Heavy Snow in Washington, January 11 to 12, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PST TUE JAN 11 2011

.SYNOPSIS...SNOW WILL DEVELOP THIS EVENING AS MOISTURE SPREADS UP
FROM THE SOUTH. SNOW WILL CHANGE TO RAIN LATE TONIGHT OR WEDNESDAY
MORNING ACROSS MOST AREAS. THE MOUNTAINS AND A FEW LOWLAND AREAS
WILL GET HEAVY SNOW. A SERIES OF WET AND WARMER SYSTEMS WILL IMPACT
THE REGION THROUGH THE WEEKEND.

&&

.SHORT TERM...LITTLE CHANGES WERE MADE TO THE PREVIOUS FORECAST.
BASED ON LATEST SATELLITE/RADAR AND MODEL TRENDS WE ARE STILL ON
TRACK TO SEE LOWLAND SNOW ACROSS WESTERN WASHINGTON TONIGHT. ALREADY
SEEING LIGHT SNOW AT KAST AND KUIL SO JUST A MATTER OF TIME.

MODELS AGREE ON A NORTHERN TRACK WITH THIS SYSTEM...WITH A 985-990
MB SURFACE LOW TRACKING NE TOWARD NORTHERN VANCOUVER ISLAND TONIGHT.
WILL SEE PRECIP IN THE FORM OF SNOW THIS EVENING AS MOISTURE
OVERRUNS THE COOL LOW LEVEL AIR MASS. TEMPS THIS AFTERNOON ARE IN
THE 30S WITH DEW POINTS IN THE TEENS AND 20S...DEFINITELY COOL
ENOUGH TO SUPPORT LOWLAND SNOW. WARMER AIR WILL RUSH INLAND LATE
THIS EVENING OR OVERNIGHT AS ONSHORE FLOW INCREASE...WITH SNOW
CHANGING OVER TO RAIN. ALL TOLD...MOST AREAS WILL JUST SEE A FEW
HOURS OF ACCUMULATING SNOW WHICH WILL LIKELY ALL WASH AWAY BY THE
RAIN EARLY WED. MODELS CONTINUE TO SHOW HEAVIEST SNOW AMOUNTS OVER
THE HOOD CANAL AREA DUE TO E/SE UPSLOPE FLOW AGAINST THE OLYMPICS.
HEAVY AMOUNTS ARE ALSO LIKELY IN THE NORTH INTERIOR AS THE
CHANGEOVER FROM SNOW TO RAIN MAY NOT OCCUR UNTIL EARLY WED
MORNING....MAINLY DUE TO COOL NE FLOW OUT OF THE FRASER.
ELSEWHERE...1 TO 3 INCHES STILL POSSIBLE WITH HIGHER AMOUNTS ON THE
HILLTOPS AND NEAR THE MOUNTAINS.

SEATTLE METRO AREA...PRECIP TIMING HAS BEEN DELAYED IN THE SEATTLE
METRO...MOST LIKELY DUE TO EASTERLY DOWNSLOPE FLOW OFF THE CASCADES.
SEA-EAT PRES GRADIENT IS UP TO -8.6 MB. HOWEVER...THE UW WRF GFS
DOES SHOW LIGHT PRECIP DURING THE EVENING RUSH HOUR COMMUTE...AROUND
00-03Z. THE 18Z NAM12 IS A LITTLE SLOWER. THE SWITCH OVER TO RAIN
WILL OCCUR OVERNIGHT 09-12Z...SO SHOULD NOT SEE ANY PROBLEMS WITH
THE WED MORNING COMMUTE. UNTIL THEN CANNOT RULE OUT AT LEAST 1 INCH
OF ACCUMULATION...WITH HIGHER AMOUNTS POSSIBLE IN SNOHOMISH CO AND
OVER IN THE KITSAP AROUND BREMERTON.

LASTLY...HEAVY SNOW IS STILL EXPECTED IN THE CASCADES WITH THIS
EVENT...1 TO 2 FEET OF SNOW EXPECTED THROUGH WED AFTERNOON. SNOW
LEVELS WILL RISE TO AROUND 3000-4000 FT WITH EASTERLY FLOW IN THE
PASSES BECOMING WEST LATE MORNING OR EARLY AFTERNOON. MAY SEE LIGHT
FREEZING RAIN AT SNOQUALMIE PASS...THEN RAIN BY THE AFTERNOON.
PRECIP AT STEVENS PASS MAY STAY AS SNOW.

BEYOND...A SERIES OF WET BUT WARMER SYSTEM WILL IMPACT WESTERN WA
THROUGH FRI. VERY LITTLE BREAK IN THE ACTION AND WILL AVOID ANY
MENTION OF BREAKS. TEMPS WILL REMAIN QUITE MILD WITH HIGHS IN THE
40S TO LOWER 50S AND LOWS STAYING ABOVE FREEZING. MODELS SHOW
SEVERAL INCHES OF RAIN IN THE OLYMPICS WHICH MAY FORCE THE SKOKOMISH
RIVER INTO FLOOD ADVISORY RANGE. 33

.LONG TERM...THE FORECAST THIS UPCOMING WEEKEND THROUGH EARLY NEXT
WEEK REMAINS ACTIVE WITH NEARLY ZONAL FLOW ALOFT...AND WAVE AFTER
WAVE OF MOISTURE IMPACTING THE REGION. THE TREND TOWARD A WETTER
FORECAST IS ALREADY IN PLACE. OVERALL PRETTY GOOD AGREEMENT BETWEEN
THE GFS AND EURO. TEMPS WILL REMAIN ABOVE CLIMO WITH HIGHS IN THE
MID 40S TO LOWER 50S...AND LOWS IN THE LOWER 40S. WE MAY SEE A BREAK
IN THE WEATHER MID TO LATE WEEK AS A STRONG RIDGE DEVELOPS OVER THE
NE PAC. 33

&&

.HYDROLOGY...WITH THE SWITCH TO A WARMER AND WETTER WEATHER PATTERN
THERE IS THE POSSIBILITY OF FLOODING ON THE SKOKOMISH RIVER AT SOME
POINT LATE THIS WEEK INTO EARLY NEXT WEEK. FOR THE REMAINDER OF THE
RIVERS...BRIEF BREAKS IN THE HEAVIER PRECIPITATION WILL ALLOW THE
RIVERS TO RECEDE IN A BIT BETWEEN SYSTEMS. FLOODING ON THE GREEN
RIVER IS NOT EXPECTED IN THE NEXT 10 DAYS.

&&

.AVIATION...MOISTURE WILL INCREASE AND CIGS WILL GRADUALLY LOWER
DOWN TO BKN-OVC040-070 THROUGH 02Z AS A FRONTAL SYSTEM APPROACHES
FROM THE W. EXPECT CIGS/VIS TO DROP TO MVFR AND POSSIBLY IFR AS
02Z-06Z AS FRONTAL PRECIP SPREADS INLAND. PRECIP WILL BEGIN AS SNOW
AND THEN CHANGE TO RAIN 06Z-10Z. BRIEF FREEZING RAIN IS POSSIBLE.

A COLD FRONT WILL REACH THE COAST AROUND 12Z AND PUGET SOUND AROUND
16Z ON WEDNESDAY.

KSEA...EXPECT VFR CONDITIONS THROUGH 04Z WITH CIGS GRADUALLY
LOWERING FROM BKN-OVC080 TO BKN-OVC035. EXPECT LOW MVFR/IFR
CONDITIONS AFTER ABOUT 04Z ONCE THE PRECIP STARTS CRANKING UP.
SURFACE WINDS WILL REMAIN E 8-12KT THEN SHIFT SE AROUND 09Z. MID
LEVEL WINDS FL050-FL100 WILL RISE TO 45-60KT 06Z-12Z THEN SHIFT SW
AND CONTINUE THROUGH 21Z.

SNOW AT KSEA...VARIOUS COMPUTER MODELS CONTINUE TO SHOW A RANGE OF
SOLUTIONS WITH IMPORTANT DETAILS WHICH LOWERS THE CONFIDENCE OF WHEN
THE SNOW WILL START AND HOW MUCH WILL ACCUMULATE. BEST GUESS AT THIS
POINT IS THAT LIGHT SNOW OR FLURRIES COULD START AS EARLY AS 02Z OR
AS LATE AS 05Z. THE BEST PERIOD FOR SNOW APPEARS TO BE SOMEWHERE
BETWEEN 04Z-09Z...WITH A TOTAL ACCUMULATION OF UP TO AN INCH. IF
TEMPERATURES REMAIN IN THE MID 30S THEN ACCUMULATIONS COULD BE
ALMOST NOTHING. EXPECT A CHANGE TO RAIN AROUND 10Z-11Z. FRONTAL
PASSAGE WILL BE AROUND 16Z WEDNESDAY. KAM

&&

.MARINE...A 995 MB SURFACE LOW ABOUT 450 NM W OF GRAYS HARBOR
WILL DEEPEN TO 991 MB ABOUT 280 NM W OF CAPE FLATTERY AROUND
06Z...THEN REACH NORTHERN VANCOUVER ISLAND AROUND 15Z-18Z WEDNESDAY.
A STRONG COLD FRONT WILL REACH THE COAST AROUND 12Z AND PUGET SOUND
AROUND 17Z WEDNESDAY. GALE WARNINGS UP FOR THE COASTAL AND INLAND
WATERS EXCEPT SCA FOR PUGET SOUND AND ADMIRALTY INLET. SCA FOR PUGET
SOUND IS FOR POST-FRONTAL S-SW WINDS WEDNESDAY. STILL EXPECTING A
WESTERLY POST-FRONTAL GALE IN THE STRAIT WEDNESDAY.

A SERIES OF FRONTAL SYSTEMS WILL AFFECT THE AREA INTO THIS WEEKEND.
CURRENT MODELS SHOW FRONTS REACHING THE AREA WEDNESDAY
NIGHT...FRIDAY...AND SATURDAY NIGHT. SOME OF THESE FEATURES COULD
ALSO BE FAIRLY VIGOROUS. WITH THE SYSTEMS ARRIVING IN QUICK
SUCCESSION DETAILS ON STRENGTH AND TIMING ARE HIGHLY UNCERTAIN.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING HOOD CANAL AREA...WESTERN WHATCOM
       COUNTY...AND NORTHERN AND CENTRAL CASCADES.
    .WINTER WEATHER ADVISORY ALL OTHER ZONES.
PZ...GALE WARNING COASTAL WATERS...ENTIRE STRAIT OF JUAN DE FUCA...
     NORTHERN INLAND WATERS.
    .SMALL CRAFT ADVISORY ADMIRALTY INLET AND PUGET SOUND.
    .SMALL CRAFT ADVISORY FOR ROUGH BAR GRAYS HARBOR BAR.

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