Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 011008
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT FRI MAY 01 2015

.SYNOPSIS...A WEAK FRONT BRUSHING WASHINGTON WILL BRING CLOUDS INTO
THE AREA THIS MORNING WITH PARTIAL AFTERNOON SUNSHINE. THERE MAY BE
A FEW STRAY SHOWERS ACROSS THE EXTREME NORTH. AN UPPER RIDGE MOVES
OVER THE REGION SATURDAY AND SUNDAY BRINGING MOSTLY SUNNY SKIES AND
WARMER TEMPERATURES. A FRONT WILL APPROACH THE REGION MONDAY WITH
INCREASING CLOUDS. LOW PRESSURE WILL BRING SHOWERS AND COOLER
TEMPERATURES BACK INTO WESTERN WASHINGTON MONDAY NIGHT THROUGH
WEDNESDAY.

&&

.SHORT TERM...IR SATELLITE IMAGE SHOWS A BAND OF MAINLY MID AND HIGH
CLOUDS WITH THE UPPER LEVEL FRONT MOVING THROUGH WRN WA EARLY THIS
MORNING. THE LAGGING SURFACE FRONT WILL DISSIPATE BEFORE REACHING
THE COAST BUT STATUS WILL INCREASE OVER MUCH THE AREA INCLUDING THE
INTERIOR DURING THE MORNING. RADAR IS ONLY PICKING UP A FEW SHOWERS
NEAR THE N COAST AND ANY BRIEF SHOWER ACTIVITY WILL BE CONFINED TO
THE EXTREME N. RESIDUAL MOISTURE WILL LINGER INTO THE AFTERNOON BUT
THE WLY FLOW PATTERN SUGGESTS PARTIAL SUNSHINE IS POSSIBLE OVER THE
INTERIOR LATE IN THE DAY. ONSHORE FLOW WILL KEEP TEMPERATURES IN
CHECK WITH HIGHS REMAINING IN THE UPPER 50S TO MID 60S.

MODELS HAVE TRENDED MORE PROGRESSIVE WITH THE TROUGH OVER BRITISH
COLUMBIA WHICH WILL BE SHIFTING EASTWARD ON SATURDAY. THE GFS40 NOW
SHOWS 500 MB HEIGHTS RISING OVER 5600M WITH A LIGHT NW SURFACE FLOW
PATTERN. ANY MORNING STRATUS SHOULD REMAIN CONFINED TO THE COAST AND
STRAIT WITH MOSTLY SUNNY SKIES ELSEWHERE. HIGHS WILL TREND WARMER IN
THIS PATTERN WITH LOW 60S COAST/N INTERIOR AND MID TO UPPER
60S OVER THE REMAINDER OF THE INTERIOR.

HIGH PRESSURE ALOFT STRENGTHENS ON SUNDAY WITH THE RIDGE AXIS
DIRECTLY OVER THE REGION. LOW LEVEL GRADIENTS REMAIN LIGHT NWLY
WHICH SHOULD AGAIN KEEP ANY MORNING STRATUS LIMITED THE
COAST/STRAIT. PLENTY OF SUNSHINE EXPECTED THROUGHOUT WRN WA WITH
HIGHS POSSIBLY REACHING THE LOW 70S AROUND PARTS OF PUGET SOUND AND
THE SW INTERIOR.

.LONG TERM...MONDAY WILL BE A TRANSITION DAY AS SW FLOW ALOFT
DEVELOPS AHEAD OF AN APPROACHING UPPER TROUGH. THE STRONGER ONSHORE
PUSH WILL PROBABLY WAIT UNTIL MONDAY NIGHT WITH MARINE CLOUDS
SURGING INLAND. EXTENDED MODELS ARE IN GOOD AGREEMENT ON BRINGING
THE UPPER LOW ACROSS THE AREA TUESDAY AND WEDNESDAY. THIS WOULD BE
AN UNSTABLE SHOWERY PATTERN WITH COOLER TEMPERATURES. THE GFS40
INDICATES LI`S AROUND -1 TO -2C DURING TUESDAY/WEDNESDAY PERIOD SO
THERE IS A THREAT OF THUNDERSTORMS JUST ABOUT ANYWHERE. THIS WILL
PROBABLY NEED TO BE INTRODUCED TO THE FORECAST SOON. MERCER

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH OVER W WA AT 09Z/2
AM...EMBEDDED IN W FLOW ALOFT...WILL CONTINUE MOVING E THIS MORNING.
SOMEWHAT MOIST W FLOW ALOFT WILL CONTINUE THROUGH 00Z/5 PM THEN
GRADUALLY TURN NW THROUGH 12Z/5 AM SATURDAY AS AN UPPER LEVEL RIDGE
APPROACHES THE REGION.

FAIRLY GOOD VFR CONDITIONS PREDOMINATE THIS MORNING WITH MAINLY
MID/HI CLOUDS...BASES 10000 FT AND HIGHER. GFS/NAM CONTINUE TO
INSIST THAT A BKN LAYER AROUND 5000 FT WILL FORM OVER THE AREA AFTER
12Z/5 AM...BUT THERE IS REALLY NO SIGN OF THIS SO FAR. HRRR MODEL
DOES NOT SUPPORT THE BKN050 LAYER. IR IMAGERY SHOWS CLOUDS OFFSHORE
WITH TOPS AROUND 700 MB BASED ON TEMPERATURE SAMPLING. THESE CLOUDS
COULD ADVECT INLAND THIS MORNING...REACHING THE CENTRAL SOUND AREA
LATER THIS MORNING SIMILAR TO THE GFS LAMP GUIDANCE. OVERALL
CONDITIONS WILL REMAIN VFR TODAY...EXCEPT FOR SOME SPOTTY MVFR CIGS
ALONG THE COAST AND OVER THE SW INTERIOR THIS MORNING.

KSEA...VFR CONDITIONS WITH JUST CIRRUS OVER THE TERMINAL THROUGH
19Z. SURFACE WINDS WILL BE NE 4-8 KT. A SCT-BKN040-050 LAYER COULD
FORM LATER THIS MORNING. SURFACE WINDS WILL BECOME N-NW 5-10 KT
AFTER 18Z. KAM

&&

.MARINE...A SURFACE HIGH OVER THE NE PACIFIC COMBINED WITH LOWER
PRESSURE OVER E WA WILL MAINTAIN W-NW ONSHORE FLOW THROUGH THIS
WEEKEND. THE ONSHORE FLOW WILL GRADUALLY INCREASE THIS MORNING
THROUGH THIS EVENING BEHIND THE UPPER LEVEL SHORTWAVE TROUGH THAT IS
EXITING THE REGION THIS MORNING. MESOSCALE GUIDANCE SHOWS THE WINDS
RISING TO 15-25 KT IN THE STRAIT THIS AFTERNOON AND TO 20-30 KT
FRIDAY EVENING. THERE IS STILL A SMALL CHANCE FOR A GALE IN THE
STRAIT TONIGHT...BUT AT THIS POINT ALL MESOSCALE GUIDANCE KEEPS THE
STRAIT BELOW GALE. ONLY THE GFS UIL-BLI 00Z GRADIENT...WHICH AT 4.5
MB SEEMS A LITTLE OVERDONE...SUGGESTS A GALE.

NW WINDS OVER THE COASTAL WATERS ARE EXPECTED TO REACH SCA LEVELS
EARLY THIS EVENING...AS ARE NW WINDS IN ADMIRALTY INLET. NW FLOW
SHOULD ALSO INCREASE TO SCA LEVELS OVER THE N INTERIOR FROM
BELLINGHAM BAY N DUE TO INCREASING NW FLOW THROUGH THE STRAIT OF
GEORGIA.

AN UPPER LEVEL RIDGE APPROACHING FROM THE W THIS WEEKEND WILL PUSH A
SURFACE RIDGE CLOSER TO THE COAST. THIS WILL MAINTAIN MODERATE W-NW
ONSHORE FLOW OVER W WA THROUGH THIS WEEKEND...WHICH SHOULD KEEP W
WINDS IN THE STRAIT IN THE SCA RANGE.

A VERTICALLY STACKED UPPER LEVEL AND SURFACE LOW WILL APPROACH THE
REGION ON MONDAY...WHICH WILL PUSH THE SURFACE RIDGE TO THE COAST
LATE MONDAY AFTERNOON...JUST AHEAD OF A WEAK COLD FRONT. SW ONSHORE
FLOW WILL INCREASE MONDAY AFTERNOON. THE UPPER LEVEL LOW SHOULD
CROSS W WA TUESDAY WHILE THE SURFACE LOW WEAKENS INTO A TROUGH. W-NW
ONSHORE FLOW SHOULD REDEVELOP TUESDAY NIGHT BEHIND THE TROUGH. KAM

&&

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