Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 252225
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
325 PM PDT MON MAY 25 2015

.SYNOPSIS...A SURFACE RIDGE OFFSHORE WILL MAINTAIN MOIST LOW LEVEL
ONSHORE FLOW ACROSS WESTERN WASHINGTON THIS WEEK. MODERATE ONSHORE
FLOW WILL KEEP SKIES MOSTLY CLOUDY THROUGH TUESDAY...BUT WEAKER
ONSHORE FLOW FROM MID WEEK ONWARD ALONG WITH HIGHER PRESSURE
BUILDING OVER THE REGION WILL ALLOW MORE AFTERNOON SUNSHINE AND A
WARMING TREND. THE AIR MASS OVER THE CASCADES WILL CONTINUE TO
BECOME SLIGHTLY UNSTABLE LATE IN THE DAY FOR A CHANCE OF SHOWERS
NEAR THE CREST.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS THE UPPER LEVEL LOW IS STILL
HANGING OUT JUST ACROSS THE CENTRAL WA BORDER OVER S CENTRAL B.C.
MOISTURE CONTINUES TO ROTATE AROUND THE LOW...BUT ASIDE FROM THE
AREA OF RAIN THAT CROSSED THE N INTERIOR THIS MORNING...NOTHING HAS
REACHED W WA. ALL MODELS EXCEPT FOR THE ARW KEEP THE N INTERIOR AND
THE REST OF THE W WA LOWLANDS DRY TONIGHT. A CHANCE OF SHOWERS WILL
CONTINUE FOR THE CASCADES TONIGHT ALONG WITH A SLIGHT CHANCE OF
EVENING THUNDERSTORMS OVER THE FAR NE CORNER. HOWEVER CONFIDENCE IS
ONLY MODERATE THAT NO SHOWERS WILL CREEP S OVER THE N INTERIOR
LOWLANDS OVERNIGHT.

WEAK ONSHORE FLOW CONTINUES THIS AFTERNOON DESPITE SURFACE PRESSURE
GRADIENTS BEING SLACK EARLY THIS AFTERNOON. ONSHORE PRESSURE
GRADIENTS WILL INCREASE THIS EVENING BRINGING IN A LITTLE MORE LOW
LEVEL MOISTURE. GFS/NAM TIME-HEIGHT SECTIONS SHOW SOME THINNING OF
THE MARINE LAYER TUESDAY AFTERNOON...BUT MEAN RH VALUES IN THE
MARINE LAYER DO NOT LOWER ENOUGH TO EXPECT MORE THAN SOME AFTERNOON
SUNBREAKS. MODELS SHOW A LITTLE LOW LEVEL WARMING...ENOUGH TO NUDGE
MAX TEMPS UP INTO THE MID 60S.

THE UPPER LEVEL LOW WILL MOVE SE OVER SE WA TUESDAY AFTERNOON THEN
CEASE TO BE A FACTOR WEDNESDAY AS IT CONTINUES MOVING E. AN UPPER
LEVEL RIDGE OFFSHORE WILL BUILD SLOWLY INLAND BEHIND THE DEPARTING
LOW. THE AIR MASS OVER THE CASCADES WILL CONTINUE BECOME UNSTABLE
EACH AFTERNOON FOR A CHANCE OF SHOWERS. I WILL LEAVE THUNDERSTORMS
OUT OF THE FORECAST FOR NOW SINCE THE CHANCE IS VERY LOW.

ONSHORE FLOW SHOULD BE WEAK ENOUGH AND WESTERLY ENOUGH TO ALLOW
PARTIAL CLEARING WEDNESDAY AFTERNOON...AND PROBABLY HOPEFULLY MOSTLY
SUNNY SKIES THURSDAY AFTERNOON. CONTINUING WEAK ONSHORE FLOW WILL
STILL PRODUCE AREAS OF MORNING LOW CLOUDS. WARMING FROM THE UPPER
LEVEL RIDGE COMBINED WITH MORE SUNSHINE SHOULD ALLOW MAX TEMPS TO
REACH INTO THE LOWER 70S ON THURSDAY. 500 MB HEIGHTS WILL APPROACH
5760 METERS OVER PUGET SOUND WITH 850 MB TEMPS CLOSE TO 14C. KAM

.LONG TERM...GFS/ECMWF/CANADIAN ALL BRING ANOTHER UPPER LEVEL LOW
DOWN THE B.C. COAST WEDNESDAY THROUGH FRIDAY MORNING...WHEN PER THE
ECMWF/CANADIAN IT WILL BE CENTERED OVER THE CENTRAL B.C. COAST. THE
GFS DOES NOT BRING THE LOW AS FAR S AND IS QUICKER IN SENDING IT
EAST OVER THE WEEKEND. NONE OF THE MODELS SHOW ANY PRECIP REACHING W
WA...BUT IT WILL PROBABLY PREVENT THE UPPER LEVEL RIDGE OVER THE
PACIFIC NW FROM BUILDING ANY HIGHER THAN 5760 METERS. ONSHORE FLOW
SHOULD REMAIN FAIRLY WEAK FOR PARTLY TO MOSTLY CLOUDY MORNINGS AND
AFTERNOON SUNSHINE. MAX TEMPS ARE EXPECTED TO TOP OUT IN THE 70S
EACH DAY THIS WEEKEND.

ALL THREE MODELS AGREE ON PROGRESSING THE RIDGE QUICKLY E SUNDAY
NIGHT...ALLOWING AN UPPER LEVEL TROUGH TO APPROACH THE REGION FROM
THE SW ON MONDAY. KAM

&&

.AVIATION...A WEAK...CLOSED UPPER LOW WILL MOVE S-SE THROUGH ERN WA
FROM TONIGHT THROUGH TUE...REACHING NE ORE LATE TUE NGT. THIS WILL
BRING LIGHT N-NW FLOW ALOFT OVER WRN WA THROUGH TUE EVNG. MODERATE
ONSHORE FLOW AT THE SFC WILL KEEP THE AIR MASS MOIST IN THE LOWER
LEVELS...AND SOMEWHAT MOIST ALOFT. WEAKLY STABLE AIR MASS...EXCEPT
UNSTABLE OVER THE CASCADES DURING THE AFTN AND EARLY EVNG HOURS.

MVFR AND VFR CIGS PREVAILING THIS EVNG...THEN LOWERING TO MAINLY IFR
CIGS OVERNIGHT AND TUE MORNING. THE TREND OF LIFTING CIGS ON TUE
AFTN WILL FOLLOW CLOSELY TO TRENDS THAT ARE BEING OBSERVED TODAY.
SIGNIFICANT SCATTERING OF CLOUDS IS NOT LIKELY FOR THE REST OF
TODAY...AND IT LOOKS UNLIKELY AGAIN ON TUE. PATCHY DRIZZLE IS
POSSIBLE ON TUE MRNG OVER THE LOWLANDS BUT ESPECIALLY ALONG THE
COAST.

KSEA...THE DEEP MARINE LAYER WILL REMAIN INTACT FOR THE NEXT 30
HOURS ALONG WITH BKN-OVC CLOUD COVERAGE. VFR CIGS NEAR 040 ARE
EXPECTED THIS EVNG...FALLING TO BELOW 020 TONIGHT AND TUE MRNG. CIGS
LIFTING AGAIN ON TUE TO NEAR 040. SW WIND 6-10 KT EXPECTED TO
PERSIST NEAR THE TERMINAL...WITH THE PUGET SOUND CONVERGENCE ZONE
GETTING ABOUT AS FAR SOUTH AS ELLIOTT BAY BETWEEN 03Z AND 06Z THIS
EVNG.     HANER

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND ALL
WEEK LONG. SMALL CRAFT ADVISORY STRENGTH WINDS OF 15 TO 25 KT WILL
OCCUR THIS EVENING AND MOST EVENINGS THIS WEEK...SIMPLY AS A
FUNCTION OF DIURNAL STRENGTHENING AND WEAKENING OF ONSHORE PRESSURE
GRADIENTS. THE LOWEST CHANCE OF SMALL CRAFT WESTERLIES WILL BE ON
THU...WHEN ONSHORE FLOW SHOULD BE WEAKEST.      HANER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT.

&&

$$

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