Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 130939
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
240 AM PDT SUN APR 13 2014

.SYNOPSIS...A STRONG UPPER LEVEL RIDGE AND OFFSHORE FLOW WILL KEEP
WESTERN WASHINGTON DRY TODAY THROUGH MONDAY. CHANGES WILL OCCUR
MONDAY NIGHT AS A MARINE PUSH USHERS IN CLOUDS AND A CHANCE OF
SHOWERS. A SERIES WEATHER SYSTEMS WILL MOVE THROUGH THE AREA TUESDAY
THROUGH THE END OF THE THE WEEK.

&&

.SHORT TERM...A STRONG UPPER LEVEL RIDGE IS SITTING ALONG 130W WHILE
A 1028 MB SURFACE HIGH SITS OVER SOUTH CENTRAL BRITISH COLUMBIA. THE
UPPER HIGH WILL SHIFT EAST ACROSS THE AREA TONIGHT WHILE THE SURFACE
DRIFTS EAST INTO FAR SOUTHEASTERN BRITISH COLUMBIA. LOW LEVEL FLOW
IS TURNING EASTERLY WITH TIME TODAY AS THE SURFACE HIGH MAKES ITS
MOVE. EXPECT QUITE SUNNY CONDITIONS TODAY WITH TEMPERATURES WARMING
WELL INTO THE 60S MOST AREAS. A FEW 70S ARE POSSIBLE OVER THE SOUTH
INTERIOR AND NEAR THE CASCADE FOOTHILLS. AREAS AROUND THE COOL
NORTHERN INLAND WATERWAYS WILL REMAIN COOLER...BUT STILL NICE IN THE
MID AND UPPER 50S.

CLEAR SKIES WILL ALLOW FOR GOOD RADIATIONAL COOLING AGAIN TONIGHT.
TEMPERATURES WILL NOT BE AS COOL AS THIS MORNING...BUT WILL FALL
INTO THE LOWER TO MID 40S...WITH MID AND UPPER 30S IN TYPICALLY
COLDER OUTLYING VALLEYS.

THE UPPER LEVEL PATTERN IS RATHER QUICKLY PROGRESSIVE...SO EXPECT
THE TRANSITION TO STRONG ONSHORE FLOW TO BEGIN MIDDAY MONDAY. THE
GFS...NAM...CANADIAN...AND ECMWF SOLUTIONS ALL SHOW KUIL-KBLI PRES
GRADIENTS INCREASING TO OVER 5 MB DURING THE AFTERNOON HOURS. THIS
WILL TRANSLATE TO LOCALLY BREEZY CONDITIONS IN THE AFTERNOON AROUND
THE STRAIT OF JUAN DE FUCA AND THROUGH THE CHEHALIS GAP. THE EARLY
SWITCH TO ONSHORE FLOW WILL MEAN HIGH TEMPERATURES WILL OCCUR AROUND
MIDDAY WITH TEMPERATURES HOLDING STEADY OR FALLING IN THE AFTERNOON
WITH THE INCREASED ONSHORE BREEZE. CLOUDS WILL ALSO SPREAD RAPIDLY
INLAND DURING THE LATE AFTERNOON HOURS AS THE FLOW TURNS ONSHORE AND
A SHORTWAVE TROUGH MOVES NORTHEASTWARD ACROSS THE AREA. BY LATE
MONDAY NIGHT EXPECT SHOWERS TO BE LIKELY OVER THE NORTH CASCADES
WITH A CHANCE OF SHOWERS ELSEWHERE. ANOTHER SHORT WAVE TROUGH WILL
PASS THE AREA FROM THE NORTHWEST ON TUESDAY MAKING CLOUDS AND
SHOWERS LIKELY AND HOLDING TEMPERATURES IN THE 50S. A PUGET SOUND
CONVERGENCE ZONE IS LIKELY ON TUESDAY. ALBRECHT

.LONG TERM...TIMING DIFFERENCES BETWEEN MODELS BEGIN TUESDAY NIGHT
AND WEDNESDAY. THE GFS SEEMS TO BRING WEAK SHORT WAVE RIDGING OVER
THE AREA FOR A BRIEF DRY PERIOD ON WEDNESDAY. THE ECMWF ON THE OTHER
HAND BRINGS WARM ADVECTION INTO THE AREA LATE TUESDAY NIGHT AND
WEDNESDAY FOR A GOOD CHANCE OF LIGHT RAIN. BOTH SOLUTIONS BRING A
FRONTAL WAVE ACROSS THE AREA ON THURSDAY OR THURSDAY NIGHT. THE
ECMWF KEEPS SHOWERS GOING FRIDAY WHILE THE GFS BRINGS ANOTHER RIDGE.
THE CANADIAN SOLUTION THIS EVENING FAVORS THE GFS. THE CURRENT
FORECAST IS A BROADBRUSH ONE WITH PRECIPITATION FAVORED TUESDAY AND
THURSDAY. THAT APPEARS QUITE REASONABLE. NO CHANGES WERE MADE TO THE
EXTENDED. ALBRECHT

&&

.HYDROLOGY...RIVER FLOODING IS UNLIKELY ON AREA RIVERS...EXCEPT THE
NORTH FORK OF THE STILLAGUAMISH EAST OF OSO...FOR THE NEXT 7 DAYS.

A FLOOD WARNING REMAINS IN EFFECT UNTIL FURTHER NOTICE FOR THE
NORTH FORK OF THE STILLAGUAMISH RIVER IN SNOHOMISH COUNTY JUST
UPSTREAM OF THE SR 530 LANDSLIDE. FLOODING CONTINUES AS A RESULT
OF WATER POOLED BEHIND THE DEBRIS DAM THAT PARTIALLY BLOCKS THE
RIVER.

THE NORTH FORK STILLAGUAMISH NEAR ARLINGTON AND THE POOL UPSTREAM OF
THE SR 530 SLIDE WILL CONTINUE FALLING THROUGH MONDAY AS A STRONG
UPPER LEVEL RIDGE BRINGS DRY WEATHER TO THE AREA. THE RIVER WILL
BEGIN RISING SOMEWHAT LATE MONDAY NIGHT AND TUESDAY AS A SHIFT TO A
WETTER WESTERLY FLOW PATTERN OCCURS. AT THIS TIME...THE MOST
SIGNIFICANT SYSTEMS ARE EXPECTED LATE MONDAY NIGHT AND
TUESDAY...GIVING AROUND A HALF INCH OF RAIN TO THE SLIDE AREA...AND
ON THURSDAY...GIVING ANOTHER HALF TO THREE QUARTERS OF AN INCH OF
RAIN TO THE SLIDE AREA. LIGHT SHOWERS AT OTHER TIMES WILL NOT GIVE
HYDROLOGICALLY SIGNIFICANT RAINFALL TO THE SLIDE AREA...BUT WILL
KEEP CONDITIONS WET. ALBRECHT

&&

.AVIATION...HIGH PRESSURE ALOFT WILL DOMINATE THE PAC NW THROUGH
TONIGHT. LIGHT N-NE FLOW ALOFT. DRY AND STABLE CONDITIONS.

KSEA...CLEAR WITH VFR CONDITIONS TODAY THROUGH TONIGHT. N-NE WIND
5-10 KT.

&&

.MARINE...THERMALLY INDUCED LOW PRESSURE WILL SET UP ALONG THE
OREGON AND FAR SRN WA COAST TODAY AND TONIGHT. THIS WILL PRODUCE
OFFSHORE FLOW WITH WINDS LESS THAN 20 KT THROUGHOUT THE WATERS.
ONSHORE FLOW WILL DEVELOP MONDAY AFTERNOON AND EVENING AS A WEATHER
SYSTEM NEARS THE AREA. THE STRONGEST WESTERLY FLOW WILL BE IN THE
STRAIT MONDAY NIGHT WITH GALE FORCE WINDS POSSIBLE. VARYING DEGREES
OF ONSHORE FLOW WILL DOMINATE THE PATTERN TUESDAY THROUGH THURSDAY.

&&

.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 WATCH MONDAY NIGHT CENTRAL STRAIT OF JUAN DE FUCA AND EAST
      ENTRANCE TO THE STRAIT.

&&

$$

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