Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 132228
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
328 PM PDT MON APR 13 2015

.SYNOPSIS...A COLD FRONT OVER WESTERN WASHINGTON WILL MOVE EAST BY
EARLY THIS EVENING. POST-FRONTAL SHOWERS WILL CONTINUE OVER THE AREA
THEN GRADUALLY DIMINISH TUESDAY AFTERNOON AND EVENING. COLD AIR WITH
THE FRONT WILL BRING SNOW TO THE MOUNTAINS WITH THE SNOW LEVEL BELOW
THE MOUNTAIN PASSES THIS EVENING. A PUGET SOUND CONVERGENCE ZONE
WILL CONTINUE THROUGH TUESDAY. AN UPPER LEVEL RIDGE WILL BRING
MAINLY DRY WEATHER WEDNESDAY AND THURSDAY. A WEAK FRONT WILL CROSS
THE AREA LATE FRIDAY. ANOTHER UPPER LEVEL RIDGE SHOULD BRING DRY
WEATHER NEXT WEEKEND.

&&

.SHORT TERM...WIND SHIFTS ON AREA OBSERVATIONS INDICATE THE COLD
FRONT WAS OVER PUGET SOUND AT 2 PM AND HEADED E OVER THE CASCADES.
THE BACK EDGE OF THE FRONTAL CLOUD BAND WAS STILL OVER THE COASTAL
WATERS AND RADAR SHOWED THE BACK EDGE OF THE RAIN BAND JUST
APPROACHING THE CENTRAL COAST. FLOW THROUGH THE STRAIT HAS SHIFTED
TO W AND A CONVERGENCE ZONE OVER N PUGET SOUND COULD FORM AT ANY
TIME.

SATELLITE AND RADAR IMAGERY SHOW SHOWERS ALREADY MOVING OVER THE
OUTER COASTAL WATERS. BOTTOM LINE IS THAT WE SHOULD BE INTO A
POST-FRONTAL SHOWERY REGIME WITH A PUGET SOUND CONVERGENCE ZONE BY
THIS EVENING. IR IMAGERY STILL SHOWS THE TWO CONVECTIVE BANDS
ASSOCIATED WITH THE TWO APPROACHING UPPER LEVEL SHORTWAVE TROUGHS.
THE FIRST ONE WAS ALONG 130W AT 21Z AND SHOULD REACH THE COAST THIS
EVENING. THE TRAILING BAND SHOULD REACH THE COAST AROUND 15Z TUESDAY
MORNING PER AN EXTRAPOLATION OF THE 20Z HRRR MODEL. MODELS DROP
LIFTED INDICES TO BELOW ZERO OVER THE COASTAL WATERS THIS
EVENING...BUT HOLD OFF FOR THE COAST AND N PUGET SOUND CONVERGENCE
ZONE REGION UNTIL AFTER 06Z. MODELS SHOW THE AIR MASS STABILIZING
TUESDAY AFTERNOON AFTER THE SECOND UPPER LEVEL TROUGH MOVES THROUGH.
THUNDERSTORM COVERAGE HAS BEEN TAILORED TO FIT THE PASSAGE OF THE
TWO TROUGHS.

SNOW HAS NOT REALLY GOTTEN GOING OVER THE CASCADES YET. THE PRECIP
BAND STILL NEAR THE COAST THIS AFTERNOON STILL HAS TO GO
THROUGH...BUT I SUSPECT IT MAY WEAKEN BEFORE IT REACHES THE
CASCADES. HURRICANE RIDGE IS NO LONGER EXPECTED TO REACH ADVISORY
AMOUNTS OF SNOW SO THE WINTER WEATHER ADVISORY HAS BEEN ENDED. THE
WINTER WEATHER ADVISORY FOR THE CASCADES ZONES REMAINS IN
EFFECT...BUT IT MAY HAVE A TOUGH TIME REACHING ADVISORY LEVELS
THROUGH TONIGHT...EVEN WITH SNOW LEVELS DROPPING TO 2000-2500 FEET
TONIGHT. THE CONVERGENCE ZONE STILL HAS A CHANCE OF BRINGING SOME
DECENT SNOW AMOUNTS TO STEVENS OR SNOQUALMIE PASSES THROUGH
TUESDAY...BUT THE CONVERGENCE ZONE WILL HAVE TO SCORE A DIRECT HIT.

SHOWERS WILL GRADUALLY DIMINISH TUESDAY AFTERNOON AS AN UPPER LEVEL
RIDGE APPROACHES AND DRIER STABLE NW FLOW ALOFT MOVES OVER THE AREA.
RESIDUAL ONSHORE FLOW COULD KEEP A CONVERGENCE ZONE GOING INTO
TUESDAY EVENING.

THE AXIS OF THE APPROACHING UPPER LEVEL RIDGE WILL MOVE OVER THE
COAST WEDNESDAY MORNING THEN SHIFT OVER THE CASCADES THURSDAY
MORNING. THIS WILL BE A RATHER WEAK RIDGE...ALLOWING WARM ADVECTION
RAIN TO PUSH THROUGH IT AND OVER VANCOUVER ISLAND WEDNESDAY THROUGH
THURSDAY. THE WARM ADVECTION WILL BE CLOSE ENOUGH TO POSSIBLY BRUSH
THE NORTHERN ZONES WITH SOME LIGHT RAIN AT TIMES...PARTICULARLY THE
N COAST AND THE FAR N CASCADES. THE REST OF W WA WILL BE DRY...BUT
LOTS OF MID AND HIGH LEVEL MOISTURE SHOULD BE FLOWING THROUGH THE
RIDGE AND ACROSS THE AREA. THE SW INTERIOR HAS THE BEST CHANCE AT A
LITTLE SUNSHINE WEDNESDAY AND THURSDAY. KAM

.LONG TERM...THE GFS AND ECMWF BRING A WEAK LOOKING UPPER LEVEL
SHORTWAVE TROUGH ACROSS THE AREA FRIDAY AFTERNOON AND EVENING. THE
TIMING HAS SLIPPED A LITTLE BIT WITH PRECIP ONLY OVER THE NW HALF OF
W WA THROUGH FRIDAY AFTERNOON. QPF WITH THE SYSTEM SHOULD BE LIGHT.
ONSHORE FLOW BEHIND THE TROUGH COULD END UP BEING STRONG. MODELS
AGREE THAT A HIGHER AMPLITUDE SHARPER UPPER LEVEL RIDGE WILL BUILD
OFFSHORE NEXT WEEKEND FOR A DRY PERIOD. THERE SHOULD BE MORE
SUNSHINE ON SUNDAY...BUT IT MAY TAKE MOST OF SATURDAY FOR CLOUDS
FROM THE POST TROUGH ONSHORE FLOW TO CLEAR. KAM

&&

.AVIATION...WESTERLY FLOW ALOFT TONIGHT WILL BECOME NORTHWESTERLY
TUESDAY AS AN UPPER LEVEL TROUGH MOVES THROUGH THE AREA. AT THE
MODERATE TO STRONG ONSHORE FLOW WILL EASE TUESDAY. A COLD FRONT NOW
MOVING THROUGH THE INTERIOR WILL CONTINUE INTO EASTERN WASHINGTON
TONIGHT. THE AIR MASS IS STABLE BUT WILL BECOME SLIGHTLY UNSTABLE
OVERNIGHT. RAIN WILL TURN TO SHOWERS THIS EVENING. A PUGET SOUND
CONVERGENCE ZONE IS LIKELY TO FORM THIS EVENING AND LAST ALL NIGHT.
VFR CEILINGS WILL BECOME MVFR 2-3K FT LATE THIS AFTERNOON.

KSEA...DISCUSSION ABOVE APPLIES. THERE IS A GOOD CHANCE A PUGET
SOUND CONVERGENCE ZONE WILL REACH THE AREA LATE THIS EVENING. SLIGHT
CHANCE OF THUNDERSTORMS AFTER 06Z. SOUTHWEST WIND 10G20KT. CHB

&&

.MARINE...A FRONT OVER THE INLAND WATERS THIS AFTERNOON WILL MOVE
EAST THIS EVENING. STRONG ONSHORE FLOW WILL BRING GALES TO THE
STRAIT TONIGHT. ELSEWHERE THE FLOW WILL BE MODERATELY ONSHORE WITH
SMALL CRAFT ADVISORIES IN EFFECT. WINDS WILL EASE TUESDAY BUT 10-12
FT SWELL ON THE COAST WILL KEEP SMALL CRAFT ADVISORIES GOING THERE.
HIGH PRES WILL THEN BUILD OVER THE AREA AND LAST THROUGH ABOUT
FRIDAY...WITH CONSIDERABLY LIGHTER WINDS IN THE FORECAST. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY FOR THE CASCADES.
PZ...GALE WARNING CENTRAL AND EAST STRAIT.
     SMALL CRAFT ADVISORY REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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