Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 222222
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
322 PM PDT FRI MAY 22 2015

.SYNOPSIS...A LARGE SURFACE HIGH OFFSHORE WILL MAINTAIN MODERATE TO
STRONG LOW LEVEL ONSHORE FLOW OVER WESTERN WASHINGTON THROUGH MID
NEXT WEEK. THIS WILL MAINTAIN THE PATTERN OF MORNING CLOUDS WITH
VARYING DEGREES OF PARTIAL AFTERNOON CLEARING. WEAK LOW PRESSURE
ALOFT WILL BRING A CHANCE OF SHOWERS...MAINLY OVER THE MOUNTAINS
AFTERNOON AND EVENINGS.

&&

.SHORT TERM...THE LARGE NE PACIFIC SURFACE HIGH WILL BE DOING ITS
DOMINANT THING FOR THE NEXT WEEK...MAINTAINING MODERATE TO STRONG
ONSHORE FLOW ACROSS W WA. THE ONSHORE FLOW DIRECTION WILL BE
PREDOMINANTLY SW SO LOW LEVEL MARINE MOISTURE WILL HAVE AN EASIER
TIME SPREADING INLAND UP THROUGH PUGET SOUND. SKIES WILL REMAIN
MOSTLY CLOUDY WHICH WILL KEEP TEMPERATURES CLOSE TO NORMAL MAY
RANGES. THERE WILL PROBABLY BE VARYING DEGREES OF PARTIAL CLEARING
FROM DAY-TO-DAY BUT OVERALL SKIES WILL REMAIN MOSTLY CLOUDY. THE
MARINE LAYER IS DEEP ENOUGH AND WITH A WEAK UPPER LEVEL TROUGH OVER
THE OFFSHORE WATERS IT SEEMS REASONABLE TO HANG ON TO THE CHANCE OF
DRIZZLE FOR THE WEST PART.

THE MAIN ISSUE THIS AFTERNOON HAS BEEN THE SHOWERS AND ISOLATED
THUNDERSTORMS OVER THE CASCADES HEADED VERY SLOWLY IN THE DIRECTION
OF THE LOWLANDS...AND SHOWERS OVER THE OLYMPICS MOVING N AND
CROSSING THE STRAIT OF JUAN DE FUCA ZONES. THERE IS STILL TIME FOR
THE SHOWERS TO WEAKEN AND DISSIPATE BEFORE REACHING THE
LOWLANDS...BUT I WILL BE CONSERVATIVE AND EXPAND THE CHANCE OF
SHOWERS OVER MORE OF THE LOWLANDS THIS EVENING.

THE WEAK UPPER LEVEL TROUGH JUST OFFSHORE WILL SHIFT OVER W WA ON
SATURDAY. THIS WILL SHIFT THE MID LEVEL FLOW OVER W WA FROM E-SE TO
SW OVER THE COAST AND S OVER THE INTERIOR. THE MAIN EFFECT SHOULD BE
TO RESTRICT CONVECTION TO THE CASCADES RATHER THAN HAVE CELLS
DRIFTING W OVER THE LOWLANDS. AN UPPER LEVEL LOW ALONG THE B.C.
COAST TODAY WILL DROP DOWN OVER SW B.C. SUNDAY AFTERNOON...SHIFTING
THE MID LEVEL FLOW TO W OVER W WA. THIS SHOULD END THE THREAT OF
THUNDERSTORMS FOR THE CASCADES...ALTHOUGH THERE COULD STILL BE SOME
SHOWERS RIGHT ALONG THE CREST.

THE 12Z ECMWF TAKES THE LOW SE OVER E WA MONDAY AFTERNOON WHILE THE
18Z GFS DROPS IT DOWN OVER W WA. THE 12Z GFS WAS SIMILAR TO THE
ECMWF SO IT APPEARS THERE COULD BE SOME UNCERTAINTY OVER THE NEXT
COUPLE OF DAYS REGARDING WHERE THE LOW GOES. THE ECMWF TRACK SHOULD
KEEP MOST SHOWERS OVER THE CASCADES AND MOST OF THE LOWLANDS
DRY...WHILE THE GFS WOULD SPREAD SHOWERS ACROSS MORE OF THE
LOWLANDS. THE FORECAST FOLLOWS THE 12Z GFS AND ECMWF. GFS MID-LEVEL
LAPSE RATES SUGGEST THE AIR MASS WILL BE TOO STABLE FOR
THUNDERSTORMS. KAM

.LONG TERM...GFS/ECMWF TAKE THE UPPER LEVEL LOW E ON TUESDAY WITH N
FLOW ALOFT DEVELOPING OVER W WA. THIS SHOULD END ANY THREAT OF
SHOWERS FOR THE LOWLANDS AND PROBABLY THE CASCADES AS WELL. THE
SURFACE RIDGE OVER THE NE PACIFIC WILL MAINTAIN THE ONSHORE FLOW
PATTERN NEXT WEEK FOR MOSTLY CLOUDY SKIES...BUT THE FLOW COULD
WEAKEN ENOUGH FOR A BIT MORE AFTERNOON SUNSHINE. KAM

&&

.AVIATION...NO MAJOR CHANGES TO THE BLOCKED UPPER LEVEL PATTERN
ARE EXPECTED OVER THE PAC NW THRU THIS WEEKEND. THE FLOW ALOFT
WILL GENERALLY BE LIGHT AND VARIABLE. THE TSTMS OVER THE CASCADES
WILL DISSIPATE AFTER 0300 UTC.

MEANWHILE...CIGS MAINLY IN THE 1-2K FT RANGE BLANKETED MUCH OF THE
LOWLANDS THIS AFTERNOON. THE MARINE LAYER WAS QUITE DEEP /TOPS
WERE RUNNING 5500 TO 6 FT/...SO NOT EXPECTING MUCH /IF ANY/
IMPROVEMENT THE REST OF TODAY. MVFR CIGS WILL PERSIST THRU
TONIGHT...ALTHOUGH THERE WILL BE AREAS OF IFR CIGS /500-1K FT/
AND VSBYS 3-5SM MAINLY OVER THE COAST. THE OLYMPICS AND CASCADES
WILL BE PARTIALLY OBSCD.

KSEA...CIGS SHOULD BE IN THE 2-3K RANGE THE REST OF TODAY...AND
WILL LOWER INTO THE 1-2K FT RANGE OVERNIGHT. MVFR CONDITIONS WILL
PERSIST THRU SATURDAY MORNING.

&&

.MARINE...
NO SIGNIFICANT CHANCES ARE ANTICIPATED IN THE WEATHER PATTERN
THROUGH THE MIDDLE OF NEXT WEEK. HIGH PRES WILL REMAIN OFFSHORE.
THIS COMBINED WITH LOWER PRES INLAND WILL RESULT IN CONTINUED
ONSHORE FLOW OF VARYING STRENGTH THROUGH NEXT WED.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IS IN EFFECT FOR THE CENTRAL AND EASTERN
     STRAIT OF JUAN DE FUCA.

&&

$$














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