Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 010402
AFDSEW

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

.SYNOPSIS...AN UPPER TROUGH WILL MOVE INTO WESTERN WASHINGTON
TONIGHT AND SATURDAY...FOR MAINLY DRY WEATHER WITH A LITTLE SUNSHINE
SATURDAY. A WEAK RIDGE WILL MOVE INLAND SATURDAY NIGHT...AND THEN A
WARM FRONT WILL BRING SOME RAIN SUNDAY AND SUNDAY NIGHT. A STRONGER
FRONTAL SYSTEM WILL BRING RAINY AND LOCALLY WINDY WEATHER MONDAY AND
TUESDAY...AND THE WET PATTERN WILL PROBABLY CONTINUE NEXT WEEK.

&&

.SHORT TERM...WE HAVE SOUTHERLY FLOW ALOFT OVER WESTERN WASHINGTON
THIS EVENING AHEAD OF AN ELONGATED UPPER TROUGH APPROACHING THE
REGION FROM THE OFFSHORE WATERS. SKIES WERE PARTLY CLOUDY TO CLOUDY
ACROSS THE FORECAST AREA AT 8 PM WITH ONLY ISOLATED SHOWERS. HOWEVER
A CELL THAT DEVELOPED OVER THE NORTH KITSAP PENINSULA EARLIER THIS
EVENING PRODUCED LOCALLY HEAVY SHOWERS AND ISOLATED THUNDER AND
LIGHTNING AS IT MOVED ACROSS NORTH SEATTLE...LAKE WASHINGTON...AND
KIRKLAND JUST AFTER 8 PM. THE TOPS ON THIS CELL WERE ONLY AROUND
13000 FT. IT SHOULD WEAKEN OVER THE NEXT 30 MINUTES AS IT CONTINUES
EAST...AND WE DO NOT EXPECT THIS ACTIVITY TO BECOME MORE WIDESPREAD.
WE ISSUED A NOWCAST FOR IT.

THE UPPER TROUGH WILL MOVE TO THE PACIFIC NORTHWEST COAST LATE
TONIGHT AND THEN INLAND ON SATURDAY. AS MENTIONED ABOVE THE TROUGH
IS ELONGATED...AND THE CORE OF IT WILL BE DIGGING SOUTHEAST ACROSS
CALIFORNIA. THE NORTHERN PORTION HEADING INTO WESTERN WASHINGTON
WILL BE WEAK...SO ANY SHOWERS IT PRODUCES SHOULD BE LIGHT AND
SPOTTY. MOST OF WESTERN WASHINGTON WILL HAVE PARTLY SUNNY SKIES ON
SATURDAY.

A FLAT UPPER RIDGE WILL MOVE INTO THE OFFSHORE WATERS SATURDAY NIGHT
AND THEN PROGRESS INLAND ON SUNDAY. A WEAK WARM FRONT IMMEDIATELY
UPSTREAM OF THE RIDGE AXIS WILL MOVE INTO THE FORECAST AREA SUNDAY
OR SUNDAY NIGHT. THE TIMING OF THIS FEATURE IS STILL SOMEWHAT
UNCERTAIN...AND IT WILL PROBABLY BE WEAKENING AS IT MOVES INLAND.
THE COAST WILL PROBABLY HAVE SOME RAINFALL ON SUNDAY BUT THE
INTERIOR COULD REMAIN DRY UNTIL LATE IN THE DAY OR EVEN SUNDAY NIGHT.

A STRONGER AND WETTER FRONTAL SYSTEM WILL MOVE INTO WESTERN
WASHINGTON MONDAY AND MONDAY NIGHT. THIS SYSTEM SHOULD RESULT IN
BREEZY TO LOCALLY WINDY CONDITIONS AND FAIRLY SIGNIFICANT
PRECIPITATION. THE SNOW LEVEL WILL RISE TO AROUND 7000 FT.   MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
FORECAST DISCUSSION -- 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...A PACIFIC FRONTAL SYSTEM WILL BRING AROUND 2 TO 4
INCHES OF RAIN...AND LOCALLY PERHAPS 5 INCHES...TO THE OLYMPIC
MOUNTAINS SUNDAY NIGHT THROUGH TUESDAY. FLOODING IS POSSIBLE ON THE
EXCEPTIONALLY FLOOD-PRONE SKOKOMISH RIVER IF THAT OCCURS.

FLOODING IS UNLIKELY ON ALL OTHER RIVERS IN OUR FORECAST AREA FOR
THE NEXT 7 DAYS.

IN ADDITION...RECENT HEAVY PRECIPITATION AMOUNTS HAVE CREATED
ANTECEDENT CONDITIONS FAVORABLE TO LANDSLIDES IN WESTERN
WASHINGTON...ACCORDING TO THE USGS LANDSLIDE HAZARDS PROGRAM. IN
FACT...PRECIPITATION FOR THE MONTH OF OCTOBER AT SEATAC IS AROUND
190 PERCENT OF NORMAL...AND THIS WILL PROBABLY BE THE THIRD WETTEST
OCTOBER ON RECORD AT SEATAC. THE THREAT OF LANDSLIDES WILL BE
SOMETHING TO WATCH AS A WET WEATHER PATTERN CONTINUES.   MCDONNAL

&&

.AVIATION...THERE WAS A TSTM OVER NRN LAKE WASHINGTON AT 830PM...WE
HAD A DOWNPOUR HERE AT THE NWS AND THERE WAS A LIGHTNING STRIKE IN
KIRKLAND. SOUTHWEST FLOW ALOFT WILL WEAKEN 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 WHICH WILL END OVERNIGHT. SLOW DRYING OF THE AIR
WILL OCCUR TONIGHT AND SATURDAY WITH PARTLY SUNNY SKIES ON SATURDAY.

KSEA...A SHOWER OVER THE NRN HALF OF LAKE WASHINGTON GENERATED A
LIGHTNING STRIKE OVER IN KIRLAND AROUND 830PM...WITH TOPS AROUND
13KFT THAT WAS PROBABLY A ONE FLASH STORM. THE SHOWERS WERE MOVING
EAST. THE CURRENT TAF LOOKS LIKE A GOOD FORECAST ALTHOUGH WITH LIGHT
PRESSURE GRADIENTS THERE IS A CHANCE THAT SOME RATHER LOW STRATUS
COULD SET UP OVERNIGHT. FOG ISN`T LIKELY BUT WE MAY SEE A PERIOD OF
3SM BR AROUND DAYBREAK.

&&

.MARINE...PRESSURE GRADIENTS ARE LIGHT THIS EVENING. THE WEAK TROUGH
JUST OFFSHORE HAS PRETTY MUCH DISSIPATED NOW. WEAK HIGH PRESSURE
WILL BUILD SATURDAY. STRONGER SYSTEMS ARE LIKELY NEXT WEEK BEGINNING
SUNDAY. SMALL CRAFT ADVISORY WINDS ARE IN THE GRIDS AND FORECASTS
FOR SEVERAL PERIODS CENTERED AROUND SUNDAY NIGHT AND AGAIN
WEDNESDAY.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

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





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