Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 290921
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
220 AM PDT SUN MAR 29 2015

.SYNOPSIS...A STALLED FRONTAL BOUNDARY WILL BRING SOME LIGHT RAIN AT
TIMES TO THE COAST AND NORTH INTERIOR THROUGH MONDAY. THE FRONT WILL
PUSH INLAND MONDAY NIGHT BRINGING RAIN TO THE REST OF THE AREA. A
TROUGH OF LOW PRESSURE WILL MOVE INTO THE REGION TUESDAY THROUGH THE
END OF NEXT WEEK. THE UPPER TROUGH WILL GIVE COOL AND SHOWERY
WEATHER TO THE AREA AND SNOW LEVELS WILL FALL TO AROUND 3000 FEET IN
THE MOUNTAINS TUESDAY NIGHT OR WEDNESDAY.

&&

.SHORT TERM...A WEAK RIDGE OF HIGH PRESSURE ALOFT IS CENTERED OVER
EASTERN WASHINGTON EARLY THIS MORNING WHILE A SURFACE HIGH IS
CENTERED OVER THE INTERIOR OF WESTERN WASHINGTON. IN THE MEAN
TIME...A SERIES OF WEAK WARM FRONTAL FEATURES ARE LIFTING
NORTHEASTWARD ACROSS THE OFFSHORE AND COASTAL WATERS INTO VANCOUVER
ISLAND. THE WARM ADVECTION AND SOUTHERLY FLOW AT THE LOWER LEVELS IS
GIVING SOME RAIN TO THE NORTH COAST...AND LIGHT RAIN OR SPRINKLES
ARE OCCASIONALLY MAKING IT ONTO THE CENTRAL COAST AND INTO THE
INTERIOR FROM ABOUT WHIDBEY ISLAND NORTH AND WEST. SATELLITE AND
RADAR LOOPS HINT THAT THE LIGHT PRECIPITATION MAY MAKE IT SOUTH AND
EAST AS FAR AS AN EVERETT TO SHELTON LINE THIS MORNING BEFORE IT
LIFTS BACK TO THE NORTH.

A WAVE OF LOW PRESSURE DEVELOPING AROUND 40N 140W LATER THIS MORNING
WILL MOVE TO THE QUEEN CHARLOTTE SOUND AS A 1004 MB LOW TONIGHT AND
EARLY MONDAY. THE WARM ADVECTION TO THE EAST OF THIS DEVELOPING
FEATURE WILL HAVE THE EFFECT OF BUILDING THE RIDGE SOMEWHAT OVER THE
INTERIOR OF WESTERN WASHINGTON...KEEPING CONDITIONS IN THE INTERIOR
FROM ABOUT EVERETT SOUTHWARD DRY. THE COAST AND THE CHEHALIS GAP
AREA WILL CONTINUE TO SEE A CHANCE OF LIGHT RAIN AT TIMES AS WARM
FRONTAL FEATURES CLIP THE COASTLINE.

TEMPERATURES TODAY THROUGH MONDAY WILL BE MILD FOR LATE MARCH WITH
HIGHS 55-65 AND LOWS AROUND THE 50 DEGREE MARK.

SHORT TERM MODELS ARE CONSISTENT IN SHOWING THE FRONT FINALLY
SURGING EASTWARD THROUGH THE INTERIOR OF WESTERN WASHINGTON ON
MONDAY NIGHT. RAIN MONDAY EVENING WILL CHANGE TO SHOWERS LATE IN THE
NIGHT BEHIND THE FRONT...AND INCREASING WEST TO SOUTHWEST GRADIENTS
WILL GIVE ONSHORE FLOW AND BREEZY TO WINDY CONDITIONS AROUND THE
EASTERN STRAIT OF JUAN DE FUCA. TEMPERATURES IN THE LOWLANDS WILL
COOL INTO THE 40S TO LOWER 50S TUESDAY BEHIND THE FRONT AND A
CONVERGENCE ZONE WILL LIKELY DEVELOP LATER MONDAY NIGHT INTO TUESDAY
AS FLOW ALOFT INTERACTS WITH THE OLYMPICS. 500 MB HEIGHTS FALL OFF
TO 545 DAM MIDDAY TUESDAY AND 850 MB TEMPERATURES DROP TO AROUND -2C
TO -3C. SNOW LEVELS WILL LIKELY FALL TO 3000-4000 FEET ON TUESDAY.
IT IS POSSIBLE THAT A WINTER WEATHER ADVISORY WILL BE NEEDED AT SOME
POINT ON TUESDAY IN THE CENTRAL CASCADES WHERE THE PUGET SOUND
CONVERGENCE ZONE SETS UP...BUT IT IS TOO EARLY TO PINPOINT WHERE
THIS WOULD OCCUR AT THIS TIME. ALBRECHT

.LONG TERM...AN UPPER LEVEL TROUGH WILL GIVE COOL AND SHOWERY
WEATHER TO THE AREA ON WEDNESDAY. THURSDAY MAY SEE SOME DECREASE IN
SHOWER ACTIVITY AND A LITTLE MORE SUNSHINE AS A SHORT WAVE RIDGE
MOVES THROUGH THE REGION.

BOTH THE ECMWF AND GFS MODELS SHOW COOL AND SHOWERY CONDITIONS
FRIDAY THROUGH NEXT WEEKEND WITH SNOW LEVELS 2500 TO 3000 FEET IN
THE MOUNTAINS. ENSEMBLES ARE RATHER CONSISTENT IN SHOWING THE CHANGE
TO THE COOLER WEATHER PATTERN WITH AN UPPER TROUGH OVER WESTERN
WASHINGTON. ALBRECHT

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT 7 DAYS.

&&

.AVIATION...WESTERLY FLOW ALOFT OVER WESTERN WASHINGTON EARLY THIS
MORNING WILL BACK SLIGHTLY TO SOUTHWEST TONIGHT...AS THE FLAT UPPER
RIDGE OVER THE PACIFIC NORTHWEST MOVES SLOWLY EAST. A FRONT WILL
REMAIN ALONG THE NORTHWEST SIDE OF THE RIDGE...EXTENDING FROM
SOUTHERN BRITISH COLUMBIA SOUTHWEST INTO THE OFFSHORE WATERS. THE
AIR MASS WILL REMAIN FAIRLY MOIST...ESPECIALLY AT MIDDLE AND UPPER
LEVELS.

VFR CONDITIONS CONTINUE ACROSS THE FORECAST AREA EARLY THIS
MORNING...WITH SKY COVER ROUGHLY SCT-BKN040 BKN-060-075 AND HIGHER
BKN-OVC LAYERS ABOVE. AREAS OF LIGHT PRECIPITATION CONTINUE OVER THE
NORTH PART OF THE AREA. ON THE WHOLE THERE WILL BE LITTLE CHANGE
THROUGH TONIGHT.

KSEA...SOUTH OR SOUTHWEST WIND 6-12 KT. BKN-OVC CEILING COULD LOWER
TO AROUND 5000 FT TONIGHT BUT REMAIN VFR.   MCDONNAL

&&

.MARINE...A RIDGE WILL REMAIN OVER WESTERN WASHINGTON THROUGH
MONDAY... WHILE A FRONT EXTENDS FROM VANCOUVER ISLAND INTO THE
OFFSHORE WATERS. THE SOUTHERLY GRADIENT ASSOCIATED WITH THIS PATTERN
IS PRODUCING WINDS OF 15 TO 30 KT...WITH THE HIGHEST WINDS GENERALLY
IN THE NORTH PART OF THE FORECAST AREA.

A COLD FRONT WILL MOVE ACROSS THE COASTAL WATERS AND WESTERN
WASHINGTON LATE MONDAY AND MONDAY NIGHT...FOLLOWED BY STRONG ONSHORE
FLOW MONDAY NIGHT AND TUESDAY. THERE IS A GOOD CHANCE THAT GALE
WESTERLIES WILL OCCUR IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST
ENTRANCE STRAIT LATER MONDAY NIGHT AND TUESDAY...AND A GALE WATCH IS
BEING ISSUED WITH THE EARLY MORNING FORECAST.

THERE WILL BE A WESTERLY SWELL OF 10 TO 12 FT OVER THE COASTAL
WATERS THROUGH TUESDAY...WITH ROUGH BAR CONDITIONS AS WELL.

A WEAK RIDGE WILL BUILD OVER THE AREA WEDNESDAY AND THURSDAY.
MCDONNAL


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT THE CENTRAL STRAIT.
     SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR.
     GALE WATCH FOR MONDAY NIGHT AND TUESDAY CENTRAL STRAIT
      AND EAST ENTRANCE STRAIT.

&&

$$

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







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