Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 140349
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
848 PM PDT SUN APR 13 2014

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE AND OFFSHORE LOW LEVEL FLOW
WILL KEEP WESTERN WASHINGTON DRY THROUGH MONDAY. ONSHORE FLOW WITH A
PAIR OF UPPER LEVEL TROUGHS WILL BRING A LITTLE RAIN TO WESTERN
WASHINGTON LATE MONDAY NIGHT THROUGH TUESDAY EVENING. A WEAK UPPER
LEVEL RIDGE MAY BRING SOMEWHAT DRIER WEATHER WEDNESDAY WITH JUST A
CHANCE OF RAIN. A STRONGER FRONTAL SYSTEM WILL BRING RAIN THURSDAY.
WESTERLY FLOW ALOFT OVER THE REGION WILL MAINTAIN AN UNSETTLED
WEATHER PATTERN INTO NEXT WEEKEND.

&&

.SHORT TERM...THE AXIS OF THE LARGE UPPER LEVEL RIDGE RESPONSIBLE
FOR THE SUNNY WARM WEATHER TODAY IS JUST NEARING THE WA COAST THIS
EVENING. THE RIDGE WILL CONTINUE MOVING STEADILY E WHILE WEAKENING
THROUGH MONDAY. THE SURFACE THERMALLY INDUCED TROUGH ALONG THE WA
COAST WILL REMAIN IN PLACE TONIGHT ALLOWING WEAK LOW LEVEL OFFSHORE
FLOW TO CONTINUE.

BIG CHANGES ARE COMING UP MONDAY AND TUESDAY. AS THE RIDGE MOVES E
THE COLD FRONT OUT ALONG 135W WILL MOVE INLAND. THE FRONT WILL
WEAKEN RAPIDLY AS IT APPROACHES MONDAY AND WILL BE WEAK AND DRY WITH
JUST A SMALL CLOUD BAND AS IT MOVES INLAND MONDAY AFTERNOON. HOWEVER
STRONG ONSHORE FLOW WILL DEVELOP JUST AHEAD OF IT LATE MONDAY
MORNING. THE ONSHORE FLOW WILL ADVECT A COOLER AND EVENTUALLY MOIST
AIR MASS INLAND MONDAY AFTERNOON AND NIGHT. IT LOOKS LIKE LOW CLOUDS
WILL REACH THE COAST MONDAY AFTERNOON BUT HOLD OFF UNTIL LATE MONDAY
EVENING BEFORE REACHING THE PUGET SOUND REGION. THIS WILL KEEP
MONDAY MOSTLY SUNNY OVER THE INTERIOR...BUT THE COOLER ONSHORE FLOW
WILL KNOCK ABOUT 5 DEGREES OFF OF TODAYS HIGHS. THE UPPER LEVEL
SHORTWAVE TROUGH ASSOCIATED WITH THE FRONT WILL HEAD NE TO THE
CENTRAL B.C. COAST AND WILL NOT BE A FACTOR.

RAIN HAS BEEN IN THE FORECAST FOR MONDAY NIGHT AND TUESDAY FOR THE
PAST COUPLE OF DAYS. IT WILL BE THE PRODUCT OF A COUPLE OF TRAILING
UPPER LEVEL SHORTWAVE TROUGHS MOVING OVER THE MOIST MARINE LAYER.
THE FIRST TROUGH...OUT ALONG 150W AT 01Z...WILL BRING THE INITIAL
RAIN TO THE AREA LATE MONDAY NIGHT AND TUESDAY MORNING. ANOTHER
UPPER LEVEL SHORTWAVE TROUGH RIGHT BEHIND IT WILL KEEP THE RAIN OR
SHOWERS GOING TUESDAY AFTERNOON AND EVENING. IN ADDITION TO THE
UPPER LEVEL TROUGHS THE ONSHORE FLOW WILL MAINTAIN A PUGET SOUND
CONVERGENCE ZONE PATTERN MONDAY EVENING THROUGH TUESDAY EVENING.
RAIN ACCUMULATION DURING THIS PERIOD SHOULD RANGE FROM 0.05-0.20
OVER THE LOWLANDS TO A HALF TO THREE QUARTERS OF AN INCH OVER THE
MOUNTAINS.

THE GFS AND ECMWF HAVE A SIGNIFICANT DIFFERENCE FOR WEDNESDAY. BOTH
DEVELOP AN UPPER LEVEL RIDGE JUST OFFSHORE...BUT THE GFS INDICATES A
SLIGHTLY HIGHER AMPLITUDE RIDGE THAN THE ECMWF. THIS HIGHER
AMPLITUDE APPEARS TO BE ENOUGH FOR THE GFS TO GO WITH A MOSTLY
DRY FORECAST FOR WEDNESDAY...WHILE THE ECMWF WITH ITS LOWER
AMPLITUDE RIDGE ALLOWS WEAK WARM ADVECTION TO PUSH THROUGH THE RIDGE
AND BRING A LITTLE LIGHT RAIN TO THE AREA. THE CURRENT FORECAST
MAINTAINS CHANCE POPS IN DEFERENCE TO THE ECMWF. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 240 PM AFD...NEXT SYSTEM
IS DUE ABOUT THURSDAY NIGHT AND IN THIS THE GFS AND EURO MORE OR
LESS AGREE. HAVE LIKELY POPS THURSDAY NIGHT EVERYWHERE. MODEL
AGREEMENT CONTINUES ON FRIDAY WITH BOTH MODELS SHOWING SCATTERED
SHOWERS AND ALSO ON SATURDAY WITH BOTH MODELS MOSTLY DRY. HAVE NOT
CUT POPS TO ZERO ANY PERIOD AS MODELS HAVE BEEN CHANGEABLE AND THIS
IS CERTAINLY THE TIME OF YEAR WHEN THE EXTENDED FORECAST IS MOST
CHANCY. ANOTHER SYSTEM IS IN THE MODELS SUNDAY OR SUNDAY NIGHT. 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.

THE NORTH FORK STILLAGUAMISH NEAR ARLINGTON AND THE POOL UPSTREAM OF
THE SR 530 SLIDE WILL CONTINUE FALLING THROUGH TUESDAY MORNING AS A
STRONG UPPER LEVEL RIDGE KEEPS THE WEATHER DRY. A SHIFT TO A WETTER
WESTERLY FLOW PATTERN WILL OCCUR BEGINNING TUESDAY.

THE RIVER AND UPSTREAM POOL ARE FORECAST TO RISE ABOUT A FOOT
TUESDAY AFTERNOON AND EVENING...THEN RESUME A SLOW DOWNWARD TREND.
AT THIS TIME...THE MOST SIGNIFICANT SYSTEMS ARE EXPECTED LATE MONDAY
NIGHT THROUGH TUESDAY MORNING...GIVING AROUND A HALF INCH OF RAIN TO
THE SLIDE AREA...AND ON THURSDAY NIGHT...GIVING ANOTHER QUARTER TO
HALF AN INCH OF RAIN TO THE SLIDE AREA. LIGHT SHOWERS AT OTHER TIMES
WILL NOT GIVE HYDROLOGICALLY SIGNIFICANT RAINFALL TO THE SLIDE
AREA...BUT WILL KEEP CONDITIONS WET. BURKE

&&

.AVIATION...UPPER LEVEL HIGH PRESSURE OVER THE PAC NW WILL SHIFT EWD
ON MONDAY AS A TROUGH REACHES THE AREA. LIGHT SE FLOW ALOFT BECOMING
MODERATE SW FLOW ALOFT ON MONDAY. DRY AND STABLE TONIGHT...THEN
INCREASING LOW LEVEL MOISTURE OVER THE COASTAL WATERS MONDAY
SPREADING INLAND MONDAY NIGHT.

KSEA...N-NE WIND 3-6 KT BECOMING LIGHT AND VARIABLE LATER TONIGHT
AND THEN LIGHT SLY WIND MONDAY...INCREASING MONDAY EVENING AS A
FRONT NEARS THE AREA.

&&

.MARINE...THERMALLY INDUCED LOW PRESSURE ALONG THE CA/OREGON COAST
WILL EXTEND NWD ALONG THE S WA COAST TONIGHT. ONSHORE FLOW WILL
DEVELOP MONDAY AND INCREASE MONDAY EVENING AS A FRONT NEARS THE
AREA. WLY GALES WILL LIKELY DEVELOP IN THE CENTRAL/E STRAIT MONDAY
AFTERNOON. ONSHORE/WLY FLOW TUESDAY WILL BECOME LIGHTER ON WED. A
WEAKER FRONT MAY AFFECT THE WATERS LATE IN THE WEEK.

&&

.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...GALE WATCH MONDAY NIGHT CENTRAL STRAIT OF JUAN DE FUCA AND EAST
      ENTRANCE TO THE STRAIT.

&&

$$

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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