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000
FXUS66 KSEW 212216
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 PM PDT TUE MAY 21 2013

.SYNOPSIS...A COLD UPPER LEVEL LOW WILL MOVE INTO OREGON THIS
EVENING THEN SPIN AROUND OVER THE PACIFIC NORTHWEST THROUGH FRIDAY
AT LEAST. 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...UPPER LEVEL LOW IS OFF THE NORTH OREGON COAST AND MOST
OF WESTERN WASHINGTON IS IN A SHOWERY PATTERN. THERE HAVE BEEN SUN
BREAKS THIS AFTERNOON AND THIS PATTERN WILL CONTINUE INTO THE
EVENING. NO THUNDERSTORMS HAVE BROKEN OUT SO REMOVED THE MINOR
MENTION OVER SOUTHERN AREAS THIS EVENING. MODELS SPIN THE UPPER LOW
AROUND OVER OREGON AND WASHINGTON OVER THE NEXT THREE DAYS SO COOL
AND SHOWERY CONDITIONS WILL REMAIN IN THE FORECAST.

ON WEDNESDAY AND THURSDAY SKIES WILL STILL BE MOSTLY CLOUDY AND
SHOWERS ARE LIKELY BUT SUN BREAKS WILL PROBABLY BE MORE NUMEROUS.
THIS COULD ALLOW A FEW THUNDERSTORMS TO DEVELOP IN THE AFTERNOONS
AND EVENINGS SO HAVE ADDED ISOLATED THUNDERSTORMS TO EACH DAY. AS IS
SO OFTEN THE CASE IN SPRING...SOME THUNDERSTORMS MAY PRODUCE SMALL
HAIL. IN FACT THERE COULD BE SOME HAIL WITHOUT THE THUNDERSTORMS.
HIGH TEMPERATURES WILL REMAIN WELL BELOW NORMAL...RISING INTO ONLY
THE LOW 60S ON THURSDAY.

ON FRIDAY THE UPPER LOW IS RIGHT OVER THE AREA BUT IT IS FILLING AND
BROADENING. THIS SHOULD DIMINISH THE SHOWERS SOMEWHAT SO POPS ARE
MOSTLY IN THE CHANCE CATEGORY. A SLOW WARMING TREND WILL CONTINUE
WITH HIGH TEMPERATURES CLIMBING A FEW DEGREES FROM THURSDAY. BURKE

.LONG TERM...MODELS DIVERGE AFTER FRIDAY BUT THE HEIGHT RISES
CONTINUE. WOULD EXPECT SLOW WARMING TO CONTINUE SATURDAY THROUGH
MONDAY WITH SOME UPPER 60S IN THE FORECAST. THE GFS IS PRETTY DRY BY
MONDAY BUT THE EURO SWINGS AN UPPER TROUGH THROUGH THE AREA SUNDAY
NIGHT. HAVE KEPT THE BROAD BRUSH CHANCE POPS. ON TUESDAY A WEAK
SYSTEM ARRIVES IN BOTH MODELS...KEEPING THE CHANCE POPS GOING BUT
CHANGING THE WEATHER FROM CHANCE OF SHOWERS TO CHANCE OF RAIN. BURKE

&&

.AVIATION...A DEEP UPPER LOW OVER THE OFFSHORE WATERS THIS AFTERNOON
WILL CONTINUE TO AFFECT THE FORECAST AREA AS IT SHIFTS SLOWLY INLAND
TONIGHT AND WEDNESDAY. THE AIR MASS WILL REMAIN MOIST AND SOMEWHAT
UNSTABLE AND SHOWERY PRECIPITATION WILL CONTINUE. ISOLATED
THUNDERSTORMS COULD DEVELOP OVER THE AREA WEDNESDAY AFTERNOON. THE
CASCADES WILL REMAIN GENERALLY OBSCURED.

LOW-END VFR TO MVFR CONDITIONS PREVAIL OVER THE FORECAST AREA THIS
AFTERNOON WITH SCATTERED SHOWERS. CLOUD COVER IS ROUGHLY SCT010
SCT-BKN030 BKN-OVC050. THESE CONDITIONS SHOULD CONTINUE GENERALLY
SPEAKING THROUGH THIS EVENING...THEN DETERIORATE SOMEWHAT TONIGHT AS
MORE SHOWERS MOVE INTO THE REGION.

KSEA...SOUTHWEST WIND 8-14 KT THIS AFTERNOON...BECOMING SOUTH
6-10 KT LATER TONIGHT AND VEERING TO SOUTHWEST AGAIN WEDNESDAY
AFTERNOON.   MCDONNAL

&&

.MARINE...SMALL CRAFT ADVISORY WEST TO NORTHWEST WINDS OVER THE
COASTAL WATERS WILL EASE TONIGHT AS A 1007 MB LOW FORMS NEAR THE
NORTHWEST END OF VANCOUVER ISLAND.

THERE IS STILL LOTS OF UNCERTAINTY...AND DIFFERENCES IN MODEL
SOLUTIONS...WITH REGARD TO THE MOVEMENT OF THE LOW AND THE WINDS IT
WILL PRODUCE. MOST LIKELY IT WILL WEAKEN AS IT MOVES SLOWLY SOUTH
ACROSS THE WASHINGTON OFFSHORE WATERS WEDNESDAY AND WEDNESDAY NIGHT
AND INTO THE OREGON OFFSHORE WATERS THURSDAY AND THURSDAY NIGHT. THE
LOW COULD PRODUCE SMALL CRAFT ADVISORY WINDS OVER THE COASTAL WATERS
AND IN SOME OF THE INLAND WATERS WEDNESDAY AND WEDNESDAY NIGHT...AN
IDEA THAT IS SUPPORTED SOME OF THE UW WRF-GFS RUNS. THE FORECAST
THAT WE ARE GOING WITH FOR NOW IS A BLEND OF THAT MODEL AND THE
WEAKER NAM SOLUTION...AND THE RESULT KEEPS WINDS 20 KT OR LESS OVER
THE MARINE ZONES.

WEAK ONSHORE FLOW WILL DEVELOP FRIDAY AND CONTINUE SATURDAY AND
SUNDAY.

&&

.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