Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151611
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT TUE APR 15 2014

.SYNOPSIS...A WARM FRONT WILL MOVE INLAND TODAY THEN STALL OVER THE
REGION ON WEDNESDAY FOR RAIN AT TIMES. A COLD FRONT WILL THEN CROSS
WESTERN WASHINGTON ON THURSDAY. DRIER WEATHER IS FORECAST ON FRIDAY
WITH WEAK HIGH PRESSURE OVERHEAD. THE NEXT FRONT WILL ARRIVE AROUND
SATURDAY.

&&

.SHORT TERM...A WARM FRONT WILL GRADUALLY INCREASE MOISTURE ACROSS
THE AREA TODAY. RAIN SHOULD DEVELOP AT THE COAST BY MID AFTERNOON...
THEN SPREAD INTO THE INTERIOR BY EARLY EVENING. THE WARM FRONT WILL
STALL ACROSS THE AREA TONIGHT THROUGH WEDNESDAY NIGHT FOR MORE RAIN
AT TIMES. A COLD FRONT WILL MOVE THROUGH THE AREA ON THURSDAY WITH
RAIN CHANGING TO SHOWERS LATE IN THE DAY. THIS WILL BE FOLLOWED BY A
MAINLY DRY PERIOD THURSDAY NIGHT AND FRIDAY AS WEAK HIGH PRESSURE
BUILDS OVER THE PACIFIC NORTHWEST.

THE TONIGHT THROUGH WEDNESDAY NIGHT PERIOD IS LOOKING WETTER
COMPARED TO YESTERDAY. WHILE THIS IS A PRETTY BIG CHANGE...THERE
SEEMS TO BE GOOD CONSENSUS IN ALL THE MODELS THAT THIS WILL BE A WET
PERIOD. THERE SHOULD NOT BE ANY SPECIFIC PERIODS OF HEAVY RAIN...BUT
THE CUMULATIVE RAINS OVER THE NEXT 48-60 HOURS ARE EXPECTED TO BRING
0.75 TO 1.50 OR AN INCH TO THE INTERIOR LOWLANDS...1-2 INCHES TO THE
COAST...AND 1.5 TO 3.5 INCHES TO THE MOUNTAINS. SCHNEIDER

.LONG TERM...PREVIOUS DISCUSSION...OUR NEXT FRONT WILL ARRIVE
SATURDAY AFTERNOON AS A COLD FRONT MOVES INLAND. THE ECMWF IS
STRONGER WITH THIS SYSTEM COMPARED TO THE GFS. HOWEVER BOTH MODELS
SHOW SIMILAR TIMING WITH THIS FRONT. WILL KEEP A BLEND OF THE MODELS
FOR NOW. AFTER THAT...THE MODELS DO NOT AGREE ON THE TIMING OF THE
NEXT SYSTEM. BOTH MODELS SHOW AN UPPER LOW SPINNING OVER THE NE
PAC...BUT THEY DIFFER ON WHEN THIS SYSTEM WILL IMPACT WESTERN WA.
CONFIDENCE IS LOW AND LITTLE CHANGES WERE MADE FROM THE PREVIOUS
FORECAST. 33

&&

.HYDROLOGY...PREVIOUS DISCUSSION...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 AND THE POOL
UPSTREAM OF THE SR 530 SLIDE ARE FORECAST TO RISE ABOUT A FOOT
TONIGHT AS A WEAK FRONT BRINGS RAIN AT TIMES.

AT THIS TIME...THE MOST SIGNIFICANT SYSTEMS ARE EXPECTED
TONIGHT...GIVING ABOUT A QUARTER OF AN INCH OF RAIN TO THE SLIDE
AREA...AND ON THURSDAY...GIVING AN INCH OF RAIN TO THE SLIDE AREA AS
A COLD FRONT MOVES IN.

LIGHT SHOWERS AT OTHER TIMES THIS WEEK ARE NOT EXPECTED TO GIVE
HYDROLOGICALLY SIGNIFICANT RAINFALL TO THE SLIDE AREA...BUT WILL
KEEP CONDITIONS WET.

&&

.AVIATION...MODERATE NORTHWEST FLOW ALOFT TODAY AND TONIGHT. A WARM
FRONT WILL MOVE NORTH ACROSS THE AREA TONIGHT. AIR MASS IS MOIST AND
STABLE. AT THE SURFACE HIGH PRESSURE OFFSHORE WILL MOVE OVER THE
AREA WEDNESDAY.

VARIABLE CEILINGS TODAY BUT GENERALLY CONDITIONS WILL BE VFR. SOME
CEILINGS LOWERING TO 2K FT AT TIMES IN RAIN. MVFR CEILINGS TONIGHT
AS RAIN INCREASES.

KSEA...SOUTHERLY WIND 6-10 KT. CEILINGS 2500-3500 FT WITH OCCASIONAL
LIGHT RAIN TODAY. RAIN TONIGHT WITH MVFR CEILINGS DOMINANT. CHB

&&

.MARINE...MODERATE ONSHORE FLOW TODAY WILL SLOWLY WEAKEN. A WARM
FRONT WILL MOVE INTO THE AREA TONIGHT. HAVE FOLLOWED THE 4KM MM5 RUN
IN HOISTING A SMALL CRAFT ADVISORY FOR THE TWO SOUTHERN OUTER COAST
ZONES THIS AFTERNOON AND TONIGHT. WINDS ARE FORECAST TO BE LESS THAN
20 KT ELSEWHERE.

HIGH PRES AND LIGHTER FLOW ON WEDNESDAY. A STRONGER FRONT WILL BRING
SMALL CRAFT ADVISORY WINDS TO MOST OF THE WATERS ON THURSDAY. LATEST
GFS INDICATES GALE FORCE WESTERLIES ARE POSSIBLE IN THE STRAIT
THURSDAY AFTERNOON AND EVENING. WINDS EASE FRIDAY AS HIGH PRES
BUILDS OVER THE AREA. CHB

&&

.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 OF JUAN DE FUCA
      AND SOUTHERN OUTER COAST ZONES.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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






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