Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 311617
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT FRI OCT 31 2014

.SYNOPSIS...A PACIFIC FRONTAL SYSTEM WILL BRING RAINFALL TO WESTERN
WASHINGTON THIS MORNING. THE COLD FRONT WILL CROSS THE CASCADES THIS
AFTERNOON WITH THE RAIN TURNING TO SHOWERS. SATURDAY SHOULD BE
PARTLY SUNNY WITH LIGHT SPOTTY SHOWERS. ANOTHER FRONT WILL APPROACH
THE AREA ON SUNDAY WITH RAIN DEVELOPING ALONG THE COAST LATE IN THE
DAY. THIS WILL BE THE FIRST IN A SERIES OF WEATHER SYSTEMS THAT WILL
BRING RAINY AND LOCALLY WINDY WEATHER TO THE AREA MOST OF NEXT WEEK.

&&

.SHORT TERM...IR SATELLITE IMAGE SHOWS THE N/S ORIENTED FRONTAL BAND
STALLED OVER THE PAC NW OVERNIGHT. RADAR AND OBS ARE INDICATING
LIGHT TO MODERATE OVERSPREADING THE AREA WHICH WILL CONTINUE THE
NEXT FEW HOURS. A SQUALL LINE ON RADAR IS ABOUT TO MOVE ONTO THE
COAST AND THIS WILL PROBABLY BE THE LAST BURST OF HEAVIER RAIN IS IT
SLOWLY MOVES INTO THE INTERIOR OVER THE NEXT FEW HOURS. IT WILL BE
WEAKLY UNSTABLE NEAR THE COAST WHERE MODEL LI`S ARE AROUND -1C.
THERE ARE STRIKES OFFSHORE SO THE FORECAST WILL BE UPDATED FOR
ISOLATED THUNDERSTORMS POSSIBLE ALONG THE COAST THIS AFTERNOON.
SATELLITE IMAGERY SHOWS A DISTINCT DRY SLOT SO A PERIOD OF MOSTLY
DRY WEATHER BY THIS EVENING. WITH THE 500 MB TROUGH AXIS STILL
OFFSHORE SO A FEW STRAY SHOWERS CANNOT BE RULED OUT.

THE TROUGH AXIS SLOWLY MOVES THROUGH ON SATURDAY. MODELS ARE
GENERALLY DRY BUT SOME ISOLATED DIURNALLY DRIVEN SHOWS ARE NOT OUT
OF THE QUESTION. POPS MAY BE TRIMMED BACK FURTHER OR REMOVED BUT
WILL MAKE THE DECISION ON THAT WITH THE AFTERNOON FORECAST
ISSUANCE.

MOSTLY DRY WEATHER WILL CONTINUE SATURDAY NIGHT BUT THE BREAK WILL
BE BRIEF. A WARM FRONTAL WAVES WILL BRING SOME RAIN BACK INTO THE
FORECAST ON SUNDAY. TIMING IS QUESTIONABLE BUT THE 06GFS/00ECMWF
INDICATE INCREASING RAIN MOSTLY DURING THE AFTERNOON. RAIN WILL
CONTINUE AT TIMES THROUGH MONDAY AHEAD THE MAIN SURFACE LOW WHICH
WILL TRACK INTO CENTRAL BRITISH COLUMBIA. HIGHS WILL CHANGE VERY
LITTLE THROUGH THE WEEKEND WITH HIGHS IN THE LOW TO MID 50S. MERCER

.LONG TERM...NO UPDATES FROM PREVIOUS DISCUSSION...EXTENDED MODELS
HAVE A GENERAL THEME OF A WET PERIOD DURING THE EXTENDED FORECAST
AND CONFIDENCE IS HIGH IN THIS GENERAL TREND. THE DEVIL IS IN THE
DETAILS. BOTH THE ECMWF AND GFS SHOW THE SYSTEM SUNDAY NIGHT INTO
MONDAY WITH ANOTHER SYSTEM RIGHT ON ITS HEELS FOR MONDAY NIGHT INTO
TUESDAY. THE SOLUTIONS START TO DIVERGE ON TUESDAY WITH THE GFS
MOVING THE FRONT THROUGH THE AREA WHILE THE ECMWF STALLS THE FRONT
OVER WESTERN WASHINGTON WITH THE PATTERN SHOWING AN ATMOSPHERIC
RIVER SETTING UP ACROSS THE EASTERN PACIFIC. THE GFS LIFTS ANOTHER
SYSTEM INTO THE AREA TUESDAY NIGHT INTO WEDNESDAY WHILE THE ECMWF
KEEPS THE LONG FETCH OF MOISTURE AIMED RIGHT AT THE AREA. THE GFS
LIFTS THE FRONT NORTH OF THE AREA WEDNESDAY NIGHT AND THURSDAY WITH
A UPPER LEVEL LOW DEVELOPING OFF THE COAST MOVING TOWARD HAIDA
GWAII. THE ECMWF IS MUCH FURTHER SOUTH WITH THIS FEATURE RESULTING
IN A SLOWER LIFTING OF THE FRONT TO THE NORTH. THE EXTENDED FORECAST
WILL HAVE RAIN OR RAIN LIKELY IN EVERY PERIOD SUNDAY NIGHT THROUGH
THURSDAY. FELTON

&&

.HYDROLOGY...A FLOOD WARNING IS IN EFFECT FOR THE SKOKOMISH RIVER
EARLY THIS MORNING. RAINFALL AMOUNTS OVERNIGHT WERE NOT VERY
IMPRESSIVE...GENERALLY UNDER 1 INCH. WITH PRECIPITATION DECREASING
AND THE UPPER FORKS OF THE RIVER ALREADY COMING DOWN...FLOODING
LOOKS UNLIKELY. WILL CONTINUE TO WATCH THIS BUT THE WARNING MAY BE
DROPPED LATER THIS MORNING. FORECASTS AND STATEMENTS SHOULD STILL BE
MONITORED.

THE OTHER RIVER WE HAVE BEEN CONCERNED ABOUT IS THE NOOKSACK RIVER.
RAINFALL AMOUNTS OVERNIGHT HAVE BEEN RUNNING BELOW THE PREDICTED
VALUES. WITH THE FRONT MOVING THROUGH LATER THIS MORNING FLOODING ON
THE NOOKSACK ALSO LOOKS UNLIKELY.

FLOODING IS NOT EXPECTED ON OTHER AREA RIVERS DURING THE NEXT 7 DAYS
ALTHOUGH IF THE ECMWF SOLUTION VERIFIES SOME OF THE MORE FLOOD PRONE
RIVERS WILL NEED WATCH IN THE MIDDLE OF THE WEEK. FELTON/MERCER

&&

.AVIATION...MODERATE SOUTHWEST FLOW ALOFT. THE AIR MASS IS STABLE
AND MOIST BUT WILL BECOMING MORE UNSTABLE TODAY AS COOLER AIR MOVES
IN ALOFT. AREAS OF RAIN OVER WESTERN WASHINGTON THIS MORNING WILL
MORE SHOWERY TODAY. THERE IS A TROUGH JUST OFFSHORE THAT SHOULD
WEAKEN AND NEAR THE COAST LATER TODAY.

KSEA...THE RAIN WILL TURN TO SHOWERS THIS MORNING. SOME PRETTY POOR
CONDITIONS THIS MORNING...IFR CEILINGS AND VISIBILITIES. THESE
SHOULD IMPROVE IN THE AFTERNOON AS THE SYSTEM MOVES SLOWLY EAST.
LIGHT SOUTHERLY WIND. 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. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...FLOOD WARNING SKOKOMISH RIVER.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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