Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 240414
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT SAT AUG 23 2014

.SYNOPSIS...HIGH PRESSURE OVER THE PACIFIC NORTHWEST WILL MAINTAIN
SUNSHINE AND WARMER TEMPERATURES ACROSS WESTERN WASHINGTON INTO THE
FIRST HALF OF NEXT WEEK. AN EXCEPTION WILL BE A WEAK WEATHER SYSTEM
MOVING SOUTHEAST THROUGH BRITISH COLUMBIA THAT MAY PRODUCE A FEW
SHOWERS OR POSSIBLE THUNDERSTORMS NEAR THE CASCADE CREST LATE
SUNDAY. A CHANGE TO MORE CLOUDS AND COOLER CONDITIONS IS LIKELY NEAR
THE END OF NEXT WEEK.

&&

.SHORT TERM...UPPER LEVEL HIGH PRESSURE IS CENTERED ALONG 140 W WITH
THE LONG WAVE TROUGH POSITIONED OVER THE WRN INTERIOR. A WEAK SHORT
WAVE RIDING OVER THE TOP OF THE RIDGE WILL CLIP THE AREA ON SUNDAY.
THE SURFACE FLOW AHEAD OF THE TROUGH WILL BE WEAK OVERNIGHT SO
STRATUS HUGGING THE COAST WILL PROBABLY ONLY SPREAD DOWN THE STRAIT
AND PARTIALLY INTO THE SW INTERIOR. THE REST OF THE INTERIOR WILL
REMAIN MOSTLY CLEAR THROUGH SUNDAY MORNING.

THE APPROACHING UPPER LEVEL LOW PRESSURE SYSTEM CAN BE PICKED OUT ON
WATER VAPOR SATELLITE IMAGERY ALONG THE B.C. COAST MOVING SE. THIS
SYSTEM IS NOT REFLECTED AT THE SURFACE AND HAS LITTLE MOISTURE
ASSOCIATED WITH IT SO MOST OF THE AREA WILL REMAIN DRY. MODELS
INDICATE ENOUGH INSTABILITY DURING THE AFTERNOON TO PRODUCE SOME
SHOWERS OVER THE HIGHER CASCADES. LI`S NEAR -2 OR -3C NEAR THE CREST
ALSO IMPLIES A CHANCE OF THUNDERSTORMS BUT THIS SHOULD BE ISOLATED
IN COVERAGE. OTHERWISE...MOSTLY SUNNY SKIES FOR THE LOWLANDS AND
SLIGHTLY MODERATING HIGH TEMPERATURES IN THE UPPER 70S.

HIGH PRESSURE OFFSHORE AMPLIFIES AND SHIFTS TOWARD THE AREA MONDAY
AND TUESDAY. 500 MB HEIGHTS RISE EACH DAY WITH LIGHT FLOW NEAR THE
SURFACE. THIS SHOULD RESULT IN DRY WEATHER AND A WARMING TREND.
TUESDAY COULD BE THE WARMEST DAY OF THE WEEK AS TEMPERATURES REACH
THE MID 80S AROUND PUGET SOUND. MERCER

.LONG TERM...NO UPDATES FROM PREVIOUS DISCUSSION...THE PROGS AGREE
THAT THE UPPER RIDGE EASES INLAND WEDNESDAY AND THURSDAY WITH 500 MB
HEIGHTS SHOWING ONLY A SLOW DECLINE. WEDNESDAY HIGHS TEMPS MAY BE
QUITE CLOSE TO TUESDAYS WITH ONLY A FEW MORE DEGREES DROP THURSDAY.

IT IS BEYOND THURSDAY THAT THE PROGS DIVERGE. THE GFS IS THE FASTEST
IN MOVING THE APPROACHING TROUGH AND ASSOCIATED PRECIPITATION
ONSHORE WHILE THE ECMWF AND CANADIAN ARE SLOWER AND IN MORE GENERAL
AGREEMENT. THE NCEP PMD DISCUSSION AGREES. SO WILL HOLD OFF THE
PRECIPITATION FRIDAY THOUGH WITH MORE CLOUD COVER AND COOLER HIGHS
BEFORE PRECIPITATION ARRIVES FRIDAY NIGHT AND SATURDAY...JUST IN
TIME FOR THE START OF THE HOLIDAY WEEKEND. SIGH....BUEHNER

&&

.AVIATION...NORTHERLY FLOW ALOFT WILL PREVAIL OVER W WA TONIGHT AND
SUNDAY...BETWEEN A LARGE UPPER LEVEL RIDGE CENTERED WELL OFFSHORE
AND AN UPPER LEVEL LOW OVER THE NORTHERN ROCKIES. SURFACE GRADIENTS
REMAINING LIGHT ONSHORE THROUGH SUNDAY.

SATELLITE IMAGERY SHOWS STRATUS ALONG THE COAST MOVING INTO THE
LOWER CHEHALIS VALLEY AND COMING DOWN THE STRAIT OF JUAN DE FUCA.
THE LAYER IS ONCE AGAIN SHALLOW WITH CEILINGS ALONG THE COAST BELOW
500 FEET. ONSHORE GRADIENTS ARE A LITTLE STRONGER TONIGHT BUT WITH
THE LAYER BEING SO SHALLOW EVEN WITH THE STRONGER GRADIENTS NOT
EXPECTING THE STRATUS TO GET EAST OF THE SOUND OVER THE CENTRAL AND
SOUTHERN PUGET SOUND. TO THE NORTH THE STRATUS COULD GET AS FAR EAST
AS WHIDBEY ISLAND. CEILINGS BELOW 500 FEET WITH THE STRATUS WITH
LOCAL VISIBILITIES 1 TO 3 SM. STRATUS WILL BURN BACK TO THE COAST
BEFORE 19Z.

AN UPPER LEVEL SHORTWAVE TROUGH OVER SOUTHERN BRITISH COLUMBIA THIS
EVENING WILL MOVE SOUTHEAST OVER WESTERN WASHINGTON SUNDAY MORNING.
THE TROUGH WILL DESTABILIZE THE AIR MASS FOR ISOLATED THUNDERSTORMS
OVER THE CASCADES SUNDAY AFTERNOON AND EVENING.

KSEA...MOSTLY CLEAR SKIES WITH NORTHEASTERLY WINDS 3 TO 6 KNOTS
BECOMING VARIABLE SUNDAY MORNING. FELTON

&&

.MARINE...A LARGE SURFACE HIGH CENTERED NEAR 140W ALONG WITH LOWER
PRESSURE EAST OF THE CASCADES WILL MAINTAIN A WEAK ONSHORE FLOW
PATTERN INTO THE MIDDLE OF NEXT WEEK. ONSHORE FLOW WILL BE TYPICALLY
STRONGEST DURING THE LATE AFTERNOON AND EVENING HOURS AND WEAKEST
DURING THE MORNING. LOW END SMALL CRAFT ADVISORY WINDS ARE EXPECTED
IN THE CENTRAL AND EAST STRAIT EACH EVENING. THE MARINE LAYER IS
SHALLOW ENOUGH THAT AREAS OF VISIBILITIES BELOW 1/2 SM WILL OCCUR
OVER THE NEAR SHORE COASTAL WATERS AS WELL AS THE STRAIT AND NEAR
THE SAN JUANS TONIGHT INTO SUNDAY MORNING. FELTON

&&

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

&&

$$

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML








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