Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251635
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT TUE AUG 25 2015...CORRECTED

.SYNOPSIS...AN OFFSHORE UPPER TROUGH WILL MAINTAIN DRY SOUTHWEST
FLOW ALOFT OVER WESTERN WASHINGTON THROUGH MIDWEEK FOR MOSTLY
SUNNY WEATHER. A SYSTEM WILL MOVE THROUGH THE UPPER TROUGH AND
ACROSS THE AREA FOR A CHANCE OF SHOWERS LATE IN THE WEEK...AND THE
UPPER TROUGH WILL BRING A BETTER CHANCE FOR PRECIPITATION THIS
WEEKEND AND EARLY NEXT WEEK.

&&

.SHORT TERM...LATEST IR SATELLITE IMAGE SHOWS AN UPPER LEVEL LOW
CENTERED WEST OF VANCOUVER ISLAND OUTSIDE 130 W. THIS SYSTEM IS
SPREADING SOME HIGH CLOUDS INTO THE NW PORTION OF WA BUT DEEP
MOISTURE REMAINS WELL OFFSHORE NEAR THE LOW. DRY AND STABLE SW FLOW
ALOFT WILL CONTINUE THROUGH AT LEAST WEDNESDAY NIGHT AS THE LOW DIGS
FURTHER SOUTHWARD.

MOSTLY SUNNY SKIES WILL PREVAIL ACROSS MOST THE AREA TODAY WITH
HIGHS REACHING INTO THE MID TO UPPER 70S AROUND PUGET SOUND. A
SURFACE RIDGE ALONG THE COAST WILL MAINTAIN LIGHT WESTERLY ONSHORE
FLOW ACROSS THE AREA THROUGH THURSDAY. PATCHY MORNING LOW CLOUDS AND
FOG WILL BE LIMITED TO THE COAST/SW INTERIOR DURING THE MORNINGS.
OTHERWISE...ANOTHER MOSTLY SUNNY DAY IS EXPECTED ON WEDNESDAY AS THE
UPPER RIDGE AMPLIFIES OVER THE INTERIOR WEST. TEMPERATURES WILL
TREND A LITTLE WARMER INTO THE LOW 80S AROUND PUGET SOUND
/FOOTHILLS/AND SW INTERIOR WITH 70S ELSEWHERE.

HIGH CLOUDS WILL INCREASE ON THURSDAY AS A SHORT WAVE LIFTS
NORTHWARD OFFSHORE. MODELS STILL SHOW THE DEEPER MOISTURE REMAINING
OFF THE COAST SO IT LOOKS TO STAY DRY AT THIS POINT. WILL LOOK AT
ALL THE 12Z RUNS BEFORE PULLING THE CHANCE POPS OUT OF THE COASTAL
ZONES. HOWEVER...A CHANGE IN THE PATTERN WILL DEVELOP AS THE TROUGH
BOTTOMS OUT THURSDAY NIGHT AND FRIDAY. MODELS CONTINUE TO ADVERTISE
THIS SYSTEM TAPPING TROPICAL MOISTURE NEAR THE HAWAIIAN ISLANDS. WET
WEATHER LOOKS INCREASINGLY POSSIBLE FRIDAY INTO THE WEEKEND. MERCER


.LONG TERM...FROM PREVIOUS DISCUSSION...AN UPPER SHORTWAVE WILL MOVE
THROUGH THE OFFSHORE TROUGH AND BRING A CHANCE OF SHOWERS TO WESTERN
WASHINGTON BEGINNING THURSDAY NIGHT OR FRIDAY. MODELS DIFFER WITH
REGARD TO DETAIL...BUT AN UPPER TROUGH WILL REMAIN JUST OFFSHORE OR
OVER THE FORECAST AREA ITSELF THIS WEEKEND AND MONDAY. IN GENERAL
THE EUROPEAN MODEL LOOKS CONSIDERABLY WETTER ESPECIALLY FOR THE
WEEKEND...AS IT SHOWS A MORE PROGRESSIVE PATTERN WITH A COUPLE OF
TROUGHS MOVING ACROSS THE PACIFIC NORTHWEST. THE GFS KEEPS THE MAIN
UPPER TROUGH OFFSHORE. MCDONNAL

&&

.AVIATION...DRY AND STABLE SW FLOW ALOFT WILL CONTINUE OVER W WA
THROUGH WEDNESDAY...BETWEEN A LARGE UPPER LEVEL LOW DEEPENING
OFFSHORE ALONG 135W AND A LARGE UPPER LEVEL RIDGE OVER THE ROCKIES.
A WEAK SURFACE RIDGE ALONG THE WA COAST WILL KEEP ONSHORE FLOW WEAK.
THE AIR MASS WILL BE DRY WITH GOOD VFR CONDITIONS EXCEPT FOR HIGH
CLOUDS MOVING ACROSS THE AREA AT TIMES...PATCHY MORNING STRATUS
MAINLY ALONG THE COAST...AND ISOLATED EARLY MORNING RIVER VALLEY
FOG.

KSEA...GOOD VFR CONDITIONS WILL CONTINUE THROUGH TONIGHT WITH JUST
HIGH CLOUDS AT TIMES. SURFACE WINDS WILL BE LIGHT THROUGH 18Z THEN
BECOME NW 3-8 KT AFTER 18Z. KAM

&&

.MARINE...A WEAK SURFACE RIDGE WILL REMAIN ALONG THE WA COAST
THROUGH FRIDAY WHILE LOWER PRESSURE CONTINUES OVER E WA. THIS WILL
KEEP THE ONSHORE FLOW WEAK THE NEXT FEW DAYS. WHILE WINDS ARE
EXPECTED TO BE BELOW SMALL CRAFT ADVISORY LEVELS OVER ALL WATERS...
LOW END SMALL CRAFT ADVISORY WEST WINDS CANNOT BE COMPLETELY RULED
OUT FOR THE CENTRAL AND EAST STRAIT EACH EVENING.

A LARGE UPPER LEVEL AND SURFACE LOW HANGING AROUND FARTHER OFFSHORE
THIS WEEK WILL MOVE E TOWARD THE COAST ON FRIDAY...THEN GET CLOSE
ENOUGH FOR S FLOW TO INCREASE ON SATURDAY. SCA WINDS AND HIGHER SEAS
ARE LIKELY OVER THE COASTAL WATERS SATURDAY AND POSSIBLE OVER SOME
OF THE INLAND WATERS THIS WEEKEND. KAM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML


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