Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 141144 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST SUN DEC 14 2014

.SYNOPSIS...AN UPPER RIDGE OVER WESTERN WASHINGTON TODAY WILL SHIFT
EAST TONIGHT. A SERIES OF WEAK SPLITTING SYSTEMS WILL MOVE ACROSS
THE AREA FOR MOST OF THE WORK WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS LOW CLOUDS AND FOG OVER THE
SOUTHWEST INTERIOR...LOWER CHEHALIS VALLEY AND GRAYS HARBOR AREA AS
WELL AS OVER THE PUGET SOUND TO NEAR SEATTLE. IN ADDITION TO THE LOW
CLOUDS A THIN BAND OF HIGH CLOUDS IS MOVING THROUGH THE AREA. PATCHY
FOG HAS REDUCED THE VISIBILITIES TO AS LOW AS 1/4 OF A MILE. WITH
THE DIFFERENCES IN CLOUD COVER THERE IS A WIDE VARIETY OF
TEMPERATURES AT 3 AM RANGING FROM NEAR FREEZING AT BELLINGHAM TO THE
MID 40S AT HOQUIAM.

LITTLE CHANGE TO THE FORECAST THIS MORNING. UPPER LEVEL RIDGE
REMAINING OVER THE AREA TODAY. CROSS CASCADE GRADIENT HAS GONE
NEGATIVE OVERNIGHT AND WILL BECOME MORE NEGATIVE AS THE DAY WEARS
ON. SANDPOINT PROFILER ALREADY SHOWS EASTERLY WINDS...ALL BE IT
FAIRLY LIGHT...RIGHT ABOVE THE GROUND. INCREASING OFFSHORE FLOW
TODAY WITH HIGH PRESSURE BUILDING OVER EASTERN WASHINGTON AND LOWER
PRESSURE OFFSHORE WITH THE APPROACHING WEAKENING FRONT WILL
DISSIPATE THE LOW CLOUDS AND FOG OVER THE SOUTHERN PORTION OF THE
AREA LEAVING MOSTLY SUNNY SKIES. HIGHS WILL BE IN THE MID 40S TO
LOWER 50S.

SYSTEM OFFSHORE NEAR 135W ALREADY SHOWING SIGNS OF SPLITTING WITH
THE PARENT LOW MOVING NORTHWEST INTO THE INTERIOR OF ALASKA WHILE
THE JET PUSHES THE SOUTHERN PORTION OF THE SYSTEM TOWARDS SOUTHERN
OREGON/NORTHERN CALIFORNIA. UPPER LEVEL RIDGE OVER WESTERN
WASHINGTON MOVING EAST TONIGHT BUT WITH THE OFFSHORE SYSTEM SLOWING
AND SPLITTING STAYING WITH THE DRY FORECAST FOR THE AREA. LOWS WILL
MOSTLY BE IN THE 30S.

WHAT IS LEFT OF THE FRONT TRYING TO MOVE INLAND ON MONDAY BUT WILL
NOT HAVE MUCH SUCCESS. BY MONDAY THE FRONT IS VERY NEGATIVELY
TILTED AND WEAK. BEST CHANCES FOR RAIN WILL BE ALONG THE COAST AND
IN THE OLYMPICS. EVEN WITH THE FRONT WEAKENING THE SYSTEM IS CLOSE
ENOUGH TO KEEP A CHANCE OF RAIN IN THE FORECAST FOR THE INTERIOR.
HIGHS WILL BE MOSTLY IN THE MID AND UPPER 40S.

UPPER LEVEL TROUGH DIGGING OFFSHORE MONDAY NIGHT INTO TUESDAY WITH
THE WEAK FRONT DISSIPATING OVER WESTERN WASHINGTON. ONCE AGAIN THE
BEST CHANCES FOR RAIN WILL BE ALONG THE COAST AND IN THE OLYMPICS
WITH JUST A CHANCE OF RAIN OVER THE INTERIOR. RAINFALL AMOUNTS WILL
BE RELATIVELY LIGHT. LOWS WILL MOSTLY BE IN THE MID 30S TO LOWER 40S
WITH HIGHS IN THE MID AND UPPER 40S ON TUESDAY.

.LONG TERM...EXTENDED MODELS INCONSISTENT EARLY ON WITH THE GFS
MAINTAINING THE SPLIT FLOW ALOFT WHILE THE ECMWF CONSOLIDATES THE
FLOW A BIT AND BRINGS A SYSTEM INTO WESTERN WASHINGTON WEDNESDAY.
GFS ENSEMBLES ARE QUITE DEFINITIVE MAINTAINING THE SPLIT. THIS
COMBINED WITH THE LACK OF CONSISTENCY BETWEEN THE MODELS WILL STAY
WITH JUST CHANCE POPS FOR WEDNESDAY. THERE IS BETTER AGREEMENT ON
THURSDAY WITH A WEAK RIDGE PASSING THROUGH THE AREA FOLLOWED BY A
MORE ORGANIZED SYSTEM THURSDAY NIGHT INTO FRIDAY. THE MODELS HAVE
HAD THIS SOLUTION FOR A FEW RUNS NOW SO HAVE GONE AHEAD AND BUMPED
THE POPS INTO THE LIKELY CATEGORY FOR THURSDAY NIGHT AND WILL
CONTINUE THE LIKELY POPS FOR FRIDAY. FLOW ALOFT BECOMING MORE
CONSOLIDATED NEXT WEEKEND BUT AT THIS POINT IT LOOKS LIKE THE NEXT
SYSTEM EMBEDDED IN THE FLOW WILL HOLD OFF UNTIL SATURDAY NIGHT OR
SUNDAY. FELTON

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED DURING THE NEXT 7 DAYS.

&&

.AVIATION...AN UPPER LEVEL RIDGE IS OVER WESTERN WASHINGTON THIS
MORNING WITH NORTHEAST TO EAST FLOW ALOFT. LOW LEVEL OFFSHORE FLOW
WILL CONTINUE AS SURFACE HIGH PRESSURE STRENGTHENS OVER INTERIOR
BRITISH COLUMBIA.

THE COMBINATION OF STRENGTHENING OFFSHORE FLOW AND HIGH LEVEL
MOISTURE FILTERING INTO THE REGION MAKES THE FOG AND STRATUS
FORECAST CHALLENGING. WITH LOW LEVEL MOISTURE ALREADY IN
PLACE...WILL KEEP IT IN THE FORECAST THROUGH LATE MORNING...THEN
SCATTER IT OUT AROUND 18Z AS EASTERLY GRADIENT HELPS TO ERODE IT.
STRONGER CROSS-CASCADE GRADIENT LATER TODAY AND TONIGHT ALONG WITH
HIGH CLOUDS SHOULD LIMIT ITS RETURN TONIGHT.

KSEA...KEEPING IFR CEILINGS AND/OR VSBYS IN THE FORECAST THROUGH
18Z...THEN SCT-BKN HIGH CLOUDS THEREAFTER. BETTER EASTERLY GRADIENT
AND HIGH CLOUDS SHOULD DECREASE POTENTIAL FOR FOG TONIGHT. WINDS
EAST TO NORTHEAST 5 TO 10 KNOTS THROUGH THE FORECAST PERIOD.

&&

.MARINE...OFFSHORE FLOW WILL PERSIST THROUGH MONDAY AS A SURFACE
RIDGE STRENGTHENS OVER SOUTHERN BRITISH COLUMBIA AND A WEAK SYSTEM
APPROACHES THE OFFSHORE WATERS. THIS WILL RESULT IN THE POTENTIAL
FOR GALE FORCE WINDS OVER THE WEST ENTRANCE TO THE STRAIT OF JUAN DE
FUCA AND ADJACENT COASTAL WATERS TONIGHT. OFFSHORE FLOW WILL WEAKEN
LATE MONDAY AS A SERIES OF SPLITTING FRONTS WEAKEN THE SURFACE RIDGE
OVER THE INLAND NORTHWEST. FORECAST MODELS BRING A STRONGER SYSTEM
TO THE WATERS AROUND THURSDAY. 27

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORIES ARE IN EFFECT FOR THE WEST ENTRANCE
     AND CENTRAL STRAIT OF JUAN DE FUCA AND MUCH OF THE COASTAL
     WATERS.

     GALE WATCH IN EFFECT THIS EVENING THROUGH LATE TONIGHT FOR THE
     NEARSHORE WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT TO 10
     NM AND THE WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA.

&&

$$

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