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 181551
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
850 AM PDT MON AUG 18 2014

.SYNOPSIS...WEAK NORTHWEST FLOW ALOFT AND A DRY AIR MASS WILL KEEP
WESTERN WASHINGTON MOSTLY SUNNY TODAY WITH TEMPERATURES A LITTLE
WARMER THAN ON SUNDAY. A SMALL WEAK UPPER LEVEL TROUGH AND STRONGER
ONSHORE FLOW WILL BRING PARTLY SUNNY AND COOLER CONDITIONS ON
TUESDAY. A LARGE UPPER LEVEL TROUGH WILL PARK OVER EASTERN
WASHINGTON WEDNESDAY THROUGH FRIDAY...AND COMBINED WITH STRONGER
ONSHORE FLOW...WILL BRING PARTLY CLOUDY AND COOLER WEATHER...WITH
SHOWERS AT TIMES OVER THE MOUNTAINS. AN UPPER LEVEL RIDGE WILL
GRADUALLY BUILD OVER THE REGION NEXT WEEKEND.

&&

.SHORT TERM...THE WEAK UPPER LEVEL TROUGH OVER THE REGION THIS
MORNING WILL DRIFT AWAY THIS AFTERNOON AS STRONGER NW FLOW ALOFT
OVER THE NE PACIFIC PUSHES EAST OVER W WA. MODELS ARE REMAINING ON
TRACK WITH THE INITIAL UPPER LEVEL SHORTWAVE TROUGH AND THE
ASSOCIATED HIGH LEVEL MOISTURE THAT SHOULD MOVE ACROSS W WA LATE
TONIGHT AND TUESDAY. THIS SYSTEM WAS OVER THE CENTRAL B.C. COAST
THIS MORNING AND EXTENDS OFFSHORE TO THE SW.

THE AIR MASS OVER W WA THIS MORNING REMAINS MOSTLY DRY...WITH
STRATUS MAINLY OVER THE CENTRAL COAST AND EXTENDING UP THE LOWER
CHEHALIS VALLEY TO SHELTON. THERE WERE SOME ISOLATED STRATUS PATCHES
OVER THE REST OF THE INTERIOR. WITH LIMITED STRATUS AND A DRY AIR
MASS MAX TEMPS SHOULD MAKE THE MID 70S TO MID 80S TODAY.

THE AIR MASS OVER THE N CASCADES WILL BECOME UNSTABLE AGAIN THIS
AFTERNOON AND EVENING...SO A FEW SHOWERS ARE POSSIBLE. W FLOW AT 700
MB SHOULD TAKE THE SHOWERS E OF THE CREST.

SLIGHTLY STRONGER ONSHORE FLOW IS EXPECTED TO DEVELOP TONIGHT. THE
APPROACHING UPPER LEVEL TROUGH IS EXPECTED TO SHIFT THE PRESSURE
GRADIENT DIRECTION FROM WESTERLY TO SOUTHWESTERLY LATE TONIGHT.
STRATUS SHOULD MAKE IT A LITTLE FARTHER EAST OVER THE N INTERIOR AND
COVER MORE OF SOUTH AND CENTRAL PUGET SOUND TUESDAY MORNING. A SW
PRESSURE GRADIENT WILL CONTINUE ON TUESDAY AND THIS MAY SLOW THE
STRATUS BURN OFF OVER THE LOWER CHEHALIS VALLEY. COOLING ALOFT FROM
THE TROUGH COMBINED WITH ONSHORE FLOW SHOULD DROP MAX TEMPS BACK
INTO THE 70S.

A LONG WAVE TROUGH WILL FORM ALONG THE WA/ID BORDER ON TUESDAY AND
WILL PERSIST INTO NEXT WEEKEND. THIS WILL MAINTAIN NW FLOW ACROSS W
WA THROUGH THE PERIOD. THE FLOW WILL BE MAINLY DRY WITH WEAK
EMBEDDED SHORTWAVE TROUGHS BRUSHING THE CASCADES WITH SOME SHOWERS
AT TIMES. MODERATE ONSHORE FLOW AND THE COOLER AIR MASS WITH THE
TROUGH WILL KEEP MAX TEMPS NEAR OR SLIGHTLY BELOW NORMAL. MORNING
STRATUS WITH AFTERNOON CLEARING WILL RESULT IN PARTLY SUNNY SKIES
OVERALL.  KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...THE MODELS
CONTINUE TO SHOW CONSISTENCY AND AGREEMENT THROUGH THE EXTENDED
FORECAST PERIOD THURSDAY THROUGH SUNDAY. SHORTWAVE ENERGY WILL CARVE
OUT A SOMEWHAT DEEPER UPPER TROUGH OVER THE REGION THURSDAY AND
FRIDAY...AND THEN THE UPPER TROUGH WILL PROGRESS SLOWLY EASTWARD
SATURDAY AND SUNDAY AHEAD OF AN OFFSHORE UPPER RIDGE. THE LOWLANDS
WILL PROBABLY CONTINUE TO HAVE DRY WEATHER...AND EVEN IN THE
MOUNTAINS -- ASIDE FROM POSSIBLY THE NORTH CASCADES -- PRECIPITATION
AMOUNTS WILL NOT BE ENOUGH TO PROVIDE WETTING RAINS.   MCDONNAL

&&

.AVIATION....A WEAK UPPER TROUGH WILL BRUSH WRN WA TONIGHT AND
TUESDAY. LIGHT WESTERLY FLOW ALOFT. AIR MASS STABLE EXCEPT SLIGHTLY
UNSTABLE NEAR THE CASCADE CREST. DRY AIR MASS TODAY EXCEPT MOIST
NEAR THE SURFACE DURING THE NIGHT AND MORNING HOURS. COASTAL
STRATUS...AND FOG IN THE CHEHALIS GAP/WRN PORTION OF THE SW INTERIOR
AND DOWN THE STRAIT OF JUAN DE FUCA WILL BURN BACK TO THE COAST THIS
AFTERNOON. INCREASING MID/UPPER LEVEL MOISTURE TONIGHT AND TUESDAY.

KSEA...VFR CONDITIONS WITH MOSTLY CLEAR SKIES. FOG AND LOW CLOUDS
ARE NOT EXPECTED THIS MORNING...AND ARE UNLIKELY TUESDAY MORNING AS
WELL. NORTHERLY WIND 4-8 KNOTS. DTM

&&

.MARINE...ONSHORE FLOW WILL PERSIST FOR THE NEXT SEVERAL DAYS WITH
HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND. THIS WILL BRING
SMALL CRAFT ADVISORY WEST WINDS TO THE STRAIT OF JUAN DE FUCA MOST
EVENINGS THIS WEEK.

AN UPPER TROUGH MOVING THROUGH TONIGHT AND TUESDAY WILL BRING AND
INCREASE TO THE LOW LEVEL ONSHORE FLOW. SMALL CRAFT ADVISORY
NORTHWEST WINDS ARE POSSIBLE AT TIMES OVER THE COASTAL WATERS
BEGINNING TONIGHT. GALE FORCE WEST WINDS ARE POSSIBLE FOR THE
CENTRAL AND EASTERN STRAIT TUESDAY EVENING. SCHNEIDER/DTM

&&

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

&&

$$

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.