High Wind in Washington, December 17 to 18, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST FRI DEC 17 2010

.SYNOPSIS...A DEEPENING FRONTAL SYSTEM OFFSHORE WILL PRODUCE LOCAL
HIGH WIND ALONG THE EAST PUGET SOUND LOWLANDS AND WEST ENTRANCE TO
THE STRAIT OF JUAN DE FUCA TONIGHT AND SATURDAY MORNING. SNOW IS
POSSIBLE AROUND HOOD CANAL SATURDAY...WITH RAIN ACROSS THE REST OF
THE LOWLANDS. A COLD UPPER TROUGH OFFSHORE WILL BRING PERIODS OF
SHOWERS TO THE AREA SATURDAY NIGHT THROUGH NEXT WEEK.

&&

.SHORT TERM...DIFFLUENT SOUTHWEST FLOW ALOFT CONTINUES OVER WESTERN
WASHINGTON THIS AFTERNOON...WITH A STRONG WESTERLY JET ALONG 37N
EXTENDING INTO CALIFORNIA AND A DEEP UPPER LOW WELL OFFSHORE NEAR
46N/144W. MUCH OF WESTERN WASHINGTON HAS HAD PARTLY TO MOSTLY SUNNY
SKIES. THE ONLY REAL EXCEPTION IS THE COAST WHERE A WEAK CONVECTIVE
BAND OFFSHORE IS BRINGING SOME CLOUD COVER AND MAYBE A SHOWER BEFORE
THE END OF THE DAY.

THE 160 KT CORE OF THE WESTERLY JET MOVING THROUGH THE SOUTH SIDE OF
THE UPPER LOW IS CAUSING A FRONTAL SYSTEM TO DEVELOP IN THE
SOUTHEAST QUADRANT OF THE UPPER LOW NEAR 39N/134W. THE MODELS HAVE
HAD A GOOD HANDLE ON THIS DEVELOPMENT. THEY SHOW THE SURFACE LOW
DEEPENING RAPIDLY -- BOMBING ACTUALLY AS IT WILL DEEPEN MORE THAN 24
MB IN 24 HOURS -- THROUGH SATURDAY MORNING AS IT CURLS ALONG A
COUNTER-CLOCKWISE ARC JUST OUTSIDE THE PACIFIC NORTHWEST OFFSHORE
WATERS. A SECONDARY SURFACE DEPRESSION CORRESPONDING TO THE TRIPLE
POINT OF THE SURFACE FRONTS WILL FURTHER TIGHTEN THE EASTERLY
PRESSURE GRADIENT ACROSS THE REGION. THIS SYSTEM WILL HAVE A COUPLE
MAJOR IMPACTS ON OUR WEATHER.

FIRST...LOCALIZED HIGH WINDS -- BY DEFINITION SUSTAINED 40 MPH OR
GUSTS 60 MPH -- ARE LIKELY TONIGHT THROUGH SATURDAY MORNING OVER THE
EAST PUGET SOUND LOWLANDS AND NEAR THE WEST ENTRANCE TO THE STRAIT
OF JUAN DE FUCA. THE FORECAST STRENGTH OF THE EASTERLY PRESSURE
GRADIENT IS ENOUGH TO JUSTIFY THIS CONCERN. BUT THERE IS STRONG
POTENTIAL FOR MOUNTAIN WAVE ACTIVITY AS WELL...WHICH MEANS THAT
DAMAGING WINDS COULD SURFACE IN THE EAST PUGET SOUND LOWLANDS IN
PLACES OTHER THAN THE LOCATIONS TYPICALLY EXPOSED TO STRONG EAST
WINDS. THE HIGH WIND WATCH WAS UPGRADED TO A WARNING THIS MORNING
FOR THE EAST PUGET SOUND LOWLANDS AND FOR THE NORTH COAST ZONE WHICH
EXTENDS NORTH TO THE WEST ENTRANCE TO THE STRAIT.

THE FRONTAL SYSTEM WILL ALSO SPREAD PRECIPITATION NORTHWARD ACROSS
THE FORECAST AREA SATURDAY. THE AMBIENT SNOW LEVEL OVER WESTERN
WASHINGTON WILL BE IN THE 2000 TO 3000 FT RANGE AS THIS SYSTEM MOVES
THROUGH...BUT EASTERLY FLOW WILL KEEP THE SNOW LEVEL AT THE SURFACE
ALONG THE WEST SLOPES OF THE CASCADES.

AN AREA OF CONCERN FOR LOW ELEVATION SNOW IS THE HOOD CANAL AREA. A
WEAK NORTHERLY COMPONENT TO THE SURFACE PRESSURE GRADIENT WILL HELP
TRAP COLD AIR ALONG THE EAST SLOPES OF THE OLYMPICS IN THIS
REGION...AND MODELS INDICATE THE AIR MASS THERE WILL BE COLD ENOUGH
FOR SNOW OR A MIX OF RAIN AND SNOW. LOCAL ACCUMULATIONS OF 2 INCHES
OR SO ARE POSSIBLE. IN MY OPINION CONFIDENCE IN THE OCCURRENCE OF
THE EVENT IS A LITTLE LOW TO ISSUE A WINTER WEATHER ADVISORY AT THIS
TIME...BUT WE MIGHT EVENTUALLY NEED ONE.

THE STEADY WIDESPREAD PRECIPITATION WITH THE FRONT WILL BECOME
SHOWERY SATURDAY NIGHT AS THE MAIN FRONTAL BAND LIFTS NORTH OF THE
FORECAST AREA. SHOWERY PRECIPITATION WILL CONTINUE SATURDAY NIGHT
THROUGH MONDAY THOUGH AS MOIST SOUTHERLY FLOW CONTINUES ALOFT AND
THE UPPER LOW MOVES SOUTHEAST INTO THE OREGON OFFSHORE WATERS.
MCDONNAL

.LONG TERM...MODELS AGREE ON THE BIG PICTURE FOR THE EXTENDED PERIOD
TUESDAY THROUGH FRIDAY...WITH A LONG WAVE TROUGH REMAINING OFF THE
WEST COAST OF NORTH AMERICA.

THE UPPER LOW OVER THE OREGON OFFSHORE WATERS MONDAY WILL WEAKEN
MOVE NORTHEAST MONDAY NIGHT...BUT ANOTHER UPPER LOW MIGRATING
SOUTHWARD FROM THE GULF OF ALASKA WILL SIMULTANEOUSLY DROP INTO THE
WATERS WELL OFFSHORE OF CALIFORNIA. THIS WILL PRESERVE THE OVERALL
LONG WAVE PATTERN OF KEEPING AN UPPER TROUGH OFFSHORE. THAT UPPER
LOW WILL PROBABLY WEAKEN AND MOVE INTO CALIFORNIA AROUND MIDWEEK...
ONLY TO BE REPLACED BY ANOTHER LARGE SCALE UPPER TROUGH BY FRIDAY.

ON THE WHOLE THE SYNOPTIC PATTERN WILL KEEP SOMEWHAT MOIST SOUTHERLY
FLOW ALOFT OVER WESTERN WASHINGTON...ALONG WITH EASTERLY FLOW ACROSS
THE CASCADES. THIS IS ALSO A LARGE SCALE PATTERN THAT COULD GENERATE
FRONTAL SYSTEMS OFFSHORE WHICH WOULD THEN MOVE INLAND ACROSS THE
PACIFIC NORTHWEST...MUCH LIKE THE SYSTEM THAT WILL AFFECT THE
FORECAST AREA LATER TONIGHT AND SATURDAY. THE MOST RECENT RUNS OF
THE LONGER RANGE MODELS DO NOT SHOW MUCH ALONG THESE LINES...BUT
PREVIOUS RUNS HAVE AND THESE ARE THE SORTS OF SMALLER SCALE DETAILS
THAT THE MODELS DISAGREE ON.  MCDONNAL

&&

.HYDROLOGY...FLOODING IS UNLIKELY ACROSS WESTERN WASHINGTON FOR THE
NEXT 10 DAYS...WITH GENERALLY LOW SNOW LEVELS AND HYDROLOGICALLY
INSIGNIFICANT PRECIPITATION. THERE ARE NO FLOODING CONCERNS FOR THE
GREEN RIVER.

&&

.AVIATION...AN UPPER LOW WILL BE OVER THE EASTERN PACIFIC WITH
SOUTHERLY FLOW ALOFT OVER WESTERN WASHINGTON TONIGHT. AN UPPER SHORT
WAVE WILL MOVE UP FROM THE SOUTH AND THROUGH WESTERN WASHINGTON
SATURDAY. AT THE SURFACE...A 990MB LOW WILL DEEPEN TO AROUND 975 MB
AND MOVE NEAR 47N 133W TONIGHT WITH INCREASING LOW LEVEL OFFSHORE
FLOW. THE LOW LEVEL OFFSHORE FLOW WILL EASE LATER IN THE DAY
SATURDAY AS THE LOW MOVES FARTHER OFFSHORE. AIR DRY AND STABLE WITH
INCREASING MID AND HIGH LEVEL MOISTURE TONIGHT...BECOMING MOIST LOW
LEVELS ON SATURDAY.

INCREASING LOW LEVEL OFFSHORE GRADIENTS SHOULD KEEP SKIES VFR
TONIGHT WITH THE EXCEPTION OF A FEW STUBBORN FOG PATCHES IN THE
SOUTHWEST INTERIOR. MID AND LOW LEVEL MOISTURE WILL BE INCREASING ON
SATURDAY FROM THE SOUTH AS AN UPPER SHORT MOVES UP FROM THE SOUTH.
RAIN WILL DEVELOP OVER THE SOUTHWEST PART OF THE FORECAST AREA IN
THE MORNING...THEN SPREAD INTO THE NORTHERN AREAS BY AROUND MIDDAY.

KSEA...EAST/SOUTHEAST WIND 10-15 KT...GUSTS 20 TO 30 KT LIKELY LATE
TONIGHT AND SATURDAY MORNING. SCHNEIDER

&&

.MARINE...A 990 MB SURFACE LOW WILL MOVE NORTH INTO THE OFFSHORE
WATERS TONIGHT. THE LOW WILL THEN DO A COUNTER-CLOCKWISE CIRCULAR
LOOP BETWEEN 130W AND 140W OVER THE NEXT FEW DAYS...RE-ENTERING THE
COASTAL WATERS FROM THE SOUTHWEST SUNDAY NIGHT OR MONDAY. THE
RESULT WILL BE OFFSHORE FLOW FOR THE COASTAL WATERS FOR MOST OF THE
NEXT FIVE DAYS. THE STRONGEST OFFSHORE FLOW LOOKS TO BE TONIGHT AND
SATURDAY MORNING AS THE LOW MAKES ITS CLOSEST PASS IN THE OFFSHORE
WATERS. GALES ARE UP FOR THE COASTAL WATERS AND WESTERN HALF OF THE
STRAIT FOR THIS. SCHNEIDER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...HIGH WIND WARNING FOR THE EAST PUGET SOUND LOWLANDS AND EXTREME
       NORTH COAST ZONE TONIGHT THROUGH MIDDAY SATURDAY.
PZ...GALE WARNING COASTAL WATERS...WEST ENTRANCE STRAIT OF JUAN DE
       FUCA...AND CENTRAL STRAIT.
    .SMALL CRAFT ADVISORY EAST ENTRANCE...ADMIRALTY INLET...AND
       NORTHERN INLAND WATERS.

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