Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 220404
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PDT THU AUG 21 2014

.SYNOPSIS...A LOW PRESSURE SYSTEM ALOFT WILL MOVE SOUTHEAST OVER THE
REGION TONIGHT BRINGING A THREAT OF SHOWERS MAINLY IN THE MOUNTAINS
THROUGH FRIDAY NIGHT. HIGHER PRESSURE ALOFT IS THEN FORECAST TO
BUILD OVER THE PACIFIC NW OVER THE WEEKEND FOR MORE SUNSHINE AND
WARMER DAYTIME TEMPERATURES...LIKELY CONTINUING WELL INTO NEXT WEEK.

&&

.SHORT TERM...UPPER LEVEL LOW PRESSURE IS IN THE PROCESS OF CLOSING
OFF OVER THE PAC NW THIS EVENING. INSTABILITY IS INCREASING AHEAD OF
AND NEAR THE LOW CENTER RESULTING IN THUNDERSTORMS OVER THE SRN B.C.
ROCKIES AND ALSO JUST E OF THE WA CASCADE CREST. A COOL MARINE AIR
MASS RESULTING FROM WEAK ONSHORE FLOW OVER THE WRN WA LOWLANDS WILL
KEEP MOST OF OUR AREA STABLE. IN FACT...AREAS OF MARINE CLOUDS ARE
EXPECTED TO DEVELOP AGAIN LATER TONIGHT AND FRIDAY MORNING BUT WITH
LESS COVERAGE THAN THIS MORNING. NLY FLOW AND COOLER AIR ALOFT
BEHIND THE LOW ON FRIDAY WILL RESULT IN UNSTABLE CONDITIONS OVER THE
MOUNTAINS. MODELS ARE IN GOOD AGREEMENT ON SOME SHOWER ACTIVITY OVER
THE CASCADES AND FOOTHILLS WITH SUB ZERO LI`S BY AFTERNOON
SUPPORTING A THREAT OF THUNDERSTORMS. THE NLY FLOW COULD ALLOW A FEW
SHOWERS TO SPILL INTO THE FAR N INTERIOR BUT PROBABLY NOT MAKING IT
INTO THE PUGET SOUND REGION. THE FORECAST WAS UPDATED THIS EVENING
TO TRIM BACK POPS SLIGHTLY TO JUST THE MOUNTAINS AND FAR N.
GRADIENTS WILL GO FLAT ON FRIDAY WITH MORE SUNSHINE EXPECTED OVER
THE LOWLANDS SO HIGH TEMPERATURES SHOULD REACH THE LOW 70S.

LOW PRESSURE WILL SHIFT EWD FRIDAY NIGHT BUT THE LONG WAVE TROUGH
POSITION WILL REMAIN CENTERED OVER THE ROCKIES. THIS KEEPS WRN WA
UNDER NW FLOW ALOFT BETWEEN THE RIDGE JUST OFFSHORE AND THE TROUGH
FURTHER INLAND. MODELS STILL SHOW 500 MB HEIGHT RISES TO AROUND
5800M BUT NOT AS HIGH AS SOME PREVIOUS RUNS. THE LACK OF ONSHORE
FLOW SHOULD RESULT IN LITTLE MORNING CLOUD COVER. UPPER HEIGHTS
UNDER THIS SCENARIO SUPPORT HIGHS NEAR 80 ON SATURDAY WHICH MATCHES
CURRENT MOS GUIDANCE. A WEAK SHORT WAVE TRAVELING DOWN THE BACK SIDE
OF THE LONG WAVE TROUGH WILL CLIP THE AREA SUNDAY. THE ONLY IMPACT
SHOULD BE TO KEEP UPPER HEIGHTS AND HIGH TEMPERATURES IN CHECK.
EXPECT ANOTHER MOSTLY CLEAR DAY WITH HIGHS NEAR 80. MERCER

.LONG TERM...NO CHANGES FROM PREVIOUS DISCUSSION...THE OVERALL TREND
IN THE GUIDANCE IS FOR THE UPSTREAM UPPER RIDGE TO MOVE ONSHORE TUE
AND THAT MAY BE THE WARMEST DAY OF THE WEEK. THEN THE FLOW ALOFT
BECOMES MORE WESTERLY OR ZONAL YET 500 MB HEIGHTS ARE RELATIVELY
HIGH. THE ACCOMPANYING LOW LEVEL ONSHORE FLOW SHOULD MAINTAIN
DAYTIME TEMPS AT OR A BIT ABOVE SEASONAL AVERAGE INTO THURSDAY
THOUGH THERE IS SOME INDICATION OF SAGGING HEIGHTS ALOFT BEFORE THE
END OF THE WEEK THAT MAY SIGNAL A GRADUAL COOLING TREND AND SOME
INCREASE IN LOW LEVEL ONSHORE FLOW. BUEHNER

&&

.AVIATION...WEAK UPPER LEVEL TROUGH OVER THE AREA TONIGHT WILL SHIFT
EAST FRIDAY MORNING. LIGHT FLOW ALOFT BECOMING NORTHERLY FRIDAY
MORNING. LOW LEVEL FLOW WILL REMAIN WEAKLY ONSHORE.

SATELLITE IMAGERY AT 0330Z/830 PM SHOWS LITTLE IN THE WAY OF STRATUS
ANYWHERE IN THE CWA. CONVECTIVE ACTIVITY REMAINS WELL TO THE NORTH
AND EAST OF THE AREA. LIGHT ONSHORE GRADIENTS CONTINUING OVERNIGHT
INTO FRIDAY MORNING. WITH THE LIGHT ONSHORE FLOW CAN NOT RULE OUT
STRATUS OVER THE INTERIOR FRIDAY MORNING. THE STRATUS THAT DOES FORM
WILL BE SHALLOW AND BURN OFF BY MIDDAY AT THE LATEST. CEILINGS WITH
THE STRATUS LOOK TO BE IN THE 1000-2000 FOOT RANGE.

KSEA...MOSTLY CLEAR SKIES WITH STRATUS REFORMING OVER THE TERMINAL
AROUND 12Z WITH CEILINGS NEAR 1500 FEET. STRATUS SCATTERING OUT BY
LATE MORNING. VARIABLE WINDS 6 KNOTS OR LESS BECOMING NORTHWESTERLY
MIDDAY FRIDAY. FELTON

&&

.MARINE...THE VERY STATIC ONSHORE FLOW PATTERN WILL CONTINUE FOR THE
NEXT SEVERAL DAYS. AS IS TYPICAL WITH THIS TYPE OF PATTERN...THE
STRENGTH OF THE ONSHORE FLOW WILL VARY DIURNALLY AS WELL AS WITH A
SERIES OF WEAK SYSTEMS THAT ARE FORECAST TO BRUSH THE REGION OVER
THE NEXT FEW DAYS. EXPECT THE STRONGEST WESTERLY WINDS OVER THE
CENTRAL AND EAST STRAIT DURING THE EVENING AND EARLY MORNING
HOURS...EASING JUST BEFORE DAY BREAK THROUGH THE LATER MORNING. OVER
THE COASTAL WATERS SOUTHERLY FLOW WILL BECOME NORTHWESTERLY FRIDAY.
SURFACE HIGH PRESSURE BUILDING INSIDE OF 140 W WILL TIGHTEN UP
SURFACE GRADIENTS SATURDAY AFTERNOON AND NIGHT OVER THE OUTER
COASTAL WATERS...BOOSTING THE NORTHWESTERLIES ABOVE SMALL CRAFT
CRITERIA. WEST TO NORTHWESTERLY SWELL 3 TO 6 FEET WITH A PERIOD
AROUND 8 SECONDS WILL PERSIST THROUGH THE WEEKEND. FELTON

&&

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

&&

$$

WWW.WEATHER.GOV/SEATTLE/

FOR THE GRAPHICAL AFD...VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML








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