Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 120937
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
238 AM PDT SAT APR 12 2014

.SYNOPSIS...AN UPPER TROUGH AND ASSOCIATED COLD FRONT MOVING BY TO
THE EAST WILL GIVE CLOUDS TO THE AREA THIS MORNING. SKIES WILL CLEAR
MIDDAY AS DRYER AIR FILTERS IN FROM THE NORTH. A STRONG UPPER LEVEL
RIDGE AND OFFSHORE FLOW WILL GIVE WARMING AND DRYING TO THE AREA
SUNDAY INTO MONDAY. CLOUDS AND AN INCREASING CHANCE OF SHOWERS ARE
EXPECTED MONDAY NIGHT THROUGH THE MIDDLE OF NEXT WEEK AS THE UPPER
RIDGE MOVES OFF TO THE EAST AND PACIFIC SYSTEMS APPROACH FROM THE
WEST.

&&

.SHORT TERM...A RATHER STRONG MID APRIL COLD FRONT ASSOCIATED WITH A
SHORT WAVE TROUGH EXTENDING FROM CENTRAL ALBERTA INTO NORTHWEST
WASHINGTON IS PUSHING SOUTHEASTWARD THROUGH EASTERN BRITISH COLUMBIA
AND ALBERTA TOWARD THE NORTHERN ROCKIES AND PLAINS EARLY THIS
MORNING. THE LOW LEVEL COLD AIR IS BOTTLED UP BY THE ROCKIES...BUT
SOME COOL AIR IS SURGING SOUTHWARD ALONG THE BRITISH COLUMBIA COAST.
LIFT ASSOCIATED WITH THIS SHORT WAVE FEATURE IS PRODUCING AREAS OF
CLOUDS OVER WESTERN WASHINGTON...BUT LIGHT SHOWERS ARE LIMITED TO
THE CASCADES NORTH OF THE CANADIAN BORDER. IN THE MEAN TIME...STRONG
UPPER LEVEL RIDGING IS BUILDING ALONG ABOUT 135W.

SHORT RANGE MODELS AGREE THAT SURFACE PRESSURES WILL BUILD OVER
CENTRAL BRITISH COLUMBIA LATER TODAY AS THE TROUGH PASSES OFF TO THE
SE OF THE AREA. THIS WILL INDUCE LOW LEVEL OFFSHORE FLOW ACROSS THE
AREA FOR CLEARING AND DRYING AND WILL GIVE INCREASING LOW LEVEL
NORTHERLY FLOW TO THE AREA. THE HIGH TEMPERATURE FORECAST IS A
LITTLE TRICKY TODAY AS THE INITIALLY COOL SHOT OF AIR MODERATES IN
THE OFFSHORE FLOW...BUT AT THE LOW LEVELS AREAS NEAR THE WATER IN
THE PUGET SOUND AREA AND ALONG THE STRAIT OF JUAN DE FUCA WILL STAY
IN THE MID 50S TO ONLY AROUND 60.

TEMPERATURES TONIGHT ARE ALSO GOING TO BE TRICKY. THE CLEAR AND DRY
AIR MASS WILL FAVOR GOOD DIURNAL COOLING OVERNIGHT. AS THE BOUNDARY
LAYER DECOUPLES...TRADITIONALLY COLDER LOCATIONS LIKE SHELTERED
RIVER VALLEYS AND THE SOUTHWEST INTERIOR COULD SEE TEMPERATURES DROP
WELL DOWN INTO THE 30S...AND LOCALLY BELOW FREEZING.

TEMPERATURES WILL WARM NICELY ON SUNDAY AS OFFSHORE FLOW AND STRONG
MID APRIL SUNSHINE ALLOWS TEMPERATURES TO RISE INTO THE 60S. A FEW
70S ARE POSSIBLE NEAR THE CASCADE FOOTHILLS AND OVER THE SOUTH
INTERIOR. DRY AND CLEAR CONDITIONS WILL MEAN ANOTHER COOL NIGHT
SUNDAY NIGHT...THOUGH NOT QUITE AS COOL AS TONIGHT.

MONDAY WILL BE A TRANSITION DAY AS THE STRONG RIDGE AXIS PASSES OFF
TO THE EAST AND LOW LEVEL FLOW TRANSITIONS ON ONSHORE. ALL OF THE
MODELS...INCLUDING THE SLOWER ECMWF...START THE TRANSITION EARLY
ENOUGH IN THE DAY THAT DAYTIME HIGH TEMPERATURES IN THE INTERIOR
WILL LIKELY NOT BE AS WARM AS EARLIER THOUGHT. FORECAST HIGH
TEMPERATURES IN THE INTERIOR ON MONDAY WERE NUDGED DOWNWARD 50
PERCENT TOWARD A MODEL AND GUIDANCE MEAN. STRONG COOLING IS EXPECTED
ALONG THE COAST ON MONDAY...ALONG WITH AN INCREASE IN CLOUD
COVER...AS FLOW BECOMES STRONGLY ONSHORE. ALBRECHT

.LONG TERM...A MORE ZONAL PATTERN BEGINS MONDAY NIGHT AND CONTINUES
THROUGH THE END OF NEXT WEEK...RETURNING THE WEATHER TO A CLOUDIER
ONE WITH COOLER HIGH TEMPERATURES AND A CHANCE OF SHOWERS. THE
LATEST GFS SOLUTION PROMISES A SHORT WAVE RIDGE ON WEDNESDAY FOR A
SUNNIER AND WARMER DAY...BUT THE ECMWF AND CANADIAN SOLUTIONS BRING
A FRONTAL WAVE IN FROM THE WEST. GIVEN THE RATHER HIGH UNCERTAINTY
ON FEATURES IMPACTING THE AREA AFTER MONDAY NIGHT OR TUESDAY...THE
EXTENDED FORECAST WAS NOT CHANGED EARLY THIS MORNING. ALBRECHT

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED ON AREA RIVERS...EXCEPT
THE NORTH FOR 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 OF THE 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. A SHIFT
TO A WESTERLY FLOW PATTERN MONDAY NIGHT AND TUESDAY WILL BRING A
CHANCE OF RAIN BACK INTO THE AREA. AT THIS TIME...RAINFALL WITH
SYSTEMS MONDAY NIGHT THROUGH THE END OF NEXT WEEK ARE NOT EXPECTED
TO BE HYDROLOGICALLY SIGNIFICANT. ALBRECHT

&&

.AVIATION...MID TO LOW LEVEL CLOUDS HAVE FILLED IN SOUTH OF EVERETT
WITH AREAS OF CLOUDS NORTH OF EVERETT. ONSHORE/WLY FLOW WEAKENING
THIS MORNING SHOULD ALLOW MOST AREAS TO CLEAR OUT BY NOON. SOME
AREAS OF LOW LEVEL CLOUDS COULD GET DOWN TO BETWEEN 1.5 AND 3K FEET.
DRY NORTHERLY FLOW WILL GIVE MOSTLY SUNNY SKIES THIS AFTERNOON AND
EVENING.

KSEA...A BROKEN DECK OF MID LEVEL CLOUDS AROUND 5K THIS MORNING MAY
LOWER TO AROUND 2.5K FEET BY SUNRISE. THE LOWER CLOUD DECK SHOULD
THEN SCATTER OUT AROUND MID TO LATE MORNING...WITH THE MID DECK ALSO
CLEARING BY NOON. WINDS EXPECTED TO REMAIN NORTHERLY TODAY. 13

&&

.MARINE...ONSHORE FLOW HAS DIMINISHED THIS MORNING WITH THE WINDS
THROUGH THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA NOW BELOW 20
KNOTS. ADMIRALTY INLET IS THE LAST LOCATION TO FALL BELOW SCA WIND
SPEEDS EARLY THIS MORNING. NORTHERLY FLOW WILL DEVELOP TODAY AND
THEN TURN EASTERLY BY SUNDAY AS A THERMALLY INDUCED SURFACE TROUGH
FORMS ALONG THE COAST. THE FLOW WILL THEN FLIP BACK TO ONSHORE
MONDAY AND MONDAY NIGHT AS THE SURFACE TROUGH SHIFTS INLAND AND EAST
OF THE CASCADES. 13

&&

.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...NONE.

&&

$$

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