Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251035
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
335 AM PDT MON MAY 25 2015

.SYNOPSIS...A RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE FLOW
ACROSS WESTERN WASHINGTON THROUGH TUESDAY...KEEPING SKIES MOSTLY
CLOUDY. 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 UPPER LEVEL LOW HAS MOVED INTO
SOUTH CENTRAL BRITISH COLUMBIA EARLY THIS MORNING. OVER WESTERN
WASHINGTON MORE OF THE SAME WITH CLOUDY SKIES ACROSS THE AREA.
TEMPERATURES AT 3 AM WERE IN THE UPPER 40S TO MID 50S.

LITTLE CHANGE IN THE FORECAST THIS MORNING. LOW LEVEL ONSHORE FLOW
CONTINUING TODAY KEEPING SKIES CLOUDY ACROSS THE AREA. TOPS OF THE
MARINE LAYER BETWEEN 5000 AND 6000 FEET SO ONCE AGAIN NO REAL
BREAKOUT EXPECTED THIS AFTERNOON. MODELS HAVE THE LOW DRIFTING SOUTH
THIS MORNING AND GIVEN THE DEEP MARINE LAYER A LITTLE EXTRA LIFT
COULD RESULT IN SOME LOCAL DRIZZLE. BY 00Z TUESDAY UPPER LEVEL LOW
OVER THE NORTHERN PORTION OF EASTERN WASHINGTON. AIR MASS OVER THE
CASCADES BECOMING SLIGHTLY UNSTABLE THIS AFTERNOON. THIS ADDED ON TO
SOME WRAP AROUND MOISTURE FROM THE LOW WILL KEEP A CHANCE OF SHOWERS
IN THE FORECAST. HIGHS IN THE LOWLANDS WILL CONTINUE TO BE A LITTLE
BELOW NORMAL...UPPER 50S TO MID 60S.

UPPER LEVEL LOW NOT MOVING MUCH TONIGHT AND TUESDAY WITH THE LOW
CENTER MODELED TO BE NEAR PASCO BY 00Z WEDNESDAY. WITH THE LOW IN
THE VICINITY THIS WILL KEEP THE THREAT OF SHOWERS GOING IN THE
CASCADES. FOR THE LOWLANDS ONSHORE FLOW CONTINUES TONIGHT AND
TUESDAY BUT BEGINS TO WEAKEN A TOUCH TUESDAY AFTERNOON INCREASING
THE CHANCES FOR A LITTLE SUN LATE IN THE DAY ON TUESDAY. WITH PLENTY
OF CLOUD COVER TONIGHT LOW TUESDAY MORNING WILL BE IN THE UPPER 40S
TO MID 50S. LOW LEVEL ONSHORE FLOW TUESDAY WILL KEEP HIGHS IN THE
UPPER 50S TO MID 60S.

UPPER LEVEL LOW KICKS OUT SLOWLY TO THE SOUTHEAST TUESDAY NIGHT AND
WEDNESDAY WITH AN UPPER LEVEL RIDGE BUILDING OFFSHORE. THE SLOW
TREND OF WEAKENING ONSHORE FLOW WILL CONTINUE ON WEDNESDAY RESULTING
IN A LITTLE SUNSHINE IN THE AFTERNOON. TEMPERATURES ALOFT DO NOT
WARM MUCH WITH THE 850 MB TEMPERATURES ONLY RISING FROM AROUND PLUS
8 ON TUESDAY TO PLUS 10 ON WEDNESDAY. ONSHORE FLOW STILL STRONG
ENOUGH FOR PLENTY OF CLOUD COVER TUESDAY NIGHT INTO AT LEAST EARLY
WEDNESDAY AFTERNOON FOR THE INTERIOR WITH THE CLOUDS HANGING TOUGH
ALONG THE COAST WEDNESDAY AFTERNOON. MOS SEEMS A LITTLE OVER
AGGRESSIVE WITH THE WARM UP WITH THE ONSHORE FLOW STILL PRESENT
WEDNESDAY AFTERNOON. WILL UNDERCUT THE GUIDANCE BY A COUPLE OF
DEGREES GOING FOR 60S AND LOWER 70S INLAND WHILE KEEPING HIGHS IN
THE LOWER 60S ALONG THE COAST. EVEN WITH THE RIDGE BUILDING OFFSHORE
THE AIR MASS OVER THE CASCADES WILL ONCE AGAIN BECOMING SLIGHTLY
UNSTABLE LATE IN THE DAY FOR A CHANCE OF SHOWERS...MAINLY NEAR THE
CREST.

.LONG TERM...EXTENDED MODELS WHICH HAVE BEEN IN GOOD AGREEMENT THE
LAST COUPLE OF NIGHTS HAVE LOST A BIT OF CONSISTENCY BOTH AMONG THE
INDIVIDUAL MODELS AND RUN TO RUN. GFS SHOWING UPPER LEVEL RIDGE
MOVING INTO WESTERN WASHINGTON ON THURSDAY. THE ECMWF IS WEAKER WITH
THIS FEATURE BECAUSE OF YET ANOTHER UPPER LEVEL LOW MOVING INTO
CENTRAL BRITISH COLUMBIA. THE GFS IS WEAKER AND FURTHER NORTH WITH
THIS FEATURE. BOTH MODELS STILL HAVE DECENT 500 MB HEIGHTS OVER
WESTERN WASHINGTON...MID 570 DMS AND WEAK LOW LEVEL ONSHORE FLOW.
WILL STAY WITH THE IDEA OF CONTINUING THE WARMING TREND ON THURSDAY
WITH PLENTY OF HIGHS IN THE 70S OVER THE INTERIOR. ECMWF CONTINUES
TO DROP THE LOW SOUTH ON FRIDAY INDUCING A STRONGER ONSHORE PATTERN
VERSUS THE GFS WHICH HAS THE LOW MUCH FURTHER NORTH. THE ECMWF DOES
KEEP THE PRECIPITATION WITH THE LOW NORTH OF THE BORDER. ON SATURDAY
THE ECMWF MOVES THE LOW BACK TO THE NORTHEAST WHILE THE GFS SOLUTION
CONTINUES TO PUT THE LOW FURTHER TO THE NORTH AND EAST. MODELS ARE
IN GENERAL AGREEMENT WITH ANOTHER WEAK UPPER LEVEL LOW DEVELOPING
NEAR HAIDA GWAII ON SUNDAY. CURRENT FORECAST FOLLOWS THE WARMER...BY
ABOUT 5 DEGREES...GFS SOLUTION FOR FRIDAY THROUGH SUNDAY WITH HIGHS
IN THE LOWER TO MID 60S ON THE COAST AND MID 60S TO MID 70S OVER THE
INTERIOR WITH THE WARMEST DAY BEING FRIDAY. WITH THE NEWER SOLUTIONS
IN THE EXTENDED RUNS THIS MORNING WILL LET THE CURRENT FORECAST RIDE
FOR ANOTHER FORECAST PACKAGE. FELTON

&&

.AVIATION...A WEAK UPPER LEVEL LOW WILL SLOWLY MOVE SOUTHEAST
ACROSS MAINLY EASTERN WASHINGTON THROUGH TUESDAY. LIGHT NW FLOW
ALOFT. AIR MASS IS MOIST IN THE LOWER LEVELS AND SOMEWHAT MOIST
ALOFT. STABLE CONDITIONS EXCEPT SLIGHTLY UNSTABLE ALONG THE CASCADE
CREST.

THE UPPER LOW WILL DEEPEN THE MARINE LAYER AND WEAK LIFT MAY PRODUCE
PATCHY DRIZZLE THIS MORNING. WIDESPREAD MVFR IS LIKELY THIS MORNING
WITH PATCHY IFR CONDITIONS WHERE DRIZZLE DEVELOPS. MVFR CEILINGS
WILL BRIEFLY SCATTER THIS AFTERNOON AND EVENING BUT SHOULD FILL BACK
IN OVERNIGHT.

 KSEA...CEILING AROUND 1-3K FT WILL RISE TO 4K FT IN THE AFTERNOON.
DRIZZLE IS POSSIBLE THIS MORNING. THE DEEP MARINE LAYER WILL REMAIN
INTACT. SW WIND 6-10 KT. CHB

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
SMALL CRAFT ADVISORY STRENGTH WINDS OF 15 TO 25 KT WILL EACH EVENING
THROUGH TUESDAY NIGHT AS ONSHORE FLOW INCREASES. ONSHORE FLOW WILL
WEAKEN WEDNESDAY THROUGH FRIDAY. CHB

&&

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

&&

$$

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