATE WEDNESDAY AS THE ATMOSPHERE APPEARS TO SHOW SOME MODIFICATION DUE MAINLY TO VARIABLE HIGH LEVEL CLOUD COVER. HOWEVER BY EVENING THE STRONG UPPER LEVEL RIDGE WILL AMPLIFY OVER THE REGION WITH THE APEX BECOMING SITUATED JUST EAST OF THE CONTINENTAL DIVIDE BY EARLY THURSDAY MORNING. THIS WILL HELP TO ENHANCE HIGH PRESSURE AT THE SURFACE AND LEAD TO IMPRESSIVE STABILITY IN THE VALLEYS AS INDICATED BY ATMOSPHERIC SOUNDINGS DURING THIS TIME FRAME. FOG WILL MOST LIKELY DEVELOP IN THE VALLEYS SOMETIME AFTER SUNSET WEDNESDAY EVENING AND POSSIBLY REMAIN FOGGY THROUGH THE BETTER PART OF THANKSGIVING DAY. THIS SCENARIO COULD CHANGE SEEING AS THOUGH IT IS ONLY MONDAY. HOWEVER THE MODELS HAVE BEEN SHOWING THE DEVELOPMENT OF A STRONG RIDGE FOR ABOUT THE PAST FIVE DAYS OR SO...GIVING THIS SCENARIO A HIGHER DEGREE OF CONFIDENCE 3 DAYS IN ADVANCE. THE POTENTIAL FOR DENSE FOG IS BETTER THAN AVERAGE AND THOSE TRAVELING WILL WANT TO KEEP MONITOR THE FORECAST FOR FUTURE UPDATES. THE GFS...ECMWF AND GFS ENSEMBLE MEAN ALL SUGGEST DELAYING THE NEXT WEATHER SYSTEM UNTIL LATE FRIDAY AND KEEP THE WEATHER PATTERN FAIRLY ACTIVE THROUGH THE WEEKEND. ON FRIDAY A TIGHTENING SOUTHERLY SURFACE PRESSURE GRADIENT SHOULD INDUCE WINDS SUFFICIENT TO GRADUALLY MIX THE ATMOSPHERE ON FRIDAY THUS MODERATING THE INTENSE VALLEY INVERSIONS ESTABLISHED EARLIER. TEMPERATURES APPEAR TO WARM AHEAD OF AN APPROACHING COLD FRONT WHICH IS DUE INTO THE NORTHERN ROCKIES REGION LATE FRIDAY AT THIS TIME. ALTHOUGH DIFFERENT IN THEIR DETAILS...THE MODEL TREND APPEARS TO BE HEADING TOWARDS QUICKLY DIGGING THESE PASSING WEATHER SYSTEMS THROUGH THE REGION AND INTO THE DESERT SOUTHWEST AND CENTRAL ROCKY MOUNTAIN REGION. THIS WILL NOT BODE WELL FOR HEAVY SNOWFALL ACROSS MONTANA AND IDAHO AS THE ENERGY APPEARS TO QUICKLY EXIT THE REGION. AND YES...YET ANOTHER HIGH AMPLITUDE RIDGE IS POTENTIALLY WAITING IN THE WINGS FOR EARLY NEXT WEEK. ALL THINGS CONSIDERED...THIS IS A VERY ATYPICAL WEATHER PATTERN FOR AN EL NINO YEAR. AIR STAGNATION...FORECAST MODELS ARE IN VERY GOOD AGREEMENT THAT VERY STRONG VALLEY INVERSIONS WILL BEGIN TO BUILD WEDNESDAY AND GROW EVEN STRONGER WEDNESDAY NIGHT INTO THANKSGIVING MORNING AS A HIGH AMPLITUDE RIDGE OF HIGH PRESSURE STRENGTHENS ALOFT. SUCH STRONG VALLEY INVERSIONS WILL LIKELY INHIBIT APPRECIABLE MIXING WHICH COULD POTENTIALLY LEAD TO STAGNANT AIR AND POTENTIAL HEALTH ISSUES DUE TO TRAPPED POLLUTION. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...NONE. ID...NONE. && $$ SHORT TERM...MEAD PREVIOUS DISCUSSION....DICKERSON/ALLEGRETTO AVIATION...ALLEGRETTO