Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 260427
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
927 PM PDT SAT JUL 25 2015

.SYNOPSIS...A LARGE UPPER LEVEL TROUGH WILL BRING SHOWERS TO WESTERN
WASHINGTON THROUGH THIS WEEKEND...ALONG WITH A CHANCE OF
THUNDERSTORMS SUNDAY AFTERNOON AND EVENING. THE TROUGH WILL MOVE
EAST OF THE CASCADES MONDAY ENDING THE SHOWERS AND BRINGING WARMER
WEATHER. AN UPPER LEVEL RIDGE WILL BUILD TUESDAY AND KEEP CONDITIONS
DRY AND WARM THROUGH THE END OF THE WEEK.

&&

.SHORT TERM...A LARGE UPPER LEVEL TROUGH IS MOVING SLOWLY ACROSS THE
REGION THIS WEEKEND. THE CENTER WAS UP OVER NORTHERN B.C. EARLY THIS
EVENING. THE NAM BUFR SOUNDING FOR KSEA SHOWS THE LOWER AIR MASS IS
STILL UNSTABLE UP THROUGH ABOUT 700 MB THIS EVENING...AND RADAR
SHOWS QUITE A FEW SHOWERS MOVING ACROSS MAINLY THE NORTHERN HALF OF
W WA. A CONVERGENCE ZONE FEATURE HAS FORMED OVER N PUGET SOUND WITH
SHOWERS ORIENTED IN A NARROW WEST-EAST BAND OVER S SNOHOMISH COUNTY.
THE 00Z NAM AND THE OLDER 12Z ARW BOTH SHOW THIS FEATURE IN THE THE
PRECIP FIELDS. THE CONVERGENCE SEEMS TO BE RELATED TO W WINDS ALOFT
FROM 850-700 MB AS THERE IS NO REAL ORGANIZED SURFACE CONVERGENCE
GOING ON. THE CURRENT FORECAST FOR SCATTERED SHOWERS TONIGHT LOOKS
ON TRACK.

MODELS REMAIN CONSISTENT IN BRINGING AN UPPER LEVEL SHORTWAVE TROUGH
ACROSS W WA SUNDAY AFTERNOON...AS THE MAIN TROUGH IS DRIFTING ACROSS
THE STATE. GFS LIFTED INDICES SUNDAY AFTERNOON DROP TO -5 OVER THE
SW INTERIOR AND -3 OVER PUGET SOUND. THE NAM IS WEAKER WITH A -2
LIFTED INDEX. THE KSEA BUFR SOUNDING SHOWS A SATURATED AIR MASS SO
WITH SO MUCH CLOUD COVER...AFTERNOON SOLAR HEATING MAY NOT INCREASE
THE INSTABILITY MUCH. MODELS SHOW THE SHOWERS DIMINISHING SUNDAY
EVENING WITH NOT MUCH ACTIVITY AFTER MIDNIGHT.

DRIER N FLOW ALOFT DEVELOPS OVER W WA ON MONDAY AS THE UPPER LEVEL
TROUGH SHIFTS OVER E WA AND AN OFFSHORE UPPER LEVEL LEVEL RIDGE
APPROACHES THE REGION. THE GFS SHOWS SOME SPOTTY SHOWER ACTIVITY BUT
THE NAM HAS HARDLY ANYTHING. THE CURRENT FORECAST LOOKS FINE KEEPING
THE LOWLANDS DRY.

MODERATE WESTERLY ONSHORE FLOW MONDAY NIGHT SHOULD PRODUCE AREAS OF
STRATUS OVER THE INTERIOR FOR A MOSTLY CLOUDY START TO TUESDAY.
HOWEVER THE BROAD UPPER LEVEL RIDGE WILL BEGIN MOVING OVER W WA ON
TUESDAY WITH 500 MB HEIGHTS RISING TO ABOVE 5800 METERS BY 18Z. THIS
WILL BE THE START OF A WARMING TREND THAT WILL PUSH MAX TEMPS WELL
INTO THE 80S LATER IN THE WEEK. HIGHS ON MONDAY WILL WARM TO THE
UPPER 60S TO MID 70S...THEN TO THE MID 70S TO LOWER 80S ON TUESDAY.
KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 PM AFD...HEIGHTS REMAIN
HIGH THROUGH THE END OF THE WEEK...IN THE 580S. THURSDAY LOOKS LIKE
THE WARMEST DAY. HIGHS EACH DAY WILL BE WELL INTO THE 80S OVER THE
INTERIOR...WITH LOW 90S OVER THE SOUTH INTERIOR. ON THURSDAY 90S
COULD REACH AS FAR NORTH AS THE SEATTLE AREA. THERE IS LITTLE CHANGE
IN THE PATTERN INTO THE WEEKEND. BURKE

&&

.AVIATION...LIGHT WLY FLOW ALOFT WITH AN UPPER TROUGH OVER THE PAC
NW THROUGH SUNDAY NIGHT BEFORE EXITING. THE AIR MASS IS MOIST AND
SOMEWHAT UNSTABLE...ENOUGH TO PRODUCE SHOWERS TONIGHT. INSTABILITY
WILL INCREASE SUBSTANTIALLY SUNDAY AFTERNOON/EVENING WITH A CHANCE
OF TSTMS.

TONIGHT...MOSTLY MID LEVEL CLOUDS THIS EVENING THEN AREAS OF MVFR
STRATUS SHOULD DEVELOP LATE TONIGHT AND SUNDAY MORNING. THE PUGET
SOUND CONVERGENCE ZONE WILL ALSO BE ACTIVE THROUGH TONIGHT BUT WILL
LIKELY REMAIN JUST TO THE N OF KSEA/KBFI AND LIKELY TO AFFECT KPAE.

KSEA...N WIND TO 8 KT...BECOMING LIGHT/VARIABLE AFTER
MIDNIGHT...THEN BECOMING LIGHT S/SW AFTER 12Z. MVFR CIGS LIKELY
BETWEEN 12Z-19Z. DTM

&&

.MARINE...ONSHORE FLOW WILL PREVAIL THROUGH TUESDAY WITH HIGH
PRESSURE OFFSHORE AND LOWER PRESSURE INLAND. SMALL CRAFT
ADVISORY WEST WINDS ARE LIKELY EACH EVENING FOR THE CENTRAL
AND EASTERN STRAIT OF JUAN DE FUCA.

ONSHORE FLOW WILL WEAKEN AND TURN MORE NORTHERLY WEDNESDAY AND
THURSDAY AS A THERMAL TROUGH BUILDS NORTH ALONG THE COAST.
SCHNEIDER/DTM

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...SMALL CRAFT ADVISORY CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.

&&

$$

WWW.WEATHER.GOV/SEATTLE

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