Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 241601
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT SUN MAY 24 2015

.SYNOPSIS...A SURFACE RIDGE OFFSHORE WILL MAINTAIN LOW LEVEL ONSHORE
FLOW OVER WESTERN WASHINGTON THROUGH MID WEEK. THIS WILL KEEP SKIES
MOSTLY CLOUDY. WEAKER ONSHORE FLOW FROM MID WEEK ONWARD 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
KEEPING A CHANCE OF SHOWERS IN THE FORECAST FOR THAT AREA.

&&

.SHORT TERM...VISIBLE SATELLITE IMAGERY SHOWS W WA UNDER CLOUDS
AGAIN THIS MORNING...WITH VARIOUS CLOUD LAYERS MOVING ACROSS THE
AREA. LOW LEVEL WINDS WILL REMAIN ONSHORE TODAY KEEPING THE MARINE
LAYER INTACT...SO AT THE BEST THERE MAY BE SOME ISOLATED SUN BREAKS
LATE THIS AFTERNOON.

AN UPPER LEVEL LOW MOVING SE OVER B.C. WILL END UP JUST N OF
VANCOUVER AT 00Z/5 PM THIS AFTERNOON. THE APPROACH OF THE LOW WILL
KEEP MID LEVEL STEERING WINDS WESTERLY WHICH WILL KEEP ANY
THUNDERSTORMS THAT FORM OVER THE CASCADES THIS AFTERNOON E OF THE
CREST. THE FORECAST STILL HAS A CHANCE OF SHOWERS NEAR THE CREST
THIS AFTERNOON AND EVENING WHICH IS REASONABLE.

THE UPPER LEVEL LOW WILL MOVE OVER THE N CASCADES NEAR THE CANADIAN
BORDER MONDAY MORNING THEN SHIFT SLIGHTLY OVER THE OKANOGAN MONDAY
AFTERNOON. THIS SEEMS TO CAUSE SOME SUBTLE BUT IMPORTANT CHANGES.
THE MARINE LAYER REMAINS INTACT AND WEAK LIFT FROM THE LOW MAY
PRODUCE AREAS OF DRIZZLE OVER THE INTERIOR MONDAY MORNING...POSSIBLY
EXTENDING INTO MONDAY AFTERNOON NEAR THE CASCADES. DRIZZLE COVERAGE
WILL HAVE TO BE EXPANDED IN THE AFTERNOON PACKAGE. DRIER N-NW FLOW
ON THE W SIDE OF THE LOW WILL WORK DOWN PAST 850 MB NEAR THE TOP OF
THE MARINE LAYER...WHICH SHOULD WEAKEN THE MARINE LAYER ENOUGH FOR
SOME SUN BREAKS TO DEVELOP MONDAY AFTERNOON. WEAK WESTERLY LOW LEVEL
ONSHORE FLOW WILL CONTINUE CLOSER TO THE SURFACE. FINALLY...WITH THE
LOW CENTER OVER OR NEAR THE N CASCADES THE AIR MASS WILL REMAIN
UNSTABLE FOR AFTERNOON CONVECTION. STEERING WINDS WILL BE VARIABLE
OR NORTHERLY WHICH WOULD OPEN MAINLY THE N CASCADES TO AFTERNOON
THUNDERSTORMS AGAIN.

ON TUESDAY...ONSHORE FLOW MONDAY NIGHT SEEMS TO HAVE A BIT MORE
WESTERLY COMPONENT AND IS WEAKER. IN ADDITION...DRIER N-NW FLOW
APPEARS TO WORK DOWN LOWER IN THE MARINE LAYER...TO AROUND 925
MB/2500 FEET. THIS WILL WEAKEN THE MARINE LAYER FURTHER...ENOUGH FOR
PARTIAL CLEARING TUESDAY AFTERNOON. THIS SHOULD ALLOW MAX TEMPS TO
WARM TO THE MID 60S TO NEAR 70. THE UPPER LEVEL LOW WILL STILL BE
HANGING OUT NEAR SPOKANE SO THE UNSTABLE AIR MASS WILL CONTINUE OVER
THE CASCADES FOR AFTERNOON CONVECTION. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 315 AM AFD...EXTENDED
MODELS AGAIN IN GOOD AGREEMENT WITH THE UPPER LEVEL LOW MOVING
SOUTHEAST ON WEDNESDAY AND AN UPPER LEVEL RIDGE BUILDING OFF THE
COAST. UPPER LEVEL RIDGE MOVING OVER THE AREA LATER THURSDAY INTO
FRIDAY AND REMAINING OVER THE AREA ON SATURDAY. 500 MB HEIGHTS RISE
INTO THE MID TO UPPER 570 DMS WITH 850 MB TEMPERATURES WARMING TO
AROUND PLUS 15C. SURFACE GRADIENTS REMAIN ONSHORE WHICH WILL TEMPER
THE WARMING TREND SOMEWHAT BUT THE ONSHORE GRADIENTS ALSO WEAKEN
LEADING TO MORE SUNSHINE AND WARMER DAYS. HIGHS WILL GET INTO THE
70S IN MANY LOCATIONS IN THE INTERIOR ON THURSDAY AND REMAIN IN THE
70S THROUGH SATURDAY. THE AIR MASS OVER THE CASCADES WILL REMAIN
SLIGHTLY UNSTABLE KEEPING THE THREAT OF LATE DAY CONVECTION IN THE
FORECAST. FELTON

&&

.AVIATION...A WEAK UPPER LEVEL LOW OVER SRN B.C. THIS MORNING WILL
DIVE SWD OVER WA LATER TODAY THROUGH MONDAY. THIS SYSTEM WILL
INCREASE ONSHORE FLOW AND DEEPEN THE MARINE LAYER. SOME PATCHY
DRIZZLE MAY DEVELOP WHEN THE CLOUD CEILINGS LIFT. BREAK OUT IS NOT
EXPECTED AT MOST TERMINALS TODAY WITH PREDOMINATELY MVFR CIGS
THROUGH AT LEAST MONDAY MORNING. THE AIR MASS IS MOIST AND WEAKLY
UNSTABLE ALOFT. THE FLOW ALOFT WILL BE LIGHT W-NWLY.

KSEA...CEILING 1-2K FT THIS MORNING WITH OCCASIONAL IFR THROUGH
17Z... GRADUALLY LIFTING TO AROUND 2500 FT IN THE AFTERNOON. LIGHT
DRIZZLE COULD OCCUR AS CEILINGS LIFT LATER TODAY. BREAKOUT IS NOT
EXPECTED. SW WIND 4-8 KT. DTM

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
WESTERLY WINDS IN THE STRAIT WILL RUN 10 TO 20 KT THROUGH EARLY-MID
AFTERNOON...THEN RISE TO SMALL CRAFT 15 TO 25 KT. SMALL CRAFT
ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING IN THE
STRAIT TONIGHT THROUGH TUESDAY NIGHT. THERE IS A POSSIBILITY OF
BRIEF GALE STRENGTH WIND TONIGHT. ONSHORE FLOW WILL WEAKEN WEDNESDAY
AND THURSDAY. CHB/DTM

&&

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

&&

$$

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