Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 240357 CCA
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT SAT MAY 23 2015

.SYNOPSIS...A RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE FLOW
OVER WESTERN WASHINGTON THROUGH NEXT WEEK. THIS WILL KEEP SKIES
MOSTLY CLOUDY AND TEMPERATURES MILD THROUGH MID WEEK. WEAKER ONSHORE
FLOW AFTER MID WEEK WILL ALLOW MORE AFTERNOON SUNSHINE AND A MINOR
WARMING TREND. WEAK LOW PRESSURE ALOFT WILL BRING A CHANCE OF
SHOWERS MAINLY OVER THE CASCADES DURING THE AFTERNOON AND EVENINGS.

&&

.SHORT TERM...THE STEERING FLOW TURNED WESTERLY THIS EVENING WITH
THUNDERSTORMS OVER THE NORTH CASCADES PUSHING EAST OF THE CREST. SO
THE FORECAST WAS UPDATED TONIGHT TO DROP THUNDERSTORMS OVER WEST
SLOPES OF THE CASCADES. MEANWHILE...LOW LEVEL ONSHORE FLOW CONTINUES
FOR CLOUDY SKIES.

AN UPPER LEVEL LOW OVER SOUTHERN B.C. WILL BRING A CHANCE OF SHOWERS
TO THE CASCADES SUNDAY AND MEMORIAL DAY AS IT MOVES INTO WASHINGTON.
THE LOWLANDS WILL BE DRY ALTHOUGH STILL MOSTLY CLOUDY AS ONSHORE
FLOW PERSISTS. TEMPS WILL BE A FEW DEGREES BELOW NORMAL WITH HIGHS
IN THE LOWER 60S.

THE UPPER LEVEL LOW WILL SLOWLY SHIFT EAST TO THE WA/ID BORDER ON
TUE. WRAP-AROUND MOISTURE MAY CLIP THE CASCADES FOR MORE SHOWERS.
HOWEVER LOOKS LIKE MOST OF THE ACTIVITY WILL BE ALONG THE EAST
SLOPES. 33

.LONG TERM...PREVIOUS DISCUSSION...THE E WA UPPER LEVEL LOW WILL
FINALLY MOVE E OVER ID WEDNESDAY...ALLOWING AN OFFSHORE UPPER LEVEL
RIDGE TO SLIDE A LITTLE CLOSER TO THE COAST. THE APPROACH OF THE
RIDGE WILL ALLOW THE AIR MASS TO WARM...BUT THE GREATER IMPACT WILL
BE THE CHANGE TO ONSHORE FLOW. ONSHORE PRESSURE GRADIENTS WILL
WEAKEN A BIT AND WILL SHIFT TO A WESTERLY DIRECTION WHICH MAKES IT
MORE DIFFICULT TO SPREAD STRATUS ACROSS PUGET SOUND. THIS WILL
RESULT IN MORE AFTERNOON CLEARING FROM WEDNESDAY ONWARD WITH MAX
TEMPS OVER THE INTERIOR WARMING INTO THE 70S. THE AIR MASS OVER THE
CASCADES WILL REMAIN UNSTABLE FOR THE STANDARD AFTERNOON AND EVENING
CONVECTION. KAM

&&

.AVIATION...A WEAK UPPER LEVEL LOW IN SOUTHERN BRITISH COLUMBIA WILL
MOVE SLOWLY SOUTHEAST THROUGH SUNDAY. THIS SYSTEM WILL INCREASE
ONSHORE FLOW AND DEEPEN THE MARINE LAYER. AT THE SURFACE ONSHORE
FLOW WILL DECREASE TONIGHT THEN INCREASE SUNDAY AFTERNOON. THE AIR
MASS IS MOIST AND WEAKLY UNSTABLE BUT ONLY ISOLATED THUNDERSTORMS
ARE EXPECTED NEAR THE CASCADE CREST.

LOW LEVEL MOISTURE AND A DEEP MARINE LAYER WILL CONTINUE RESULTING
IN WIDESPREAD MVFR CEILINGS LATER TONIGHT AND SUNDAY MORNING.
CEILINGS WILL LIFT TO VFR AFTER MIDDAY BUT BREAKOUT IS UNLIKELY AT
MOST TERMINALS.

KSEA...CEILING 2500 FT FALLING TO 1-2K FT AFTER MIDNIGHT. SIMILAR
CONDITIONS SUNDAY WITH CEILING GRADUALLY LIFTING TO 3-4K FT IN THE
AFTERNOON BUT BREAKOUT IS NOT EXPECTED. SW WIND 4-8 KT. CHB

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
SMALL CRAFT ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING
IN THE STRAIT TONIGHT THROUGH MONDAY NIGHT. ONSHORE FLOW WILL WEAKEN
TUESDAY THROUGH THURSDAY. CHB

&&

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

&&

$$

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