Weather Service Report


499 
FXUS66 KSEW 280542
AFDSEW

AREA FORECAST DISCUSSION 
NATIONAL WEATHER SERVICE SEATTLE WA
940 PM PST FRI FEB 27 2015

.SYNOPSIS...NORTHERLY FLOW ALOFT AND LOW LEVEL OFFSHORE FLOW HAVE 
DEVELOPED AND ARE USHERING A MUCH DRIER AIR MASS INTO WESTERN 
WASHINGTON TONIGHT. AS A RESULT...SHOWERS HAVE TAPERED OFF...AND 
SKIES WILL CONTINUE CLEARING OVERNIGHT. A WEAK SYSTEM WILL MOVE 
THROUGH SUNDAY NIGHT AND MONDAY...WITH DRY WEATHER AGAIN TUESDAY AND 
WEDNESDAY.

&&

.SHORT TERM...UPPER LOW OVER OVER NW ORE THIS EVNG MOVE SOUTH TO THE 
BAY AREA BY LATE SAT...ALLOWING A LONGWAVE RIDGE AXIS NEAR 140W TO 
EXPAND INTO THE PAC NW. SFC HIGH PRES HAS ALREADY SETTLED INTO THE 
INTERIOR OF B.C...WITH THE KBLI-CYWL GRADIENT GETTING AS STRONG AS 
-12 MB AS OF 03Z. AS A RESULT...A SHORT-LIVED FRASER OUTFLOW EVENT 
IS UNDERWAY. THE AIR MASS COMING OUT OF B.C. IS NOT TERRIBLY COLD. 
HOWEVER...THE AIR MASS IS DRY...WITH THE DEWPOINT AT KBLI NOW DOWN 
TO 23F. DRIER AIR AND LARGE-SCALE SUBSIDENCE WILL SPREAD ACROSS THE 
FCST AREA OVERNIGHT AND EARLY SAT MRNG...CAUSING THE FEW REMAINING 
SPRINKLES TO DISSIPATE WITH SKIES CLEARING FROM NORTH TO SOUTH. 

HIGH PRESSURE OVER B.C. WILL MOVE OUT EAST OF THE ROCKIES ON 
SUNDAY...WITH PRESSURE GRADIENTS OVER WRN WA RELAXING ON SAT NGT. 
WINDS WILL BECOME MUCH LIGHTER. 

THE FLOW ALOFT WILL BACK TO NW ON SUNDAY AS A WEATHER SYSTEM 
APPROACHES FROM THE NORTHWEST. THIS WEATHER SYSTEM WILL DROP DOWN 
FROM THE NW SUN NIGHT AND MON...BRINGING CLOUDS. THE ECMWF AND NAM 
SHOW RAIN ALL AREAS WITH THIS FEATURE BUT THE GFS JUST HAS SPOTTY 
PRECIPITATION AND THE GEM IS DRY. THE FORECAST IS A BLEND OF THESE 
MODELS FOR NOW WHICH MEANS MOSTLY CLOUDY WITH A CHANCE OF RAIN.      
HANER

.LONG TERM...FROM PREVIOUS DISCUSSION...AN UPPER RIDGE WILL BE OVER 
THE NORTHEAST PACIFIC TUESDAY AND WEDNESDAY...GIVING DRY NORTHERLY 
FLOW ALOFT TO WESTERN WASHINGTON. THERE WILL BE SOME BREEZY NORTH 
WINDS TUESDAY AS A SURFACE HIGH MOVES INTO SOUTHERN BRITISH 
COLUMBIA...THEN LESS WIND WEDNESDAY AS THE HIGH SHIFTS SOUTHEAST. 

MODELS SHOW THE UPPER RIDGE WEAKENING A BIT THURSDAY WITH A WARM 
FRONT MOVING OVER THE TOP. THIS WILL BRING SOME CLOUDS TO THE AREA 
AND PERHAPS A CHANCE OF RAIN...WITH THE BEST CHANCES COAST AND FAR 
NORTH. A COLD FRONT WILL APPROACH FROM THE WEST ON FRIDAY. THIS DAY 
COULD WIND UP BEING DRY BUT A CHANCE OF RAIN CANNOT BE COMPLETELY 
RULED OUT FOR NOW. SCHNEIDER 

&&

.HYDROLOGY...FLOODING IS UNLIKELY FOR THE NEXT 7 DAYS.

&&

.AVIATION...AN UPPER LEVEL LOW OVER OREGON WILL CONTINUE TRACKING 
SOUTH TONIGHT. THE FLOW ALOFT OVER WESTERN WA IS NORTHERLY. LIGHT 
SHOWERS ARE LIMITED TO THE SOUTHERN PORTION OF THE CWA THIS 
EVENING...MAINLY SOUTH OF SEATAC AS THIS SYSTEM PULLS FARTHER SOUTH. 
MEANWHILE MODERATE NORTH TO NE SURFACE WINDS WILL SCOUR OUT THE LOW 
LEVEL CLOUDS WITH SKIES BECOMING CLEAR FROM NORTH TO SOUTH TONIGHT. 
ALTHOUGH KCLM WILL SEE A CEILING WITH NE UPSLOPE WINDS. PATCHY 
SHALLOW FOG WILL BE LIMITED TO KOLM OVERNIGHT. EXPECT VFR CONDITIONS 
ON SAT WITH OFFSHORE FLOW. 33

KSEA...N WINDS WILL PREVAIL TONIGHT TO 10-15 KT. SKIES CLEARING BY 
09Z WITH VFR CONDITIONS EXPECTED. 33

&&

.MARINE...NORTH TO NORTHEAST FLOW WILL CONTINUE ACROSS WESTERN WA 
TONIGHT AS A 1008 MB SURFACE LOW TRACKS SOUTH ALONG THE SOUTHERN 
OREGON COAST. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR ALL WATERS. 
HIGH PRESSURE OVER INTERIOR B.C. WILL MAINTAIN OFFSHORE ON SATURDAY. 
A WEAK SURFACE TROUGH WILL CROSS THE WATERS ON SUN FOR BRIEF ONSHORE 
WINDS. NORTH TO NE FLOW WILL DEVELOP MONDAY NIGHT THROUGH TUESDAY AS 
ANOTHER SURFACE RIDGE BUILDS OVER B.C. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS. 

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT 
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML



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