Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 192154
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
254 PM PDT TUE AUG 19 2014

.SYNOPSIS...AN UPPER LEVEL TROUGH OVER THE PACIFIC NORTHWEST WILL
MAINTAIN NORTHWEST FLOW ALOFT AND MODERATE LOW LEVEL ONSHORE FLOW
OVER WESTERN WASHINGTON THIS WEEK. SMALL WEAK UPPER LEVEL TROUGHS
BRUSHING THE AREA MAY BRING A FEW SHOWERS TO MAINLY THE CASCADES AT
TIMES. THE ONSHORE FLOW WILL BRING MOSTLY CLOUDY SKIES TO THE
LOWLANDS WITH PARTIAL AFTERNOON CLEARING. THE UPPER LEVEL TROUGH
WILL SHIFT EAST THIS WEEKEND ALLOWING AN UPPER LEVEL RIDGE TO BUILD
INLAND FOR A SMALL WARMING AND DRYING TREND.

&&

.SHORT TERM...AN UPPER LEVEL LONGWAVE TROUGH HAS SET UP OVER E WA
TODAY...WHICH WILL GIVE W WA NW FLOW ALOFT FOR THE NEXT COUPLE DAYS.
MESOSCALE MODELS STILL SHOW SOME CONVECTION DEVELOPING OVER THE
OLYMPICS THIS AFTERNOON AND EVENING AND SATELLITE IMAGERY SHOW
CUMULUS HAS BEGUN TO INCREASE. INCREASING SW ONSHORE FLOW IS
BEGINNING TO DEVELOP OVER S PUGET SOUND AND NW FLOW COMING DOWN FROM
THE EASTERN STRAIT COULD ADD SOME CONVERGENCE TO THE MIX. I WILL
KEEP PARTLY CLOUDY SKIES AND A CHANCE OF SHOWERS OVER CENTRAL PUGET
SOUND THIS EVENING...ALTHOUGH CONFIDENCE IS NOT VERY HIGH.

THE AIR MASS OVER THE CASCADES HAS BECOME UNSTABLE THIS AFTERNOON
WITH CUMULUS DEVELOPING. A FEW SHOWERS ARE EXPECTED. CONVECTION WILL
PROBABLY BE DEEPER OVER THE FAR NE CASCADES SO AN ISOLATED
THUNDERSTORM WOULD NOT BE SURPRISING BUT IS STILL UNLIKELY.

THE NW FLOW ALOFT WILL BE MAINLY DRY THROUGH FRIDAY...BUT SMALL
WEAK UPPER LEVEL SHORTWAVE TROUGHS EMBEDDED IN THE NW FLOW MAY BRING
A FEW SHOWERS TO THE CASCADES AT TIMES AS THEY BRUSH THE AREA. THE
UPPER LEVEL TROUGH WILL DEEPEN INTO AN UPPER LEVEL LOW OVER CENTRAL
WA THURSDAY NIGHT AND FRIDAY. NE FLOW ALOFT AROUND THE LOW ON FRIDAY
COULD BRING A MORE UNSTABLE AIR MASS AND MORE WIDESPREAD SHOWERS
OVER THE CASCADES BY FRIDAY AFTERNOON. IT IS POSSIBLE THAT SOME
SHOWERS COULD SPREAD W OUT OVER THE LOWLANDS BUT THE MODELS DO NOT
SHOW THAT JUST YET. FRIDAY AFTERNOON AND EVENING WILL PROBABLY BE
THE BEST BET FOR THUNDERSTORMS OVER THE CASCADES. IT IS JUST A BIT
EARLY TO JUMP ON THE THUNDERSTORMS...BUT IF THE MODELS REMAIN
CONSISTENT FOR A COUPLE MORE RUNS THEY WILL BE ADDED TO THE
FORECAST.

THE OTHER IMPACT OF THE UPPER TROUGH WILL BE TO MAINTAIN A SW
DIRECTION TO THE LOW LEVEL ONSHORE FLOW THE NEXT COUPLE OF DAYS. A
DEEPER MARINE LAYER WILL FORM TONIGHT SO STRATUS WEDNESDAY MORNING
SHOULD COVER JUST ABOUT ALL W WA...AND SHOULD BE DEEP ENOUGH SO THAT
SOME DRIZZLE IS POSSIBLE. WEAK CONVERGENCE OVER CENTRAL PUGET SOUND
SHOULD INCREASE THE CHANCE OF DRIZZLE THERE. NOT MUCH BREAKOUT IS
EXPECTED WEDNESDAY AFTERNOON...BUT THERE COULD BE SOME LATE
AFTERNOON SUN BREAKS. ONSHORE FLOW WEDNESDAY NIGHT WILL NOT BE AS
STRONG BUT STRATUS IN THE DEEPER MARINE LAYER SHOULD ONCE AGAIN
COVER THE W WA LOWLANDS. THE STRATUS IS EXPECTED TO BREAK UP MORE ON
THURSDAY AND FRIDAY FOR PARTLY TO MOSTLY SUNNY CONDITIONS IN THE
AFTERNOON.

THE UPPER LEVEL TROUGH AND ONSHORE FLOW COMBINED WILL COOL MAX TEMPS
BACK TO THE LOWER 70S OVER THE INTERIOR...WHICH IS NEAR OR SLIGHTLY
BELOW NORMAL. KAM

.LONG TERM...THE UPPER LEVEL LOW OVER THE PACIFIC NW ON FRIDAY WILL
SHIFT E OVER ID BY SATURDAY MORNING...ALLOWING AN OFFSHORE UPPER
LEVEL RIDGE TO BUILD EASTWARD OVER THE AREA. WITH WEAK ONSHORE FLOW
FRIDAY AND SATURDAY NIGHTS AND DRIER NW FLOW ALOFT ON
SATURDAY...SKIES WILL BE MOSTLY SUNNY WITH MAX TEMPS WARMING BACK TO
THE 70S TO LOWER 80S. BOTH THE GFS AND ECMWF BRING AN UPPER LEVEL
SHORTWAVE TROUGH DOWN OVER W WA ON SUNDAY. AT THIS POINT THE
SHORTWAVE LOOKS MAINLY DRY WITH JUST AN INCREASE IN MID/HI CLOUD
COVER EXPECTED. ONSHORE FLOW WILL PROBABLY INCREASE SUNDAY NIGHT AND
MONDAY. KAM

&&

.AVIATION...A WEAK UPPER LEVEL TROUGH WILL DIG SE THROUGH THE
INTERIOR PAC NW TONIGHT. LIGHT WLY FLOW ALOFT...BECOMING NWLY ON
WED. ATMOSPHERE GENERALLY STABLE...AND SOMEWHAT MOIST IN THE LOWER
AND MID LEVELS.

STRONGER SOUTHWEST ONSHORE FLOW IS EXPECTED TONIGHT WITH MORE
EXTENSIVE LOW CLOUDS FOR THE INTERIOR BY WEDNESDAY MORNING. EXPECT
AREAS OF IFR OR LOW MVFR CEILINGS TO DEVELOP...THEN SCATTER ACROSS
THE INTERIOR DURING THE AFTERNOON.

A PUGET SOUND CONVERGENCE ZONE MAY DEVELOP OVER FAR N KING OR
SOUTHERN SNOHOMISH COUNTY EARLY THIS EVENING. THE KPAE TERMINAL HAS
THE BEST CHANCE OF PRECIPITATION AND BRIEF MVFR CIGS...THOUGH
CONFIDENCE IN THESE DETAILS ARE LOW.

KSEA...A PUGET SOUND CONVERGENCE ZONE MAY RESULT IN SOME SCT-BKN
CLOUD COVER AROUND 5K FT DEVELOPING THIS EVENING. ANY PRECIPITATION
MOST LIKELY WILL REMAIN N OF THE TERMINAL. LOW CLOUDS ARE EXPECTED
LATER TONIGHT WITH THE MARINE PUGH. SOUTH TO SOUTHWEST WIND 4-8
KNOTS.

&&

.MARINE...STRONGER SOUTHWESTERLY ONSHORE FLOW WILL PREVAIL TONIGHT
BEHIND A WEAK UPPER LEVEL DISTURBANCE. RACE ROCKS REACHED 32 KT AT 2
PM SO A GALE WARNING WILL BE IN EFFECT FOR THE CENTRAL AND EAST
STRAIT LATE THIS AFTERNOON AND THIS EVENING. THE FLOW ADJACENT TO
THE EAST ENTRANCE TONIGHT SUPPORTS SMALL CRAFT WINDS IN THE NRN
PORTION OF ADMIRALTY INLET AND THE SRN PORTION OF THE N INTERIOR. IN
ADDITION...NORTHWESTERLY GRADIENT WILL TIGHTEN TONIGHT THROUGH
WEDNESDAY MORNING SUPPORTING SMALL CRAFT WINDS OVER THE OUTER
COASTAL WATERS.

SMALL CRAFT ADVISORY WEST WINDS ARE LIKELY EACH EVENING FOR THE NEXT
SEVERAL DAYS OVER THE CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
THE WINDS IN THE STRAIT SHOULD BE STRONGEST TONIGHT.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING CENTRAL AND EASTERN STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY OUTER COASTAL WATERS...ADMIRALTY INLET...
     AND NORTHERN INLAND WATERS.

&&

$$

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