Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 301028
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
328 AM PDT SUN AUG 30 2015

.SYNOPSIS...A BROAD UPPER TROUGH WILL REMAIN OVER THE REGION FOR
THE NEXT SEVERAL DAYS...AND SMALLER SYSTEMS MOVING THROUGH THE
TROUGH WILL BRING RATHER COOL WET AND LOCALLY BREEZY WEATHER TO
WESTERN WASHINGTON TODAY THROUGH AT LEAST THE MIDDLE OF THE WEEK.
THE SNOW LEVEL IN THE MOUNTAINS WILL RANGE FROM AROUND 6000 TO
8000 FEET OVER THE NEXT SEVERAL DAYS.

&&

.SHORT TERM...AN UPPER TROUGH IS MOVING TOWARD THE PACIFIC
NORTHWEST COAST EARLY THIS MORNING...WITH ITS TROUGH AXIS OVER THE
COASTAL WATERS. LIGHT TO MODERATE SHOWERS CONTINUE OVER MUCH OF
WESTERN WASHINGTON...BUT THE WINDS ACROSS THE FORECAST AREA HAVE
BEEN MOSTLY 20 MPH OR LESS OVERNIGHT.

THE UPPER TROUGH WILL MOVE INLAND TODAY...CROSSING THE CASCADES
CREST MIDDAY AND MOVING QUICKLY ACROSS EASTERN WASHINGTON THIS
AFTERNOON. SHOWERS WILL CONTINUE AHEAD OF THE TROUGH AXIS...BUT
THEY SHOULD GENERALLY LET UP IN ITS WAKE WITH SOME SUN BREAKS.
THERE HAVE BEEN SOME THUNDERSTORMS IN THE PRECIPITATION BANDS
AHEAD OF THE UPPER TROUGH AS WELL...SO WE WILL HAVE A CHANCE THIS
MORNING. HIGHS SHOULD BE IN THE MID 60S TO LOWER 70S...WITH THE
SNOW LEVEL AROUND 7000 FT.

MOIST WESTERLY FLOW ALOFT WILL CONTINUE TONIGHT AND MONDAY...AS A
BROAD UPPER TROUGH REMAINS OVER THE REGION. AN UPPER SHORTWAVE IN
THE LARGER TROUGH WILL MOVE THROUGH SOUTHERN BRITISH COLUMBIA AND
THE NORTH PART OF WASHINGTON LATER TONIGHT AND MONDAY. THIS
FEATURE SHOULD BRING A HALF INCH TO AN INCH OF PRECIPITATION TO THE
NORTH COAST...OLYMPIC MOUNTAINS...NORTH INTERIOR...AND NORTH
CASCADES. MOST OF THE PUGET SOUND REGION AND SOUTHWEST INTERIOR
WILL RECEIVE JUST A TENTH TO A QUARTER OF AN INCH.

ANOTHER SYSTEM WILL MOVE THROUGH THE FORECAST AREA MONDAY NIGHT
AND TUESDAY. IT WILL ALSO BRING MOST OF THE PRECIPITATION TO THE
NORTH COAST...OLYMPICS...NORTH INTERIOR...AND NORTH CASCADES. THE
INTERIOR LOWLANDS FROM AROUND EVERETT SOUTHWARD...ESPECIALLY THE
SEATTLE AREA...SHOULD HAVE LESS RAINFALL. HIGHS MONDAY AND
TUESDAY WILL BE IN THE 60S TO LOWER 70S...AND THE SNOW LEVEL WILL
BE AROUND 8000 FT. MCDONNAL

.LONG TERM...MODELS HAVE BEEN FAIRLY CONSISTENT WITH THE IDEA THAT
THE LARGE UPPER TROUGH WILL PROGRESS SLOWLY TOWARD THE AREA
WEDNESDAY...WITH AN UPPER SHORTWAVE DEEPENING THE TROUGH AS IT
MOVES INLAND OVER THE FORECAST AREA ON THURSDAY. THIS WILL KEEP
SHOWERS OVER WESTERN WASHINGTON...ESPECIALLY AT THE COAST AND IN
THE MOUNTAINS. THE SNOW LEVEL WILL FALL TO AROUND 6000 FT.

CONFIDENCE IN THE FORECAST FOR FRIDAY AND SATURDAY IS NOT VERY
HIGH. EARLIER MODEL RUNS SHOWED DRIER NORTHWEST FLOW ALOFT OVER
THE FORECAST AREA FRIDAY AND SATURDAY...IN THE WAKE OF THE UPPER
TROUGH DISCUSSED JUST ABOVE. HOWEVER THE MOST RECENT RUNS OF THE
GFS...EUROPEAN...AND CANADIAN MODELS SHOW ANOTHER UPPER TROUGH
DROPPING OVER WESTERN WASHINGTON ON FRIDAY FOR CONTINUED COOL
SHOWERY WEATHER. OUR CURRENT FORECAST HINTS AT ONLY SLIGHTLY DRIER
WEATHER...SO IT SHOULD SUFFICE UNTIL FORECAST CONFIDENCE
IMPROVES. MCDONNAL

&&

.CLIMATE...THE 0.73 INCHES OF RAIN THAT FELL AT SEA-TAC AIRPORT
DURING THE LAST THREE HOURS OF THE DAY PUSHED THE RAINFALL TOTAL FOR
SATURDAY TO 1.28 INCHES. THIS IS THE 5TH WETTEST AUGUST DAY ON
RECORD AT SEA-TAC. WITH THE 1.20 INCHES THAT WAS RECORDED ON THE
14TH...NOW TIED FOR THE 7TH WETTEST DAY IN AUGUST AT SEA-TAC...THIS
IS THE FIRST TIME INCLUDING THE FEDERAL BUILDING RECORDS THAT GO
BACK TO 1891 THAT THERE HAS BEEN TWO DAYS IN AUGUST WITH 1 INCH OR
MORE OF RAIN. THROUGH THE 29TH THE RAINFALL TOTAL FOR THE MONTH AT
SEA-TAC IS 2.88 INCHES...TIED FOR THE 5TH WETTEST AUGUST ON RECORD.
FELTON

&&

.AVIATION...FRONTAL SYSTEM THAT WAS OVER THE AREA SATURDAY EVENING
HAS MOVED EAST OF THE CASCADES EARLY THIS MORNING. UPPER LEVEL
TROUGH OFFSHORE MOVING OVER WESTERN WASHINGTON THIS MORNING  KEEPING
THE AIR MASS MOIST AND SOMEWHAT UNSTABLE. IN GENERAL MULTIPLE CLOUD
LAYERS BETWEEN 2000 AND 8000 FEET. CEILINGS IN THE 3500-4500 FOOT
RANGE WITH SHOWER ACTIVITY LOWERING THE CEILINGS DOWN TO NEAR 2000
FEET WITH VISIBILITIES AS LOW AS 3 SM. THERE IS A CHANCE OF
THUNDERSTORMS ACROSS THE AREA THIS MORNING WITH THE BEST CHANCES
ALONG THE COAST. THE CHANCE FOR THUNDERSTORMS IS NOT HIGH ENOUGH TO
INCLUDE IN THE TERMINAL FORECASTS. TROUGH MOVING EAST OF THE AREA
THIS AFTERNOON WITH SHOWER CHANCES DECREASING FOR THE LOWLANDS. A
WEAK FRONT WILL APPROACH THE AREA FROM THE NORTHWEST TONIGHT.

KSEA...CEILINGS IN THE 3500-4500 FOOT RANGE LOWERING TO NEAR 2500
FEET IN SHOWERS THIS MORNING. SOUTHERLY WINDS 8 TO 12 KNOTS.
FELTON

&&

.MARINE...WINDS HAVE EASED ACROSS ALL THE WATERS THIS MORNING AS THE
COLD FRONT MOVED EAST OF THE AREA. SOUTHERLY GRADIENTS WILL INCREASE
AGAIN TONIGHT AS ANOTHER FRONTAL SYSTEM APPROACHES FROM THE
NORTHWEST WITH SMALL CRAFT ADVISORY WINDS EXPECTED OVER MOST OF THE
WATERS BY LATE TONIGHT. SMALL CRAFT ADVISORY WINDS WILL CONTINUE
OVER THE INTERIOR WATERS MONDAY MORNING BEFORE EASING MONDAY
AFTERNOON. A COUPLE OF WEAK SYSTEMS WILL MOVE THROUGH THE AREA
TUESDAY AND WEDNESDAY. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...NONE.

PZ...SMALL CRAFT ADVISORIES COAST...ENTRANCES TO THE STRAIT...
     ADMIRALTY INLET AND THE INLAND WATERS.
&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS DISCUSSION AT
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