FXUS65 KBOI 141655
AFDBOI
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
955 AM MST MON DEC 14 2009
.DISCUSSION...HAVE CANCELLED DENSE FOG ADVISORY FOR THE TREASURE
VALLEY. VSBY CURRENTLY ALL ABOVE ONE MILE. TEMPERATURE FORECAST
DIFFICULT AGAIN TODAY WITH EXTENSIVE STRATUS DECK OVER THE AREA.
FOCUS NOW TURNS TO LARGE STORM INBOUND TO THE WEST COAST AT THIS
TIME. STORM TARGETING AREAS NORTH OF SAN FRANCISCO AND THEN OUR
AREA. WINTER STORM WATCHES WILL MOST LIKELY BE UPGRADED ON AFTERNOON
PACKAGE AND OTHERS ADDED. NO UPDATES AT THIS TIME.
&&
.AVIATION...IFR FOG NEAR KEUL AND KONO IS EXPECTED TO DISSIPATE BY
18Z THIS MORNING. ELSEWHERE...PREDOMINANTLY VFR THROUGH 00Z WITH
SCT-BKN CLOUD DECKS BETWEEN 8K AND 11K FT MSL. LIGHT SURFACE WINDS
WITH MODERATE WESTERLY WINDS ALOFT.
TONIGHT...A POTENT WINTER STORM WILL MOVE INTO THE FORECAST AREA
FROM THE NORTHWEST. EXPECT WIDESPREAD IFR IN SNOW AND GUSTY SE-SW
SURFACE WINDS AFTER 04Z /9 PM MST/ TONIGHT.
&&
.PREV DISCUSSION...TODAY THROUGH TUESDAY...VERY STRONG AND WET
SYSTEM HEADED OUR WAY FOR LATE TONIGHT INTO TUESDAY NIGHT. MAIN
PROBLEM WITH THIS WILL BE DECIDING WHEN THE VALLEYS WILL TRANSITION
FROM SNOW TO RAIN. WITH THE COLD AIR STILL AROUND FROM THE RECENT
ARCTIC AIRMASS...THERE IS NO DOUBT THAT THE PRECIPITATION WILL BEGIN
LATE MONDAY NIGHT AND EARLY TUESDAY AS ALL SNOW. HOWEVER...THE WARM
AIR ADVECTION WITH THIS SYSTEM IS QUITE STRONG...AND THE LOWER
ELEVATIONS WILL TURN FROM SNOW TO A MIX TO ALL RAIN DURING THE DAY
TUESDAY. EXACTLY WHEN AND WHERE WILL DETERMINE HOW DEEP THE SNOW
GETS AND WHAT AREAS NEED TO BE IN A WINTER WEATHER WATCH..OR OTHER
HEADLINE. FOR NOW...HAVE ADDED ZONE 33 TO THE WATCH THAT WAS ISSUED
DURING THE DAY SUNDAY. THIS ZONE APPEARS SET TO GET BETWEEN 2 AND 6
INCHES OF SNOW TUESDAY BEFORE TURNING TO A SLOSHY MESS TUESDAY
NIGHT. OTHERWISE HAVE LEFT THE WATCH ALONE SO THE DAYSHIFT CAN
ASSESS THE LATEST GUIDANCE BEFORE MAKING FINAL DECISIONS LATER
TODAY. SOME OF THE LOWER ELEVATIONS MAY NEED A SHORT ADVISORY TO
DESCRIBE THE MODERATE SNOWFALL EXPECTED EARLY TUESDAY. FOR TEMPS
TODAY TRIED TO ACCOUNT FOR THE LOCATIONS OF THE CURRENT SNOW PACK IN
LOWERING GUIDANCE TOWARD SOMETHING HOPEFULLY MORE REALISTIC.
HOWEVER...WINDS MAY MIX IN WARMER AIR AND RUIN THIS EFFORT...BUT
THIS IS NOT THE FAVORED SOLUTION. AS THE NEXT SYSTEM APPROACHES
TONIGHT...SE WINDS WILL INCREASE IN THE SNAKE RIVER VALLEY AND HELP
KEEP TEMPS FROM DROPPING TOO FAR...EVEN WITH THE SNOW ON THE GROUND.
FOR TUESDAY...TEMPS WILL WARM INTO THE EVENING HOURS AS THE
PREVIOUSLY REFERENCES WARM AIR ADVECTION CONTINUES...MEANING THE
SO-CALLED DAYTIME MAX MAY OCCUR TOWARD 8 PM LOCAL TIME. ONE LAST
NOTE...THE PREVIOUS SYSTEM PASSED BY TO OUR SOUTH AND WE DID NOT END
UP GETTING AS MUCH PRECIPITATION THROUGHOUT OUR AREA AS WE HAD
EXPECTED. THIS SYSTEM IS HEADED RIGHT FOR US...AND WE WILL GET
NOTICEABLY MORE PRECIPITATION THIS TIME. THE MAIN QUESTION FOR THE
POPULATED VALLEYS WILL BE THE TIMING OF TRANSITION FROM SNOW TO RAIN.
LONG TERM...TUESDAY NIGHT THROUGH SUNDAY...MOIST SOUTHWEST FLOW WILL
CONTINUE TUESDAY NIGHT AND WEDNESDAY. FORECAST CHALLENGE REMAINS TO
BE PRECIPITATION CHANGING OVER TO RAIN EVENT IN LOWER ELEVATIONS.
MODELS HAVE CONSISTENTLY SHOWN WARM ADVECTION PATTERN THE PAST
SEVERAL RUNS FOR VALLEY RAIN OVERNIGHT TUESDAY AND WEDNESDAY. FLOW
WILL TRANSITION TO MORE OF A ZONAL PATTERN WITH ANOTHER IMPULSE TO
EXTEND PRECIP CHANCES MAINLY ACROSS NORTHERN ZONES THROUGH LATE
THURSDAY. BEYOND THAT AN UPPER LEVEL RIDGE WILL AMPLIFY OVER THE
PACIFIC NORTHWEST BEGINNING THURSDAY NIGHT FOR DRIER CONDITIONS THAT
SUGGEST A RETURN TO INVERTED TEMPERATURES AND ASSOCIATED AVIATION
FORECAST CHALLENGES THROUGH THE WEEKEND.
&&
.BOI WATCHES/WARNINGS/ADVISORIES...
ID...WINTER STORM WATCH FROM THIS EVENING THROUGH TUESDAY AFTERNOON
FOR IDZ033.
WINTER STORM WATCH FROM LATE TONIGHT THROUGH TUESDAY EVENING
FOR IDZ011-IDZ013-IDZ029.
OR...WINTER STORM WATCH FROM THIS EVENING THROUGH TUESDAY
AFTERNOON FOR ORZ062.
&&
$$
WEATHER.GOV/BOISE
DISCUSSION...GS
PREV DISCUSSION...SP
LONG TERM....CD
AVIATION.....MT
