FXUS66 KSEW 220551
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST MON DEC 21 2009

.SYNOPSIS...AN UPPER LEVEL LOW OVER NORTHWEST WASHINGTON WILL DROP 
SOUTHEAST INTO OREGON THROUGH TUESDAY. A COOLER AIR MASS OVER THE 
AREA SUPPORTS A MIX OF ISOLATED RAIN AND SNOW SHOWERS TO NEAR 
SEA LEVEL. A PUGET SOUND CONVERGENCE ZONE OVERNIGHT WILL FOCUS SOME 
OF THE SHOWER ACTIVITY INTO SNOHOMISH COUNTY WITH SOME LIGHT SNOW 
ACCUMULATION POSSIBLE. CLOUDS AND ISOLATED SHOWERS WILL LINGER INTO 
TUESDAY...THEN DRYING DEVELOPS LATER TUESDAY NIGHT AND WEDNESDAY 
WITH COLDER TEMPERATURES. HIGH PRESSURE WILL KEEP WESTERN WASHINGTON 
GENERALLY DRY THROUGH THE WEEKEND. BY MONDAY...A SYSTEM MAY BRING 
SOME LIGHT RAIN BACK INTO THE AREA.   

&&

.SHORT TERM...RADAR SHOWS A POORLY ORGANIZED PUGET SOUND CONVERGENCE 
ZONE EXTENDING FROM ADMIRALTY INLET AND WHIDBEY ISLAND INTO 
NORTHWEST SNOHOMISH COUNTY. THERE HAS BEEN SOME MIXED PRECIPITATION 
IN THE HEAVIER SHOWERS IN SNOHOMISH COUNTY THIS EVENING BUT WITHOUT 
A PERSISTENT CONVERGENCE BAND IT WILL REMAIN MARGINAL FOR 
ACCUMULATING SNOW. THIS THREAT IS HANDLED WELL IN CURRENT FORECAST 
PACKAGE. ELSEWHERE ACROSS TEH LOWLANDS THE SHOWERS REMAIN RATHER 
ISOLATED AND TRANSIENT FOR LITTLE THREAT OF ACCUMULATING SNOW.

THE MESO MODELS SUPPORT THE IDEA THAT THE PSCZ WILL REMAIN 
DISORGANIZED OVERNIGHT AND THEN BREAK UP AS THE FLOW SHIFTS TO 
OFFSHORE LATER TOMORROW. THIS WEAKENING IS LIKELY ASSOCIATED 
WITH A GRADUAL DROP-OFF OF WINDS AT ALL LEVELS AS THE UPPER LEVEL 
LOW SHIFTS SOUTHEAST. HOWEVER WITH THE RATHER UNSTABLE AIR MASS 
THERE IS ALWAYS A CHANCE THAT THE HEATING FROM THE PUGET SOUND AND 
STRAIT WILL HELP ORGANIZE SHOWERS TOWARD MORNING THAT COULD THEN 
DRIFT OVER LAND AREAS FOR A MIX OF RAIN AND SNOW SHOWERS IN THE 
MORNING. SO AM HESITANT TO DRY THINGS OUT TOO QUICKLY.

THE OTHER SHORT TERM CONCERN WILL BE THE TRANSITION TO COOLER 
OFFSHORE FLOW TOMORROW AND TOMORROW EVENING. THERE COULD BE A FEW 
SHOWERS ON THE LEADING EDGE OF THE COOLER AIR MASS AS IT SHIFTS 
SOUTWARD THROUGH PUGET SOUND TOMORROW EVENING. ANOTHER AREA THAT HAS 
SOME THREAT OF LINGERING SHOWER ACTIVITY IS THE NORTHEAST OLYMPIC 
PENINSULA WITH SOME UPSLOPE NORTHEASTERLY FLOW.

BY WEDNESDAY MORNING HIGHER PRESSURE IS BUILDING STRONGLY ALOFT AND 
AT THE SURFACE TO OUR EAST. BY FRIDAY MORNING THE LOW LEVEL EASTERLY 
FLOW OVER THE CASCADES REACHES 20 TO 30 KT AND THE UPPER LEVEL FLOW 
IS FROM THE NORTHWEST. THERE WILL ALSO BE A STRONG INVERSION NEAR 
THE CASCADE CREST AS THE WARM UPPER LEVEL RIDGE SHIFTS INLAND. THESE 
ARE ALL INGREDIENTS FOR SOME ENHANCED EASTERLY WINDS NEAR THE WEST 
SLOPES OF THE CASCADES. COLMAN

.LONG TERM...FROM PREVIOUS DISCUSSION...MODELS HOLD HIGH PRES OVER 
WA THROUGH AT LEAST FRIDAY FOR DRY WEATHER. THE ECMWF/CANADIAN 
MODELS MAINTAIN A BLOCKED HIGH OVER LOW PATTERN THROUGH THE WEEKEND 
WHICH WOULD CONTINUE THE DRY TREND. THE GFS IS THE ODD MODEL OUT 
TRYING TO BREAK SOME ENERGY THROUGH THE RIDGE. WE HAVE DISCOUNTED 
THIS MODEL GIVEN THE LARGE SCALE SPLIT WHICH SHOULD SEND SYSTEM WELL 
SWD INTO CA. LEFT SOME LOW POPS ALONG OUR COAST AND FAR SOUTH BORDER 
WITH OREGON TO REMAIN CONSISTENT AND HEDGE SLIGHTLY IN CASE THE GFS 
ENDS UP BEING RIGHT.

MERCER

&&

.HYDROLOGY...THE SKOKOMISH RIVER...WHICH CRESTED EARLIER TODAY JUST 
ABOVE FLOOD STAGE...CONTINUES TO FALL AND IS NOW SEVERAL INCHES 
BELOW FLOOD STAGE. THERE ARE NO OTHER HYDRO CONCERNS THROUGH THE 
WEEKEND. 

THERE IS NO THREAT OF RIVER FLOODING IN THE FORECAST AREA FOR THE 
NEXT WEEK...INCLUDING THE GREEN RIVER.

COLMAN

.AVIATION...STRONG POST FRONTAL ONSHORE FLOW TONIGHT WILL GRADUALLY 
RELAX...THEN TURN LIGHT NELY TUE AS HIGH PRES BUILDS OVER BRITISH 
COLUMBIA. CONVERGENCE ZONE OVER THE EAST ENTRANCE OF THE STRAIT 
EXTENDING INTO WESTERN SKAGIT COUNTY WILL REMAIN NEARLY STATIONARY 
AND DISSIPATE BETWEEN 09Z-15Z.  

WEAK UPPER LEVEL LOW MOVING THROUGH WESTERN WASHINGTON OVERNIGHT 
BRINGING A THREAT OF SHOWERS TO THE COAST AND SOUTHWEST INTERIOR. 

MULTIPLE CLOUD LAYERS OVER WESTERN WASHINGTON OVERNIGHT INTO TUESDAY 
MORNING WITH THE MAIN CLOUD DECKS NEAR 2000 FEET AND 4000-5000 FEET. 
2000 FOOT LAYER PREDOMINATELY BROKEN TO OVERCAST THROUGH THE MORNING 
HOURS BECOMING SCATTERED TUESDAY AFTERNOON. HIGHER CLOUD DECK 
SCATTERING OUT TUESDAY EVENING. FELTON

KSEA...CEILINGS NEAR 2000 FEET THROUGH TUESDAY MORNING WITH CEILINGS 
RISING TO THE 4000 TO 5000 FOOT RANGE LATE TUESDAY MORNING INTO THE 
EARLY AFTERNOON HOURS. NO PRECIPITATION IS EXPECTED AT THE TERMINAL 
WITH THE CONVERGENCE ZONE STAYING TO THE NORTH AND THE PRECIPITATION 
FROM THE UPPER LEVEL LOW REMAINING TO THE SOUTH. SOUTH WIND 10-14 KT 
WILL DIMINISH TO 7-10 KT AFTER 09Z THEN BECOME VARIABLE TUE MORNING. 
FELTON

&&

.MARINE...STRONG ONSHORE FLOW OVER THE WATERS WILL DIMINISH TONIGHT. 
HIGH PRES WILL BUILD OVER BRITISH COLUMBIA TUE THEN SLIDE SLOWLY SE 
INTO EASTERN WA LATE THU INTO FRI...GIVING LIGHT TO MODERATE 
OFFSHORE FLOW. SOME WEAK FRASER OUTFLOW IS EXPECTED TO IMPACT THE 
NORTH INLAND WATERS LATER TUE INTO WED. ALBRECHT/FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST AND WEST ENTRANCE TO THE STRAIT. 
     SMALL CRAFT ADVISORY FOR ROUGH BAR CONDITIONS GRAYS HARBOR.

$$

WEATHER.GOV/SEATTLE