Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 312223
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI OCT 31 2014

.SYNOPSIS...A TROUGH WILL LINGER OVER THE PACIFIC NORTHWEST TONIGHT.
SATURDAY AND SATURDAY NIGHT SHOULD BE MOSTLY DRY AS THE TROUGH
SHIFTS SOUTH OF THE REGION. ANOTHER FRONT WILL APPROACH THE AREA ON
SUNDAY WITH RAIN DEVELOPING. THIS WILL BE THE FIRST IN A SERIES OF
WEATHER SYSTEMS THAT WILL BRING RAINY AND BREEZY WEATHER TO THE AREA
THROUGH NEXT WEEK.

&&

.SHORT TERM...THE FRONTAL BAND IS WEAKENING AND SHIFTING EWD WITH
DECREASING SHOWERS ACROSS WRN WA LATE THIS AFTERNOON. SATELLITE
IMAGERY INDICATES THE TROUGH IS IN THE PROCESS OF SPLITTING SO THE
SHOWER COVERAGE SHOULD DECREASE CONSIDERABLY THROUGH THE EVENING.
THE EXCEPTION COULD BE AROUND WRN SNOHOMISH COUNTY NWD IF A
CONVERGENCE ZONE FORMS AS ADVERTISED BY THE HRRR. THIS IS HIGHLY
UNCERTAIN GIVEN THE LACK OF FLOW THROUGH THE STRAIT BUT THE HRRR
DOES SHOW CONVERGING WINDS AT 925 MB. THE 4KM WRF-GFS/NAM-12 ALSO
SHOW SOME LOCALIZED QPF OVER THE N INTERIOR EARLY IN THE EVENING.
GIVEN THAT THE NRN BRANCH OF THE UPPER TROUGH IS STILL OFFSHORE
TONIGHT AND MODELS SHOW SOME SPOTTY QPF...IT SEEMS REASONABLE TO
KEEP THE CHANCE POPS IN PLACE.

THE GFS/ECMWF INDICATE THE UPPER TROUGH WILL CONTINUE TO SPLIT WITH
ALMOST ALL THE ENERGY TO THE N AND S OF WRN WA. FORECAST POPS WERE
REMOVED FOR MOST THE INTERIOR SATURDAY AND SATURDAY NIGHT. SOME LOW
CHANCE POPS WERE MAINTAINED ALONG THE COAST. THERE SHOULD BE SOME
PARTIAL SUNSHINE DURING THE DAY BUT THE COOLER AIR MASS WILL KEEP
HIGHS LIMITED TO THE LOW TO MID 50S.

MODELS ARE IN GOOD AGREEMENT THAT THE NEXT SYSTEM WILL ARRIVE
MONDAY. RAIN SHOULD REACH THE COAST DURING THE MORNING OR NEAR
MIDDAY...THEN SPREAD INLAND LATE IN THE DAY. WARM ADVECTION RAIN
WILL PERSIST MONDAY NIGHT THROUGH TUESDAY OVER THE ENTIRE AREA.
HIGHER POPS ARE ALREADY IN PLACE DURING THIS PERIOD.

.LONG TERM...GLOBAL MODELS AGREE ON A WET PATTERN NEXT WEEK. THE
WARM FRONT MOVING THROUGH ON MONDAY WILL BE FOLLOWED BY A MOIST COLD
FRONT. SOME MODEL RUNS INCLUDING THE 12Z GFS SHOW THE MAIN FRONTAL
BAND STALLING OVER WRN WA MONDAY NIGHT INTO TUESDAY WITH LOCALLY
HEAVY RAIN. SEE HYDRO DISCUSSION BELOW.

MODELS DIVERGE ON THE PATTERN BY WEDNESDAY. THE ECMWF MAINTAINS THE
WET SWLY FLOW PATTERN WHILE THE GFS IS DRY WITH MORE RIDGING. OTHER
MODELS LIKE THE CANADIAN SHOW SOME BREAK IN PRECIPITATION BUT BRING
RAIN BACK INTO THE AREA LATER IN THE DAY. REGARDLESS...WET WEATHER
WILL RESUME THURSDAY AND FRIDAY AS A STRONGER AREA OF LOW PRESSURE
TRACKS NEAR OR TO THE N OF WRN WA. BOTH THE CANADIAN/ECMWF SHOW A
DEEPER LOW CLOSER TO OUR AREA WHICH BARES WATCHING AS BREEZY OR
WINDY CONDITIONS ARE POSSIBLE.  MERCER

&&

.HYDROLOGY...THE THREAT OF RIVER FLOODING HAS ENDED AND THE WARNING
FOR THE SKOKOMISH WAS DROPPED EARLIER TODAY. A LULL IN PRECIPITATION
IS EXPECTED TONIGHT THROUGH EARLY SUNDAY SO FLOODING IS NOT
EXPECTED.

RAIN WILL DEVELOP LATE SUNDAY AND PERSIST AT TIMES THROUGH THE END
OF NEXT WEEK. THE MONDAY THROUGH TUESDAY PERIOD MAY HAVE LOCALLY
HEAVY RAIN...ESPECIALLY ALONG S/SW FACING MOUNTAIN SLOPES. SOME OF
THE MORE FLOOD PRONE RIVERS SUCH AS THE SKOKOMISH RIVER WILL NEED TO
BE WATCHED IN THIS PATTERN.

IN RELATION TO THE RECENT RAIN AND EXPECTED UPCOMING WET
PATTERN...THE THREAT OF LANDSLIDES WILL ALSO NEED TO BE MONITORED.
THE LATEST USGS 3/15 DAY PRECIP THRESHOLD WAS SURPASSED BUT 24
PRECIPITATION AMOUNTS WERE LESS THAN A COUPLE INCHES AT SEA-TAC
WHICH MITIGATES BUT DOES NOT ELIMINATE THE THREAT OF LANDSLIDES.
MORE RAIN IS ANTICIPATED NEXT WEEK SO THE LANDSLIDE THREAT NEEDS TO
BE MONITORED AS SOILS WILL APPROACH NEAR SATURATION. MERCER

&&

.AVIATION...MODERATE SOUTHWEST FLOW ALOFT WILL WEAKEN TONIGHT AND
BECOME LIGHT NORTHWESTERLY SATURDAY AFTERNOON AS A WEAK UPPER TROUGH
MOVES THROUGH THE AREA. THE AIR MASS IS SLIGHTLY UNSTABLE AND MOIST
WITH SCATTERED SHOWERS. DRYING WILL OCCUR TONIGHT AND SATURDAY WITH
PARTLY SUNNY SKIES ON SATURDAY.

KSEA...VFR CONDITIONS THIS EVENING THOUGH A CHANCE OF A SHOWER COULD
BRIEFLY BRING CEILINGS DOWN TO MVFR. THERE WILL PROBABLY BE SEVERAL
HOURS OF LOW END MVFR OR IFR LATER TONIGHT BEFORE IMPROVING TO VFR
SATURDAY AFTERNOON. LIGHT SOUTHERLY WIND BECOMING LIGHT NORTHERLY
OVERNIGHT. CHB

&&

.MARINE...PRESSURE GRADIENTS ARE LIGHT TODAY AND THERE ARE NO
ADVISORIES IN EFFECT. THE TROUGH JUST OFFSHORE IS FORECAST TO BE
WEAK BY THE TIME IT MOVES ASHORE TONIGHT...SO NO ADVISORIES FOR THAT
EITHER. WEAK HIGH PRESSURE WILL BUILD SATURDAY.

A SERIES OF STRONGER SYSTEMS IS LIKELY FOR NEXT WEEK BEGINNING
SUNDAY. SMALL CRAFT ADVISORY WINDS ARE IN THE GRIDS AND FORECASTS
FOR SEVERAL PERIODS CENTERED AROUND SUNDAY NIGHT AND AGAIN
WEDNESDAY. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.
PZ...NONE.

&&

$$

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