Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 232151
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
252 PM PDT SAT MAY 23 2015

.SYNOPSIS...A SURFACE RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE
FLOW OVER WESTERN WASHINGTON THROUGH NEXT WEEK. THIS WILL KEEP SKIES
MOSTLY CLOUDY AND TEMPERATURES MILD THROUGH MID WEEK. WEAKER ONSHORE
FLOW AFTER MID WEEK WILL ALLOW MORE AFTERNOON SUNSHINE AND A MINOR
WARMING TREND. WEAK LOW PRESSURE ALOFT WILL BRING A CHANCE OF
SHOWERS MAINLY OVER THE CASCADES DURING THE AFTERNOON AND EVENINGS.

&&

.SHORT TERM...AN AREA OF WEAK SHOWERS CAUSED BY A WEAK MID LEVEL
SHORTWAVE TROUGH MIGRATED UP OVER THE N INTERIOR THIS AFTERNOON AND
SHOULD DISSIPATE BY LATE AFTERNOON.

CONVECTION OVER THE CASCADES HAS BEEN SLOW STARTING THIS
AFTERNOON...BUT BY 1 PM HAD STARTED TO FIRE UP E OF THE CREST NEAR
THE CANADIAN BORDER. BASED ON CLOUD COVER AND A RECENT HRRR MODEL IT
LOOKS LIKE THERE MAY ONLY BE MINOR ACTIVITY OVER THE N CASCADES NEAR
THE CREST THIS AFTERNOON AND EVENING. WATER VAPOR IMAGERY SHOWS AN
UPPER LEVEL TROUGH EXTENDING SE ACROSS NW WA...FROM THE LOW OVER
HAIDA GWAII. AS THIS FEATURE SHIFTS SLOWLY E...IT IS TURNING THE MID
LEVEL STEERING FLOW OVER W WA FROM E-SE TO SOUTHERLY. IT HAS ALREADY
TURNED THE SHOWERS OVER THE N INTERIOR TO THE NORTH...AND WILL ALSO
PREVENT ANY CELLS OVER THE CASCADES FROM DRIFTING W OVER THE W WA
LOWLANDS THIS EVENING.

THE HAIDA GWAII UPPER LEVEL LOW WILL MOVE DOWN OVER SW B.C. ON
SUNDAY. THIS WILL SHIFT THE MID LEVEL FLOW OVER W WA TO WESTERLY
WHICH WILL CONFINE ANY CONVECTION DEVELOPING OVER THE CASCADES TO
THE CREST AND E SLOPES. JUST A CHANCE OF SHOWERS SUNDAY AFTERNOON
NEAR THE CASCADE CREST LOOKS ABOUT RIGHT.

THE UPPER LEVEL LOW MOVES OVER E WA ON MONDAY AND TUESDAY WITH
NORTHERLY MID LEVEL FLOW DEVELOPING OVER W WA. THE AIR MASS WILL
REMAIN UNSTABLE OVER THE CASCADES FOR MORE AFTERNOON AND EVENING
CONVECTION.

FOR THE REST OF W WA...THE SURFACE RIDGE ALONG 135W ABOUT 500 NM
OFFSHORE WILL MAINTAIN MAINLY MODERATE SW OR WSW ONSHORE FLOW ACROSS
W WA THROUGH TUESDAY. THIS WILL KEEP THE MARINE LAYER OVER THE
LOWLANDS FAIRLY DEEP WITH MOSTLY CLOUDY SKIES CONTINUING. AT BEST
THERE MAY BE SOME SMALL LATE AFTERNOON SUN BREAKS. TEMPERATURES WILL
REMAIN NEAR NORMAL WITH LOWS IN THE 50S AND AND HIGHS IN THE 60S. KAM

.LONG TERM...THE E WA UPPER LEVEL LOW WILL FINALLY MOVE E OVER ID
WEDNESDAY...ALLOWING AN OFFSHORE UPPER LEVEL RIDGE TO SLIDE A LITTLE
CLOSER TO THE COAST. THE APPROACH OF THE RIDGE WILL ALLOW THE AIR
MASS TO WARM...BUT THE GREATER IMPACT WILL BE THE CHANGE TO ONSHORE
FLOW. ONSHORE PRESSURE GRADIENTS WILL WEAKEN A BIT AND WILL SHIFT TO
A WESTERLY DIRECTION WHICH MAKES IT MORE DIFFICULT TO SPREAD STRATUS
ACROSS PUGET SOUND. THIS WILL RESULT IN MORE AFTERNOON CLEARING FROM
WEDNESDAY ONWARD WITH MAX TEMPS OVER THE INTERIOR WARMING INTO THE
70S. THE AIR MASS OVER THE CASCADES WILL REMAIN UNSTABLE FOR THE
STANDARD AFTERNOON AND EVENING CONVECTION. KAM

&&

.AVIATION...A WEAK UPPER LEVEL LOW ALONG THE B.C. COAST WILL MOVE SE
OVER WA ON SUNDAY. THIS SYSTEM WILL INCREASE ONSHORE FLOW AND DEEPEN
THE MARINE LAYER. WEAKLY UNSTABLE ATMOSPHERE ALOFT THROUGH THIS
EVENING BUT THUNDERSTORMS ARE NOT EXPECTED. MOIST AT ALL LEVELS THIS
EVENING...THEN DRYING IN THE MID/UPPER LEVELS LATER TONIGHT AND
SUNDAY EXCEPT OVER THE CASCADES. LOW LEVEL MOISTURE AND A DEEP
MARINE LAYER WILL CONTINUE...RESULTING IN WIDESPREAD MVFR CIGS BY
LATER TONIGHT AND SUNDAY MORNING. CIGS WILL AGAIN LIFT TO A MIX OF
MVFR AND VFR AFTER MIDDAY BUT BREAK OUT IS UNLIKELY AT MOST
TERMINALS.

KSEA...CEILINGS 2500 FT RISING TO 3000 TO 3500 FT THIS EVENING. MVFR
CIGS 1-2K FT DEVELOPING AGAIN AFTER AROUND MIDNIGHT. SIMILAR
CONDITIONS SUNDAY WITH CIGS GRADUALLY LIFTING TO 2500-3500 FT IN THE
AFTERNOON BUT BREAK OUT IS NOT EXPECTED. W WIND 4-7 KT...BECOMING
S-SW 3-5 KT AFTER 06Z. DTM

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
SMALL CRAFT ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING
IN THE STRAIT TONIGHT THROUGH MONDAY NIGHT. ONSHORE FLOW WILL WEAKEN
TUE THROUGH THU WITH LIGHTER FLOW INCLUDING THE STRAIT. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT THROUGH LATE
     TONIGHT.

&&

$$

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