Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 182220
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
320 PM PDT FRI APR 18 2014

.SYNOPSIS...DRY WEATHER IS EXPECTED TONIGHT AS WEAK HIGH PRESSURE
MOVES ACROSS THE AREA. A COLD FRONT WILL MOVE ACROSS THE AREA
SATURDAY GIVING ANOTHER SHOT OF RAIN TO THE AREA. SHOWERS WILL
GRADUALLY TAPER OFF SATURDAY NIGHT INTO SUNDAY. ANOTHER FRONT WILL
STALL AS IT MOVES INTO THE COASTAL WATERS SUNDAY NIGHT INTO MONDAY.
A SERIES OF FRONTAL SYSTEMS WILL MOVE THROUGH THE AREA GIVING
PRECIPITATION AT TIMES TUESDAY THROUGH THE END OF NEXT WEEK.

&&

.SHORT TERM...A WEAK HIGH PRESSURE SYSTEM WILL MOVE ACROSS THE AREA
TONIGHT GIVING GENERALLY DRY...BUT MOSTLY CLOUDY CONDITIONS TO THE
AREA. LOW TEMPERATURES WILL RANGE FROM THE UPPER 30S TO THE MID 40S.

A FRONTAL SYSTEM CURRENTLY SITTING FROM ABOUT 50N 135W SSW TO AROUND
37N 140W...ASSOCIATED WITH A 975 MB VERTICALLY STACKED LOW NEAR 47N
145W...WILL MOVE THROUGH THE AREA SATURDAY AFTERNOON. FOR A MID TO
LATE APRIL SYSTEM...THIS FRONT LOOKS TO BE FAIRLY VIGOROUS AS A
35-40 KT SSW WIND AT 5000 FT INTERACTS WITH THE TERRAIN GIVING A
NICE MESOLOW TO THE NE OF THE OLYMPICS ON SATURDAY. EXPECT BREEZY
CONDITIONS OVER THE LOWLAND ZONES WITH WINDY CONDITIONS IN THE NORTH
INTERIOR AND ALONG THE COAST. THIS FRONT HAS ENTRAINED SOME
SUBTROPICAL MOISTURE FROM THE WESTERN PACIFIC...SO EXPECT BRIEFLY
MODERATE TO LOCALLY HEAVY PRECIPITATION WITH THE FRONT AND IN THE
POST FRONTAL ONSHORE FLOW. PRECIPITATION AMOUNTS FROM 12Z SAT
THROUGH 12Z SUN COULD REACH 0.75 TO NEARLY 1.0 INCHES AROUND THE
LANDSLIDE EAST OF ARLINGTON AND 1-2 INCHES IN THE NORTH CASCADES.
THE FRONT MOVES ACROSS THE AREA DURING THE LATE AFTERNOON HOURS AND
THE AIR MASS DESTABILIZES SOMEWHAT...BUT WITH LIFTED INDICES ONLY
AROUND +1 AND CAPE VALUES AROUND 250 J/KG PER THE WRFGFS 4KM...WILL
LEAVE TSTMS OUT OF THE FORECAST SAT EVENING FOR NOW.

POST FRONTAL SHOWERS SAT EVENING WILL TAPER OFF LATE SAT NIGHT INTO
SUNDAY AS SURFACE HIGH PRES AND AN ASSOCIATED UPPER RIDGE BUILDS
EAST ACROSS THE AREA THEN INTO EASTERN WASHINGTON SUN AFTERNOON.

THE ECMWF AND GFS SOLUTIONS BOTH SHOW YET ANOTHER FRONTAL SYSTEM
MOVING INTO THE COASTAL WATERS LATE SUN NIGHT INTO MON MORNING. BUT
WITH UPPER RIDGING BUILDING OVER EASTERN WASHINGTON...THE FRONT IS
EXPECTED TO STALL ALONG THE COAST AND PIVOT INTO A N-S ORIENTATION.
THIS GIVES A GOOD CHANCE OF PRECIPITATION TO THE COAST BUT ONLY
SMALL CHANCES OVER THE INTERIOR...ESPECIALLY SEATTLE SOUTHWARD.
THERE WILL BE A THREAT OF SHOWERS MONDAY OVER THE AREA AS DIFFLUENT
S/SW FLOW ALOFT DEVELOPS OVER THE REGION.  ALBRECHT

.LONG TERM...LONG TERM MODELS GENERALLY SHOW PROGRESSIVE SHORT WAVE
SYSTEMS MOVING THROUGH THE AREA TUE AND WED GIVING A CHANCE OF
SHOWERS AT TIMES. A MORE WELL DEFINED FRONT APPEARS THAT IT WILL
MOVE ACROSS THE AREA THU OR THU NIGHT. WHILE THERE WILL BE A
TENDENCY FOR CONDITIONS TO BE UNSETTLED...THE TIMING OF INDIVIDUAL
FEATURES IS UNCERTAIN WITH THE ECMWF AND GFS SHOWING SOME TIMING
DIFFERENCES. WILL SHOW HIGHER POPS FOR THU THAN THE OTHER DAYS IN
THE EXTENDED...BUT OVERALL THE LONG TERM FORECAST WILL BE A BROAD
BRUSH. ALBRECHT

&&

.HYDROLOGY...RIVER FLOODING IS UNLIKELY FOR THE NEXT 7 DAYS.

THE EXCEPTION IS THE NORTH FORK OF THE STILLAGUAMISH RIVER EAST OF
THE SR 530 LANDSLIDE NEAR OSO WHERE A FLOOD WARNING REMAINS IN
EFFECT UNTIL FURTHER NOTICE. FLOODING CONTINUES FOR THIS RIVER AS A
RESULT OF WATER POOLED BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS
THE RIVER.

AT 3 PM...THE NORTH FORK OF THE STILLAGUAMISH NEAR ARLINGTON IS AT
5.6 FT...OR 4200 CFS AND IS FALLING. IT IS EXPECTED TO FALL TO
AROUND 4.5 FT EARLY SAT AFTERNOON THEN RISE BACK UP TO 5.2 FT MIDDAY
SUNDAY BEFORE RESUMING A FALL BACK DOWN TO AROUND 4 FT EARLY NEXT
WEEK. SIMILAR TRENDS ARE EXPECTED ON THE NORTH FORK OF THE
STILLAGUAMISH NEAR OSO AND AT THE SR 530 SLIDE POOL TO THE EAST OF
THE LANDSLIDE.  ALBRECHT

&&

.AVIATION...WEAK UPPER LEVEL RIDGE OFFSHORE MOVING TOWARDS THE COAST
THIS AFTERNOON AND INLAND THIS EVENING. NORTHWESTERLY FLOW ALOFT
TODAY WILL SHIFT TO SOUTHWESTERLY SATURDAY MORNING.

SCT CLOUDS TYPICALLY AT THE 3-4K FT RANGE THIS AFTERNOON WITH BKN
DECK AOA 15K FT OVER PORTIONS OF NW WASHINGTON. WILL SEE THIS
CONTINUE THROUGH THE REMAINDER OF THE AFTERNOON AND INTO THE EVENING
BEFORE CIGS BEGIN TO LOWER SLOWLY LATE TONIGHT. CIGS WILL LOWER TO
3-5K FT AS FRONT BEGINS TO MOVE THROUGH LATE TOMORROW MORNING/EARLY
TOMORROW AFTERNOON...WITH WINDS ALSO PICKING UP AREAWIDE WITH THE
FRONTAL PASSAGE. PRECIP ASSOCIATED WITH THIS FRONT WILL ALSO START
UP IN THIS TIME FRAME AS WELL. FRONT EXPECTED TO PASS THROUGH BY
LATE SATURDAY EVENING...ALLOWING WINDS TO SUBSIDE AND PRECIP TO
BECOME MORE SCATTERED.

KSEA...CURRENT CONDITIONS EXPECTED TO PERSIST THIS AFTERNOON INTO
THIS EVENING...WITH SCT CLOUDS AT 3-4K FT AND BKN AT AROUND 20K FT.
CIGS EXPECTED TO LOWER AFTER MIDNIGHT TONIGHT TO AROUND 1000 FT.
SOUTHWEST WINDS 5 TO 10 KTS WILL REMAIN IN PLACE THIS AFTERNOON AND
EVENING...SHIFTING TO NORTH TO NORTHEAST LATE TONIGHT 4 TO 8
KTS...THEN SOUTHEASTERLY BY SATURDAY MORNING. SMR

&&

.MARINE...SURFACE HIGH PRESSURE OVER THE WATERS TODAY WILL SHIFT
EAST TONIGHT. A STRONG COLD FRONT WILL MOVE THROUGH THE WATERS ON
SATURDAY WITH SMALL CRAFT ADVISORY WINDS EXPECTED OVER ALL WATERS.
NEW ADVISORY WILL GO OUT WITH AFTERNOON PACKAGE. HIGH PRESSURE WILL
REBUILD ON SUNDAY. A WEAKER SYSTEM WILL ARRIVE LATER MONDAY OR
MONDAY NIGHT.

A LARGE SWELL TRAIN BEHIND THE COLD FRONT ON SATURDAY WILL ARRIVE
ALONG THE WASHINGTON COAST SATURDAY NIGHT AND CONTINUE INTO SUNDAY.
SWELLS IN THE 16 TO 19 FOOT RANGE ARE EXPECTED. THIS IS A RAZOR CLAM
DIG WEEKEND FOR TWIN HARBORS...COPALIS AND MOCLIPS BEACHES.
FELTON/SMR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING REMAINS IN EFFECT FOR THE NORTH FORK OF THE
      STILLAGUAMISH RIVER EAST OF THE SR 530 LANDSLIDE.

PZ...SMALL CRAFT ADVISORY ALL WATERS LATE TONIGHT THROUGH SATURDAY
      NIGHT.

&&

$$

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