Winter Storm Warning 4, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST THU FEB 21 2013

.SYNOPSIS...WINDY CONDITIONS AND HEAVY MOUNTAIN SNOW ARE EXPECTED
FRIDAY AND FRIDAY NIGHT AS A STRONG COLD FRONT IMPACTS THE AREA.
SHOWERS WILL DECREASE SATURDAY IN A BREAK BETWEEN SYSTEMS. ANOTHER
WARM FRONT WILL REACH THE REGION SUNDAY FOLLOWED BY A COLD FRONT ON
MONDAY.

&&

.SHORT TERM...A VIGOROUS FRONTAL SYSTEM WILL MOVE THROUGH THE AREA
FRIDAY. THIS FRONT WILL BRING HEAVY SNOW TO THE MOUNTAINS AND WINDY
CONDITIONS TO THE LOWLANDS. ONE TO TWO FEET OF NEW SNOW LOOKS LIKELY
FOR THE MOUNTAINS BY SATURDAY MORNING WITH THE HEAVIEST SNOW IN THE
AFTERNOON AND PERHAPS EVENING HOURS FRIDAY. LOCALLY HIGHER
ACCUMULATIONS ARE POSSIBLE. THE CURRENT WINTER STORM WATCH WILL BE
UPGRADED TO A WARNING FOR THE CASCADES. THE OLYMPIC MOUNTAINS LOOK
LIKE THEY WILL GET HIGH END ADVISORY AMOUNTS DURING THIS TIME. FOR
TONIGHT...EXPECTED SNOW ACCUMULATIONS FOR THE CASCADES LOOK LIKE
THEY WILL MIGHT FALL SLIGHTLY SHORT OF ADVISORY AMOUNTS.
HOWEVER...IT WILL BE CLOSE. TO AVOID CONFUSION...THE EXISTING
ADVISORY FOR TONIGHT WILL MAINTAINED...WITH A TRANSITION INTO A
WARNING ON FRIDAY WHEN THE HEAVIER SNOWFALL ARRIVES.

ALL AREAS SHOULD SEE BREEZY TO WINDY SOUTHERLY WINDS FRIDAY AS THE
FRONT MOVES ONSHORE. THE COAST AND NORTH INTERIOR SHOULD GET THE
STRONGEST WINDS. A WIND ADVISORY WILL BE ISSUED FOR THESE AREAS
ALTHOUGH THE EVENT LOOKS MARGINAL.

IN GENERAL...THINGS WILL BE WINDING DOWN FRIDAY NIGHT AS THE FRONT
MOVES INLAND WITH RAIN CHANGING TO SHOWERS. SNOW WILL CONTINUE OVER
THE MOUNTAINS IN MOIST ONSHORE FLOW WITH SNOW LEVELS FALLING TO 1000
TO 1500 FEET. WITH SNOW LEVELS THIS LOW...A FEW OF THE HIGHEST
HILLTOPS IN THE LOWLANDS COULD GET A LITTLE SNOW SATURDAY MORNING
BUT IT DOES NOT LOOK LIKE A LOWLAND SNOW EVENT FOR MOST AREAS. WINDS
WILL BE EASING ALL AREAS BUT STILL BREEZY AND GUSTY IN STRONG LOW
LEVEL ONSHORE FLOW. THERE COULD BE MORE WIND ADVISORY LEVEL WEST
WINDS DURING THIS PERIOD IN THE STRAIT OF JUAN DE FUCA AND POSSIBLY
THE COAST.

DRIER NORTHWEST FLOW ALOFT WILL DEVELOP SATURDAY. SHOWERS WILL
LINGER...MOSTLY OVER THE MOUNTAINS AND WINDS WILL REMAIN BREEZY IN
CONTINUED MODERATE TO STRONG LOW LEVEL ONSHORE FLOW. ONSHORE FLOW
WILL EASE SATURDAY NIGHT...THEN A WARM FRONT WILL BRUSH THE AREA
SUNDAY WITH RAIN BECOMING LIKELY WEST PART AND A CHANCE OF RAIN
FARTHER INLAND. SCHNEIDER

.LONG TERM...MONDAY THROUGH THURSDAY WILL REMAIN RELATIVELY ACTIVE
WITH FRONTS MOVING THROUGH THE AREA EVERY OTHER DAY OR SO. THE
MODELS ARE STILL STRUGGLING WITH THE EXACT DETAILS. IN GENERAL...IT
LOOKS LIKE A FRONT WILL MOVE ONSHORE SUNDAY NIGHT OR MONDAY...WITH A
BREAK IN BETWEEN SYSTEMS TUESDAY...FOLLOWED BY ANOTHER FRONT TUESDAY
NIGHT OR WEDNESDAY. BOTH THE GFS AND ECMWF ARE BEGINNING TO HINT AT
A BIT OF A PINEAPPLE EXPRESS STARTING THURSDAY. SCHNEIDER

&&

.HYDROLOGY...RIVER FLOODING IS NOT EXPECTED THE NEXT 7 DAYS.

&&

.AVIATION...NORTHWEST FLOW ALOFT WILL CONTINUE OVER THE PACIFIC
NORTHWEST TONIGHT. A VIGOROUS FRONTAL SYSTEM WILL AFFECT WESTERN
WASHINGTON LATE TONIGHT AND FRIDAY...WITH ITS WARM FRONT REACHING
THE AREA LATE TONIGHT AND THE COLD FRONT MOVING THROUGH FRIDAY. THE
UPPER TROUGH FOLLOWING THE FRONT WILL MOVE INTO THE COASTAL WATERS
LATE FRIDAY. THE AIR MASS WILL BE MOIST WITH THE MOUNTAINS GENERALLY
OBSCURED. THE AIR MASS WILL BECOME WEAKLY UNSTABLE LATE FRIDAY.

CEILINGS ACROSS WESTERN WASHINGTON REMAINED MOSTLY 015-030 BKN-OVC
INTO THIS AFTERNOON...WITH RAIN DECREASING FOR THE TIME BEING. THERE
WILL BE A RELATIVE LULL IN PRECIPITATION THIS EVENING...THEN
PRECIPITATION WILL INCREASE AGAIN LATER TONIGHT AND FRIDAY.
CONDITIONS WILL REMAIN MOSTLY MVFR. SOUTHERLY WINDS IN THE 2000 TO
6000 FT LAYER WERE AROUND 30 KT THIS AFTERNOON AND THEY WILL RISE TO
AROUND 50 KT FRIDAY.

KSEA...CEILINGS WILL MAINLY BE 015-030 BKN-OVC THROUGH TONIGHT.
PERIODS OF HEAVIER RAIN ON FRIDAY WILL OCCASIONALLY LOWER VISIBILITY
TO 3-5SM RA BR. SOUTHERLY WINDS IN THE 2000 TO 6000 FT LAYER WILL
RISE FROM AROUND 30 KT THIS AFTERNOON TO AROUND 50 KT FRIDAY...WITH
SURFACE WINDS OCCASIONALLY GUSTING TO 30 KT.   MCDONNAL

&&

.MARINE...A SMALL SURFACE RIDGE OVER WESTERN WASHINGTON THIS EVENING
WILL WEAKEN AND MOVE EAST OF THE CASCADES TONIGHT. MARGINAL SMALL
CRAFT ADVISORY CONDITIONS WILL CONTINUE OVER THE MARINE ZONES MOST
OF THE NIGHT.

A VIGOROUS FRONTAL SYSTEM WILL MOVE THROUGH THE FORECAST AREA FROM
THE WEST LATE TONIGHT AND FRIDAY...WITH THE COLD FRONT CROSSING THE
PUGET SOUND REGION MIDDAY. STRONG SOUTHEAST PRESSURE GRADIENTS WILL
PRECEDE THE FRONT...THEN STRONG SOUTHWEST GRADIENTS AND ONSHORE FLOW
WILL FOLLOW THE FRONT. GALE FORCE WINDS ARE LIKELY IN EVERY MARINE
ZONE AT SOME POINT ON FRIDAY.

STRONG SOUTHWEST PRESSURE GRADIENTS WILL GRADUALLY EASE FRIDAY NIGHT
AND SATURDAY AS SURFACE HIGH PRESSURE MOVES TOWARD THE PACIFIC
NORTHWEST FROM THE OFFSHORE WATERS. GALE FORCE WESTERLIES WILL
CONTINUE OVER THE COASTAL WATERS FRIDAY NIGHT...AND WESTERLIES GALES
WILL CONTINUE IN THE CENTRAL STRAIT OF JUAN DE FUCA AND EAST
ENTRANCE STRAIT SATURDAY.

IN ADDITION...WESTERLY SWELLS WILL BUILD TO 22 TO 24 FT ON SATURDAY
IN THE COASTAL WATERS AND UP TO 18 FEET AT THE WEST ENTRANCE TO THE
STRAIT OF JUAN DE FUCA.

ANOTHER WEAKER FRONTAL SYSTEM WILL AFFECT WESTERN WASHINGTON SUNDAY
THROUGH MONDAY...AND ANOTHER FRONT WILL APPROACH FROM OFFSHORE ON
TUESDAY.   MCDONNAL

&&

.AVALANCHE...A STRONG PACIFIC FRONTAL SYSTEM TAKING AIM AT THE
PACIFIC NORTHWEST FRIDAY SHOULD DELIVER 1-2 FEET OR MORE OF SNOWFALL
FRIDAY AND FRIDAY NIGHT. WHILE THE DEEP SNOWPACK OF MID-WINTER HAS
BEEN RELATIVELY STABLE DURING THE MONTH OF FEBRUARY...HEAVY
SNOWFALL...STRONG WESTERLY WINDS...RAPID LOADING AND A GRADUAL
WARMING TREND BEGINNING FRIDAY SHOULD CAUSE AVALANCHES TO OCCUR
MOSTLY IN THE NEW STORM SNOW.

MORE MODEST AMOUNTS OF SNOW AND WINDS ARE EXPECTED FIRST ON THURSDAY
WITH SNOW LEVELS BELOW PASS ELEVATIONS. STORM AND WIND SLAB
AVALANCHES THAT RELEASE FRIDAY MAY STEP DOWN AND ENTRAIN SNOWFALL
FROM THURSDAY SNOWFALL AS WELL. INCREASINGLY THICK NEW WIND SLABS
ARE LIKELY TO FORM ON LEE SLOPES AT HIGHER ELEVATIONS.

BACKCOUNTRY TRAVEL IN AVALANCHE TERRAIN IS NOT RECOMMENDED FRIDAY
AND FRIDAY NIGHT.

ALTHOUGH WARNING CONDITIONS ARE NOT CURRENTLY FORECASTED ALONG THE
CASCADE EAST SLOPES DUE TO A LITTLE LESS EXPECTED SNOWFALL...A
SIGNIFICANT INCREASE IN THE DANGER IS LIKELY IN THAT AREA AS WELL.

PLEASE VISIT NWAC.US FOR DETAILS. FERBER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY FOR THE NORTH AND CENTRAL CASCADES
     THROUGH FRIDAY MORNING.
    .WINTER STORM WARNING FOR THE NORTH AND CENTRAL CASCADES FRIDAY
     MORNING THROUGH SATURDAY MORNING.
    .WINTER WEATHER ADVISORY FOR THE OLYMPICS FRIDAY MORNING THROUGH
     FRIDAY EVENING.
    .WIND ADVISORY FOR THE COAST AND NORTH INTERIOR FRIDAY.
PZ...SMALL CRAFT ADVISORY ALL WATERS TONIGHT.
    .GALE WARNING COASTAL WATERS...ENTRANCES TO THE STRAIT...
     ADMIRALTY INLET...NORTHERN INLAND WATERS...PUGET SOUND AND HOOD
     HOOD CANAL.
    .GALE WATCH CENTRAL STRAIT OF JUAN DE FUCA.

&&