Winter Storm Watch 3, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
315 AM PST THU JAN 19 2012

.SYNOPSIS...A WARM FRONT MOVING ACROSS NORTHERN OREGON WILL BRING A
WINTRY MIX OF PRECIPITATION TO THE SOUTH PART OF THE FORECAST AREA
TODAY...WITH A CHANCE OF RAIN OR SNOW OVER THE NORTH. A STRONGER...
WET...AND WARMER FRONTAL SYSTEM WILL ARRIVE FRIDAY AND FRIDAY NIGHT.
THE ACTIVE WEATHER PATTERN WILL CONTINUE SATURDAY THROUGH THE MIDDLE
OF NEXT WEEK.

&&

.SHORT TERM... THE NEXT DAY OR SO WILL CONTINUE TO BE A TRICKY AND
COMPLICATED PRECIPITATION FORECAST.

A WARM FRONT WILL BRUSH THE SOUTH PART OF THE FORECAST AREA TODAY.
MODELS DISAGREE WITH HOW FAR NORTH THE PRECIPITATION WILL REACH WITH
THE GFS FARTHER NORTH THAN THE NAM. BASED ON RECENT RADAR AND
SATELLITE TRENDS...THE GFS...WITH PRECIPITATION UP TO CENTRAL PUGET
SOUND...APPEARS TO BE THE BETTER CHOICE. TEMPERATURES WILL BE COLD
ENOUGH FOR SNOW OR FREEZING RAIN THIS MORNING. QPF AMOUNTS
SUGGEST 1-3 INCHES OF SNOW COULD FALL OVER THE SOUTHWEST INTERIOR
THIS MORNING WITH THE HIGHER AMOUNTS SOUTH OF OLYMPIA. FARTHER
NORTH...TO ABOUT SEATTLE...QPF WILL BE LIGHTER BUT THERE WILL BE A
RENEWED THREAT OF FREEZING RAIN OR DRIZZLE FOR THE MORNING COMMUTE.
WITH ALL OF THIS IN MIND...WILL ISSUE A WINTER WEATHER ADVISORY FOR
AREAS FROM ABOUT SEATTLE SOUTH FOR THIS MORNING. AREAS FARTHER NORTH
SHOULD STAY COLD BUT MAINLY DRY TODAY. TEMPERATURES WILL GRADUALLY
MODERATE TODAY AND SHOULD RISE ABOVE FREEZING BY AROUND MIDDAY IN
THE PLANNED ADVISORY AREA WITH SNOW/FREEZING RAIN CHANGING TO MAINLY
RAIN AND TAPERING IN THE AFTERNOON.

A SECOND UPPER LEVEL DISTURBANCE WILL MOVE THROUGH THE AREA
TONIGHT...BRINGING MORE PRECIPITATION. QPF AMOUNTS LOOKS RELATIVELY
LIGHT BUT THERE WILL PROBABLY BE ENOUGH TO GET ADVISORY AMOUNTS OF
SNOW AGAIN OVER THE NORTHWEST INTERIOR. THIS IS STILL MORE THAN 12
HOURS OUT AND WILL LET THE DAY SHIFT TAKE ONE MORE LOOK AT THE MODEL
FORECASTS BEFORE DECIDING IF AN ADVISORY IS NEEDED.

TEMPERATURES WILL CONTINUE TO MODERATE TONIGHT...GRADUALLY RISING
DURING THE NIGHT MANY AREAS. A VIGOROUS FRONTAL SYSTEM WILL MOVE
ONSHORE LATER IN THE DAY FRIDAY. BY THIS TIME...ALL LOWLAND AREAS
SHOULD BE WARM ENOUGH FOR RAIN. BOTH THE GFS AND ECMWF SHOW POSSIBLE
HEAVY SNOW FOR THE CASCADES FRIDAY NIGHT AND SATURDAY MORNING. SNOW
LEVELS WILL RISE TO 4000 FEET OR SO DURING THIS TIME BUT EAST WINDS
IN THE PASSES WILL KEEP SNOW DOWN TO THE SURFACE. WILL ISSUE A
WINTER STORM WATCH TO COVER THIS. THERE WILL ALSO BE SOME BREEZY TO
WINDY CONDITIONS WITH THE FRONT...MAINLY COAST AND NORTHWEST
INTERIOR. WIND ADVISORIES WILL MAY EVENTUALLY BE NEEDED FOR THIS
EVENT FOR THESE AREAS. LASTLY...THE QPF OVER THE SOUTHWEST PART OF
THE FORECAST AREA WITH THIS SYSTEM...COMBINED WITH SNOW MELT...COULD
BE ENOUGH TO CAUSE SOME FLOODING CONCERNS. AN ESF IS CURRENTLY OUT
FOR THIS. SEE THE HYDROLOGY SECTION BELOW FOR MORE DETAILS.

SATURDAY WILL BE A POST FRONTAL SHOWERY DAY WITH SNOW TAPERING OVER
THE MOUNTAINS AND BREEZY WINDS ALL AREAS IN STRONG ONSHORE FLOW.
SCHNEIDER

.LONG TERM...THE ACTIVE WEATHER PATTERN WILL CONTINUE SUNDAY THROUGH
WEDNESDAY. LONG RANGE MODELS ARE IN RELATIVE AGREEMENT SHOWING
WEATHER SYSTEMS MOVING THROUGH AROUND SUNDAY NIGHT...TUESDAY...AND
WEDNESDAY. SCHNEIDER

&&

.HYDROLOGY...PREVIOUS DISCUSSION...MINOR URBAN AND SMALL STREAM
FLOODING IS POSSIBLE FRIDAY AND FRIDAY NIGHT AS THE SNOW MELTS AND A
FRONTAL SYSTEM BRINGS RAIN TO THE AREA. THERE COULD BE HEAVY ENOUGH
RAIN FRIDAY NIGHT...SO THAT COMBINED WITH LOWLAND SNOW MELT...SOME
RIVER FLOODING COULD OCCUR IN LOW ELEVATION RIVER BASINS OVER THE
SOUTHERN PART OF THE AREA. THE SERIES OF WARMER AND WET WEATHER
SYSTEMS WILL CONTINUE INTO MID NEXT WEEK...WITH AN INCREASING CHANCE
OF FLOODING AS EACH SYSTEM ADDS MORE RAIN TO THE RIVERS. COMPUTER
MODELS SHOW A PARTICULARLY MOIST AND STRONG SYSTEM AFFECTING THE
AREA TUESDAY AND WEDNESDAY. A FLOOD POTENTIAL OUTLOOK HAS BEEN
ISSUED WITH MORE SPECIFICS. KAM

&&

.AVIATION...STRONG WESTERLY FLOW ALOFT WILL CONTINUE THROUGH
TONIGHT. AT THE SURFACE HIGH PRESSURE OVER BRITISH COLUMBIA WILL
GRADUALLY WEAKEN TODAY...WHILE A FRONTAL SYSTEM MOVES TO THE OREGON
COAST LATE THIS MORNING AND THEN INLAND THIS AFTERNOON. THIS WILL
MAINTAIN WEAKER NORTHERLY SURFACE FLOW ACROSS WESTERN WASHINGTON.
THE SURFACE FLOW WILL SWITCH TO SOUTHERLY TONIGHT...AS THE LEADING
EDGE OF THE NEXT FRONTAL SYSTEM REACHES THE AREA. THE AIR MASS WILL
REMAIN MOIST AND STABLE.

THERE IS A MIX OF IFR AND LOW-END MVFR CONDITIONS ACROSS WESTERN
WASHINGTON EARLY THIS MORNING...WITH AREAS OF LIGHT FREEZING DRIZZLE
STILL OCCURRING IN A NARROW BAND OVER THE INTERIOR LOWLANDS FROM
SEATAC TO OLYMPIA. A SHARP TRANSITION ZONE EXISTS ACROSS EXTREME
SOUTH WESTERN WASHINGTON...WITH TEMPERATURES NEAR OR JUST BELOW
FREEZING OVER OUR FORECAST AREA AND A RATHER MILD AIR MASS
IMMEDIATELY TO THE SOUTH. AT 2 AM CHEHALIS WAS STILL REPORTING 32
DEGREES WHILE VANCOUVER WASHINGTON WAS 47 DEGREES.

MOST OF THE PRECIPITATION WITH THE FRONTAL SYSTEM MOVING THROUGH THE
REGION TODAY WILL BE FROM THE SOUTH PUGET SOUND REGION SOUTHWARD.
PRECIPITATION NORTH OF A LINE FROM HOQUIAM TO SEATTLE TO STEVENS
PASS WILL PROBABLY AMOUNT TO 0.10 INCH OR LESS. DETERMINING THE TYPE
OF PRECIPITATION THAT WILL FALL IS ANOTHER PROBLEM...AS THE
TRANSITION ZONE MENTIONED ABOVE WILL SHIFT SLOWLY NORTHWARD INTO THE
PUGET SOUND REGION. WELL NORTH OF THE TRANSITION ZONE PRECIPITATION
-- IF IT OCCURS AT ALL -- WILL BE MAINLY SNOW. SOUTH OF THE
TRANSITION ZONE IT WILL BE RAIN OF COURSE...AND WITHIN THE
TRANSITION ZONE IT COULD BE SNOW OR FREEZING RAIN OR DRIZZLE.
CONFIDENCE IN THE DETAILS FOR INDIVIDUAL TAFS IS NOT VERY HIGH.

KSEA...NORTHERLY WIND 6-10 KT. IFR CONDITIONS SHOULD CONTINUE THIS
MORNING...WITH MODEST IMPROVEMENT THIS AFTERNOON. VERY LIGHT
FREEZING RAIN OR DRIZZLE SHOULD CONTINUE OCCASIONALLY THIS
MORNING...POSSIBLY CHANGING TO VERY LIGHT SNOW AT TIMES. THE
TEMPERATURE SHOULD RISE JUST ABOVE FREEZING AROUND NOON TODAY...WITH
A FEW HOURS OF MIXED RAIN AND SNOW POSSIBLE BEFORE CHANGING TO JUST
RAIN. AT MOST THERE WILL BE AROUND 0.10 INCH WATER EQUIVALENT
PRECIPITATION TODAY. THE TEMPERATURE WILL GRADUALLY WARM TONIGHT
WITH OCCASIONAL LIGHT RAIN.   MCDONNAL

&&

.MARINE...HIGH PRESSURE WILL WEAKEN OVER BRITISH COLUMBIA
TODAY...WHILE A 998 MB LOW MOVES ACROSS THE OREGON COASTAL WATERS
THIS MORNING AND THEN INLAND THIS AFTERNOON. THE PREVAILING
NORTHEAST FLOW WILL RESULT IN SMALL CRAFT ADVISORY WINDS OVER THE
COASTAL WATERS...WEST ENTRANCE STRAIT OF JUAN DE FUCA...AND NORTHERN
INLAND WATERS.

A VIGOROUS FRONTAL SYSTEM WILL SWITCH THE FLOW TO SOUTHERLY AS IT
MOVES THROUGH THE FORECAST AREA FRIDAY THROUGH SATURDAY. THIS
FEATURE WILL PROBABLY BRING A SOLID ROUND OF GALE FORCE WINDS TO AT
LEAST THE COASTAL WATERS...ENTRANCES TO THE STRAIT...AND NORTHERN
INLAND WATERS. A GALE WATCH FOR THESE ZONES HAS BEEN POSTED WITH THE
EARLY MORNING FORECAST ISSUANCE. THE FORECAST CALLS FOR JUST BELOW
GALES AT ADMIRALTY INLET AND PUGET SOUND AND HOOD CANAL ZONES...BUT
GALES COULD CERTAINLY OCCUR THERE TOO. IN ADDITION THERE WILL
PROBABLY BE A SURGE OF GALE WESTERLIES IN THE CENTRAL STRAIT AND
EAST ENTRANCE FOLLOWING THE COLD FRONT ON SATURDAY.

ANOTHER STRONG FRONTAL SYSTEM WILL MOVE THROUGH THE AREA SUNDAY
THROUGH MONDAY...AND THIS WILL PROBABLY BRING ANOTHER ROUND OF
GALES.

IN ADDITION...A LARGE WESTERLY SWELL WILL MOVE INTO THE COASTAL
WATERS THIS WEEKEND WITH SWELLS OF 20 TO 25 FT LIKELY.   MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY CENTRAL COAST...SOUTHWEST
     INTERIOR...AND PUGET SOUND REGION THROUGH NOON.
    .WINTER STORM WATCH NORTH AND CENTRAL CASCADES FRIDAY AFTERNOON
     THROUGH SATURDAY MORNING.
PZ...SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE
     STRAIT...NORTHERN INLAND WATERS.
    .GALE WATCH FOR FRIDAY AND FRIDAY NIGHT FOR COASTAL
     WATERS...ENTRANCES TO THE STRAIT...NORTHERN INLAND WATERS.
    .SMALL CRAFT ADVISORY THROUGH FRIDAY GRAYS HARBOR FOR ROUGH
     BAR CONDITIONS.

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

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