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000
FXUS66 KSEW 211606
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PDT TUE MAY 21 2013

.SYNOPSIS...A COLD UPPER LEVEL LOW OFF VANCOUVER ISLAND THIS MORNING
WILL MOVE SOUTHEAST INTO NORTHWESTERN OREGON THIS AFTERNOON THEN
INTO WESTERN WASHINGTON TONIGHT THROUGH THURSDAY. THE UPPER LOW WILL
GIVE COOL AND SHOWERY WEATHER TO THE AREA THROUGH THURSDAY. AFTER
THAT TEMPERATURES WILL SLOWLY RISE AND SHOWERS WILL DECREASE.

&&

.SHORT TERM...RADAR INDICATES A FRONT OF SORTS PASSED THROUGH PUGET
SOUND THIS MORNING AND IS NOW BREAKING UP. A BATCH OF SHOWERS IS
MOVING ONTO THE COAST AND SHOWERS ARE LIKELY EVERYWHERE TODAY.
SATELLITE IMAGERY SHOWS QUITE A LARGE CLOUD SHIELD OFF THE COAST.
THERE ARE A FEW BREAKS IN THE CLOUDS SO AFTERNOON SUN BREAKS ARE NOT
OUT OF THE QUESTION. THE COOLER AIR MASS HAS DEFINITELY MOVED INTO
WESTERN WASHINGTON. FREEZING LEVEL AT FORKS WAS 3900 FEET THIS
MORNING. HIGH TEMPERATURES WILL BE IN THE LOW TO MID 50S TODAY.

THE UPPER LEVEL LOW REMAINS IN THE AREA WEDNESDAY AND THURSDAY WHICH
WILL KEEP CONDITIONS COOL...CLOUDY...AND SHOWERY. IT DOES SEEM LIKE
ISOLATED AFTERNOON AND EVENING THUNDERSTORMS WITH SOME SMALL HAIL
CANNOT BE RULED OUT...BUT FOR NOW HAVE KEPT THEM LIMITED TO THE FAR
SOUTH TODAY. BURKE

.LONG TERM...PREVIOUS DISCUSSION...THE UPPER LOW WILL CONTINUE TO
WOBBLE AROUND THE AREA THURSDAY NIGHT THROUGH THE MEMORIAL DAY
WEEKEND. AS THE UPPER LOW SLOWLY FILLS AND THE AIR MASS GRADUALLY
MODERATES...EXPECT SHOWER ACTIVITY TO GRADUALLY DIMINISH IN COVERAGE
AND INTENSITY AND HIGH TEMPERATURES TO RISE INTO THE UPPER 50S TO
MID 60S. SOME SUNSHINE CAN BE EXPECTED IN THE LOWLANDS DURING THE
AFTERNOON HOURS AS WE MOVE INTO THE UPCOMING WEEKEND. ALBRECHT

&&

.AVIATION...A DEEP UPPER LOW...CENTERED OVER THE WASHINGTON OFFSHORE
WATERS THIS MORNING...WILL MOVE SLOWLY SOUTHEAST INTO OREGON
TONIGHT. SOUTHERLY FLOW ALOFT WILL BACK TOWARD EASTERLY. A
NORTH-SOUTH ORIENTED FRONT OVER THE EAST PART OF THE FORECAST AREA
WILL MOVE SLOWLY EASTWARD INTO THE CASCADES THIS AFTERNOON. THE AIR
MASS WILL REMAIN MOIST AND WEAKLY UNSTABLE...WITH WEAK ONSHORE LOW
LEVEL FLOW...AND SHOWERY PRECIPITATION WILL CONTINUE THROUGH
TONIGHT. THE CASCADES WILL REMAIN GENERALLY OBSCURED.

MVFR TO IFR CONDITIONS PREVAIL OVER MUCH OF THE FORECAST AREA THIS
MORNING WITH CLOUD COVER SCT-BKN010 BKN-OVC030 OVC050 AND AREAS OF
VISIBILITY AOB 3SM -RA BR. THERE SHOULD BE SOME IMPROVEMENT THIS
AFTERNOON AND EVENING TO LOW-END VFR AND LOCAL MVFR CONDITIONS.

KSEA...SOUTH WIND 6-10 KT BECOMING SOUTHWEST 8-14 KT THIS AFTERNOON
THEN SOUTH 6-10 KT AGAIN LATER TONIGHT. MCDONNAL

&&

.MARINE...ONSHORE FLOW TODAY WILL WEAKEN TONIGHT AS A 1007 MB LOW
FORMS NEAR THE NORTH END OF VANCOUVER ISLAND. SMALL CRAFT ADVISORY
NORTHWEST WINDS OVER THE COASTAL WATERS WILL WEAKEN OVERNIGHT.

THERE IS LOTS OF UNCERTAINTY ON THE MOVEMENT OF THE LOW NEAR THE
NORTH END OF VANCOUVER ISLAND...AND HOW MUCH WIND IT WILL PRODUCE.
MOST LIKELY IT WILL WEAKEN AS IT MOVES SOUTH ACROSS THE WASHINGTON
OFFSHORE WATERS WEDNESDAY AND INTO THE OREGON OFFSHORE WATERS
WEDNESDAY NIGHT AND THURSDAY. FOR NOW I WILL KEEP THE WINDS 20 KT OR
LESS OVER THE MARINE FORECAST AREA. BUT THIS PATTERN COULD PRODUCE
SMALL CRAFT ADVISORY WINDS OVER PARTS OF THE MARINE FORECAST AREA
WEDNESDAY AND WEDNESDAY NIGHT...ESPECIALLY OVER THE COASTAL WATERS
AND IN THE STRAIT OF JUAN DE FUCA AND ADJACENT INLAND WATERS.

WINDS SHOULD BE GENERALLY 20 KT OR LESS THURSDAY AS THE LOW WEAKENS
TO OUR SOUTH...AND AGAIN FRIDAY THROUGH SATURDAY AS WEAK ONSHORE
FLOW PREVAILS.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS THROUGH MIDNIGHT...EXCEPT
        FOR THE NORTHERN INNER COASTAL WATERS ZONE.

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML