FXUS66 KSEW 141724
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 AM PST MON DEC 14 2009

.SYNOPSIS...RAIN WILL DEVELOP THIS AFTERNOON AS A WARM FRONT 
APPROACHES FROM THE SOUTHWEST. PRECIPITATION WILL START AS SNOW IN 
THE NORTH INTERIOR AND HOOD CANAL AREA AND MAY NOT CHANGE TO RAIN 
BEFORE THE PRECIPITATION ENDS LATE TONIGHT. HEAVY SNOW WILL FALL IN 
THE MOUNTAINS THROUGH TUESDAY. A SERIES OF FRONTAL SYSTEMS WILL 
AFFECT THE REGION THROUGH LATE THIS WEEK.

&&

.SHORT TERM...SOME ICY SIDEWALKS AND ROADS NOTED THIS MORNING BY THE 
STAFF COMING IN...SO ISSUED A SHORT TERM FORECAST TO COVER FREEZING 
DRIZZLE OR FREEZING LIGHT RAIN. TEMPERATURES SHOULD RISE ENOUGH THIS 
MORNING TO END THE THREAT OF FREEZING PRECIPITATION. HIGHS TODAY 
WILL BE IN THE UPPER 30S TO AROUND 40...EXCEPT IN THE FAR NORTH AND 
POSSIBLY ALONG HOOD CANAL WHERE COLDER AIR WILL LINGER.

THE WARM FRONT MOVING IN THIS AFTERNOON WILL SPREAD PRECIPITATION 
OVER THE AREA TONIGHT. THIS MAY BEGIN AS RAIN AND SNOW BUT SHOULD 
CHANGE TO RAIN MOST PLACES. AGAIN THE EXCEPTIONS WILL BE THE HOOD 
CANAL AREA...WHERE COLD AIR IS LIKELY TO REMAIN TRAPPED...AND THE 
FAR NORTH...WHERE COLD AIR OUTFLOW FROM THE FRASER RIVER CANYON WILL 
CONTINUE ALL NIGHT. THERE IS ALREADY A WINTER WEATHER ADVISORY FOR 
THE HOOD CANAL AREA TONIGHT AND WILL PROBABLY ADD WESTERN WHATCOM TO 
THAT ADVISORY LATER THIS MORNING. BOTH AREAS COULD SEE 1-3 INCHES OF 
SNOW. PRECIPITATION WILL END LATE TONIGHT.

ANOTHER SYSTEM IS DUE TUESDAY NIGHT. THIS ONE SHOULD SCOUR OUT ANY 
REMAINING COLD AIR THOUGH THERE IS A CHANCE THE PRECIPITATION IN THE 
FAR NORTH COULD START AS SNOW AGAIN. BY WEDNESDAY MORNING THE 
PATTERN SHOWS A RETURN TO NORMAL...SYSTEMS EVERY FEW DAYS BRINGING 
RAIN...WITH THE SNOW LEVEL SOMEWHERE AROUND 4000 FEET.

UNTIL THEN THE CASCADES AND OLYMPICS WILL GET SNOW...HEAVY AT 
TIMES...AND A WINTER STORM WARNING IS IN EFFECT. BURKE
    
.LONG TERM...PREVIOUS DISCUSSION...NOT MUCH CHANGE TO THE LONG TERM 
PACKAGE. WE STAY IN AN ACTIVE PATTERN THROUGH THU...THEN A POSSIBLE 
BREAK IN THE WEATHER ON FRI AS AN UPPER LEVEL RIDGE BUILDS OVER THE 
PAC NW. THE RIDGE MAY AMPLIFY OVER THE WEEKEND BUT BOTH THE GFS AND 
ECMWF SHOW MOISTURE CLIPPING THE REGION...MAINLY AFFECTING THE COAST 
OR NORTH PART OF THE CWA. THE RIDGE MAY BREAK DOWN EARLY NEXT WEEK 
AS A TROUGH DIGS OFFSHORE AND SHIFTS INLAND. THE GFS IS A BIT MORE 
PROGRESSIVE WITH THIS NEXT WAVE AND WILL HOLD POPS/TEMPS NEAR CLIMO. 
33 

&&

.HYDROLOGY...PREVIOUS DISCUSSION...A TRANSITION TO A WETTER PATTERN 
WILL BEGIN TONIGHT THROUGH AT LEAST THE MIDDLE OF THE WORK WEEK AS A 
SERIES OF PACIFIC FRONTAL SYSTEMS SWEEP THROUGH WESTERN WASHINGTON. 
INITIALLY SNOW LEVELS REMAIN LOW WITH HEAVY SNOW EXPECTED TONIGHT 
THROUGH TUESDAY AFTERNOON IN THE MOUNTAINS. SNOW LEVELS WILL RISE TO 
AROUND 4000 FT ON WEDNESDAY AS ANOTHER SYSTEMS CLIPS THE REGION. THE 
FLOOD POTENTIAL LOOKS LOW AT THE MOMENT AS RIVERS HAVE BEEN RUNNING 
LOW WITH LITTLE TO NO PRECIP ACROSS THE REGION SINCE LATE NOV. NO 
FLOOD CONCERNS FOR THE GREEN RIVER. 

MODELS TRY TO BUILD AN UPPER LEVEL RIDGE ACROSS THE PAC NW LATE THIS 
WEEK INTO THE WEEKEND...WITH A BREAK IN THE WEATHER POSSIBLE. THE 
RIDGE WILL BREAK DOWN EARLY NEXT WEEK BUT EXACT TIMING REMAINS 
UNCLEAR AS BOTH THE GFS AND ECMWF DIFFER. 33

&&

.AVIATION...LOW LEVEL COLD AND DRY AIR FLOWING OUT THROUGH THE 
FRASER IS LIMITED TO WESTERN WHATCOM COUNTY TODAY WHILE THE 
REMAINDER OF THE AREA IS SEEING WEAK WARM ADVECTION OVER THE TOP OF 
A COLD GROUND. WITH THE EXCEPTION OF MVFR STRATUS OVER THE FAR NORTH 
INTERIOR...MOST OF WESTERN WASHINGTON IS SEEING ABUNDANT LOW STRATUS 
WITH CIGS LESS THAN 010 AND AREAS OF DRIZZLE AND FOG REDUCING VIS TO 
1-3SM. WITH SOME INCREASE IN LOW LEVEL E/SE FLOW AHEAD OF THE 
INCOMING WARM FRONT...EXPECT SURFACE VIS TO IMPROVE SOMEWHAT THIS 
AFTERNOON BUT CIGS TO REMAIN NEAR 020.

WARM ADVECTION PRECIPITATION...NOW SPREADING ONSHORE...WILL PUSH 
INLAND 23Z-00Z THIS AFTERNOON. EXPECT WIDESPREAD IFR VIS AND CIG 
DURING THE WARM ADVECTION EVENT TONIGHT. EXPECT SNOW TO BE LIMITED 
TO THE AREA AROUND THE FRASER OUTFLOW IN THE N INTERIOR TONIGHT AND 
AROUND HOOD CANAL THROUGH ABOUT MIDNIGHT. HOWEVER...PRECIPITATION 
AND WET BULB COOLING MAY LOCALLY DRAG THE SNOW LEVEL DOWN TO BELOW 
1000 FT THIS EVENING AS THE PRECIPITATION BEGINS. STATISTICAL 
GUIDANCE DEWPOINT FORECASTS AND SNOW ACCUMULATION FORECASTS OFF THE 
GFS SUGGESTS THAT THERE COULD BE SNOWFALL ACCUMULATIONS AT SEVERAL 
INTERIOR TERMINALS KSEA NORTHWARD. AT THIS POINT WILL FOLLOW 
MESOSCALE GUIDANCE BUT ONLY WITH MODERATE CONFIDENCE.

KSEA...CEILINGS HAVE IMPROVED TO AROUND 900 FT AND WILL LIKELY LIFT 
TO AROUND 2000 FT BY NOON. PRECIPITATION WILL START AROUND 23Z. 
GIVEN HIGH STATIC STABILITY AND RAIN FALLING INTO A COLD BOUNDARY 
LAYER EXPECT CIGS 005-007 WITH 3SM -RA BR BY 02Z AND CONTINUING 
THROUGH 12Z TUE. PRECIPITATION COULD BEGIN AS WET SNOW...BUT WITH 
TEMPERATURES ABOVE FREEZING...EXPECT ANY BRIEF WET SNOW TO NOT 
ACCUMULATE. LIGHT WIND WILL TREND 090-120 7-9 KT THROUGH TONIGHT. 
WINDS AT 050 WILL INCREASE TO S 40-50 KT AROUND 00Z. THE WARM FRONT 
WILL PASS AROUND 10Z TONIGHT WITH PRECIPITATION BECOMING SPOTTY AND 
VIS IMPROVING. BUT CIGS REMAINING 015-020 THRU TUE. ALBRECHT  

&&

.MARINE...HIGH PRESSURE OVER SOUTHERN BRITISH COLUMBIA WILL WEAKEN 
TODAY. A WARM FRONT WILL MOVE INTO THE AREA FROM THE SOUTHWEST AND 
PASS THROUGH THE INLAND WATERS LATE TONIGHT. E TO SE WINDS WILL 
INCREASE TO GALE FORCE AHEAD OF THE WARM FRONT LATE THIS AFTERNOON 
OR THIS EVENING IN THE COASTAL WATERS AND AT THE EAST ENTRANCE LATER 
THIS EVENING. SMALL CRAFT ADVISORY OUTFLOW IS EXPECTED IN THE NORTH 
INLAND WATERS THROUGH TONIGHT. 

THE NEXT FRONT...ASSOCIATED WITH A 988 MB LOW MOVING INTO NORTHERN 
VANCOUVER ISLAND WILL MOVE THROUGH TUESDAY NIGHT WITH GALES POSSIBLE 
AGAIN OVER THE COASTAL WATERS BEGINNING TUESDAY AFTERNOON.  

A SERIES OF FAST MOVING FRONTAL SYSTEMS WILL MOVE ACROSS THE WATERS 
THROUGH THE REMAINDER OF THE WEEK WITH SMALL CRAFTS OVER PORTIONS OF 
THE INTERIOR AT TIMES AND NEAR GALE WINDS AT TIMES OVER THE COASTAL 
WATERS AND SEAS BETWEEN 12 AND 19 FEET. THE PASSAGE OF THESE FRONTAL 
SYSTEMS ARE DIFFICULT TO TIME AT THIS POINT SO THE LATTER HALF OF 
THE FORECAST PERIOD ARE BROAD BRUSHED. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...A WINTER WEATHER ADVISORY IS IN EFFECT FOR THE HOOD             
     CANAL AREA FOR MONDAY AFTERNOON THROUGH MONDAY EVENING.

     A WINTER STORM WARNING IS IN EFFECT FOR THE OLYMPICS AND        
     CASCADES FOR MONDAY AFTERNOON THROUGH TUESDAY. 

     AN AVALANCHE WATCH IS IN EFFECT FOR THE OLYMPICS AND
     CASCADES FOR MONDAY NIGHT.

PZ...GALE WARNING COAST AND EAST ENTRANCE TO THE STRAIT.
     SMALL CRAFT ADVISORY WEST ENTRANCE TO THE STRAIT...CAMANO       
     ISLAND TO POINT ROBERTS AND ADMIRALTY INLET.

$$