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 171558
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
858 AM PDT SUN AUG 17 2014

.SYNOPSIS...A MOSTLY DRY AIR MASS OVER WESTERN WASHINGTON WILL BRING
SUNNY AND WARM WEATHER TODAY AND MONDAY...AFTER AREAS OF LOW
CLOUDS AND FOG CLEAR THIS MORNING. A VERY WEAK UPPER LEVEL TROUGH
MOVING SOUTH ALONG THE COAST COULD PRODUCE SOME AFTERNOON AND
EVENING THUNDERSTORMS OVER THE NORTH CASCADES. AN UPPER LEVEL TROUGH
AND STRONGER ONSHORE FLOW WILL BRING PARTLY SUNNY DAYS...MOSTLY
CLOUDY NIGHTS...AND A CHANCE OF SHOWERS TUESDAY THROUGH SATURDAY.

&&

.SHORT TERM...A WEAK DRY UPPER LEVEL TROUGH OVER THE N WA COAST THIS
MORNING WILL MOVE SLOWLY S OVER THE N OREGON COAST BY THIS EVENING.
THE TROUGH IS PRETTY MUCH CLOUD FREE BUT DOES SHOW UP NICELY ON
WATER VAPOR IMAGERY. THE ONLY IMPACT THE TROUGH MAY PRODUCE IS TO
BRING A CHANCE OF THUNDERSTORMS TO THE NORTH CASCADES THIS AFTERNOON
AND EVENING. THE AIR MASS ALOFT IS QUITE UNSTABLE BUT IS LACKING
MOISTURE. VISIBLE SATELLITE IMAGERY SHOWS LOW CLOUDS OVER THE NORTH
AND N CENTRAL CASCADES THIS MORNING WHICH COULD BE A MOISTURE
SOURCE...BUT ANY SURFACE BASED CONVECTION WILL HAVE TO GET PAST A
LOW LEVEL STABLE AREA THAT SHOWS UP ON THE SMP FORECAST BUFR
SOUNDING.

THE LOW LEVEL AIR MASS OVER W WA WAS MOIST ENOUGH TO ALLOW AREAS OF
LOW CLOUDS AND FOG TO FORM THIS MORNING. THE LOW CLOUDS AND FOG OVER
THE LAND AREAS SHOULD DISSIPATE AROUND 10 AM OR SO WHILE THE
MOISTURE OVER THE MARINE AREAS WILL PROBABLY LAST UNTIL EARLY
AFTERNOON. AFTER THE MORNING MOISTURE CLEARS...SUNNY SKIES THIS
AFTERNOON SHOULD PUSH MAX TEMPS IN TO THE 70S TO NEAR 80. ONSHORE
FLOW SHOULD BE WEAK ENOUGH TONIGHT AND THE LOWER AIR MASS DRIER...SO
THAT STRATUS MONDAY MORNING SHOULD BE MOSTLY ALONG THE COAST AND
PART WAY INLAND. MONDAYS MAX TEMPS ARE STILL EXPECTED TO BE A LITTLE
WARMER THAN TODAY.

MODELS STILL AGREE ON STRONGER NW FLOW ALOFT SPREADING INLAND OVER W
WA MONDAY NIGHT AND TUESDAY AS AN UPPER LEVEL SHORTWAVE TROUGH PLOWS
SE ACROSS THE AREA. THE FLOW WILL CARVE OUT A MEAN UPPER LEVEL
LONGWAVE TROUGH THAT WILL PERSIST BEYOND TUESDAY. MODELS SOLUTIONS
HAVE BEEN INCONSISTENT IN HANDLING MOISTURE WITH THE INITIAL
SHORTWAVE TROUGH WITH THE MOST RECENT SOLUTIONS BACKING OFF A
LITTLE ON PRECIP. SHOWERS OVER THE LOWLANDS WILL BE IFFY BOTH
TUESDAY AND TUESDAY NIGHT. THERE WILL BE A SLIGHTLY BETTER CHANCE OF
SHOWERS OVER THE MOUNTAINS BUT THE CHANCES OF MEASURABLE RAIN ARE
STILL 50 PERCENT OR LOWER...WITH EMPHASIS ON THE LOWER. KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 3 AM AFD...THE MODELS ARE
REACHING BETTER CONSISTENCY AND AGREEMENT NOW FOR THE EXTENDED
FORECAST PERIOD WEDNESDAY THROUGH SATURDAY. WEST-NORTHWEST FLOW
ALOFT WILL PREVAIL WEDNESDAY BEHIND THE TROUGH DISCUSSED JUST ABOVE.
STRONGER ONSHORE FLOW WILL DEVELOP AS WELL...AND A DEEPER MARINE
LAYER WILL PROBABLY PUSH INLAND TUESDAY NIGHT AND WEDNESDAY MORNING.

A SHORTWAVE IN THE UPPER FLOW WILL DEEPEN AS IT MOVES INTO THE
PACIFIC NORTHWEST ON THURSDAY. THE TROUGH WILL SLOWLY MOVE EAST ON
FRIDAY...AND AN OFFSHORE UPPER RIDGE WILL START MOVING TOWARD THE
AREA ON SATURDAY. FOR ALL OF THAT...THE FORECAST FOR WEDNESDAY
THROUGH SATURDAY IS STILL A BROAD BRUSH PARTLY SUNNY DAYS...MOSTLY
CLOUDY NIGHTS...AND A CHANCE OF SHOWERS. BUT SIGNIFICANT
PRECIPITATION LOOKS INCREASINGLY UNLIKELY...ESPECIALLY FOR THE
LOWLANDS.   MCDONNAL

&&

.AVIATION...THE WEAK UPPER LEVEL TROUGH OVER THE AREA TODAY WILL DIG
SOUTH ALONG THE WASHINGTON AND OREGON COASTS THROUGH MONDAY MORNING.
LIGHT NORTHWESTERLY FLOW ALOFT WILL BECOME EASTERLY OVERNIGHT...THEN
WESTERLY DURING THE DAY ON MONDAY. ONSHORE LOW LEVEL FLOW WILL
CONTINUE WITH HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND. THE
PASSING UPPER LEVEL LOW AND AFTERNOON HEATING MAY BE JUST ENOUGH TO
SUPPORT A CHANCE OF THUNDERSTORMS OVER THE NORTH CASCADES THIS
AFTERNOON...OTHERWISE THE AIR MASS WILL REMAIN DRY AND GENERALLY
STABLE THROUGH MONDAY. LOW FOG AFFECTING PARTS OF THE INTERIOR THIS
MORNING WILL BURN OFF QUICKLY OVER THE NEXT HOUR OR SO...FOLLOWED BY
STRATUS OVER THE CHEHALIS GAP...COAST...STRAIT AND INTERIOR WATERS.
SUNNY SKIES AND VFR CONDITIONS THIS AFTERNOON WILL PERSIST THROUGH
MONDAY FOR MOST OF THE INTERIOR. STRATUS WILL REFORM OVER THE COAST
AND STRAIT MONDAY MORNING...CLEARING BY THE AFTERNOON.

KSEA...VFR CONDITIONS THROUGH MONDAY. LIGHT WIND BECOMING
NORTHWESTERLY 4-8 KNOTS TODAY.

&&

.MARINE...ONSHORE FLOW WILL PERSIST FOR THE NEXT SEVERAL DAYS WITH
HIGH PRESSURE OFFSHORE AND LOWER PRESSURE INLAND. THIS WILL BRING
MARGINAL SMALL CRAFT ADVISORY WEST WINDS TO THE STRAIT OF JUAN DE
FUCA THE NEXT COUPLE OF EVENINGS. IN ADDITION...SMALL CRAFT ADVISORY
NORTHWEST WINDS ARE EXPECTED AT TIMES OVER MAINLY THE OUTER COASTAL
WATERS OVER THE NEXT FEW DAYS.

AN UPPER LEVEL DISTURBANCE PASSING TO THE NORTH MONDAY NIGHT AND
TUESDAY WILL CAUSE LOW LEVEL ONSHORE FLOW TO INCREASE. GALE FORCE
WEST WINDS REMAIN POSSIBLE FOR THE CENTRAL AND EASTERN STRAIT
TUESDAY EVENING.

&&

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

&&

$$

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.