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000
FXUS66 KSEW 232231 AAA
AFDSEW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE SEATTLE WA
331 PM PDT WED APR 23 2014

UPDATED HYDROLOGY SECTION AND ADDED AVALANCHE SECTION.

.SYNOPSIS...A WARM FRONT WILL SPREAD RAIN INTO WESTERN WASHINGTON
FROM THE SOUTH TONIGHT. A COLD FRONT WILL MOVE INLAND THURSDAY
MORNING. AN UPPER LEVEL TROUGH DIGGING INTO THE PACIFIC NORTHWEST ON
FRIDAY WILL KEEP SCATTERED SHOWERS IN THE FORECAST. A THIRD FRONTAL
SYSTEM WILL BRING ANOTHER ROUND OF MORE ORGANIZED RAINFALL TO THE
AREA SATURDAY NIGHT INTO SUNDAY MORNING.

&&

.SHORT TERM...RAIN WILL INCREASE ACROSS WESTERN WASHINGTON THIS
EVENING AS A WARM FRONT LIFTS NORTH THROUGH THE REGION. THE LEADING
EDGE OF THE PRECIP SHIELD IS NOW UP TO KING COUNTY...RAIN SHOULD
REACH THE NW INTERIOR BY MIDNIGHT TONIGHT. THE COLD FRONT WILL PUSH
INLAND THURSDAY MORNING FOR MORE SHOWERS AND BREEZY TO WINDY
CONDITIONS. SNOW LEVELS WILL RISE TO AROUND 5000 FT WITH THE WARM
FRONT...THEN DROP BACK DOWN TO AROUND 4000 FT BEHIND THE COLD FRONT
ON THURSDAY.

THE AIR MASS WILL REMAIN COOL AND UNSTABLE ON FRIDAY AS AN UPPER
LEVEL LOW OVER THE NE PAC MOVES INLAND. EXPECT ISOLATED TO SCATTERED
SHOWERS WITH TEMPS IN THE 50S. ISOLATED T-STORMS ARE POSSIBLE FRI
AFTERNOON BUT CONFIDENCE IS LOW. THE UPPER LOW WILL WEAKEN AND EXIT
EAST FRI NIGHT WITH SHOWERS DECREASING ACROSS THE AREA.

LOOK FOR A BREAK IN THE WEATHER ON SATURDAY WITH WEAK RIDGING OVER
WESTERN WASHINGTON. THE TIMING OF THE NEXT FRONT IS A LITTLE SLOWER
IN THE MODELS...WHICH IS NOW SLATED TO MOVE INLAND SATURDAY NIGHT
FOR ANOTHER WET PERIOD OF WEATHER. 33

.LONG TERM...MOIST WESTERLY FLOW WILL KEEP A CHANCE OF SHOWERS IN
THE FORECAST ON SUNDAY. MODELS SHOW A WEAK UPPER LEVEL DISTURBANCE
TRACKING THROUGH WESTERN WA FOR SCATTERED ACTIVITY. SHOWERS WILL
DECREASE SUN NIGHT AS A RIDGE START TO BUILD OVER THE PAC NW.

A WEAK WARM FRONT MAY CLIP WESTERN WASHINGTON ON MONDAY FOR A CHANCE
OF RAIN. THE MODELS THEN STRENGTHEN THE RIDGE OVER THE PAC NW TUE
AND WED FOR WARMER AND DRIER WEATHER. THE FORECAST LEANS TOWARD THE
ECMWF SOLUTION WHICH SHOWS AN H5 580 RIDGE OVER WA. THE LOW LEVEL
FLOW IS ALSO TURNING OFFSHORE AND WE SHOULD SEE HIGHS IN THE UPPER
60S TO 70S IF THIS PANS OUT. 33

&&

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

THE EXCEPTION REMAINS 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 WAS
AT 4.6 FT...OR 2840 CFS. THE NORTH FORK OF THE STILLAGUAMISH RIVER
NEAR OSO WAS AT 216.4 FT AT 1850 CFS. THE POOL EAST OF THE SLIDE WAS
282.3 FT THIS AFTERNOON.

A WARM FRONT WILL SPREAD RAIN INTO THE AREA TONIGHT...FOLLOWED BY A
COLD FRONT THURSDAY MORNING. THIS SYSTEM WILL BRING AROUND 1.00 INCH
OF RAINFALL TO THE AREA THROUGH THURSDAY AFTERNOON. THIS IS EXPECTED
TO PRODUCE RISES ON THE STILLAGUAMISH RIVER BEGINNING THIS EVENING
THROUGH THURSDAY. THE LATEST HYDROGRAPH SHOWS THE STILLAGUAMISH NEAR
ARLINGTON CRESTING LATE THURSDAY MORNING AROUND 5 FEET.

&&

.AVALANCHE...FURTHER HEAVY SNOW AND THEN POTENTIAL RAIN SHOULD BE
SEEN IN THE MT RAINIER TO MT HOOD AREA WEDNESDAY NIGHT AND THURSDAY
MORNING. THIS IS VERY LIKELY TO CAUSE WET LOOSE...STORM SLAB AND
WIND SLAB AVALANCHE CONDITIONS IN THIS AREA. AN AVALANCHE WARNING
WILL BE ISSUED FOR THE MT RAINIER TO MT HOOD AREA FOR WEDNESDAY
NIGHT AND THURSDAY MORNING. BACK COUNTRY TRAVEL IN AVALANCHE TERRAIN
IS THIS AREA IS NOT RECOMMENDED WEDNESDAY NIGHT AND THURSDAY
MORNING.

SIGNIFICANT SNOW AND AVALANCHE CONDITIONS ARE ALSO EXPECTED IN OTHER
AREAS BUT THAT ARE SLIGHTLY LESS THAN WARNING CRITERIA. AVALANCHES
ARE ALSO LIKELY IN THESE OTHER AREAS AS WELL.

PLEASE VISIT NWAC.US FOR DETAILS. GKF

&&

.AVIATION...A WARM FRONT WILL MOVE SOUTH TO NORTH THROUGH WASHINGTON
TONIGHT. UPPER FLOW WILL TURN SOUTHWESTERLY. WIDESPREAD RAIN WILL
SPREAD NORTH THIS AFTERNOON AND TONIGHT AS THE WARM FRONT MOVES IN.
AT THE SURFACE A LOW PRES CENTER WILL APPROACH THE COAST TONIGHT
THEN MOVE NORTHEAST INTO VANCOUVER ISLAND TONIGHT...GIVING MODERATE
TO STRONG SOUTHEAST WINDS.

CLOUDS WILL CONTINUE TO INCREASE AND SPREAD NORTH. CEILINGS WILL
LOWER THIS AFTERNOON TO MVFR AS RAIN MOVES IN FROM THE SOUTH. MVFR
CEILINGS ARE LIKELY ALL NIGHT INTO THURSDAY MORNING.

KSEA...MVFR CEILINGS 2-3K FT. RAIN WILL REDUCE VISIBILITY TO 3SM AT
TIMES TONIGHT. SOUTHERLY WIND 10-15 KT. CHB

&&

.MARINE...WARM FRONTAL RAIN BAND IS MOVING SLOWLY NORTH. WINDS HAVE
BEEN 15-20 KT OVER INLAND AREAS...MORE IN A FEW SPOTS...SO HAVE
ISSUED SMALL CRAFT ADVISORIES FOR ALL INLAND WATERS TONIGHT THROUGH
THURSDAY. MODELS HAD SOME LOW END GALE WINDS ON THE COAST SO HAVE
ISSUED A GALE WARNING THERE. GENERALLY IT WILL BE A GUSTY PERIOD
TONIGHT AND THURSDAY.

AFTER A LULL IN THE ACTION FRIDAY...ANOTHER FRONT WILL SPREAD SMALL
CRAFT ADVISORY STRENGTH WINDS OVER THE AREA SATURDAY NIGHT. YET
ANOTHER SYSTEM ARRIVES AROUND TUESDAY. 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...GALE WARNING COAST.
     SMALL CRAFT ADVISORY ALL OTHER WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML   (ALL LOWERCASE)