Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 292204 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI AUG 29 2014

.SYNOPSIS...LOW PRESSURE ALOFT WILL DOMINATE THE WEATHER PATTERN
OVER THE REGION THROUGH THIS WEEKEND FOR COOL AND UNSETTLED
CONDITIONS. A SERIES OF WEATHER SYSTEMS ARE EXPECTED TO MOVE ONSHORE
THROUGH THE HOLIDAY WEEKEND. ANOTHER SYSTEM IS FORECAST TO MOVE
ONSHORE TUESDAY BEFORE HIGHER PRESSURE ALOFT BUILDS FOR A WARMING
AND DRYING TREND TOWARD THE END OF NEXT WEEK.

&&

.SHORT TERM...ZONAL FLOW PATTERN ACROSS THE NORTHERN PACIFIC WITH A
DECENT JETSTREAM FOR LATE AUGUST. PEAK OF THE JET NEAR THE DATELINE
WAS ABOUT 135 KNOTS EARLY THIS AFTERNOON. SO EMBEDDED SYSTEMS SHOULD
MOVE RIGHT ALONG AND THAT IS WHAT THE LATEST PROGS SHOW THROUGH THE
HOLIDAY WEEKEND AND INTO NEXT WEEK.

THE OFFSHORE UPPER TROUGH WAS MOVING INSIDE 130W THIS AFTERNOON AND
SHOULD MOVE ONSHORE TONIGHT WITH LIMITED LIGHT SHOWERS. THE
ASSOCIATED VERY WEAK COLD FRONT MOVED INLAND EARLIER TODAY AND
PRODUCED QUITE LIGHT AMOUNTS OF RAIN - MAINLY TRACE AMOUNTS.

THE NEXT EMBEDDED SHORTWAVE HAD PROGRESSED TO JUST INSIDE 160W EARLY
THIS AFTERNOON. PROGS ARE SIMILAR IN BRINGING THIS SYSTEM ONSHORE
SATURDAY PM FOR ANOTHER INCREASE IN SHOWERS. THE FOLLOWING SHORTWAVE
WAS MOVING ACROSS THE DATELINE AND AGAIN THE PROGS ARE RATHER
SIMILAR IN BRINGING THIS ONE ONSHORE LATE SUNDAY FOR ANOTHER SURGE
OF SHOWERS...PERHAPS BEING THE WETTEST OF THE SERIES IN A RELATIVE
MANNER.

AMOUNTS IN GENERAL SHOULD BE RATHER LIGHT THROUGHOUT THE HOLIDAY
WEEKEND WITH THE GREATEST TOTALS IN OROGRAPHIC MOUNTAIN AREAS. IN
ADDITION...EACH SYSTEM MAY INCLUDE A PUGET SOUND CONVERGENCE ZONE AS
WELL - NOTED IN THE MESOSCALE MODEL OUTPUT AND IN THE GRAPHICAL
FORECASTS. HEIGHTS RISE MONDAY FOR A DRYING TREND WITH LINGERING
SHOWERS MAINLY IN THE MOUNTAINS AND CONVERGENCE ZONE. DAYTIME TEMPS
SHOULD REMAIN A BIT BELOW SEASONAL AVERAGES. BUEHNER

.LONG TERM...THE MEDIUM RANGE GUIDANCE - FOREIGN AND DOMESTIC - HAS
GENERALLY COME TOGETHER REGARDING DAYS 4 THRU 7 OF THE FORECAST
PERIOD. ALL DIG A SHORTWAVE INTO THE PACIFIC NW AS THE ZONAL FLOW
PATTERN AMPLIFIES THROUGH THE WEEK. PRECIPITATION WITH THE SHORTWAVE
ARRIVES TUESDAY AND EXITS BY LATE WEDNESDAY - A BIT QUICKER THAN
PREVIOUS GUIDANCE. THEN RISING HEIGHTS ALOFT AND LOW LEVEL FLOW
TURNS MORE OFFSHORE AS THERMALLY INDUCED LOW PRESSURE NOSES NORTH
FROM NORTHERN CAL AND WESTERN OREGON. THERE HAS BEEN SOME
SPECULATION THAT PERHAPS WE MAY NOT SEE 80 DEGREES AGAIN THIS YEAR.
THIS PATTERN CHANGE MAY PRODUCE SOME 80S WHEN IT IS ALL SAID AND
DONE.

OF INTEREST...KSEA HAS HAD 39 DAYS OF 80 DEGREES OR BETTER THIS YEAR
MAKING IT THE 6TH MOST NUMBER OF WARM DAYS ON RECORD. THE MOST IS 47
DAYS OF 80 OR WARMER SET IN 1958. IN SHARP CONTRAST...1954 HAD ONLY
2 DAYS OF 80 OR BETTER - WHEW! BUEHNER

&&

.AVIATION...SKIES ARE MOSTLY CLOUDY OVER WESTERN WA AS A WEAK COLD
FRONT BREAKS UP OVER WRN WA THIS AFTERNOON. WITH A DEEP MARINE LAYER
IN PLACE TONIGHT...LOW CLOUDS WILL SPREAD OUT OVERNIGHT...THEN LIFT
AND BREAK UP IN TYPICAL FASHION SATURDAY. LOW LEVEL ONSHORE FLOW
WILL CONTINUE THROUGH SATURDAY. THE FLOW ALOFT IS WESTERLY.

KSEA...THE CLOUDS WERE BREAKING UP THIS AFTERNOON AS THE WEAK COLD
FRONT DISSIPATED. WITH A MARINE LAYER IN PLACE LOW CLOUDS WILL
REDEVELOP OVERNIGHT AND THEN BREAK UP AGAIN MIDDAY SAT. PRESSURE
GRADIENTS ARE LIGHT AND THE WIND THIS EVENING MAY TURN FROM SW TO
JUST LIGHT AND VARIABLE FOR A TIME.

&&

.MARINE...A WEAK COLD FRONT BROKE UP OVER WRN WA THIS AFTERNOON.
ONSHORE FLOW SHOULD INCREASE BEHIND THE FRONT THIS EVENING AND THE
FCST IS FOR SMALL CRAFT ADVISORY WINDS IN THE CENTRAL AND EAST
STRAIT OF JUAN DE FUCA. ANOTHER WEAK SURFACE TROUGH WILL MOVE INLAND
ON SUNDAY. HIGH PRES OVER THE NE PAC WILL MAINTAIN ONSHORE FLOW
ACROSS WESTERN WA THROUGH EARLY NEXT WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY IN EFFECT UNTIL 5 AM SATURDAY FOR THE
     CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

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