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628
FXUS66 KSEW 210027
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
425 PM PST THU NOV 20 2014

.SYNOPSIS...THE AREA WILL BE BETWEEN SYSTEMS TONIGHT. A PACIFIC
STORM WILL IMPACT THE REGION ON FRIDAY AND FRIDAY NIGHT WITH LOCALLY
HEAVY RAIN AND GUSTY WINDS...ALONG WITH DECENT SNOWFALL IN THE
CASCADES FROM STEVENS PASS NORTHWARD. THE WEATHER PATTERN WILL
REMAIN WET AND UNSETTLED THIS WEEKEND WITH ANOTHER WEATHER SYSTEM
IMPACTING THE AREA ON SUNDAY. MUCH UNCERTAINTY EXISTS IN THE
FORECAST FROM MONDAY ONWARD.

&&

.SHORT TERM...THE RADAR IS SHOWING SOME SHOWERS CONTINUING...IN
AGREEMENT WITH THE MOST RECENT MODEL RUNS...SO I HAVE ADDED SHOWERS
INTO THE FORECAST FOR THIS EVENING. ANY REMAINING SHOWERS SHOULD
TAPER OFF OVERNIGHT.

ON FRIDAY AND FRIDAY NIGHT...A RELATIVELY STRONG FRONTAL SYSTEM
STEMMING FROM A DEEP LOW OVER THE ALEUTIAN ISLANDS WILL MOVE THROUGH
WESTERN WASHINGTON. THE FRONT WILL REACH THE COAST IN THE EARLY
EVENING AND MOVE INLAND OVERNIGHT. WIND ADVISORIES HAVE BEEN ISSUED
FOR THE NORTH AND CENTRAL COAST AS WELL AS THE SAN JUANS AND WESTERN
WHATCOM COUNTY. WINDS AHEAD OF THE FRONT WILL BE FROM THE SOUTH AND
THEN THE WINDS WILL SWITCH AROUND TO THE WEST AND WEAKEN SLIGHTLY AS
THE FRONT MOVES THROUGH.

A WINTER WEATHER ADVISORY FOR SNOW HAS BEEN ISSUED FOR THE CASCADES
OF KING COUNTY NORTHWARDS FOR ELEVATIONS ABOVE 4000 FEET...AS THERE
WILL BE QUITE A BIT OF PRECIPITATION WITH THE FRONT AND SNOW LEVELS
IN THE NORTH CASCADES WILL BE FAIRLY LOW. HIGHER SURFACE PRESSURE
EAST OF THE CASCADES WILL KEEP COOLER AIR FROM EASTERN WASHINGTON
FLOWING THROUGH THE PASSES AT LOWER LEVELS...LOWERING THE SNOW LEVEL
TO GROUND LEVEL AT STEVENS. NEARLY ALL OF THE PRECIPITATION AT
STEVENS ON FRIDAY AND FRIDAY NIGHT IS LIKELY TO BE IN THE FORM OF
SNOW. HOWEVER...SINCE THE MOUNT BAKER SKI AREA IS FURTHER WEST AND
IS NOT IMPACTED AS MUCH BY AIR FLOWING OVER FROM EASTERN
WASHINGTON...IT MAY SEE A MIX OF RAIN AND SNOW ON FRIDAY BEFORE
SWITCHING OVER TO SNOW LATER FRIDAY AS THE SNOW LEVELS DROP A BIT.

ON SATURDAY AND SATURDAY...MOISTURE BEHIND THE FRONT WILL CONTINUE
TO IMPACT THE AREA...SO I HAVE KEPT A GOOD CHANCE OF PRECIPITATION
IN THE FORECAST. SNOW LEVELS WILL LIKELY DROP TO BELOW 3000 FEET
ACROSS WESTERN WASHINGTON...SO CONTINUING SNOW AT STEVENS AND MOUNT
BAKER...AS WELL AS SNOWFALL AT WHITE PASS...HURRICANE RIDGE...AND MT
RAINIER...IS LIKELY. ACCUMULATIONS MAY APPROACH WINTER WEATHER
ADVISORY CRITERIA IN SOME AREAS.

ON SUNDAY...A SHORTWAVE TROUGH WILL IMPACT THE AREA AND BRING MORE
RAIN AND SNOW TO THE MOUNTAINS. SNOW LEVELS WILL REMAIN BELOW 3000
FEET SO A GOOD AMOUNT OF SNOW ACCUMULATION IS LIKELY IN THE
MOUNTAINS. JSMITH

&&

.LONG TERM...MONDAY THROUGH WEDNESDAY...BOTH THE GFS AND ECMWF SHOW
MOISTURE BEING TRANSPORTED TO THE PACIFIC NW VIA THE JET
STREAM...HOWEVER THE GFS HAS THE MOISTURE HITTING SW BC FOR THE MOST
PART AND US STAYING DRY...WHEREAS THE ECMWF HAS A WETTER SOLUTION
FOR US WITH THE MOISTURE HITTING US DIRECTLY. THEREFORE MUCH
UNCERTAINTY EXISTS IN THE FORECAST. THURSDAY LOOKS DRY WITH A RIDGE
BUILDING IN ON BOTH MODELS...BUT IT IS TOO EARLY TO TELL WITH MUCH
CERTAINTY. JSMITH

&&

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

&&

.AVIATION...WEST FLOW ALOFT WILL INCREASE TONIGHT AS A WEAK UPPER
LEVEL TROUGH MOVES E OF THE AREA AND A STRONG FRONTAL SYSTEM
APPROACHES THE COAST. THE AIR MASS IS ALREADY MOIST AND STABLE FROM
THE TROUGH...WITH SPOTTY SHOWERS AND CIGS MAINLY BKN-OVC030-060.
RAIN FROM THE APPROACHING FRONT WILL SPREAD INLAND AROUND 15Z. FROPA
ON THE COAST SHOULD BE AROUND 00Z...AND OVER PUGET SOUND AROUND 04Z.
GUSTY S-SE WINDS ARE EXPECTED TO DEVELOP ALONG THE COAST AND OVER
PARTS OF THE NORTH INTERIOR ON FRIDAY. CIGS AND VISIBILITY WILL
LOWER TO MVFR AFTER 15Z.

KSEA...THE MOIST AIR MASS OVER THE REGION WILL KEEP CIGS IN THE
BKN-OVC025-040 RANGE THROUGH 08Z. WINDS WILL REMAIN S 5-10 KT. RAIN
AND MVFR CONDITIONS WILL PREDOMINATE ON FRIDAY. KAM

&&

.MARINE...THE NEXT FRONTAL SYSTEM OVER THE NE PACIFIC WAS ALONG 135W
AT 22Z/2 PM. IT WILL APPROACH THE REGION TONIGHT THEN MOVE
ACROSS THE AREA FRIDAY/FRIDAY EVENING. MODELS REMAIN FAIRLY
CONSISTENT IN SHOWING GALES OVER THE COASTAL WATERS STARTING FRIDAY
AFTERNOON AND OVER THE EAST ENTRANCE AND NORTH INLAND WATERS MAINLY
FRIDAY AFTERNOON AND EVENING. SMALL CRAFT ADVISORY WINDS ARE
EXPECTED OVER THE REMAINING WATERS. FROPA ALONG THE COAST SHOULD BE
AROUND 00Z...THEN AROUND 04Z OVER PUGET SOUND. A WESTERLY PUSH
BEHIND THE FRONT SHOULD PRODUCE SCA WINDS THROUGH THE STRAIT AND
OVER THE N INLAND WATERS.

WESTERLY FLOW ALOFT OVER THE NORTHEAST PACIFIC SATURDAY THROUGH
MONDAY WILL BRING SEVERAL WEAKER SURFACE TROUGHS ACROSS THE AREA.
CONFIDENCE IN THE TIMING AND STRENGTH OF EACH OF THESE TROUGHS IS
LOW...BUT THEY COULD PRODUCE SOLID SCA WINDS AND PERHAPS SPOTTY
GALES. SWELLS IN THE 15-20 FOOT RANGE ARE FORECAST TO REACH THE
COAST SUNDAY. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY NORTH AND NORTH CENTRAL CASCADES.
     WIND ADVISORY NORTH AND CENTRAL COAST...THE SAN JUAN
     ISLANDS...AND WESTERN WHATCOM COUNTY.
PZ...GALE WARNING COAST...EAST ENTRANCE...NORTH INLAND WATERS.
     SMALL CRAFT ADVISORY ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS FORECAST DISCUSSION AT
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