FXUS65 KBOI 050418
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
918 PM MST FRI DEC 4 2009

.DISCUSSION...VERY COLD SYSTEM MOVING NEARLY DUE SOUTH THIS
EVENING WILL PUSH RAPIDLY THROUGH OVERNIGHT...BRINGING A COLD
FRONT BUT NOT MUCH IN THE WAY OF PRECIPITATION. TEMPERATURES WILL
BECOME COLDER DAY BY DAY THROUGH TUESDAY. NORTHWEST WINDS SATURDAY
AFTERNOON WILL MAKE IT FEEL LIKE IT IS IN THE LOWER 20S IN THE
SNAKE RIVER VALLEY...WITH TEENS AND SINGLE DIGITS IN THE
MOUNTAINS. NO CHANGES TO THE FORECAST AT THIS TIME.

&&

.AVIATION...VFR CONDITIONS TONIGHT INTO SATURDAY. CLOUDS WILL 
INCREASE OVERNIGHT AS A WEATHER SYSTEM FROM BRITISH COLUMBIA MOVES 
OVER OUR MONTANA. WIDELY SCATTERED SNOW SHOWERS WILL SPREAD SOUTH 
DURING THE NIGHT BRINGING LOCAL MOUNTAIN OBSCURATION THROUGH 
SATURDAY. SATURDAY NIGHT AND SUNDAY A SECOND IMPULSE FROM CANADA 
WILL BRINGING WIDELY SCATTERED SNOW SHOWERS. SURFACE WINDS WILL BE 
LIGHT TONIGHT THEN INCREASE TO MODERATE NORTHWEST SATURDAY. WINDS 
ALOFT WILL BE MODERATE NORTHERLY.

&&

.PREV DISCUSSION...
SHORT TERM...TONIGHT THROUGH SUNDAY...THERE WILL BE TWO UPPER LOWS 
TO CONTEND WITH OVER THE WEEKEND...NEITHER OF WHICH HAS MUCH 
MOISTURE TO WORK WITH FOR SNOWFALL PRODUCTION. THE REASON WHY IS 
THAT THE SOURCE REGION IN INTERIOR WESTERN CANADA IS DRY AND LACKING 
A MOISTURE FEED OFF THE PACIFIC. THE FIRST SYSTEM WILL END UP 
TRACKING JUST OFF TO THE EAST THROUGH SATURDAY...BUT IT WILL DRIVE A 
COLD FRONT THROUGH THE REGION TONIGHT FOLLOWED BY COLDER 
TEMPERATURES ON SATURDAY. THE SECOND SYSTEM WILL TAKE A SIMILAR 
TRACK INITIALLY...DIVING SOUTH THROUGH WESTERN CANADA...BUT WILL 
THEN TAKE A TURN TO THE SOUTHWEST SATURDAY NIGHT AND SUNDAY. THIS 
SECOND SYSTEM IS IN A BETTER POSITION TO BRING SNOW SHOWERS TO SE 
OREGON BASED ON A TRACK THROUGH WESTERN OREGON...BUT QPF/SNOW 
AMOUNTS WILL STILL BE LIGHT. IT WILL ALSO START TO DELIVER EVEN 
COLDER TEMPERATURES TO THE REGION ON SUNDAY. HIGHS BOTH DAYS WILL 
STRUGGLE TO GET INTO THE 30S IN THE VALLEYS.

LONG TERM...SUNDAY NIGHT THROUGH FRIDAY...MODELS AGREE QUITE WELL 
THROUGH TUESDAY NIGHT...KEEPING US UNDER COLD DRY NORTHERLY FLOW AT 
ALL LEVELS WITH TEMPERATURES AROUND 10 DEGREES BELOW NORMAL. FOR 
SUNDAY NIGHT THROUGH TUESDAY WE LOWERED TEMPERATURES ACROSS THE 
BOARD...BASED ON BLENDS WEIGHTED TOWARD THE COLDER SOLUTIONS. POPS 
ARE BELOW CLIMATOLOGY THROUGH TUESDAY NIGHT BASED ON LACK OF 
MOISTURE AND DYNAMICS PLUS THE PRECIPITATION INHIBITING INFLUENCE OF 
STRONG LOW LEVEL COLD AIR ADVECTION. MODELS BEGIN TO DIVERGE ON 
WEDNESDAY...WITH THE OPERATIONAL GFS...DGEX...AND CANADIAN KEEPING 
US UNDER THE COLD DRY NORTHERLY BRANCH...WHILE THE ECMWF AND GFS 
ENSEMBLE MEAN BRING THE SOUTHERN BRANCH OF WESTERLIES WITH ITS 
ATTENDANT MOISTURE INLAND AS FAR NORTH AS OUR SOUTHERN ZONES. DUE TO 
THIS UNCERTAINTY WE RAISED POPS TO NEAR CLIMATOLOGY FOR WEDNESDAY... 
BUT KEPT TEMPERATURES COLD BASED ON THE IDEA THAT THE LOW LEVEL COLD 
AIR WOULD BE HELD IN PLACE BY WARMING ALOFT AND LACK OF LOW LEVEL 
FLOW. FOR THURSDAY AND FRIDAY MODELS ALL FAVOR A PATTERN CHANGE TO  
PROGRESSIVE WESTERLY FLOW. SO WE RAISED POPS TO NEAR OR ABOVE 
CLIMATOLOGY AND WARMED TEMPERATURES BACK TO NEAR SEASONAL NORMALS. 

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
OR...NONE.
&&

$$

WEATHER.GOV/BOISE

DISCUSSION...SP
AVIATION.....JS
PREV SHORT TERM...BW
PREV LONG TERM....JT/BB