Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 312156
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
256 PM PDT MON AUG 31 2015

.SYNOPSIS...A FRONTAL SYSTEM WILL BRING RAIN AT TIMES TO WESTERN
WASHINGTON TONIGHT AND TUESDAY AS IT MOVES SLOWLY SOUTH ACROSS THE
AREA. A LARGE UPPER LEVEL LOW WILL SETTLE OVER THE REGION WEDNESDAY
THROUGH FRIDAY FOR COOL SHOWERY WEATHER. THE UPPER LEVEL LOW WILL
WEAKEN AND MOVE EAST EARLY THIS WEEKEND FOR A GRADUAL DRYING AND
WARMING TREND.

&&

.SHORT TERM...A BROAD UPPER LEVEL LONGWAVE TROUGH EXTENDS FROM THE
NE PACIFIC ACROSS WESTERN CANADA AND THE PACNW THIS AFTERNOON...WITH
FAIRLY STRONG W FLOW ALOFT OVER W WA FOR LATE AUGUST. A FRONTAL
SYSTEM IS HUNG UP OVER SW B.C. WITH THE COLD FRONT EXTENDING WSW
FROM ABOUT CENTRAL VANCOUVER ISLAND. RADAR STILL SHOWS AREAS OF RAIN
TRAVERSING THE NORTHERN HALF OF W WA THIS AFTERNOON.

WATER VAPOR IMAGERY SHOWS A WELL DEVELOPED SHORTWAVE TROUGH OFFSHORE
THAT HAS MOVED SE TO AROUND 47N/136W THIS AFTERNOON AND WILL
COMEDIAN HEADING SE TOWARD N CA. AS THIS SHORTWAVE MOVES SE IT WILL
WILL DRAG THE SW B.C. COLD FRONT SLOWLY SE ACROSS W WA LATE TONIGHT
THROUGH TUESDAY. FOR W WA THIS MEANS PERIODS OF LIGHT RAIN THROUGH
TUESDAY AND BREEZY WINDS AT TIMES.

THE BROAD UPPER LEVEL LONGWAVE TROUGH OVER THE REGION...MENTIONED
ABOVE...WILL GRADUALLY DEEPEN AND GAIN AMPLITUDE OVER THE REGION
THIS WEEK. A SMALL UPPER LEVEL LOW OVER THE GULF OF AK TODAY WILL
MOVE SE ALONG THE B.C. COAST THROUGH TUESDAY...EVENTUALLY BECOMING
THE CENTER OF THIS LARGE DEEPENING LONGWAVE TROUGH OVER CENTRAL B.C.
ON WEDNESDAY. THIS WILL BE A COOL SHOWERY PERIOD FOR W WA. THE AIR
MASS WILL COOL AS COLD AIR ALOFT WITH THE TROUGH MOVES DOWN OVER THE
AREA. THIS WILL LOWER SNOW LEVELS OVER THE MOUNTAINS TO 5000-6000
FEET LATE TUESDAY NIGHT THROUGH THURSDAY NIGHT.

WHILE THE LARGE UPPER LEVEL TROUGH IS OVER THE REGION A NUMBER OF
SHORTWAVE TROUGHS EMBEDDED IN THE FLOW AROUND THE LOW WILL MOVE
ACROSS W WA. THE GFS SHOWS A SHORTWAVE TROUGH ASSOCIATED WITH THE
DEVELOPING LOW CENTER CROSSING W WA TUESDAY NIGHT...BRINGING AND
INCREASE IN SHOWERS. THE 06Z GFS RUN SHOWED ANOTHER WEAK SHORTWAVE
CROSSING W WA WEDNESDAY...BUT THIS FEATURE HAS DROPPED OUT OF THE
12Z RUN. MOIST W FLOW ALOFT WILL CONTINUE OVER THE AREA
WEDNESDAY...AND THERE COULD STILL BE A WEAK SHORTWAVE PROVIDING SOME
EXTRA LIFT...SO SCATTERED SHOWERS WILL CONTINUE. A LARGER MORE
DEVELOPED SHORTWAVE TROUGH IS EXPECTED TO MOVE ACROSS THE AREA ON
THURSDAY. SW PRESSURE GRADIENTS WILL REMAIN STRONG ENOUGH SO THAT
LOCALLY BREEZY WINDS ARE LIKELY AS EACH SHORTWAVE CROSSES THE AREA.
KAM

.LONG TERM...THE CENTER OF THE LARGE UPPER LEVEL TROUGH OVER B.C.
WILL FORM A LOW WHICH WILL MOVE S OVER W WA ON FRIDAY. WHILE THE LOW
IS OVERHEAD FRIDAY AFTERNOON...THE GFS HAS LIFTED INDICES DROPPING
BELOW ZERO. THERE COULD BE ENOUGH INSTABILITY FOR ISOLATED
THUNDERSTORMS...BUT I WILL HOLD OFF INCLUDING THEM IN THE FORECAST
UNTIL THE MODELS BECOME A LITTLE MORE CONSISTENT OR MORE UNSTABLE.

FRIDAY NIGHT AND SATURDAY THE GFS BEGINS MOVING THE WEAKENING TROUGH
TO THE EAST...WHILE THE ECMWF AND CANADIAN MOVE IT S OVER OREGON
SATURDAY THEN EAST SATURDAY NIGHT. THE CURRENT FORECASTS STILL
FOLLOW THE GFS SOLUTION WHICH ENDS THE SHOWERS OVER W WA A LITTLE
SOONER. ONCE THE TROUGH MOVES EAST A WARMING TREND WILL
DEVELOP...EITHER STARTING SATURDAY OR HOLDING OFF UNTIL SUNDAY. THE
MODEL SOLUTIONS DIFFER ON MONDAY AS WELL SO CONFIDENCE IN THE MOSTLY
SUNNY FORECAST IS A LITTLE SHAKY. KAM

&&

.AVIATION...A FRONTAL SYSTEM OVER THE AREA THROUGH TONIGHT WILL MOVE
INLAND LATER TUESDAY. RAIN AT TIMES THROUGH TONIGHT SHOULD TAPER OFF
TO SCATTERED SHOWERS LATER TUE. THE MOUNTAINS AND HIGHER TERRAIN
WILL REMAIN OBSCURED AND THE VSBY WILL DROP AT TIMES IN THE PRECIP
AND MIST. THE AREAS OF BREEZY SOUTHERLY WIND WILL DECREASE OVERNIGHT
AND TUESDAY MORNING AS THE FRONT WEAKENS.

KSEA...CLOUDY WITH RAIN AT TIMES AND A SLY BREEZE CONTINUES.

&&

.MARINE...A FRONTAL SYSTEM IS OVER THE AREA BUT IT WILL MOVE INLAND
TUESDAY. THE SOUTHERLY GRADIENTS TODAY WHICH ARE STRONG ENOUGH FOR
BREEZY WEATHER AND SMALL CRAFT ADVISORIES SHOULD GIVE OUT OVERNIGHT
OR TUESDAY MORNING. A BROAD TROUGH IN THE NORTHEAST PACIFIC WILL
GRADUALLY SAG OVER THE PACNW AND WEAKEN OVER THE NEXT SEVERAL DAYS
AND THEN WEAK HIGH PRESSURE WILL BUILD LATE IN THE WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COAST...EAST ENTRANCE TO THE STRAIT...
      ADMIRALTY INLET...NORTHERN INLAND WATERS...PUGET SOUND.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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