Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211618
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PDT SAT MAR 21 2015

.SYNOPSIS...AN UPPER LEVEL TROUGH WILL MOVE ACROSS WESTERN
WASHINGTON TODAY BRINGING SHOWERS AND A PUGET SOUND CONVERGENCE
ZONE. A COLD FRONT WILL SPREAD RAIN PARTWAY INLAND SUNDAY MORNING
THEN ACROSS ALL OF WESTERN WASHINGTON SUNDAY AFTERNOON. ANOTHER
UPPER LEVEL TROUGH WILL BRING SHOWERS ON MONDAY. A GENERALLY WET
WEATHER PATTERN WILL CONTINUE OVER THE AREA THROUGH MID NEXT WEEK.

&&

.SHORT TERM...WATER VAPOR IMAGERY SHOWS A CIRCULATION CENTER
ASSOCIATED WITH THE 500 MB TROUGH AXIS MOVING THROUGH WRN WA THIS
MORNING. SHOWERS ARE NUMEROUS ON RADAR WITH SOME CELLS PRODUCING
BRIEF HEAVY RAIN AND ISOLATED LIGHTING. THE 12Z KUIL SOUNDING WAS
WEAKLY UNSTABLE EARLIER WITH AN LI JUST BELOW ZERO AND CAPE OF 130
J/KG. INSTABILITY HAS SINCE INCREASED WITH THE HRRR SHOWING A BROAD
AREA OF -1C TO -3C AND CAPE OF 200 TO 400 J/KG THROUGH ABOUT MIDDAY.
ONCE THE UPPER TROUGH PASSES THIS AFTERNOON...THE BEST INSTABILITY
WILL BE FOCUSED FROM AROUND SEATTLE NORTHWARD WHERE A CONVERGENCE
ZONE IS LIKELY TO DEVELOP. MODELS SHOW A SURFACE RIDGE BUILDING NWD
ALONG THE COAST WHICH SHOULD PRODUCE ONSHORE GRADIENTS FAVORABLE FOR
A PUGET SOUND CONVERGENCE ZONE IN THE SEATTLE-EVERETT CORRIDOR. THE
GFS AND NAM-12 BOTH MAINTAIN THE CONVERGENCE ZONE INTO THE
EVENING...THEN SHIFT IT NWD AS IT DISSIPATES TOWARD MIDNIGHT. WINDS
HAVE ALSO PICKED UP WITH BREEZY SWLY CONDITIONS THROUGH THIS
AFTERNOON. ZONES WERE UPDATED EARLIER TO ACCOUNT FOR SUSTAINED SW
WINDS OF 15 TO 25 MPH.  GUSTS COULD EXCEED 40 MPH IN A FEW
PLACES...ESPECIALLY NEAR STRONGER SHOWERS OR ISOLATED THUNDERSTORMS.
COLDER AIR WILL DROP SNOW LEVELS DOWN TO NEAR 4000 FEET WITH A FEW
INCHES OF SNOW POSSIBLE. HIGHEST AMOUNTS UP TO ABOUT 4 INCHES WILL
BE WITHIN ANY CONVERGENCE ZONE OVER KING/SNOHOMISH COUNTIES TODAY.
THESE AMOUNTS ARE BELOW ADVISORY LEVEL SO WILL JUST MONITOR IT AT
THIS POINT.

THE FLOW LIFTS NWD TONIGHT AHEAD OF THE NEXT APPROACHING SYSTEM WITH
PRECIPITATION AND WINDS DIMINISHING. RAIN WILL DEVELOP AGAIN ALONG
THE COAST BY EARLY SUNDAY MORNING ACCORDING TO THE NEW 12Z GFS. A
SURFACE TROUGH WILL MOVE THROUGH DURING THE DAY BRINGING RAIN AT
TIMES...MOSTLY IN THE AFTERNOON. CURRENT TIMING AND POPS LOOK PRETTY
GOOD. SNOW LEVELS WILL REMAIN RELATIVELY LOW AROUND 4000 TO 4500 FT
BUT ACCUMULATIONS LOOK TO REMAIN UNDER 2 OR 3 INCHES.

A TRAILING UPPER TROUGH EMBEDDED IN WLY FLOW WILL SPLIT AS IT MOVES
ACROSS THE PAC NW ON MONDAY. THE BULK OF MOISTURE AND LIFT WILL BE
TO THE S OF OUR AREA OVER OREGON AND NRN/CA. SHOWERS ARE STILL
EXPECTED OVER WRN WA ON MONDAY AS THE WEAKER NRN BRANCH TROUGH
TRAVERSES THE AREA. IT LOOKS LIKE A HIGH POP/LOW QPF PATTERN. HIGHS
WILL REMAIN COOL OVER THE NEXT FEW DAYS IN THE 50S.  MERCER

.LONG TERM...FROM PREVIOUS DISCUSSION...THE GFS AND ECMWF SHOW MOIST
NW FLOW ALOFT CONTINUING OVER THE NE PACIFIC AND W WA THROUGH
TUESDAY NIGHT. HOWEVER CURRENT IR IMAGERY SHOWS JUST A VERY LARGE
AREA OF OPEN CELL CUMULUS OFFSHORE TRAILING SUNDAYS COLD FRONT. I
SUSPECT THE MODELS ARE OVERDOING THE MOISTURE AGAIN THAT FAR OUT AND
THAT A FORECAST OF SHOWERS LIKELY COAST AND SCATTERED SHOWERS INLAND
WOULD WORK.

MODELS BUILD A MODEST AMPLITUDE PROGRESSIVE UPPER LEVEL RIDGE
OFFSHORE ON WEDNESDAY BUT WEAK ENOUGH SO THAT WARM ADVECTION WILL BE
PUSHING THROUGH THE RIDGE. CONFIDENCE IS MODERATE THAT A WARM FRONT
WILL BRING RAIN TO W WA ON WEDNESDAY AND THAT THE WARM FRONT WILL
SHIFT N OVER B.C. ON THURSDAY. THE TRAILING COLD FRONT MAY END UP
COMING THROUGH ON FRIDAY. IF THE 12Z MODELS STICK WITH THIS
EVOLUTION THEN LESS OF A BROAD BRUSH FORECAST MAY BE WORKABLE IN THE
AFTERNOON PACKAGE. KAM

&&

.HYDROLOGY...FLOODING IS UNLIKELY OVER THE NEXT 7 DAYS. BURKE

&&

.AVIATION...AN UPPER LEVEL TROUGH WILL MOVE THROUGH WESTERN
WASHINGTON TODAY...THEN WEAK HIGH PRES WILL BUILD ALOFT TONIGHT. AT
THE SURFACE MODERATE ONSHORE FLOW THIS MORNING WILL EASE IN THE
AFTERNOON. HIGH PRES WITH LIGHT WINDS WILL PREVAIL TONIGHT. THE AIR
MASS IS MOIST AND UNSTABLE BUT WILL STABILIZE TONIGHT. A PUGET SOUND
CONVERGENCE ZONE SHOULD BE ACTIVE TODAY AND ISOLATED THUNDERSTORMS
ARE POSSIBLE.

CEILINGS ARE VFR WITH OCCASIONAL DIPS TO MVFR IN SHOWERS. THIS
PATTERN WILL LAST TODAY THEN SOME CLEARING IS POSSIBLE TONIGHT.
PATCHY FOG IS POSSIBLE SUNDAY MORNING. ANOTHER FRONT WILL BRING
CLOUDS AND RAIN SUNDAY AFTERNOON.

KSEA...SHOWERS ARE LIKELY THIS MORNING BUT WILL TAPER THIS AFTERNOON
AND EVENING. WITH ONSHORE FLOW SKIES WILL BE MOSTLY CLOUDY WITH
BREEZY SOUTHWEST WIND 12G22KT EASING IN THE AFTERNOON. CHB

&&

.MARINE...GUSTY SOUTHWEST WINDS ARE OVER MOST OF THE AREA THIS
MORNING IN THE WAKE OF A COLD FRONT THAT PASSED LAST NIGHT. WINDS
WILL EASE IN THE AFTERNOON. SMALL CRAFT ADVISORIES ARE IN EFFECT FOR
MOST WATERS BUT ARE FORECAST TO END THIS AFTERNOON.

HIGH PRES WILL BUILD OVER THE AREA TONIGHT...THEN ANOTHER FRONT WILL
MOVE INTO THE COASTAL WATERS SUNDAY MORNING AND INTO WESTERN
WASHINGTON SUNDAY AFTERNOON. SOME SMALL CRAFT ADVISORY STRENGTH
WINDS ARE POSSIBLE WITH THIS FRONT AS WELL.

ANOTHER WEAK FRONT WILL ARRIVE MONDAY. A STRONGER FRONT IS FORECAST
TO AFFECT THE AREA TUESDAY NIGHT. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT ADMIRALTY INLET.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

$$

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











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