Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152240
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
340 PM PDT FRI AUG 15 2014

.SYNOPSIS...AN UPPER LEVEL LOW WILL KEEP SKIES MOSTLY CLOUDY TONIGHT
AND SATURDAY...WITH A CHANCE OF SHOWERS MAINLY OVER THE MOUNTAINS.
WEAK HIGH PRESSURE WILL BUILD OVER THE AREA SATURDAY AFTERNOON AND
SUNDAY AS THE LOW MOVES EAST...ALLOWING CLOUDS TO SLOWLY DECREASE
THROUGH SUNDAY MORNING. MOSTLY SUNNY AND WARMER WEATHER IS EXPECTED
SUNDAY AFTERNOON AND MONDAY. ANOTHER UPPER LEVEL TROUGH WILL BRING
MORE CLOUDS AND A CHANCE OF SHOWER FROM TUESDAY ONWARD.

&&

.SHORT TERM...THE UPPER LEVEL LOW OVER THE REGION CONTINUES TO MOVE
SLOWLY NE AND WAS CENTERED OVER E WA AT 21Z/2 PM. SHOWER ACTIVITY
OVER W WA HAS BEEN MINIMAL SO FAR THIS AFTERNOON...ALTHOUGH A SMALL
AREA OF SHOWERS OVER VANCOUVER ISLAND WAS HEADED TOWARD THE N COAST
AT 245 PM. THE LATEST RUC13 MODEL SHOWED NW STEERING WINDS OVER THE
CASCADES LATE THIS AFTERNOON AND EVENING WITH SUB-ZERO LIFTED
INDICES RESTRICTED TO JUST ALONG THE CREST OF THE N CASCADES. GIVEN
THOSE FACTORS THE CHANCE OF THUNDERSTORMS WAS DROPPED FROM THE
CASCADES THIS EVENING. THE LOWER AIR MASS REMAINS UNSTABLE ENOUGH TO
KEEP A CHANCE OF MOUNTAIN SHOWERS TONIGHT.

THE UPPER LEVEL LOW WILL BE OVER W MT BY 12Z/5 AM SATURDAY WITH A
WEAK UPPER LEVEL RIDGE BUILDING BRIEFLY OVER W WA BEHIND IT. FLOW
ALOFT WILL BE WEAK W-NW. THE AIR MASS WILL STILL BE RATHER
MOIST...SO IT WILL TAKE A WHILE FOR THE CLOUD COVER TO DIMINISH.
SATURDAY MORNING WILL BE CLOUDY...BUT THERE SHOULD BE ENOUGH
CLEARING FOR PARTLY SUNNY SKIES SATURDAY AFTERNOON.

AN UPPER LEVEL TROUGH IS OFFSHORE ALONG ROUGHLY 140W THIS AFTERNOON
WITH AN ASSOCIATED WEAK FRONTAL SYSTEM MOVING E TOWARD 130W AT 21Z/2
PM. THE MODELS ARE IN GENERAL AGREEMENT THAT THE WEAK FRONTAL SYSTEM
WILL FALL APART OFFSHORE ON SATURDAY. HOWEVER THE MUCH WEAKENED
UPPER LEVEL TROUGH WILL SLIDE E OVER W WA BY 12Z/5 AM SUNDAY MORNING
BRINGING A LITTLE LEFTOVER MID/HIGH LEVEL MOISTURE WITH IT. THE
TROUGH MOISTURE COMBINED WITH LEFTOVER STRATUS WILL KEEP SKIES
MOSTLY CLOUDY SUNDAY MORNING...BUT SKIES SHOULD CLEAR OFF FOR A
SUNNY AFTERNOON.

MONDAY WILL BE A TRANSITION DAY AS THE WEAK UPPER LEVEL TROUGH
SHIFTS E AND S OF THE AREA...AND WESTERLY FLOW ALOFT OFFSHORE PUSHES
A UPPER LEVEL SHORTWAVE TROUGH TOWARD THE REGION. THE AIR MASS
SHOULD BE PRETTY DRY AT THIS POINT SO THE ADDED SUNSHINE SHOULD PUSH
MAX TEMPS UP TO THE LOWER 80S IN THE WARMEST SPOTS. THIS COULD BE
THE WARMEST DAY THIS WEEK. KAM

.LONG TERM...RECENT MODEL RUNS HAVE BEEN SPEEDING UP THE ARRIVAL OF
THE OFFSHORE UPPER LEVEL SHORTWAVE TROUGH EMBEDDED IN W FLOW ALOFT.
THE LATEST GFS/ECMWF AGREE THAT THE UPPER LEVEL TROUGH WILL MOVE
OVER THE COAST 12Z/5 AM TUESDAY MORNING. SO FAR THE MODELS ARE
MOSTLY DRY WITH THE TROUGH...WITH JUST INCREASED CLOUD COVER OVER
THE AREA AND SOME PRECIP OVER THE MOUNTAINS. THE CLOUD COVER FOR
TUESDAY HAS BEEN NUDGED UP WITH A CHANCE OF SHOWERS FOR THE COAST
AND MOUNTAINS.

THE GFS AND ECMWF ARE IN POOR AGREEMENT FROM TUESDAY NIGHT ONWARD
WITH REGARD TO MOISTURE AND PRECIP. BOTH MODELS MAINTAIN NW FLOW
ALOFT OVER W WA BUT THE GFS 12Z AND 18Z RUNS ARE QUITE WET WHILE THE
12Z ECMWF IS MAINLY DRY. THE ECMWF SOLUTION IS PREFERRED BUT FOR NOW
I WILL LEAVE THE COMPROMISE CHANCE OF SHOWERS IN THE FORECAST FOR
EACH DAY. KAM

&&

.AVIATION...WEAK NWLY FLOW ALOFT WILL CONTINUE AS AN UPPER LOW MOVES
NE OUT OF THE AREA AND A WEAK UPPER RIDGE MOVES IN. LOW LEVEL
ONSHORE FLOW WILL CONTINUE. THE AIR MASS IS MOIST AND SLIGHTLY
UNSTABLE BUT WILL DRY AND STABILIZE A BIT OVERNIGHT AND INTO
SATURDAY. A SLIGHT CHANCE OF ISOLATED TSTORMS REMAINS THIS EVENING
FOR THE CASCADES.

VFR/MVFR CIGS TODAY WILL DROP TO MVFR OR IFR LATE TONIGHT AND THEN
RISE TO VFR IN MOST LOCATIONS LATE SATURDAY MORNING. VIS WILL LIKELY
DROP TO MVFR OR LOWER IN PRONE LOCATIONS AS WELL.

KSEA...CIGS WILL LIKELY LOWER TO MVFR...AND POSSIBLY IFR...LATE
TONIGHT. VIS WILL LIKELY STAY UNRESTRICTED AT SEATAC...BUT THERE IS
A SLIGHT CHANCE THAT IT WILL LOWER IF MARINE FOG ADVECTS INTO THE
AREA. LIGHT SURFACE WINDS WILL REMAIN FROM THE SOUTHWEST THROUGH
THIS EVENING BEFORE SWITCHING TO LIGHT AND VARIABLE OVERNIGHT AND
THEN NORTHERLY SATURDAY AFTERNOON. JSMITH

&&

.MARINE...SMALL CRAFT ADVISORY TONIGHT FOR THE CENTRAL AND EASTERN
STRAIT IS A MARGINAL CASE...BUT EXPANDED THE ADVISORY TO THE EASTERN
STRAIT BECAUSE HIGH RESOLUTION MODELS SHOW MARGINAL SCA STRENGTH
WINDS THERE TOO. HIGH PRES OFFSHORE AND LOWER PRES E OF THE CASCADES
WILL RESULT IN ONSHORE FLOW OF VARYING STRENGTH THROUGH THE MIDDLE
OF NEXT WEEK. SOME STRENGTHENING OF THE FLOW WILL OCCUR THIS WEEKEND
THROUGH TUE. THERE IS A POSSIBILITY OF WINDS REACHING GALE FORCE
OVER THE CENTRAL OR EASTERN STRAIT OF JUAN DE FUCA SOMETIME EARLY
NEXT WEEK. JSMITH

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT FROM 3 PM THIS AFTERNOON
     TIL 2 AM SATURDAY FOR THE CENTRAL AND EAST STRAIT OF JUAN DE
     FUCA.

&&

$$

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











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