FXUS66 KSEW 221125
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST TUE DEC 22 2009

.SYNOPSIS...SPOTTY LIGHT RAIN OR SNOW SHOWERS WILL FALL ACROSS 
WESTERN WASHINGTON TODAY AS COLDER AIR FILTERS INTO THE REGION FROM 
THE NORTH. AN UPPER RIDGE WILL KEEP THE REGION DRY WEDNESDAY THROUGH 
FRIDAY WHILE THE COLD AIR MASS REMAINS OVER THE AREA. THE UPPER 
RIDGE WILL WEAKEN AND MOVE EAST LATE THIS WEEKEND OR EARLY NEXT WEEK 
AND ALLOW A WEAK WEATHER SYSTEM TO REACH THE AREA.

&&

.SHORT TERM...A DEEP 5330 METER UPPER LEVEL LOW HAS BEEN TRACKING 
SOUTHEAST ACROSS WESTERN WASHINGTON DURING THE NIGHT. MOST OF THE 
SHOWER ACTIVITY DIRECTLY ASSOCIATED WITH THE UPPER LOW IS LOCATED 
OVER EXTREME SOUTHWEST WASHINGTON AND NORTHWEST OREGON -- IE JUST TO 
THE SOUTH OF OUR FORECAST AREA. IN OUR AREA THERE ARE SHOWERS IN THE 
REMNANTS OF THE WEAK PUGET SOUND CONVERGENCE ZONE THAT FORMED MONDAY 
EVENING AND ALONG THE NORTHEAST SLOPE OF THE OLYMPIC MOUNTAINS. A 
COLDER AIR MASS IS MOVING INTO THE REGION...BUT TEMPERATURES EARLY 
THIS MORNING ARE A LITTLE ABOVE FREEZING EXCEPT IN THE NORTHWEST 
INTERIOR.

THE UPPER LOW...WHICH WILL BE LOCATED OVER THE WEST SLOPES OF THE 
SOUTH WASHINGTON CASCADES BY DAYBREAK...WILL CONTINUE ITS 
SOUTHEASTWARD TRACK AND END UP OVER NEVADA LATE TODAY. FAIRLY DRY 
NORTHWEST FLOW WILL DEVELOP ALOFT OVER WESTERN WASHINGTON AND 
CONTINUE THROUGH TONIGHT AHEAD OF AN UPPER RIDGE OFFSHORE AS IT 
PROGRESSES ACROSS 130W. A 1035 MB SURFACE HIGH MOVING SOUTH THROUGH 
BRITISH COLUMBIA -- THE REFLECTION OF AN ARCTIC AIR MASS MIGRATING 
SOUTHWARD -- WILL PUSH COLDER AIR INTO WESTERN WASHINGTON TODAY AND 
TONIGHT.

SHOWERS TODAY WILL BE RATHER SPOTTY AND LIGHT...AMOUNTING TO ONLY A 
FEW HUNDREDTHS OF AN INCH WATER EQUIVALENT AT MOST WHERE SHOWERS DO 
OCCUR. THE ONE AREA WHERE PRECIPITATION AMOUNTS MIGHT BE A LITTLE 
HEAVIER IS ALONG THE EASTERN STRAIT OF JUAN DE FUCA...AS NORTHEAST 
LOW LEVEL FLOW UPSLOPES AGAINST THE NORTHEAST SIDE OF THE OLYMPIC 
MOUNTAINS. THE FORECAST FOR THAT ZONE WILL MENTION POSSIBLE 
ACCUMULATIONS OF 1 OR 2 INCHES THIS MORNING. FOR WESTERN WASHINGTON 
AS A WHOLE THE COLDER AIR MASS MOVING INTO THE REGION WILL BE 
FIGHTING WITH SOME DAYTIME WARMING...SO WE WILL GENERALLY WIND UP 
WITH RAIN OR SNOW SHOWERS AND AMOUNTS TOO LIGHT TO RESULT IN SNOW 
ACCUMULATIONS.

PRECIPITATION SHOULD BE OVER BY THE END OF THE DAY BUT CLOUDS WILL 
PROBABLY NOT BE TOO QUICK TO CLEAR OUT TONIGHT. TEMPERATURES TONIGHT 
WILL FALL INTO THE MID 20S TO LOWER 30S.

THE UPPER RIDGE WILL SLOWLY PROGRESS EASTWARD WEDNESDAY THROUGH 
THURSDAY...WHILE OFFSHORE FLOW CONTINUES AT LOW LEVELS. THE RIDGE 
WILL KEEP THE FORECAST AREA DRY AND CLOUD COVER SHOULD GIVE WAY TO 
PARTLY SUNNY AND EVENTUALLY MOSTLY SUNNY SKIES. WITH LESS CLOUD 
COVER WEDNESDAY NIGHT...WE SHOULD SEE THE COLDEST TEMPERATURES OF 
THIS SPELL EARLY THURSDAY MORNING WITH MOST SPOTS IN THE 20S. 
MCDONNAL

.LONG TERM...THE UPPER RIDGE WILL KEEP WESTERN WASHINGTON DRY ON 
FRIDAY. CONFIDENCE IN THAT PART OF THE EXTENDED PERIOD IS HIGH. 
MODEL AGREEMENT FOR SATURDAY THROUGH MONDAY IS AGAIN NOT TOO STRONG 
AS MODEL SOLUTIONS ARE WAFFLING ON HOW LONG THE UPPER RIDGE WILL 
KEEP THE AREA DRY. THE CURRENT FORECAST...WHICH KEEPS THE AREA DRY 
THROUGH SUNDAY...MAY BE A LITTLE OPTIMISTIC. HOWEVER ANY SYSTEMS 
THAT DO REACH THE AREA THIS WEEKEND OR EARLY NEXT WEEK WILL BE 
WEAK. MCDONNAL 

&&

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

THE SKOKOMISH RIVER...WHICH CRESTED JUST ABOVE FLOOD STAGE ON MONDAY 
...WILL CONTINUE TO FALL SLOWLY. MCDONNAL

&&

.AVIATION...CONVERGENCE BOUNDARY BETWEEN THE INCOMING NORTHERLY FLOW 
AND THE RESIDUAL WEAKENING ONSHORE FLOW HAS PRODUCED SHOWERS ALONG A 
LINE FROM JUST N OF KBFI WILL CONTINUE TO MOVE SOUTHWEST AND 
EVENTUALLY DISSIPATE BY LATE MORNING. ALONG THE BOUNDARY CEILINGS 
NEAR 1000 FEET WITH VISIBILITIES 3-5SM IN PRECIPITATION. 
TEMPERATURES WARM ENOUGH AND PRECIPITATION NOT HEAVY ENOUGH FOR 
STRAIGHT SNOW SHOWERS. SOME MIXED RAIN AND SNOW POSSIBLE. THE EXCEPT 
TO THIS WILL BE NEAR PORT ANGELES WHERE THE UPSLOPE FLOW INTO THE 
NORTHERN OLYMPICS WILL ENHANCE THE PRECIPITATION ENOUGH THIS MORNING 
FOR BRIEF SNOW SHOWERS. OUTSIDE OF THE CONVERGENCE SC DECK AROUND 
5000 FEET STILL PRESENT OVER THE AREA EXCEPT UP NEAR KBLI WHERE 
SKIES ARE CLEARING OUT. UPPER LEVEL RIDGE BUILDING OFFSHORE WITH 
NORTHERLY FLOW ALOFT OVER THE ENTIRE AREA THIS AFTERNOON. FLOW IN 
THE LOWER LEVELS IS WEAK AND WITH THE SURFACE GRADIENTS LIGHT IT 
WILL BE HARD TO GET RID OF THE LOW LEVEL MOISTURE ALREADY IN PLACE. 
WILL KEEP CEILINGS IN THE 5000 FOOT RANGE SEATTLE SOUTHWARD THIS 
AFTERNOON INTO THE EVENING HOURS. FELTON

KSEA...CEILINGS NEAR 1000 FEET THIS MORNING WITH LIGHT RAIN SHOWERS 
UNTIL AROUND 14Z. NO SNOW ACCUMULATION EXPECTED. CEILINGS LIFTING UP 
TO 2000-3000 FEET THIS AFTERNOON AND TO AROUND 5000 FEET BY LATE 
AFTERNOON/EARLY EVENING. SOUTHERLY WIND 4 TO 8 KNOTS SWITCHING TO 
NORTHERLY MIDDAY. FELTON 

&&

.MARINE...A WEAK TROUGH OF LOW PRESSURE OVER THE NORTHERN PORTION OF 
THE AREA WILL DISSIPATE TODAY. HIGH PRESSURE WILL BUILD OVER 
SOUTHERN BRITISH COLUMBIA TODAY...THEN SLIDE SLOWLY SOUTHEAST INTO 
EASTERN WASHINGTON THURSDAY AND FRIDAY. A SPLITTING FRONT WILL 
APPROACH THE COAST ON SATURDAY. SWELL IN THE LOW TEENS THIS MORNING 
ALONG THE COAST WILL SLOWLY SUBSIDE DOWN TO SINGLE DIGITS BY LATER 
TONIGHT. 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.
 
$$

WWW.WEATHER.GOV/SEATTLE

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WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.