AREA FORECAST DISCUSSION

 
 FXUS65 KMSO 231206 AAA
 AFDMSO
 
 AREA FORECAST DISCUSSION...UPDATED
 NATIONAL WEATHER SERVICE MISSOULA MT
 506 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.
 
 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.
 
 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...ALLEGRETTO
 LONG TERM....DICKERSON
 AVIATION...ALLEGRETTO
 AIR STAGNATION...DICKERSON