FXUS65 KBOI 150334
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
834 PM MST MON DEC 14 2009

.DISCUSSION...NEW MODEL RUNS CONFIRM EARLIER THINKING ON WINTER
STORM POISED TO MOVE INTO THE AREA. WINTER STORM CONDITIONS WILL
BEGIN SHORTLY AFTER MIDNIGHT IN NORTHERN HARNEY COUNTY AND IN
BAKER COUNTY OREGON AND SPREAD INTO THE WEST CENTRAL AND BOISE
MOUNTAINS OF IDAHO AROUND DAWN ON TUESDAY. HEAVY SNOW WILL OCCUR
IN THESE AREAS. THE FORECAST FOR THE TREASURE VALLEY IS A LITTLE
MORE UNCERTAIN. SOUTHEAST WINDS HAVE HELPED KEEP TEMPERATURES
ABOVE FREEZING IN BOISE SO FAR THIS EVENING. HOWEVER...EVAPORATIVE
COOLING SHOULD ALLOW TEMPERATURES TO FALL INTO THE UPPER 20S AT
THE ONSET OF PRECIPITATION. SNOW IS EXPECTED TO BEGIN AROUND 7 AM
IN THE TREASURE VALLEY AND CONTINUE INTO THE EARLY AFTERNOON. THE
ONSET OF PRECIPITATION IN THE TREASURE VALLEY IS A LITTLE SLOWER
THAN EARLIER INDICATED BUT STILL EARLY ENOUGH TO AFFECT RUSH HOUR
TRAFFIC. FORECAST TEMPERATURE PROFILES FROM THE 00Z NAM MODEL
CONTINUE TO INDICATE A CHANGE OVER TO RAIN IN THE AFTERNOON IN
BOISE WHICH WILL LIMIT SNOW ACCUMULATIONS IN THE UPPER TREASURE
VALLEY. HOWEVER...COLD AIR WHICH HAS POOLED IN THE LOWER TREASURE
VALLEY IN THE VICINITY OF ONTARIO WILL ALLOW SNOW TO CONTINUE ALL
DAY RESULTING IN HEAVIER ACCUMULATIONS THERE. SNOW CHANGING TO
RAIN IS ALSO LIKELY TOMORROW IN THE MAGIC VALLEY INCLUDING THE
TWIN FALLS AREA. MINOR UPDATES TO WATCHES AND ADVISORIES HAVE BEEN
MADE TO ADJUST TIMING AND SNOW AMOUNTS.

&&

.AVIATION...VFR CONDITIONS WILL CONTINUE ON THE IDAHO SIDE OF THE 
BORDER THIS EVENING WITH SCT-BKN DECKS ABOVE 15K FT MSL. CONDITIONS 
WILL DETERIORATE ON THE OREGON SIDE WITH MVFR CEILINGS AND 
VISIBILITIES IN SNOW AT KBKE AND KBNO BY 06Z-08Z. MOUNTAIN 
OBSCURATION WITH AREAS OF MVFR CONDITIONS IN THE VALLEYS...AND LOCAL 
IFR/LIFR... WILL SPREAD EAST ACROSS THE WEST CENTRAL IDAHO AND BOISE 
MOUNTAINS AFTER 06Z REACHING KBOI BY 12Z AND KTWF/KJER BY 18Z. 
SURFACE WINDS WILL BE LIGHT. WINDS ALOFT WILL BE MODERATE WESTERLY. 

&&

.PREV DISCUSSION...
SHORT TERM...A LARGE PACIFIC STORM SYSTEM IS POISED TO MOVE INTO 
THE AREA. THIS SYSTEM HAS A LONG PLUME OF HIGH PWAT STREAMING FAR TO 
THE SOUTHWEST TO JUST NORTH OF HAWAII. UNLIKE THE LAST SYSTEM THIS 
WEEKEND...THIS ONE IS AIMED DIRECTLY AT OUR CENTRAL MOUNTAINS AND 
AREAS ON THE WEST COAST DUE WEST OF BOISE. STORM HAS WARMER AIR 
ENTRAINED INTO IT AND THEREIN LIES THE DIFFICULTY IN THE FORECAST. 
WHAT HAPPENS EARLY TONIGHT TEMPERATURE WISE WILL HAVE A BIG EFFECT 
ON WHAT TRANSPIRES DURING TUESDAY MORNING WHEN THE FIRST PART OF THE 
STORM IS UNDERWAY ACROSS THE AREA. CURRENTLY...MODEL SOUNDINGS 
INDICATING AN ALL SNOW EVENT THROUGH MID DAY TUESDAY EVEN FOR THE 
TREASURE VALLEY. BY AFTERNOON SNOW LEVELS AND WARMER AIR BEGIN TO 
WORK IN PRODUCING A MIXTURE OF SNOW AND RAIN AT LOWER ELEVATIONS BUT 
STILL ALL SNOW IN THE MOUNTAINS AND HIGHLANDS. HAVE GONE OUT WITH 
MULTIPLE HIGHLIGHTS FOR THIS ONE WITH POSSIBLE CHANGES OR 
ADJUSTMENTS ON THE EVENING SHIFT AS THE EVENT NEARS. AT ANY 
RATE...THE MOUNTAINS OF BAKER COUNTY...WEST CENTRAL MOUNTAINS AND 
OWYHEES LOOK TO GET JUST WHAT SANTA HAS PROMISED...A GOOD DUMPING OF 
SNOW! EVEN IN THE LOWER ELEVATIONS AND HIGHLANDS OF SOUTHEAST OREGON 
AND SOUTHWEST IDAHO...A GOOD AMOUNT OF SNOW IS EXPECTED TO FALL 
BEFORE WARMER AIR GETS HERE. THE REAL TRICK IS THE TREASURE VALLEY 
WHERE TEMPERATURE PROFILES OVERNIGHT ARE CRITICAL TO WHAT TYPE 
PRECIP AND HOW MUCH QPF FALLS. SEE SPECIFIC WSWBOI PRODUCTS FOR 
DETAILS ON SNOW AMOUNTS AND TIMING.       

LONG TERM...WEDNESDAY NIGHT THROUGH MONDAY...A WEAK SHORTWAVE WILL 
TRANSITION ACROSS NORTH IDAHO EARLY THURSDAY - CONTINUING CHANCES 
FOR PRECIPITATION ACROSS NORTHERN TERRAIN. ON FRIDAY...MODELS AGREE 
UPON AN AMPLIFIED UPPER RIDGE OVER THE PAC NW BUT LOCATION OF RIDGE 
AXIS REMAINS UNCERTAIN. THIS PROVES PROBLEMATIC IN FORECASTING THE 
LOCATION /AND THUS TIMING/ OF THE NEXT PACIFIC SYSTEM. FOR 
NOW...HAVE MAINTAINED SLIGHT CHANCE TO CHANCE POPS ACROSS SE OREGON 
AND HIGHER TERRAIN OF SW ID ON FRIDAY/SATURDAY. ID VALLEYS MORE 
LIKELY TO REMAIN DRY AND STABLE AND HAVE MINIMIZED POPS EAST OF 
BOISE. BY SUNDAY...CONFIDENCE DECREASES AS ECMWF MAINTAINS WRN CONUS 
RIDGE AND GFS DIGS PACIFIC TROUGH INLAND. THEREFORE...HAVE TRENDED 
SUNDAY/MONDAY TEMPERATURES AND POPS TOWARD CLIMO. 

AVIATION...PREDOMINANTLY VFR THROUGH 00Z WITH SCT-BKN CLOUD DECKS 
AROUND 10K TO 12K FT MSL. LIGHT SURFACE WINDS WITH MODERATE WESTERLY 
WINDS ALOFT. THIS EVENING...CEILINGS WILL THICKEN AND LOWER /FROM 
THE NW/ IN ADVANCE OF THE NEXT STORM SYSTEM. EXPECT WIDESPREAD IFR 
IN SNOW AND FOG...MOUNTAIN OBSCURATION...AND GUSTY SE-SW SURFACE 
WINDS AFTER 04Z /9 PM MST/ TONIGHT.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...WINTER STORM WARNING FROM 5 AM TO 5 PM MST TUESDAY FOR IDZ011-
     IDZ013-IDZ029-IDZ033.
     WINTER WEATHER ADVISORY FROM 5 AM TO 2 PM MST TUESDAY FOR IDZ012-
     IDZ014-IDZ015-IDZ016-IDZ028-IDZ030.
OR...WINTER WEATHER ADVISORY FROM 5 AM MST /4 AM PST/ TO 2 PM MST /1 
     PM PST/ TUESDAY FOR ORZ063.
     WINTER STORM WARNING FROM 5 AM TO 2 PM MST TUESDAY FOR ORZ064.
     WINTER STORM WARNING FROM 10 PM THIS EVENING TO 4 PM PST TUESDAY 
     FOR ORZ061-ORZ062.
&&

$$

WEATHER.GOV/BOISE

DISCUSSION...JB
AVIATION.....JT
PREV SHORT TERM...GS
PREV LONG TERM....MT/JS