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 202155
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PDT SUN JUL 20 2014

.SYNOPSIS...A WEAK UPPER TROUGH WILL MOVE EAST TONIGHT. THE AIR MASS
WILL DRY AND ONSHORE FLOW WILL WEAKEN MONDAY AND TUESDAY FOR PARTLY
SUNNY SKIES. AN UPPER LOW WILL BRING WIDESPREAD SHOWERS WEDNESDAY.
HIGH PRESSURE WILL BUILD OVER THE AREA LATE IN THE WEEK AND NEXT
WEEKEND FOR SUNNIER WARMER WEATHER.

&&

.SHORT TERM...AN UPPER TROUGH IS JUST OFF THE PACIFIC NORTHWEST
COAST THIS AFTERNOON...WITH MOIST ONSHORE FLOW AT LOW LEVELS.
WESTERN WASHINGTON REMAINS GENERALLY CLOUDY WITH A FEW HUNDREDTHS OF
AN INCH OF RAIN HERE AND THERE. THE CAMANO ISLAND RADAR HAS BEEN
DOWN SINCE THIS MORNING DUE TO A FAILURE OF THE WIDEBAND
COMMUNICATIONS LINE...BUT THE MESOWEST OBSERVATIONS HINT AT A VERY
WEAK CONVERGENCE ZONE OVER THE CENTRAL PUGET SOUND REGION.

THE UPPER TROUGH WILL MAKE ITS WAY QUICKLY INLAND AND EAST OF THE
CASCADES TONIGHT. SKIES WILL PROBABLY REMAIN CLOUDY IN MOIST ONSHORE
FLOW...BUT THE LITTLE SHOWER ACTIVITY WE HAVE WILL DECREASE.

THE UPPER FLOW WILL BACK FROM WESTERLY TO SOUTHWEST ON MONDAY...AS
THE FOUR CORNERS HIGH BUILDS NORTHWARD AND AN UPPER LOW DIGS
SOUTHEAST OFF THE COAST OF BRITISH COLUMBIA. ONSHORE FLOW WILL
WEAKEN AND THE AIR MASS WILL DRY. SKIES SHOULD BE MOSTLY CLOUDY
MONDAY MORNING BUT GIVE WAY TO PARTLY TO MOSTLY SUNNY SKIES IN THE
AFTERNOON.

THE UPPER LOW DIG TO A POSITION WEST OF NORTH VANCOUVER ISLAND
TUESDAY AFTERNOON. THE FORECAST FOR WESTERN WASHINGTON IS FAIRLY
UNCERTAIN. THE AIR MASS WILL BE FAIRLY DRY AND ONSHORE FLOW WILL BE
LIGHT. HOWEVER THE AIR MASS WILL DESTABILIZE A BIT...AND MID AND
HIGH LEVEL MOISTURE WILL INCREASE IN THE AFTERNOON. FOR NOW WE WILL
STAY WITH STAY WITH PARTLY SUNNY AND KEEP A LOW-END CHANCE OF
AFTERNOON SHOWERS IN THE FORECAST.

THE UPPER LOW WILL SWING THROUGH THE COASTAL WATERS WEDNESDAY AND
INLAND ACROSS WASHINGTON WEDNESDAY NIGHT. IT SHOULD BRING WIDESPREAD
SHOWERY PRECIPITATION...AND A GOOD PORTION OF THE FORECAST AREA
COULD RECEIVE 0.25 INCH OR MORE. THE SNOW LEVEL WILL BE 7000 TO
8000 FT. WE HAVE DECIDED TO LEAVE THE MENTION OF THUNDERSTORMS OUT
OF THE FORECAST FOR NOW. THAT DECISION IS PARTLY BASED ON THE FACT
THAT THE MODELS DO NOT HAVE THIS SYSTEM NAILED DOWN. FOR ONE THING
THEY SEEM TO BE DELAYING ITS ARRIVAL WITH SUCCESSIVE MODEL RUNS.
MCDONNAL

.LONG TERM...THE UPPER LOW WILL LIFT NORTHEAST INTO ALBERTA ON
THURSDAY...AND AN UPPER RIDGE WILL START BUILDING OFFSHORE. CLOUDY
SKIES AND A FEW SHOWERS COULD LINGER INTO THURSDAY MORNING...WITH A
PARTLY TO MOSTLY SUNNY AFTERNOON. LONGER RANGE MODELS AGREE AND HAVE
BEEN FAIRLY CONSISTENT WITH THE IDEA THAT THE UPPER RIDGE WILL BUILD
AND SLOWLY PROGRESS INTO THE PACIFIC NORTHWEST FRIDAY THROUGH SUNDAY
FOR SUNNIER WARMER DAYS. THE FORECAST FOR THOSE DAYS...ESPECIALLY
THE WEEKEND DAYS...HAS BEEN NUDGED TOWARD THAT IDEA.   MCDONNAL

&&

.AVIATION...AN UPPER LEVEL TROUGH OVER WESTERN WASHINGTON WILL SHIFT
INLAND TONIGHT WITH WEST FLOW ALOFT BECOMING SOUTHWEST ON MONDAY. AT
THE SURFACE...HIGH PRESSURE WILL BE OFFSHORE WITH LOWER PRESSURE
INLAND. AIR MASS MOIST AND STABLE...WITH DRYING ON MONDAY.

MOSTLY 4000-6000 FOOT CEILINGS THIS EVENING. LOWER CEILINGS ARE
EXPECTED TO GRADUALLY DEVELOP TONIGHT AND MONDAY MORNING UNDER
CONTINUED ONSHORE FLOW. THE LOW LEVELS WILL DRY ON MONDAY AND MOST
AREAS SHOULD SCATTER BY AFTERNOON.

KSEA...CEILINGS 3500-4500 THIS EVENING. CEILINGS AROUND 1500 FEET
ARE EXPECTED TO DEVELOP LATE TONIGHT...THEN LOW CLOUDS SHOULD
SCATTER AROUND MIDDAY MONDAY. WEST TO SOUTHWEST WIND 4-8 KNOTS THIS
EVENING...BECOMING SOUTH AND EASING AFTER MIDNIGHT...THEN BECOMING
NORTHWEST 5-10 KNOTS MONDAY AFTERNOON. SCHNEIDER

&&

.MARINE...HIGH PRESSURE WILL BE OFFSHORE WITH LOWER PRESSURE INLAND
FOR THE NEXT SEVERAL DAYS...RESULTING IN VARYING DEGREES OF ONSHORE
FLOW. THIS WILL GENERALLY PRODUCE SMALL CRAFT ADVISORY WEST WINDS
FOR THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA EACH EVENING INTO
THE EARLY MORNING HOURS. SCHNEIDER

&&

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

&&

$$

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.