Weather Service Report


506 
FXUS66 KSEW 020419
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT TUE PEP 1 2015

.SYNOPSIS...A LARGE UPPER LEVEL LOW WILL MOVE SLOWLY SOUTH OVER 
WESTERN WASHINGTON TONIGHT THROUGH THURSDAY. THE LOW WILL BRING 
SHOWERS AT TIMES TO THE AREA ALONG WITH A CHANCE OF THUNDERSTORMS 
MAINLY WEDNESDAY AFTERNOON AND EVENING. THE UPPER LEVEL LOW WILL 
BEGIN SHIFTING SOUTHEASTWARD ON FRIDAY FOR A GRADUAL DRYING AND 
WARMING TREND OVER THE WEEKEND. A WEAK SYSTEM MAY BRING A CHANCE OF 
SHOWERS EARLY NEXT WEEK. 

&&

.SHORT TERM...AN UPPER LEVEL LOW CENTERED ALONG THE CENTRAL B.C. 
COAST WILL DRIVE SOUTHEASTWARD TONIGHT AND MEANDER OVER THE PAC NW 
WEDNESDAY AND THURSDAY. THE AIR MASS WILL BECOME INCREASINGLY 
UNSTABLE WITH SHOWERS CONTINUING ACROSS THE AREA. AN EMBEDDED SHORT 
WAVE AND ASSOCIATED FRONTAL BAND IS PRODUCING SOME BETTER ORGANIZED 
RAIN BANDS THIS EVENING ACROSS WRN WA. A LULL IN SHOWERS IS 
PROBABLE LATER TONIGHT BEFORE THE NEXT SHORT WAVE AND MAIN UPPER  
LOW MOVE OVERHEAD.

THE -25C COLD POOL AT 500 MB MOVES OVER WRN WA WEDNESDAY AFTERNOON. 
LI'S DROP OFF TO -2 TO -3C INDICATING ENOUGH DIURNAL INSTABILITY 
AND A THREAT OF THUNDERSTORMS. VORTICITY ADVECTION WILL AID IN LIFT 
BUT OTHER LARGER SCALE FORCING MECHANISMS ARE NOT PRESENT. 
THEREFORE...THUNDERSTORMS SHOULD GENERALLY REMAIN ISOLATED EXCEPT 
CLOSER TO THE LOW NEAR THE COAST WHERE SCATTERED COVERAGE IS 
POSSIBLE. SHOWERS COULD BE BRIEFLY HEAVY. QPF AMOUNTS WILL VARY 
CONSIDERABLY THE NEXT COUPLE DAYS WITH THE MOUNTAINS PICKING UP THE 
MOST...OVER A HALF INCH THROUGH THURSDAY. HIGHER ELEVATIONS ABOVE 
ABOUT 5500 TO 6000 FEET COULD PICK UP A FEW INCHES OF SNOW WITH OVER 
6 INCHES POSSIBLE ON THE HIGHER PEAKS OVER THE CASCADES AND 
OLYMPICS. A SPECIAL WEATHER STATEMENT REMAINS IN EFFECT HIGHLIGHTING 
THIS CHANGE IN THE PATTERN WITH EARLY SEASON SNOWFALL POSSIBLE AT 
HIGHER ELEVATIONS. 

A FEW SHOWERS MAY LINGER INTO FRIDAY BUT THE UPPER TROUGH AXIS WILL 
START TO SHIFT EAST AWAY FROM THE REGION. DECREASING MOISTURE AND 
LIFT MEANS LESS SHOWER ACTIVITY AND A BIT MORE SUNSHINE.
TEMPERATURES WILL STAY BELOW AVERAGE ACROSS THE AREA THROUGH FRIDAY 
WITH HIGHS MAINLY IN THE MID 60S.

.LONG TERM...THE GFS ECMWF AND CANADIAN ALL AGREE THAT THE UPPER 
LEVEL LOW WILL BE OVER W OREGON 18Z FRIDAY AND WILL THEN HEAD 
GENERALLY E FRIDAY NIGHT...ENDING UP OVER SE OREGON 12Z SATURDAY. 
WITH DRIER N FLOW ALOFT DEVELOPING OVER W WA FRIDAY NIGHT...SHOWERS 
WERE PHASED OUT...LEADING TO A MOSTLY SUNNY DAY ON SATURDAY. MAX 
TEMPS WILL REBOUND...BUT JUST UP TO AROUND 70. 

THE DEPARTING LOW ALLOWS NW FLOW ALOFT OVER THE NE PACIFIC TO SPREAD 
IN OVER W WA SUNDAY AND MONDAY. THE MODELS ARE SHOWING MAYBE A 
COUPLE OF WEAK UPPER LEVEL SHORTWAVE TROUGHS REACHING W WA SUNDAY 
THROUGH MONDAY. THE MODELS JUST TEASE THE AREA WITH THE 0.01 PRECIP 
CONTOUR...SO I HAVE KEPT THE DRY FORECAST FOR THE TIME 
BEING...ALTHOUGH IT MAY BE TEMPORARY. CLOUDS WERE INCREASED FOR 
PARTLY SUNNY SKIES. A STRONGER SHORTWAVE REACHES THE AREA MONDAY 
NIGHT OR TUESDAY SO A CHANCE OF SHOWERS WAS ADDED FOR THIS PERIOD. 
KAM

&&

.AVIATION...SHOWERS CONTINUE TO LINGER BEHIND THE FRONTAL SYSTEM 
THAT WORKED THROUGH THE AREA TUESDAY. MULTIPLE CLOUD LAYERS BETWEEN 
2000 AND 10000 FEET. IN GENERAL CEILINGS 5000-7000 FEET EXCEPT NEAR 
2000 FEET IN THE VICINITY OF SHOWERS. AN UPPER LEVEL LOW OVER 
NORTHERN BRITISH COLUMBIA WILL SAG SOUTH TONIGHT WHILE A WEAK 
SECONDARY SURFACE FRONT WILL MOVE THROUGH THE AREA BY 12Z. EXPECT 
EVENING SHOWERS TO TRANSITION BACK TO RAIN AND LOWER CEILINGS AS 
THIS FEATURE WORKS INLAND. ABUNDANT LOW LEVEL MOISTURE AND LINGERING 
SHOWERS AND A CHANCE OF THUNDERSTORMS OVER MUCH OF THE AREA BY 
WEDNESDAY AFTERNOON AS THE UPPER LEVEL TROUGH CONTINUES TO WORK 
SOUTHWARD.  
 
KSEA...CEILINGS AOA 5000 FEET LOWERING DOWN TO 2000-3000 FEET AFTER 
09Z. MVFR CEILINGS CONTINUING THROUGH WEDNESDAY. THUNDERSTORMS ARE 
POSSIBLE WEDNESDAY AFTERNOON AS THE UPPER LEVEL LOW MOVES OVERHEAD. 
SOUTHERLY WINDS 8 TO 14 KNOTS. FELTON 

&&

.MARINE...SHOWERS WILL CONTINUE OVER AREAS WATERS THIS EVENING 
BEHIND THE FRONTAL SYSTEM THAT MOVED THROUGH EARLIER TODAY. AN UPPER 
LEVEL LOW OVER NORTHERN BRITISH COLUMBIA WILL MOVE SOUTH OVER THE 
PACIFIC NORTHWEST TONIGHT AND WEDNESDAY...HELPING BRING INSTABILITY 
AND A CHANCE OF THUNDERSTORMS TO THE COASTAL WATERS LATE TONIGHT AND 
INLAND WATERS BY WEDNESDAY AFTERNOON. A WEAK SURFACE FRONT WILL 
ALSO MOVE SOUTH INTO THE COASTAL WATERS LATER THIS EVENING BRINGING 
SMALL CRAFT ADVISORY SOUTHERLIES TO PARTS OF THE COASTAL WATERS AND 
THE PUGET SOUND AND HOOD CANAL. THE UPPER LEVEL TROUGH WILL LINGER 
OVER THE PACIFIC NORTHWEST INTO FRIDAY FOLLOWED BY WEAK HIGH 
PRESSURE LATER FRIDAY INTO SATURDAY. FELTON 

&&

.SEW WATCHES/WARNINGS/ADVISORIES... 
WA...NONE.              
PZ...SMALL CRAFT ADVISORIES NORTHERN AND CENTRAL OUTER COASTAL 
     WATERS...PUGET SOUND AND HOOD CANAL...GRAYS HARBOR BAR.

&&

$$

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