000 FXUS65 KFGZ 271042 CCA AFDFGZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE FLAGSTAFF AZ 330 AM MST FRI NOV 27 2009 .SYNOPSIS...HIGH PRESSURE WILL BRING ONE MORE DAY OF MOSTLY SUNNY SKIES AND MILD HIGH TEMPERATURES. A STRONG LOW PRESSURE SYSTEM WILL IMPACT NORTHERN ARIZONA SATURDAY AND SUNDAY WITH COLDER TEMPERATURES...BREEZY CONDITIONS...MOUNTAIN SNOW AND VALLEY RAIN. && .DISCUSSION...VERY DRY AIR IN PLACE OVER THE INTERMOUNTAIN WEST AND OVER THE ROCKIES. THE HIGH PRESSURE RIDGE THAT HAS RESIDED OVER THE WESTERN U.S. WILL BEGIN TO MOVE EAST TODAY...MAKING ROOM FOR AN APPROACHING UPPER LEVEL LOW PRESSURE SYSTEM THAT IS FORECAST TO DROP SOUTH THROUGH CALIFORNIA SATURDAY AND THEN INTO BAJA SUNDAY AND THE REST OF NORTHERN MEXICO ON MONDAY. BOTH THE GFS AND THE ECMWF HAVE BEEN CONSISTENT ON THE TRACK OF THE LOW OVER THE LAST SEVERAL MODEL RUNS. GOES SOUNDER DPI SHOWING SOME WEAK ADVECTION OF MOISTURE INTO EXTREME SOUTHEAST ARIZONA...WITH THE REST OF THE STATE AND THE INTERMOUNTAIN WEST AND ROCKIES REMAINING EXTREMELY DRY. THE DRY AIR WILL PLAY A ROLL IN LOWERING SNOW LEVELS ON SATURDAY OVER WHAT IS INDICATED BY THE GFS...WARMER WITH HIGHER SNOW LEVELS. MIDLEVEL MOISTURE AND QPF PRESENTED BY THE MODELS WILL GO INTO MOISTENING THE LOWER LEVELS. SO QPF AMOUNTS ON SATURDAY AS ADVERTISED BY THE MODELS CAN BE CUT BACK AND SNOW LEVELS WERE LOWERED CONSIDERABLY OVER PREVIOUS FORECAST FOR CONSIDERATION OF WET BULB FACTORS OF PRECIP INTO DRY AIR. THE CLOSEST THE UPPER LOW WILL GET TO ARIZONA WILL BE LATE SATURDAY AFTERNOON AND EVENING...BEFORE DROPPING SOUTH INTO MEXICO. CURRENT THINKING IS FOR THE BEST CHANCES OF PRECIPITATION TO OCCUR WILL BE LATE SATURDAY INTO EARLY SUNDAY MORNING. ONCE THE LOW BEGINS TO DROP SOUTH INTO MEXICO THE FLOW WILL BECOME EAST NORTHEAST...THUS ADVECTING THE DRY LOW TO MID-LEVEL AIR FROM OVER UTAH AND COLORADO BACK INTO AREAS NORTH OF THE MOGOLLON RIM. THE 00Z GFS WAS INDICATING THE DRY ADVECTION INTO AREAS NORTH OF THE MOGOLLON RIM ON SUNDAY...AND CHC'S OF PRECIP WERE LOWERED FROM THE MOGOLLON RIM NORTH. AGAIN SNOW LEVELS SHOULD BE LOWER THAN ADVERTISED BY THE GFS DUE TO THE INFLUENCES OF THE DRY ADVECTION AND WET BULBING AS THE PRECIPITATION FALLS THROUGH DRIER AIR. BY MONDAY MORNING THE UPPER LOW IS FORECAST TO BE NEAR GUAYMAS AND IS FORECAST TO MOVE EAST - SOUTHEAST ACROSS MEXICO WITH THE BEST MOISTURE AND LIFT FOR NORTHERN ARIZONA NOW CONFINED TO THE EASTERN RIM SOUTH INTO NORTHERN GILA AND THE WHITE MOUNTAINS. THE LOW EXITS OFF TO THE SOUTHEAST LATE MONDAY WITH A STRONG HIGH PRESSURE RIDGE ALOFT BUILDING INTO THE SOUTHWEST U.S. BY TUESDAY MORNING. IT APPEARS THAT THE BEST SNOW ACCUMULATION WILL BE OVER THE WHITE MOUNTAINS WITH POSSIBLE 3 DAY TOTALS (SATURDAY THROUGH MONDAY) OF AROUND 6 TO 8 INCHES WITH 1 TO 2 INCHES OR LESS OVER THE ELEVATION AREAS ABOVE 5500 FEET FOR NORTHERN ARIZONA. FOR THE REMAINDER OF THE WEEK NORTHERN ARIZONA WILL RESIDE IN NORTHERLY FLOW WITH THE HIGH PRESSURE RIDGE TO OUR WEST AND THE LOW PRESSURE TROUGH OVER THE CENTRAL U.S. THIS WILL BRING DRY CONDITIONS WITH NEAR SEASONABLE HIGH TEMPERATURES FOR THIS TIME OF THE YEAR. BEYOND MONDAY MODEL CONSISTENCY OF BOTH THE GFS AND ECMWF IS LOW WITH MAJOR CHANGES FROM TROUGH TO RIDGES WITH EACH MODEL RUN. THE ECMWF APPEARS TO BE MORE STABLE THAN THE GFS BEYOND MONDAY AND THAT IS WHAT THE REST OF THE DIGITAL FORECAST REFLECTS. && .AVIATION...FOR THE 12Z PACKAGE...EXPECT VFR CONDITIONS UNTIL 06Z SATURDAY. AFTER 06Z...INCREASING LOW LEVEL CLOUD COVER IS EXPECTED ALONG THE MOGOLLON RIM WITH LOCAL MVFR CEILINGS POSSIBLE. AVIATION DISCUSSION NOT UPDATED FOR AMENDMENTS. && .FGZ WATCHES/WARNINGS/ADVISORIES...NONE. && $$ PUBLIC.................TC AVIATION...............DJO VISIT WEATHER.GOV/FLAGSTAFF FOR NORTHERN ARIZONA WEATHER INFORMATION.