Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 010320
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
820 PM PDT FRI JUL 31 2015

.SYNOPSIS...AN UPPER RIDGE WILL GRADUALLY WEAKEN OVER THE NEXT
COUPLE OF DAYS. ONSHORE FLOW WILL RETURN EARLY NEXT WEEK FOR
COOLER WEATHER. AN UPPER TROUGH WILL BRING A CHANCE OF SHOWERS
LATER IN THE WEEK.

&&

.SHORT TERM...SKIES WILL BE CLEAR TONIGHT EXCEPT AT THE COAST
WHERE A LITTLE STRATUS OR FOG COULD FORM LATER TONIGHT UNDER WEAK
LOW LEVEL ONSHORE FLOW.

SATURDAY WILL BE ANOTHER VERY WARM DAY FOR THE INTERIOR WITH AN
UPPER RIDGE IN CONTROL. UPPER HEIGHTS ARE FORECAST TO FALL SLIGHTLY
BUT THE 850 MB TEMPERATURES ACTUALLY WARM A LITTLE AND LOW LEVEL
ONSHORE FLOW WILL REMAIN WEAK AND NORTHWESTERLY. HIGHS SHOULD
ONCE AGAIN BE IN THE LOWER TO MID 90S FOR PARTS OF THE INTERIOR. A
HEAT ADVISORY REMAINS IN EFFECT FOR SEATTLE SOUTHWARD.

UPPER HEIGHTS WILL CONTINUE TO FALL OVER THE AREA ON SUNDAY AS AN
UPPER TROUGH APPROACHES FROM THE WEST. AT THE SAME TIME...ONSHORE
FLOW WILL TURN MORE WESTERLY AND BEGIN TO STRENGTHEN. IT WILL BE
ANOTHER WARM DAY FOR THE INTERIOR...BUT PROBABLY A FEW DEGREES
COOLER COMPARED TO SATURDAY FOR MOST SPOTS.

A MORE SIGNIFICANT AND NOTICEABLE COOL-DOWN WILL OCCUR MONDAY. AN
UPPER TROUGH WILL MOVE OVERHEAD AND LOW LEVEL ONSHORE FLOW WILL
BECOME STRONG AND MORE SOUTHWESTERLY. HIGHS SHOULD COOL SEVERAL
DEGREES...BACK TO WITHIN A FEW DEGREES OF NORMAL. SCHNEIDER

.LONG TERM...PREVIOUS DISCUSSION...ON TUESDAY MODELS HAVE BEEN
FAIRLY CONSISTENT IN SHOWING A SHORTWAVE TROUGH APPROACHING THE
AREA. EXPECT CONDITIONS SIMILAR TO MONDAY...BUT PERHAPS A FEW
DEGREES COOLER.

WEDNESDAY THROUGH FRIDAY THE MODELS DIVERGE...WITH THE ECMWF AND
CANADIAN MODELS SHOWING AN UPPER LEVEL CLOSED LOW DROPPING DOWN
ALONG THE COAST FROM HAIDA GWAII AND MOVING DIRECTLY OVER WESTERN
WASHINGTON ON THURSDAY. IN THIS CASE...THE WHOLE AREA COULD SEE
SHOWERS ON THURSDAY AND HIGHS COULD FALL SLIGHTLY BELOW NORMAL. THE
TWO MOST RECENT GFS RUNS HAVE BEEN OUTLIERS...BRINGING THE LOW WELL
TO THE NORTH OF US ACROSS NORTHERN BC. IN THIS CASE...THE AREA WOULD
NOT SEE ANY SHOWERS AT ALL AND SEE NORMAL TO SLIGHTLY ABOVE NORMAL
TEMPERATURES. AS WE GET INTO NEXT WEEK...HOPEFULLY THE MODELS WILL
BEGIN TO CONVERGE ON A SINGLE SOLUTION. JSMITH

&&

.AVIATION...A BROAD UPPER LEVEL RIDGE WILL REMAIN OVER THE PACIFIC
NORTHWEST THROUGH THIS WEEKEND WITH LIGHT W-SW FLOW ALOFT. EXCEPT
FOR AREAS OF LATE NIGHT AND MORNING STRATUS MAINLY OVER WATER
AREAS...THE AIR MASS WILL REMAIN VERY WARM...DRY AND STABLE. THE
DENSITY ALTITUDE WILL BE HIGHER THAN NORMAL DURING THE AFTERNOON
HOURS.

AREAS OF LATE NIGHT AND MORNING STRATUS ARE EXPECTED TO FORM AFTER
09Z ALONG THE COAST AND PARTWAY INLAND BETWEEN KHQM AND KSHN...AND
IN THE WESTERN STRAIT OF JUAN DE FUCA...BRINGING LOCAL IFR
CONDITIONS. THE STRATUS WILL BURN OFF AGAIN BY 18Z SATURDAY MORNING.
OTHERWISE CLEAR SKIES WILL CONTINUE ACROSS THE W WA.

KSEA...CLEAR SKIES FOR GOOD VFR CONDITIONS. SURFACE WINDS WILL
REMAIN N-NE 3-7 KT TONIGHT THEN RISE TO N-NW 5-10 KT AFTER 18Z
SATURDAY. KAM

&&

.MARINE...A LARGE SURFACE RIDGE OFFSHORE WILL COMBINE WITH LOWER
PRESSURE OVER E WA TO MAINTAIN VARYING DEGREES OF ONSHORE FLOW
THROUGH NEXT WEEK. THE ONSHORE FLOW IS EXPECTED TO BECOME MODERATE
TO STRONG DURING THE LATE AFTERNOON AND EVENING HOURS THIS WEEKEND
WITH POSSIBLE GALES IN THE STRAIT OF JUAN DE FUCA. SMALL CRAFT
ADVISORY NORTHWEST WINDS ARE LIKELY OVER THE OUTER COASTAL WATERS
TONIGHT THROUGH AT LEAST SATURDAY NIGHT AND POSSIBLY SUNDAY AS WELL.
THESE SCA NW WINDS MAY REACH THE NEAR SHORE COASTAL WATERS SATURDAY
NIGHT.

THERE IS A LITTLE UNCERTAINTY REGARDING A POSSIBLE GALE IN THE
STRAIT SATURDAY EVENING. THE 18Z GFS SHOWED ABOUT THE SAME UIL-BLI
PRESSURE GRADIENT SATURDAY EVENING AS THIS EVENING...WHICH SO FAR
HAS ONLY PRODUCED 29 KT AT RACE ROCKS. THE U.S. MESO MODELS ALL SHOW
WINDS WELL BELOW GALE. THE 18Z CANADIAN 2.5 KM LAM ONLY SHOWED A
BRIEF TINY AREA OF POSSIBLE GALE WINDS. FOR NOW THE GALE WATCH WILL
CONTINUE FOR SATURDAY EVENING. SOMEWHAT WEAKER ONSHORE FLOW SHOULD
PREVAIL NEXT WEEK. KAM

&&

.FIRE WEATHER...PREVIOUS DISCUSSION...A CRITICAL PERIOD FOR FIRE
WEATHER CONTINUES OVER ALL OF WESTERN WASHINGTON...WITH A VERY DRY
AIR MASS IN PLACE. THE IMMEDIATE COAST HAS SEEN SOME RELIEF THIS
AFTERNOON FROM SEA BREEZES WHICH HAVE RAISED DEWPOINTS INTO THE
UPPER 50S AT HOQUIAM AND QUILLAYUTE. ELSEWHERE ACROSS WESTERN
WASHINGTON...DRY AIR PERSISTS.

IN ADDITION...ELEVATED MOUNTAIN LOCATIONS ARE FEELING THE ADDED
EFFECT OF SOMEWHAT UNSTABLE MID-LEVEL AIR...WITH BOTH MID-LEVEL AND
HIGH-LEVEL HAINES 5 CONDITIONS CONTINUING THROUGH SATURDAY EVENING.
VISIBLE SATELLITE IMAGERY SHOWED A BURST OF SMOKE THIS AFTERNOON
COMING FROM THE PARADISE FIRE...SO IT APPEARS THE FIRE IS FEELING
THE EFFECTS OF THE DRY AND UNSTABLE AIR MASS. WILL KEEP THE RED
FLAG WARNING GOING FOR ELEVATIONS ABOVE 2000 FEET UNTIL SATURDAY
EVENING FOR THE DRY AND UNSTABLE AIR MAS. LARGE FIRE GROWTH IN
WESTERN WASHINGTON HISTORICALLY TAKES PLACE UNDER SUCH CONDITIONS.
HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HEAT ADVISORY UNTIL 8 PM PDT SUNDAY FOR BELLEVUE AND VICINITY-
     EAST PUGET SOUND LOWLANDS-SEATTLE AND VICINITY-SOUTHWEST
     INTERIOR-TACOMA AREA.

     RED FLAG WARNING UNTIL 10 PM PDT SATURDAY FOR CENTRAL CASCADES-
     EAST PORTION OF THE OLYMPIC MOUNTAINS-NORTH CASCADES-WEST
     PORTION OF THE OLYMPIC MOUNTAINS.

PZ...SMALL CRAFT ADVISORY UNTIL 6 AM PDT SUNDAY FOR COASTAL WATERS
     FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-COASTAL
     WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO 60 NM-
     COASTAL WATERS FROM POINT GRENVILLE TO CAPE SHOALWATER 10
     TO 60 NM.

     SMALL CRAFT ADVISORY UNTIL 3 AM PDT SATURDAY FOR CENTRAL U.S.
     WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S. WATERS STRAIT
     OF JUAN DE FUCA.

     GALE WATCH FROM SATURDAY EVENING THROUGH LATE SATURDAY NIGHT FOR
     CENTRAL U.S. WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE
     U.S. WATERS STRAIT OF JUAN DE FUCA.


&&

$$

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