Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 242217
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
318 PM PDT SUN MAY 24 2015

.SYNOPSIS...A SURFACE RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE
FLOW OVER 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...VISIBLE SATELLITE IMAGERY SHOWS MOST OF W WA STILL
COVERED IN CLOUDS THIS AFTERNOON...BUT THERE ARE A FEW MORE THIN
SPOTS AROUND THE INTERIOR...AND BIG BREAKS OVER THE SAN JUANS AND
STRAIT OF JUAN DE FUCA. PRESSURE GRADIENTS REMAIN WEAKLY ONSHORE
THIS AFTERNOON. CONVECTION IS DEVELOPING E OF THE CASCADE CREST AND
IT LOOKS LIKE STEERING WINDS ARE TAKING THE CLOUD ELEMENTS TO THE
NE. DEVELOPMENT W OF THE CREST DOES NOT LOOK TOO PROMISING...HOWEVER
IT IS STILL WORTHWHILE TO HANG ON TO A CHANCE OF SHOWERS OVER THE
CASCADES THIS EVENING.

WATER VAPOR IMAGERY SHOWS THE APPROACHING UPPER LEVEL LOW IS ON
TRACK AND ALMOST OVER VANCOUVER B.C. THIS AFTERNOON. MODEL
SOLUTIONS ARE FAIRLY CLOSE IN MOVING THE LOW OVER THE CANADIAN
BORDER JUST E OF THE CASCADE CREST MONDAY AFTERNOON. WITH A MOIST
MARINE LAYER STILL IN PLACE OVER W WA...WEAK LIFT PROVIDED BY THE
LOW SHOULD PRODUCE AREAS OF DRIZZLE OVER W WA LATE TONIGHT AND
MONDAY MORNING. THE MOST LIKELY DRIZZLE LOCATIONS WOULD BE CLOSER TO
THE LOW...LIKE THE N INTERIOR...AND THE GREATER PUGET SOUND REGION.
MODELS STILL INDICATE THE MARINE LAYER WILL BREAK UP A LITTLE BIT
MONDAY AFTERNOON. THIS MAY RESULT IN PARTLY SUNNY SKIES LATE IN THE
AFTERNOON OVER THE N INTERIOR AND SOME AFTERNOON SUNBREAKS
ELSEWHERE.

WITH THE LOW STILL JUST E OF THE CREST THE AIR MASS OVER THE
CASCADES WILL BECOME UNSTABLE AGAIN MONDAY AFTERNOON. IT LOOKS LIKE
THE MOST LIKELY AREA FOR CONVECTION IS ALONG THE CANADIAN BORDER
JUST W OF THE CREST. N STEERING WINDS COULD ALSO SEND NEARBY
CANADIAN CELLS ACROSS THE BORDER.

THE UPPER LEVEL LOW WILL MOVE E OVER THE ID BORDER BY TUESDAY
AFTERNOON WITH N TO NE FLOW OVER THE CASCADES. THE AIR MASS OVER THE
CASCADES WILL BECOME WEAKLY UNSTABLE TUESDAY AFTERNOON FOR A CHANCE
OF SHOWERS. ONSHORE FLOW MONDAY NIGHT AND TUESDAY MORNING WILL BE
WEAKER WITH SOMEWHAT DRIER N-NW FLOW ALOFT MIXING INTO THE TOP OF
THE MARINE LAYER. THE STRATUS SHOULD BREAK UP ENOUGH FOR PARTLY
SUNNY SKIES TUESDAY AFTERNOON AND SLIGHTLY WARMER TEMPERATURES. THE
WARMEST SPOTS COULD REACH NEAR 70.

ON WEDNESDAY THE UPPER LEVEL LOW WILL BE OVER ID AND NO LONGER A
FACTOR. AN UPPER LEVEL RIDGE OFFSHORE WILL BE SLOWLY PUSHING INLAND
OVER W WA. A WEAKLY UNSTABLE AIR MASS OVER THE CASCADES IN THE
AFTERNOON STILL MERITS A CHANCE OF SHOWERS. WEAK WESTERLY ONSHORE
FLOW TUESDAY NIGHT SHOULD KEEP THE MARINE LAYER DIFFUSE ENOUGH FOR
CLEARING WEDNESDAY AFTERNOON. THE INCREASED SUNSHINE COMBINED WITH
GENERAL AIR MASS WARMING FROM THE RIDGE SHOULD PUT MAX TEMPS IN THE
UPPER 60S TO MID 70S OVER THE INTERIOR. KAM

.LONG TERM...THE OFFSHORE UPPER LEVEL RIDGE AXIS WILL MOVE INLAND
OVER W WA ON THURSDAY THEN SHIFT E OVER MT ON FRIDAY. HOWEVER THE
RIDGE WILL BE BROADENING AT THE SAME TIME SO THAT W WA WILL REMAIN
UNDER ITS INFLUENCE. A SURFACE RIDGE WILL REMAIN ABOUT 500 NM
OFFSHORE ALONG ROUGHLY 135W...BUT THE ONSHORE PRESSURE GRADIENTS
OVER W WA WILL BE WEAK AND PREDOMINANTLY WESTERLY. THIS WILL LIMIT
THE AMOUNT OF MARINE MOISTURE SPREADING INLAND OVER W WA EACH
EVENING...RESULTING IN AFTERNOON CLEARING AFTER AREAS OF MORNING LOW
CLOUDS. ADDITIONAL WARMING FROM THE RIDGE WITH AFTERNOON SUNSHINE
SHOULD KICK MAX TEMPS INTO THE 70S OVER THE INTERIOR. ONSHORE FLOW
WILL KEEP THE COAST IN THE 60S. KAM

&&

.AVIATION...A WEAK UPPER LEVEL LOW WILL SLOWLY MOVE SE ACROSS WA THE
NEXT COUPLE DAYS. LIGHT NW ALOFT. AIR MASS MOIST IN THE LOWER
LEVELS...AND SOMEWHAT MOIST ALOFT. STABLE CONDITIONS EXCEPT ALONG
THE CASCADE CREST WHERE THUNDERSTORMS ARE POSSIBLE THE NEXT COUPLE
AFTERNOON/EVENINGS...MAINLY N. THE UPPER LOW WILL DEEPEN THE MARINE
LAYER AND WEAK LIFT MAY PRODUCE PATCHY DRIZZLE...ESPECIALLY MONDAY
MORNING. MVFR CIGS MAY BRIEFLY SCATTER EARLY THIS EVENING BUT SHOULD
FILL BACK IN OVERNIGHT. WIDESPREAD MVFR IS EXPECTED MONDAY MORNING
WITH PATCHY IFR CONDITIONS WHERE DRIZZLE DEVELOPS.

KSEA...CIGS BKN 1500-2500 FT MAY BRIEFLY SCATTER EARLY THIS EVENING.
1-2K FT CIGS ARE EXPECTED TO DEVELOP AFTER 05Z. PATCHY DRIZZLE IS
POSSIBLE MONDAY MORNING AS A WEAK LOW MOVES SE OVER CENTRAL/ERN WA.
CIGS SHOULD LIFT LATE MONDAY AFTERNOON BUT AGAIN...ANY SCATTERING
WILL BE SHORT LIVED AS THE DEEP MARINE LAYER REMAINS INTACT. DTM

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
SMALL CRAFT WINDS OF 15 TO 25 KT WILL INCREASE TO 20 TO 30 KT DURING
THE EVENING THROUGH EARLY MONDAY MORNING AS ONSHORE FLOW INCREASES.
SMALL CRAFT ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING
IN THE STRAIT TONIGHT THROUGH AT LEAST TUESDAY NIGHT. THERE IS A LOW
PROBABILITY OF BRIEF GALE STRENGTH WINDS TONIGHT IN THE STRAIT.
ONSHORE FLOW WILL WEAKEN WEDNESDAY THROUGH FRIDAY. DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT LATE THIS
     AFTERNOON THROUGH LATE TONIGHT.

&&

$$

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