Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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 231608
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
908 AM PDT SAT MAY 23 2015

.SYNOPSIS...A SURFACE RIDGE OFFSHORE WILL MAINTAIN MODERATE LOW
LEVEL ONSHORE FLOW OVER WESTERN WASHINGTON THROUGH NEXT WEEK. THIS
WILL KEEP SKIES MOSTLY CLOUDY AND TEMPERATURES MILD WITH PARTIAL
AFTERNOON CLEARING ON SOME DAYS. WEAK LOW PRESSURE ALOFT WILL BRING
A CHANCE OF SHOWERS MAINLY OVER THE MOUNTAINS DURING THE AFTERNOON
AND EVENINGS.

&&

.SHORT TERM...A SMALL WEAK UPPER LEVEL SHORTWAVE TROUGH EMBEDDED IN
MID LEVEL E FLOW ALOFT IS BRINGING AN AREA OF LIGHT SHOWERS ACROSS
THE CASCADES TO THE GREATER PUGET SOUND REGION THIS MORNING.
RAINFALL ACCUMULATIONS SHOULD RANGE FROM A TRACE TO A FEW HUNDREDTHS
OF AN INCH. THE SHOWERS SHOULD BE DISSIPATING LATE THIS MORNING BUT
COULD LAST UNTIL NOON. THE DIFFUSE BACK EDGE TO THE AREA OF SHOWERS
WAS JUST CROSSING THE CASCADE CREST AT 830 AM.

A LARGER WEAK UPPER LEVEL TROUGH EXTENDS FROM AN UPPER LEVEL LOW
OVER THE CENTRAL B.C. COAST DOWN ACROSS W WA THIS MORNING. THE
TROUGH AXIS IS STILL EXPECTED TO SHIFT OVER THE CASCADES THIS
AFTERNOON...WHICH WILL CHANGE THE MID LEVEL FLOW FROM E TO SE THIS
MORNING...TO VARIABLE TO S THIS AFTERNOON. THIS WILL MAKE IT LESS
LIKELY THAT CONVECTION OVER THE CASCADES THIS AFTERNOON AND EVENING
WILL MAKE IT W OVER THE LOWLANDS. GFS LIFTED INDICES OVER THE
CASCADES THIS AFTERNOON RANGE FROM -1 DOWN BY MT RAINIER TO -4 OVER
THE FAR N PART OF THE CASCADES SO ISOLATED THUNDERSTORMS ARE
EXPECTED AGAIN. THE AIR MASS IS MORE UNSTABLE JUST E OF THE CREST.

MODELS BRING THE B.C. LOW DOWN OVER THE SW PART OF B.C. SUNDAY
AFTERNOON. MID LEVEL STEERING FLOW OVER W WA WILL SHIFT FROM SW TO
W...WHICH SHOULD SHIFT THE THREAT OF THUNDERSTORMS E OF THE CASCADE
CREST. THIS WILL PROBABLY LIMIT SHOWERS W OF THE CREST AS WELL.

THE LOW IS EXPECTED TO MOVE SE OVER CENTRAL OR EASTERN WA BY MONDAY
AFTERNOON...PUTTING W WA UNDER N FLOW ALOFT. THIS WOULD EXPOSE W WA
TO ANY SHOWERS OVER B.C. MOVING S IN THE FLOW...BUT AT THIS POINT
THE MODELS ARE NOT REALLY SHOWING ANY PRECIP DEVELOPING.

THE REAL DOMINANT FACTOR IN THE WEATHER PATTERN FOR THIS NEXT WEEK
IS QUITE BORING AND PRETTY TYPICAL FOR LATE SPRING AND EARLY SUMMER.
AN AREA OF HIGH PRESSURE ALWAYS SHIFTS N TO A POSITION OFFSHORE OF
THE PACIFIC NW THIS TIME OF YEAR...PUTTING W WA UNDER A PERSISTENT
LOW LEVEL ONSHORE FLOW PATTERN. FOR THE NEXT WEEK A SURFACE RIDGE
WILL BE PRESENT ALONG ROUGHLY 135W OR ABOUT 500 NM W OF THE WA
COAST. THE SURFACE RIDGE WILL MAINTAIN THE ONSHORE FLOW...WITH THE
STRENGTH AND DIRECTION INFLUENCED BY THE UPPER LEVEL PATTERN.
THROUGH MONDAY THE FLOW WILL BE MODERATE AND FROM A SW OR WSW
DIRECTION...WHICH EASILY PUSHES MARINE MOISTURE INLAND AND NORTH
ACROSS PUGET SOUND. SKIES OVER W WA WILL REMAIN MOSTLY CLOUDY
THROUGH MONDAY...WITH ONLY SUN BREAKS OR PARTIAL LATE AFTERNOON
CLEARING EXPECTED. TEMPERATURES WILL REMAIN MILD WITH LOWS IN THE
50S AND HIGHS IN THE 60S. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 320 AM AFD...THE GFS AND
ECMWF IN PRETTY GOOD AGREEMENT THIS MORNING. BOTH OF THE MODELS ARE
HINTING AT A LITTLE UPPER LEVEL RIDGING IN THE LATTER HALF OF NEXT
WEEK. BEFORE THAT THE UPPER LEVEL LOW OVER EASTERN WASHINGTON VERY
SLOWLY DRIFTS OFF TO THE SOUTHEAST LEAVING WESTERN WASHINGTON
WITH...WAIT FOR IT...WEAK LOW PRESSURE ALOFT AND LOW LEVEL ONSHORE
FLOW TUESDAY AND WEDNESDAY. EVEN WITH THE WEAK UPPER LEVEL RIDGING
THURSDAY AND FRIDAY LOW LEVEL FLOW REMAINS ONSHORE SO NOT EXPECTING
A HEAT WAVE BUT WARMER TEMPERATURES ALOFT AND SOME MORE SUN IN THE
AFTERNOON WILL PUSH TEMPERATURES INTO THE MID 60S TO MID 70S FOR THE
INTERIOR BY THURSDAY AND FRIDAY AND DIMINISH THE THREAT OF AFTERNOON
AND EVENING SHOWERS OVER THE CASCADES. FELTON

&&

.AVIATION...A WEAK UPPER LEVEL LOW ALONG THE CENTRAL B.C. COAST WILL
MOVE SE OVER WA ON SUNDAY. THE MAIN EFFECT FROM THIS SYSTEM WILL BE
TO INCREASE ONSHORE FLOW AND DEEPEN THE MARINE LAYER. WEAKLY
UNSTABLE ATMOSPHERE ALOFT THROUGH THIS EVENING WITH THUNDERSTORMS
POSSIBLE OVER THE CASCADES. MOIST AT ALL LEVELS TODAY...THEN DRYING
IN THE MID/UPPER LEVELS TONIGHT AND SUNDAY EXCEPT OVER THE CASCADES.
A DEEP MARINE LAYER OVER WRN WA TODAY WILL KEEP CEILINGS FIRMLY MVFR
THIS MORNING WITH SOME IMPROVEMENT AFTER 21Z. LOCALIZED IFR CIGS ARE
POSSIBLE ALONG THE COAST AND STRAIT THROUGH LATE THIS MORNING.

KSEA...CEILINGS 1-2K FT THIS MORNING WITH SCATTERED LIGHT SHOWERS.
CIGS SHOULD LIFT TO VFR BY LATE THIS AFTERNOON WITH BROKEN CIGS BY
AROUND 4-5K FT BY 22Z. S-SW WIND 4-8 KT WILL CONTINUE. DTM

&&

.MARINE...HIGH PRES WILL REMAIN OFFSHORE WITH LOWER PRES INLAND.
SMALL CRAFT ADVISORY STRENGTH WESTERLY WINDS ARE LIKELY EACH EVENING
IN THE STRAIT TONIGHT THROUGH MONDAY NIGHT. ONSHORE FLOW WILL
CONTINUE TUESDAY AND WEDNESDAY. DTM/CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EAST STRAIT FROM NOON PDT
     TODAY THROUGH LATE TONIGHT.

&&

$$

WWW.WEATHER.GOV/SEATTLE

YOU CAN SEE AN ILLUSTRATED VERSION OF THIS 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.