Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KSEW 281021
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PDT TUE APR 28 2015

.SYNOPSIS...A WEAK COLD FRONT WILL MOVE THROUGH WESTERN WASHINGTON
TODAY. AN UPPER TROUGH WILL FOLLOW THE FRONT TUESDAY NIGHT AND
WEDNESDAY. A WEAK SYSTEM COULD BRUSH NORTHERN PORTIONS THE AREA
THURSDAY. DRY AND SOMEWHAT WARMER WEATHER IS EXPECTED FRIDAY THROUGH
MONDAY.

&&

.SHORT TERM...IR SATELLITE IMAGE SHOWS A WEAK AND DIFFUSE FRONTAL
BAND JUST INSIDE 130 W AS IT APPROACHES WRN WA. THE KLGX COASTAL
RADAR IS NOT CURRENTLY PICKING UP ANY RAIN SO PRECIPITATION MUST BE
CONFINED ALONG THE FRONTAL BOUNDARY JUST OUT OF RADAR RANGE. MODELS
ALL SHOW A SKINNY MOISTURE BAND WITH THE FRONT REACHING THE COAST BY
DAY BREAK AND PUSHING THROUGH MOST THE INTERIOR BY MIDDAY. QPF WITH
THE FRONT WILL GENERALLY BE LESS THAN A TENTH BUT MESO MODELS
INDICATE SOME AREAS NEAR THE COAST AND OLYMPICS COULD REACH A
QUARTER INCH. IN CONTRAST...SOME SPOTS AROUND PUGET SOUND AND THE SW
INTERIOR MAY JUST TRACE TODAY. THE SYSTEM WILL PUSH E OF THE
CASCADES BY THE EVENING WITH POST FRONTAL SHOWERS. ONSHORE FLOW
TODAY SHOULD RESULT IN MUCH COOLER HIGH TEMPERATURES...ONLY AROUND
60 FOR MUCH OF THE LOWLANDS.

A PUGET SOUND CONVERGENCE ZONE IS LIKELY TO DEVELOP BEHIND THE FRONT
LATER THIS AFTERNOON AND TONIGHT. MODELS SHOW SURFACE RIDGING ALONG
THE COAST WITH SW FLOW OVER S/CENTRAL PUGET SOUND AND A BURST OF
WESTERLIES IN THE STRAIT. THIS SHOULD SET UP A CONVERGENCE ZONE OVER
WRN SNOHOMISH/SRN SKAGIT COUNTY...POSSIBLY DRIFTING S TO ALONG THE
KING/SNOHOMISH COUNTY LINE LATER TONIGHT. WITH THE TRAILING 500 MB
TROUGH TURNING THE UPPER FLOW MORE WLY...THE CONVERGENCE ZONE SHOULD
STAY CONFINED FROM N SEATTLE TO AROUND THE EVERETT VICINITY BEFORE
DISSIPATING WEDNESDAY MORNING.

MODELS SHOW A TRAILING SHORT WAVE MOVING ACROSS WRN WA LATER
WEDNESDAY MORNING AND AFTERNOON. WEAK UPPER LEVEL DYNAMICS WILL
ENHANCE SHOWER ACTIVITY ACROSS THE N. DIURNAL INSTABILITY WILL
MAINTAIN SOME SHOWERS ELSEWHERE ACROSS THE AREA. WILL KEEP CHANCE
POPS INDICATED FROM AROUND PUGET SOUND SWD WITH LIKELY POPS
ELSEWHERE. FORECAST LI`S ARE JUST ABOVE ZERO WITH MARGINAL LAPSE
RATES FOR THUNDERSTORMS. WILL LEAVE OUT ANY MENTION FOR NOW.

ANOTHER BRIEF CONVERGENCE ZONE COULD EMERGE WEDNESDAY
EVENING...PERHAPS SAGGING FURTHER S INTO KING COUNTY AS THE FLOW
TURNS MORE NWLY. MOIST ONSHORE FLOW AND A WEAKLY UNSTABLE AIR MASS
WILL ALSO MAINTAIN SHOWERS BUT BECOMING MORE ISOLATED OVERNIGHT AS
THE ATMOSPHERE GRADUALLY BECOMES STABLE. UPPER HEIGHTS WILL RISE
SLIGHTLY WITH A SOMEWHAT DRIER W-NW ZONAL PATTERN. A FEW SHOWERS
COULD LINGER INTO THURSDAY AS A WEAK SHORT WAVE BRUSHES THE N. KEPT
SOME LOW CHANCE POPS IN THE FORECAST N OF PUGET SOUND. HIGHS SHOULD
REBOUND SLIGHTLY INTO THE LOW 60S FROM PUGET SOUND TO THE SW
INTERIOR.

.LONG TERM...ZONAL FLOW DOMINATES THROUGH FRIDAY NIGHT WITH MODELS
TAKING ANY SYSTEMS ON A MORE NLY TRACK THROUGH BRITISH COLUMBIA. 500
MB HEIGHTS GRADUALLY RISE THIS THE WEEKEND WITH RIDGING AND DRY
WEATHER OVER THE PAC NW. THE FLOW WILL BE ONSHORE BUT WEAKER WHICH
SHOULD ALLOW FOR SOMEWHAT WARMER HIGH TEMPERATURES. A MOS BLEND
GIVES HIGHS MOSTLY IN THE MID/UPPER 60S OVER THE WEEKEND ACROSS
GREATER PUGET SOUND BUT COOLER TOWARD THE COAST AND N WHERE ANY
ONSHORE FLOW WILL HOLD TEMPERATURES BACK.

THE FORECAST SHOULD REMAIN DRY THROUGH MONDAY BUT A TROUGH WILL BE
APPROACHING. TIMING OF THE ONSHORE PUSH IS UNCERTAIN AND WILL
INFLUENCE HIGH TEMPERATURES CONSIDERABLY. FOR NOW IT APPEARS MONDAY
COULD BE THE WARMEST DAY...AT LEAST ACROSS THE INTERIOR WHERE HIGHS
MAY PUSH 70. ONSHORE FLOW SHOULD USHER IN COOLER MARINE AIR MONDAY
NIGHT AND TUESDAY. A WEAK TROUGH COULD BRING A RETURN TO SHOWERS
ACROSS WRN WA BY THAT TIME.  MERCER

&&

.AVIATION...A COLD FRONT ASSOCIATED WITH A LOW MOVING INTO WESTERN
BRITISH COLUMBIA WILL SWEEP ACROSS THE AREA THIS MORNING INTO
MIDDAY. MODERATE POST FRONTAL ONSHORE FLOW WILL GIVE A PUGET SOUND
CONVERGENCE ZONE LATER THIS AFTERNOON INTO THIS EVENING BETWEEN KPAE
AND KBFI. THE AIR MASS IS STABLE AND WILL BECOME SOMEWHAT UNSTABLE
TUESDAY AFTERNOON.

MID LEVEL DECK OUT AHEAD OF THE FRONT HAS JUST REACHED THE COAST AS
OF 09Z. FRONT WILL REACH THE COAST AROUND 15Z AND THROUGH THE
INTERIOR OF WESTERN WASHINGTON ABOUT 18Z. CEILINGS WILL INITIALLY
LOWER DOWN TO THE LOW END OF VFR WITH SOME MVFR CEILINGS...2000-2900
FEET RIGHT WITH AND JUST BEHIND THE FRONT. CEILINGS WILL IMPROVE
BACK UP INTO THE 4000-5000 FOOT RANGE LATE THIS AFTERNOON INTO THE
EVENING HOURS. THE EXCEPTION WILL BE IN THE CONVERGENCE ZONE WITH
CEILINGS NEAR 2000 FEET CONTINUING. CONVERGENCE ZONE CONTINUING INTO
EARLY WEDNESDAY MORNING.

KSEA...MID LEVEL DECK MOVING OVER THE TERMINAL BY 15Z WITH CEILINGS
LOWERING QUICKLY AFTER THAT DOWN TO 3000-4000 FEET. POSSIBILITY OF
CEILINGS A LITTLE LOWER RIGHT WITH AND BEHIND THE FRONT. THE FRONTAL
PASSAGE IS TIMED AROUND 18Z. CEILINGS IMPROVING BACK UP TO 4000-5000
FEET BY LATE AFTERNOON. SOUTH WIND 8 TO 12 KNOTS. WINDS WILL TRY TO
COME AROUND TO NORTHWESTERLY LATE AFTERNOON INTO THE EVENING HOURS.
FELTON

&&

.MARINE...A COLD FRONT WILL MOVE THROUGH THE WATERS THIS MORNING.
EXPECT SMALL CRAFT ADVISORY SOUTHERLY WINDS AHEAD OF THE FRONT WITH
POST FRONTAL SMALL CRAFT ADVISORY WESTERLY WINDS IN THE STRAIT OF
JUAN DE FUCA CONTINUING THROUGH TONIGHT. WINDS WILL EASE THIS
AFTERNOON AND EVENING OVER THE REMAINDER OF THE WATERS.

ANOTHER COLD FRONT WILL MOVE THROUGH THE WATERS ON WEDNESDAY
AFTERNOON. THIS FRONT STILL LOOKS WEAKER THAN THE FRONT THIS MORNING
SO WIND SPEEDS ARE EXPECTED TO REMAIN BELOW SMALL CRAFT ADVISORY.

HIGH PRES WILL BUILD INTO THE COASTAL WATERS ON THURSDAY AND WILL
GRADUALLY SHIFT OFFSHORE FRIDAY INTO SATURDAY. SMALL CRAFT ADVISORY
NORTHWEST WINDS ARE POSSIBLE FRIDAY AND SATURDAY OVER THE COASTAL
WATERS. FELTON

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT WEST ENTRANCE TO THE
     STRAIT AND GRAYS HARBOR BAR.

&&

$$

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








USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.