Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 222152
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT FRI AUG 22 2014

.SYNOPSIS...AN UNSTABLE AIR MASS ASSOCIATED WITH AN UPPER LOW OVER
THE INTERIOR OF THE PACIFIC NORTHWEST WILL GENERATE AND CIRCULATE A
FEW SHOWERS AND THUNDERSTORMS THIS EVENING OVER THE CASCADES.
OTHERWISE HIGHER PRESSURE ALOFT IS FORECAST TO NOSE INTO THE REGION
THIS WEEKEND FOR MORE SUNSHINE AFTER AREAS OF MORNING CLOUDS AND
WARMER DAYTIME TEMPERATURES. THIS WARMING TREND IS EXPECTED TO
CONTINUE INTO THE MIDDLE OF NEXT WEEK.

&&

.SHORT TERM...THE UPPER RIDGE IS NOW INSIDE 140W. WATER VAPOR IMAGES
SHOW THE SUNDAY SHORTWAVE BEGINNING TO TRACK OVER THE CROWN OF THE
RIDGE MOVING INSIDE 150W IN THE GULF OF ALASKA.

THE UPPER LOW IN THE INTERIOR OF THE PACIFIC NW HAS AN UNSTABLE AIR
MASS CIRCULATING AROUND IT. THE BULK OF THE CONVECTION SHOULD REMAIN
EAST OF WESTERN WA BUT A FEW SHOWERS OR THUNDERSTORMS MAY OCCUR IN
THE CASCADES THIS EVENING. THE UPSTREAM UPPER RIDGE SHOULD THEN
NUDGE THE UPPER LOW FURTHER INLAND BY SATURDAY TO CONTINUE THE
WARMING TREND. MARINE CLOUDS ALONG THE COAST THIS AFTERNOON WILL
LIKELY SURGE INLAND TO SOME DEGREE - DO NOT EXPECT WIDESPREAD MARINE
CLOUDS SATURDAY...PRIMARILY COAST...STRAIT AND THRU THE CHEHALIS
GAP.

THE PROGS ARE RATHER SIMILAR ON THE GULF OF ALASKA SHORTWAVE
TRACKING IT THROUGH BC IN THE NW FLOW ALOFT. MAY BE ENOUGH
ASSOCIATED INSTABILITY WITH IT SUNDAY PM AND INTRODUCED A THREAT OF
SHOWERS IN THE CASCADES. ALSO COOLED MAX TEMPS DOWN A BIT FROM
SATURDAY HIGHS AS HEIGHTS DIP A BIT AND LOW LEVEL ONSHORE FLOW RISES
A BIT MORE.

ON MONDAY RIDGING RESUMES AND BY TUESDAY...THE UPPER RIDGE MOVES
OVER THE REGION FOR PROBABLY THE WARMEST DAY OF THE WEEK. BUEHNER

.LONG TERM...THE UPPER RIDGE AXIS BEGINS TO SHIFT INLAND ON
WEDNESDAY AND HEIGHTS FALLING A BIT. THE PROGS START TO DIVERGE AT
THIS POINT WITH THE GFS THE MOST AGGRESSIVE IN BRINGING A TROUGH
OFFSHORE FRIDAY SPREADING PRECIP ONSHORE. THE CANADIAN AND ECMWF
ALSO SAG HEIGHTS OFFSHORE BUT HOLD PRECIP AT BAY. SO FOR NOW...WILL
COOL DAYTIME TEMPERATURES A BIT WEDNESDAY THROUGH FRIDAY WITH SOME
INCREASE IN CLOUD COVER BY FRIDAY. WILL ALSO INTRODUCE A SMALL
THREAT OF SHOWERS FRIDAY AND WOULD NOT BE SURPRISED WE ADJUST TO
HOLD OFF ANY PRECIP UNTIL - OF COURSE - THE HOLIDAY WEEKEND. BUEHNER

&&

.AVIATION...AN UPPER LEVEL LOW OVER EASTERN WA AT 21Z/2 PM WILL MOVE
SE OVER S ID BY 06Z/11 PM TONIGHT AND THEN OVER SW MT ON SATURDAY.
THE AIR MASS WILL BE UNSTABLE OVER THE CASCADES THOUGH ABOUT 06Z
WHILE THE LOW IS IN THE AREA...WITH TOWERING CU AND ISOLATED
THUNDERSTORMS EXPECTED OVER THE CASCADES. ISOLATED LIGHTNING HAS
ALREADY BEEN DETECTED EAST OF THE CASCADE CREST.

ONSHORE FLOW IS SLIGHTLY STRONGER THAN AT 21Z YESTERDAY. STRATUS
BURNED BACK TO THE IMMEDIATE COASTLINE THIS AFTERNOON BUT IS POISED
TO QUICKLY MOVE BACK INLAND THIS EVENING. HOWEVER ONSHORE PRESSURE
GRADIENTS SHOULD WEAKEN QUICKLY LATE THIS EVENING SO STRATUS IS ONLY
EXPECTED TO ADVECT EAST THROUGH THE STRAIT TO WHIDBEY ISLAND...AND
UP THE LOWER CHEHALIS A LITTLE PAST SHELTON INTO SW PUGET SOUND.
PATCHY FOG WILL ALSO DEVELOP OVER SW PUGET SOUND AND THE SW
INTERIOR. IFR/MVFR CONDITIONS WILL RESULT FROM THE STRATUS/FOG.
OTHERWISE CONDITIONS ARE EXPECTED TO REMAIN GOOD VFR. BURN-OFF
SATURDAY MORNING SHOULD OCCUR BY 19Z/12 PM.

KSEA...GOOD VFR CONDITIONS ARE EXPECTED THROUGH TONIGHT. STRATUS IS
NOT EXPECTED TO REACH THE TERMINAL SATURDAY MORNING. SURFACE WINDS
WILL BE N 8-12 KT THROUGH 03Z...THEN DIMINISH TO N-NE 3-8 KT AFTER
03Z. KAM

&&

.MARINE...A LARGE SURFACE HIGH OVER THE NE PACIFIC COMBINED WITH
LOWER PRESSURE OVER EASTERN WA WILL MAINTAIN AN ONSHORE FLOW PATTERN
ACROSS W WA INTO EARLY NEXT WEEK. ONSHORE FLOW WILL BE TYPICALLY
STRONGEST DURING THE LATE AFTERNOON AND EVENING HOURS AND WEAKEST
DURING THE MORNING. SMALL CRAFT ADVISORY WINDS CAN BE EXPECTED IN
THE CENTRAL AND EAST STRAIT EACH EVENING DUE TO THE ONSHORE FLOW. AN
UPPER LEVEL TROUGH WILL MOVE SE ACROSS SW B.C. SATURDAY NIGHT AND
SUNDAY MORNING. SURFACE PRESSURE GRADIENTS ALONG THE COAST WILL
INCREASE AHEAD OF THE TROUGH AND SHOULD GENERATE SCA WINDS OVER THE
OUTER COASTAL WATERS SATURDAY EVENING. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT OF JUAN DE FUCA
TONIGHT.

&&

$$

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