Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 231021
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 AM PDT SAT MAY 23 2015

.SYNOPSIS...HIGH PRESSURE OFFSHORE WILL MAINTAIN MODERATE TO STRONG
LOW LEVEL ONSHORE FLOW OVER WESTERN WASHINGTON THROUGH THE MIDDLE OF
NEXT WEEK. WEAK LOW PRESSURE ALOFT WILL BRING A CHANCE OF SHOWERS
MAINLY OVER THE MOUNTAINS DURING THE AFTERNOON AND EVENINGS.

&&

.SHORT TERM...SHORT VERSION OF THE DISCUSSION...ONSHORE FLOW...
CLOUDY SKIES...WEAK LOW PRESSURE ALOFT FOR A CHANCE OF SHOWERS IN
THE MOUNTAINS.

SATELLITE IMAGERY SHOWS CLOUDY SKIES OVER WESTERN WASHINGTON THIS
MORNING. SHOWER ACTIVITY OVER MOST OF THE AREA HAS COME TO AN END
BUT A WEAK UPPER LEVEL DISTURBANCE MOVING WESTWARD IS JUST REACHING
THE CENTRAL CASCADES AT 10Z. WITH THE CLOUDY SKIES TEMPERATURES AT 3
AM ARE IN A NARROW RANGE...LOWER TO MID 50S.

LITTLE CHANGE IN THE PATTERN BOTH SURFACE AND ALOFT TODAY WITH HIGH
PRESSURE OFFSHORE GIVING LOW LEVEL ONSHORE FLOW AND WEAK LOW
PRESSURE ALOFT KEEPING THE AIR MASS SLIGHTLY UNSTABLE OVER THE
MOUNTAINS ESPECIALLY IN THE AFTERNOON. THE MARINE LAYER IS FAIRLY
DEEP WITH TOPS APPROACHING 5000 FEET THIS MORNING. WITH THE ONSHORE
FLOW CONTINUING THROUGHOUT THE DAY NOT MUCH IN THE WAY OF CLEARING
EXPECTED. WEAK UPPER LEVEL DISTURBANCE MOVING INTO THE CENTRAL
CASCADES WILL CONTINUE TO MOVE WEST THIS MORNING AND WEAKEN. SOME OF
THE PRECIPITATION ECHOES HAVE ALREADY MADE IT WEST OF THE CREST.
GOOD CHANCE THE SHOWERS WILL NOT MAKE IT ALL THE WAY INTO THE
LOWLANDS BUT WITH THE LOWER LAYER OF THE AIR MASS SO MOIST IT WILL
NOT TAKE MUCH TO SQUEEZE OUT A SHOWER OR TWO. HAVE ADDED A CHANCE OF
SHOWERS TO THE FORECAST THIS MORNING FROM SEATTLE SOUTHWARD. SAME
STORY THIS AFTERNOON OVER THE CASCADES THAT HAS BEEN THE CASE FOR
THE LAST FEW DAYS. THE AIR MASS LATE IN THE DAY IS SOMEWHAT UNSTABLE
SO THERE CONTINUES TO BE A CHANCE OF SHOWERS OR THUNDERSTORMS IN THE
FORECAST. WITH ALL THE CLOUD COVER HIGH TEMPERATURES WILL BE ABOUT 5
DEGREES BELOW NORMAL...MID 50S TO MID 60S.

SOME SMALL CHANGES TO THE UPPER LEVEL PATTERN TONIGHT AND SUNDAY
WITH THE UPPER LEVEL LOW NEAR HAIDA GWAII TODAY SLOWLY MOVING SOUTH
TO JUST NORTH OF BELLINGHAM BY 00Z MONDAY. EVEN WITH THIS SLIGHT
CHANGE IN THE UPPER PATTERN NO CHANGES TO THE FORECAST WITH THE LOW
LEVEL ONSHORE FLOW REMAINING INTACT KEEPING CLOUDY SKIES OVER THE
AREA. MOST OF THE SHOWER ACTIVITY WITH THE LOW CONFINED OVER
SOUTHERN BRITISH COLUMBIA ON SUNDAY. ONCE AGAIN THERE WILL BE A
CHANCE OF SHOWERS IN THE CASCADES DURING THE AFTERNOON WITH THE
SLIGHTLY UNSTABLE AIR MASS. HIGHS WITH THE CLOUD COVER WILL REMAIN
IN THE MID 50S TO MID 60S.

LOW CONTINUES TO SAG SOUTHEAST INTO EASTERN WASHINGTON ON MEMORIAL
DAY. IF ANYTHING THIS WILL SLIGHTLY INCREASE THE ONSHORE FLOW OVER
WESTERN WASHINGTON KEEPING PLENTY OF CLOUDS IN THE FORECAST. WITH
WESTERN WASHINGTON BEING ON THE BACK SIDE OF THE LOW BEST CHANCES
FOR SHOWERS ON MEMORIAL DAY WILL AGAIN BE IN THE CASCADES. HIGHS
WILL AGAIN BE IN THE MID 50S TO MID 60S.

.LONG TERM...THE GFS AND ECMWF IN PRETTY GOOD AGREEMENT THIS
MORNING. BOTH OF THE MODELS ARE HINTING AT A LITTLE UPPER LEVEL
RIDGING IN THE LATTER HALF OF NEXT WEEK. BEFORE THAT THE UPPER LEVEL
LOW OVER EASTERN WASHINGTON VERY SLOWLY DRIFTS OFF TO THE SOUTHEAST
LEAVING WESTERN WASHINGTON WITH...WAIT FOR IT...WEAK LOW PRESSURE
ALOFT AND LOW LEVEL ONSHORE FLOW TUESDAY AND WEDNESDAY. EVEN WITH
THE WEAK UPPER LEVEL RIDGING THURSDAY AND FRIDAY LOW LEVEL FLOW
REMAINS ONSHORE SO NOT EXPECTING A HEAT WAVE BUT WARMER TEMPERATURES
ALOFT AND SOME MORE SUN IN THE AFTERNOON WILL PUSH TEMPERATURES INTO
THE MID 60S TO MID 70S FOR THE INTERIOR BY THURSDAY AND FRIDAY AND
DIMINISH THE THREAT OF AFTERNOON AND EVENING SHOWERS OVER THE
CASCADES. FELTON

&&

.AVIATION...LIGHT FLOW ALOFT WILL CONTINUE TODAY THEN BECOME
WESTERLY TONIGHT. AT THE SURFACE ONSHORE FLOW WILL INCREASE AGAIN
THIS AFTERNOON. THE AIR MASS IS MOIST AND SLIGHTLY UNSTABLE. DEEP
MARINE LAYER OVER WESTERN WASHINGTON WILL KEEP CEILINGS FIRMLY MVFR
THIS MORNING WITH SOME IMPROVEMENT AFTER 21Z.

KSEA...CEILINGS 1-2K FT THIS MORNING. SOME IMPROVEMENT IS LIKELY THIS
AFTERNOON. SOUTHERLY WIND 4-8 KT WILL CONTINUE. 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
CONTINUE TUESDAY AND WEDNESDAY. CHB

&&

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

&&

$$

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