Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 130400
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT SAT APR 12 2014

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE AND OFFSHORE FLOW WILL KEEP
WESTERN WASHINGTON DRY SUNDAY AND MONDAY. A SERIES OF WEAK WEATHER
SYSTEMS WILL REACH THE AREA THROUGH NEXT WEEK...BEGINNING MONDAY
NIGHT. CONDITIONS WILL BE COOLER WITH WET WEATHER LIKELY TUESDAY AND
THURSDAY...AND A CHANCE OF SHOWERS ON OTHER DAYS.

&&

.SHORT TERM...SKIES HAVE CLEARED OUT ACROSS W WA THIS EVENING. WITH
A STRONG UPPER LEVEL RIDGE MOVING ACROSS THE REGION THE NEXT TWO
DAYS AND LOW LEVEL OFFSHORE FLOW DEVELOPING TONIGHT...SUNNY WEATHER
WILL CONTINUE THROUGH MONDAY. THERE COULD BE SOME ISOLATED VALLEY
FOG OVER THE SW INTERIOR SUNDAY MORNING. TONIGHTS OFFSHORE FLOW WILL
HELP BOOST MAX TEMPS ON SUNDAY WELL INTO THE 60S...WITH EVEN A FEW
READINGS IN THE 70S POSSIBLE OVER THE SW INTERIOR AND NEAR THE
CASCADES.

THE UPPER LEVEL RIDGE AXIS SHIFTS E OF THE AREA BY MIDDAY MONDAY AND
AN UPPER LEVEL TROUGH TRAILING RIGHT BEHIND THE RIDGE WILL HEAD
TOWARD THE CENTRAL B.C. COAST. WHILE THE PRECIP FROM THE TROUGH WILL
MISS W WA ON MONDAY...WE WILL GET NAILED BY THE CLOUDS AND ONSHORE
FLOW. THE FRONT TRAILING S OF THE LOW IS EXTREMELY WEAK AND THE
MODELS INDICATE ONLY A SMALL AMOUNT OF MID AND HIGH CLOUD WILL CROSS
W WA ON MONDAY. HOWEVER...STRONG SW LOW LEVEL ONSHORE FLOW WILL BE
DEVELOPING JUST AHEAD OF THE FRONT ALONG THE WA COAST BY LATE MONDAY
MORNING. THIS WILL EFFECTIVELY SET UP A MARINE PUSH INTO THE W WA
INTERIOR WITH THE MOISTURE ALL BELOW 850 MB. THE STRENGTH OF THE
ONSHORE FLOW AND THE FAST DEVELOPING NATURE OF THE PUSH SUGGESTS
THAT THE LOW CLOUDS WILL COME IN AT MVFR LEVELS...PERHAPS 2000-3000
FEET...WITH CLOUD LAYERS RISING OVERNIGHT AS THE MARINE LAYER
DEEPENS. BREAKS IN THE OVERCAST ARE ALSO COMMON THE NEXT DAY AFTER
STRONG FAST PUSHES. SCATTERED SHOWERS WILL DEVELOP BEHIND THE FRONT
MONDAY NIGHT AS THE S PART OF THE B.C. UPPER LEVEL TROUGH MOVES
ACROSS W WA.

ON TUESDAY...NW FLOW ALOFT WILL BE OVER W WA WITH ONE OR MORE WEAK
UPPER LEVEL SHORTWAVE TROUGHS CROSSING THE AREA. MODERATE ONSHORE
FLOW WILL CONTINUE AND SHOWERS ARE LIKELY. TEMPERATURES WILL COOL
BACK DOWN TO NEAR NORMAL. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 PM AFD...THERE IS BRIEF
RIDGING AT THE SURFACE AND ALOFT ON WEDNESDAY. MODELS STILL HAVE A
FEW DRIBS AND DRABS OF RAIN BUT WEDNESDAY COULD TURN OUT MOSTLY DRY.
HAVE POPS IN THE LOW CHANCE CATEGORY FOR WEDNESDAY. ANOTHER SYSTEM
ARRIVES THURSDAY IN BOTH THE GFS AND THE EURO AND HAVE INCREASED
POPS TO LIKELY FOR THAT SYSTEM AS WELL. MODELS DIVERGE AFTER THAT
WITH THE GFS NOTABLY DRY AND THE EURO WET...HAVE BROAD BRUSH CHANCE
POPS FRIDAY AND SATURDAY. BURKE

&&

.HYDROLOGY...RIVER FLOODING IS UNLIKELY ON AREA RIVERS...EXCEPT THE
NORTH FORK OF THE STILLAGUAMISH EAST OF OSO...FOR THE NEXT 7 DAYS.

A FLOOD WARNING REMAINS IN EFFECT UNTIL FURTHER NOTICE FOR THE
NORTH FORK OF THE STILLAGUAMISH RIVER IN SNOHOMISH COUNTY JUST
UPSTREAM OF THE SR 530 LANDSLIDE. FLOODING CONTINUES AS A RESULT
OF WATER POOLED BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS THE
RIVER.

A SHIFT TO A WETTER WESTERLY FLOW PATTERN WILL OCCUR MONDAY NIGHT.
RAIN OR SHOWERS ARE IN THE FORECAST EACH DAY FOR THE REST OF NEXT
WEEK. PRECIPITATION WILL NOT BE HYDROLOGICALLY SIGNIFICANT FOR AREA
RIVERS...EXCEPT FOR THE NORTH FORK STILLAGUAMISH AND THE SR 530
LANDSLIDE AREA.

THE NORTH FORK STILLAGUAMISH NEAR ARLINGTON AND THE POOL UPSTREAM OF
THE SR 530 SLIDE WILL CONTINUE FALLING THROUGH MONDAY AS A STRONG
UPPER LEVEL RIDGE BRINGS DRY WEATHER TO THE AREA. A SHIFT TO A
WETTER WESTERLY FLOW PATTERN MONDAY NIGHT WILL CONTINUE THROUGH NEXT
WEEK. PERIODS OF RAIN MAY BRING RISES TO THE POOL LEVEL AND
DOWNSTREAM AREAS NEXT WEEK. THERE IS POTENTIAL FOR AROUND A HALF
INCH OF RAIN TO FALL MONDAY NIGHT THROUGH TUESDAY...AND ANYWHERE
FROM 0.30 TO 0.80 WEDNESDAY NIGHT THROUGH THURSDAY EVENING.
CONFIDENCE IS NOT HIGH REGARDING THE TIMING OF THE RAIN OR THE
AMOUNTS...BUT ACCUMULATIONS COULD BE SIGNIFICANT FOR OPERATIONS IN
THE SLIDE AREA. KAM

&&

.AVIATION...A STRONG UPPER RIDGE WILL DOMINATE THE PAC NW THROUGH
SUNDAY NIGHT. LIGHT NLY FLOW ALOFT. DRY AND STABLE CONDITIONS.

KSEA...CLEAR TONIGHT AND SUNDAY. NE WIND 5-10 KT...BECOMING NLY
MONDAY AFTERNOON. VFR CONDITIONS THROUGH SUNDAY NIGHT.

&&

.MARINE...THERMALLY INDUCED LOW PRESSURE WILL SET UP ALONG THE
OREGON AND FAR SRN WA COAST LATE TONIGHT AND SUNDAY. THIS WILL
PRODUCE OFFSHORE FLOW WITH WINDS LESS THAN 20 KT THROUGHOUT THE
WATERS. ONSHORE FLOW WILL DEVELOP MONDAY AFTERNOON AND EVENING AS A
WEATHER SYSTEM NEARS THE AREA. THE STRONGEST WESTERLY FLOW WILL BE
IN THE STRAIT MONDAY NIGHT WITH GALE FORCE WINDS POSSIBLE.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
     STILLAGUAMISH RIVER EAST OF THE SR 530 LANDSLIDE.

PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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








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