Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 181611
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
910 AM PDT FRI APR 18 2014

.SYNOPSIS...ISOLATED SHOWERS...MAINLY IN THE MOUNTAINS AND ALONG THE
COAST...WILL COME TO AN END MIDDAY. DRIER WEATHER IS EXPECTED
THIS AFTERNOON THROUGH 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. ANOTHER FRONT WILL STALL AS IT
MOVES INTO THE COASTAL WATERS SUNDAY INTO MONDAY. A BROAD UPPER
LEVEL TROUGH WILL BRING SHOWERY WEATHER TO THE AREA TUESDAY AND
WEDNESDAY.

&&

.SHORT TERM...THE FRONTAL SYSTEM AND UPPER TROUGH THAT BROUGHT
PRECIPITATION TO THE AREA ON THURSDAY AND LAST NIGHT IS NOW WELL
EAST OF THE AREA. POST FRONTAL ONSHORE FLOW CONTINUES TO PRODUCE
ISOLATED SHOWERS IN THE MOUNTAINS AND ALONG THE COAST. A PUGET SOUND
CONVERGENCE ZONE THAT WAS GIVING SOME SHOWERS FROM ABOUT STEVENS
PASS WESTWARD INTO NORTH KING AND SOUTH SNOHOMISH COUNTY EARLIER
THIS MORNING HAS PRETTY MUCH DISSIPATED. ONSHORE GRADIENTS ARE
GRADUALLY RELAXING THIS MORNING. EXPECT CONDITIONS TO CONTINUE TO
DRY AND IMPROVE THIS AFTERNOON AND TO REMAIN DRY TONIGHT AS A RIDGE
OF HIGH PRESSURE AT THE SURFACE AND ALOFT MOVES ACROSS THE AREA.

THE NEXT FRONTAL SYSTEM SITS FROM ABOUT 50N 140W SOUTHWESTWARD TO
35N 160W WITH ITS ASSOCIATED VERTICALLY STACKED LOW NEAR 48N 148W.
PRECIPITATION ASSOCIATED WITH THE FRONT WILL MOVE ONSHORE DURING THE
EARLY MORNING HOURS THEN INTO THE CASCADES BY ABOUT NOON. LONG
SATELLITE LOOPS SHOW THIS FRONT HAVING ENTRAINED SUBTROPICAL
MOISTURE FROM THE WESTERN PACIFIC. THIS CORRELATES WELL WITH MODEL
FORECASTS OF BRIEF MODERATE PRECIPITATION AMOUNTS TO THE AREA
SATURDAY AFTERNOON AND EVENING. PRECIPITATION AMOUNTS FROM 18Z SAT
THROUGH 12Z SUN COULD REACH 0.75 TO NEARLY 1.0 INCHES AROUND THE
LANDSLIDE EAST OF ARLINGTON. THE UPPER TROUGH ASSOCIATED WITH THE
FRONT WILL PASS ACROSS THE AREA LATE IN THE AFTERNOON AND THE GFS
SHOWS LIFTED INDICES FALLING TO BELOW 0C WHILE MOS TSTM CONDITIONAL
PROBABILITIES RISE TO AROUND 20 PERCENT. AT THIS POINT WILL ADJUST
POPS AND QPF FOR SATURDAY BUT WILL WAIT UNTIL THE FULL SUITE OF 12Z
GUIDANCE COMES IN BEFORE ADDING A CHANCE OF TSTMS TO THE FORECAST
SAT AFTN AND EVENING.

ANOTHER COLD FRONT WILL REACH THE COAST ON SUNDAY THEN WEAKEN AS
PIVOTS AND BECOMES NORTH-SOUTH ORIENTATED OFFSHORE. MODELS KEEPS THE
INTERIOR MAINLY DRY AS THE FRONT STALLS OVER THE COASTAL WATERS..

AN UPPER LEVEL LOW OVER THE NE PAC WILL START TO SHIFT INLAND LATE
MONDAY OR MONDAY NIGHT...RESULTING IN SCATTERED SHOWERS ACROSS THE
REGION.

FORECASTS WILL BE UPDATED FOR INCREASED POPS AND QPF IN THE INTERIOR
ON SATURDAY. ALBRECHT

.LONG TERM...FROM THE PREVIOUS LONG TERM DISCUSSION...THE AIR MASS
WILL REMAIN COOL AND UNSTABLE ON TUE AS AN UPPER LEVEL LOW CROSSES
THE PAC NW. MODELS CONTINUE TO SHOW A FEW SHOWERS ACROSS WESTERN WA
OVER THIS PERIOD. A WEAK RIDGE MAY BRING DRIER WEATHER NEXT
WEDNESDAY...BUT THEN ANOTHER FRONTAL SYSTEM IS SLATED TO ARRIVE WED
NIGHT OR THURSDAY. TIMING OF THIS NEXT SYSTEM IS STILL UNCLEAR IN
THE MODELS. MODELS ARE SHOWING ANOTHER COOL/SHOWERY WEATHER PATTERN
NEXT WEEKEND AS ANOTHER UPPER LEVEL TROUGH FORMS OVER THE PAC NW. 33

&&

.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.

THE NORTH FORK OF THE STILLAGUAMISH NEAR ARLINGTON CRESTED AT 7.4
FEET EARLY THIS MORNING AND IS NOW FALLING. THE RIVER LEVEL WILL
CONTINUE FALLING TODAY AS HIGH PRESSURE MOVES OVER THE AREA. A
FRONT WILL BRING ANOTHER BATCH OF RAIN SATURDAY AFTERNOON AND
EVENING. THIS IS EXPECTED TO CAUSE THE RIVER TO RISE ABOUT A FOOT
SATURDAY NIGHT. THE RIVER WILL BEGIN FALLING AGAIN SUNDAY AND
CONTINUE TO FALL EARLY NEXT WEEK AS DRIER WEATHER PREVAILS.

&&

.AVIATION...WEAK UPPER LEVEL RIDGE OFFSHORE MOVING TOWARDS THE COAST
THIS AFTERNOON AND INLAND THIS EVENING. NORTHWESTERLY FLOW ALOFT
BECOMING SOUTHWESTERLY SATURDAY MORNING. CONVERGENCE ZONE THAT
IMPACTED THE AREA EARLIER THIS MORNING HAS MOVED OVER THE CASCADES.

CEILINGS ARE EXPECTED TO LIFT AND CLOUDS SCATTERING OUT FOR MUCH OF
WESTERN WASHINGTON THIS MORNING. SOME CIGS RANGING 2000 TO 4000 FEET
WILL LINGER NEAR WHERE THE CONVERGENCE ZONE WAS THROUGH 16-18Z THIS
MORNING BEFORE AN UPPER LEVEL RIDGE OFF THE COAST WILL WORK TO LIFT
CIGS SIGNIFICANTLY BEFORE CLOUDS SCATTER OUT FOR THE AFTERNOON. MID
LEVEL CLOUD DECK OUT AHEAD OF THE NEXT SYSTEM ARRIVING SATURDAY
MORNING.

KSEA...CLOUDS WILL CONTINUE TO SCATTER OUT FOR THE REMAINDER OF THE
MORNING AND INTO THE AFTERNOON HOURS. SOUTHWEST WIND 6 TO 10 KNOTS
BECOMING LIGHT MIDDAY. WINDS GOING NORTHWESTERLY 4 TO 8 KNOTS LATE
AFTERNOON AND EVENING HOURS BEFORE BECOMING NORTHEASTERLY LATE
TONIGHT. FELTON/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 OF THE
WATERS. 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 CENTRAL AND EASTERN STRAIT THROUGH
      NOON.
     SMALL CRAFT ADVISORY COASTAL WATERS AND WEST ENTRANCE TO
      THE STRAIT FOR HAZARDOUS SEAS THROUGH NOON

&&

$$

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