Heavy Snow in Washington, December 19 to 20, 2012

AREA FORECAST DISCUSSION...UPDATED TO ADD AVALANCHE SECTION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST WED DEC 19 2012

.SYNOPSIS...A STRONG FRONTAL SYSTEM WILL MOVE INTO WESTERN
WASHINGTON TODAY BRINGING WIND AND RAIN...AS WELL AS SOME LOWLAND
SNOW THIS MORNING. HEAVY SNOW WILL FALL IN THE MOUNTAINS THROUGH
TONIGHT. COOL SHOWERY CONDITIONS WILL PERSIST THURSDAY AND INTO THE
WEEKEND AS AN UPPER TROUGH MOVES OVER THE AREA.

&&

.SHORT TERM...A MINOR UPPER LEVEL RIDGE MOVED ACROSS WESTERN
WASHINGTON TO THE CASCADES OVERNIGHT. THE VIGOROUS PACIFIC FRONTAL
SYSTEM THAT WE HAVE BEEN ADVERTISING IS IMMEDIATELY UPSTREAM OF THE
RIDGE...AND PRECIPITATION IS SPREADING ACROSS THE FORECAST AREA
EARLY THIS MORNING. THE FRONTAL SYSTEM WILL MOVE ACROSS WESTERN
WASHINGTON TODAY AND TONIGHT. ITS MAIN IMPACT WILL BE FELT THIS
MORNING AND EARLY AFTERNOON THOUGH...AS THE WARM FRONT OR OCCLUSION
MOVES THROUGH.

MARGINAL HIGH WINDS -- BY DEFINITION 40 MPH SUSTAINED OR 60 MPH
GUSTS -- STILL LOOK LIKELY AT THE COAST AND ESPECIALLY IN THE NORTH
INTERIOR ZONES. THE WINDS WILL PROBABLY HIT THEIR PEAK AT THE COAST
AROUND DAYBREAK...AND MID MORNING TO EARLY AFTERNOON IN THE NORTH
INTERIOR. A HIGH WIND WARNING REMAINS IN EFFECT FOR THOSE AREAS...
WHICH WILL BE UPDATED WITH THE MORNING FORECAST PACKAGE.

STORM TOTAL PRECIPITATION IN THE MOUNTAINS FOR TODAY AND TONIGHT
WILL BE IN THE 1 TO 3 INCH LIQUID EQUIVALENT RANGE. THE SNOW LEVEL
WILL RISE FROM BELOW 500 FT EARLY THIS MORNING TO AROUND 2500 FT AS
THE SYSTEM MOVES THROUGH...SO PRECIPITATION WILL FALL AS SNOW EVEN
AT THE LOWEST MOUNTAIN PASSES THE ENTIRE TIME. SNOWFALL TOTALS OF 1
TO 2 FT ARE LIKELY. A WINTER STORM WARNING REMAINS IN EFFECT FOR THE
CASCADES AND OLYMPICS...WHICH WILL BE UPDATED WITH THE MORNING
FORECAST PACKAGE.

OF COURSE THE TRICKIEST ASPECT OF THE FORECAST TODAY IS THE
POTENTIAL FOR SNOW IN THE WESTERN WASHINGTON LOWLANDS. THE HOOD
CANAL AREA IS THE MOST SUSCEPTIBLE ZONE TO HEAVY SNOW IN THIS
PATTERN...BECAUSE COLD AIR REMAINS TRAPPED THERE A BIT LONGER AND
PRECIPITATION IS RELATIVELY HEAVY. THE ADVISORY THERE FOR 1 TO 3
INCHES IS PROBABLY FINE...BUT HEAVIER SNOWFALL IS CERTAINLY NOT OUT
OF THE QUESTION. I WILL ALSO ADD A WINTER WEATHER ADVISORY FOR THIS
MORNING TO THE COAST...LOWER CHEHALIS VALLEY...AND SOUTHWEST
INTERIOR ZONES MAINLY BECAUSE THOSE AREAS WILL GET MORE SIGNIFICANT
PRECIPITATION SOONER.

THE UPPER LOW WILL MOVE TO AROUND 46N/138W IN THE FAR OFFSHORE
WATERS ON THURSDAY...AND ANOTHER SHORTWAVE OR FRONTAL WAVE WILL MOVE
THROUGH THE FORECAST AREA. THE SNOW LEVEL WILL BE AROUND 1500 TO
2500 FT...AND 3 TO 8 INCHES OF ADDITIONAL SNOW WILL FALL IN THE
MOUNTAINS. FOR THE LOWLANDS IT WILL BE A COOL AND SHOWERY DAY.

THE OFFSHORE UPPER LOW WILL REMAIN NEARLY STATIONARY THURSDAY NIGHT
THROUGH FRIDAY...BUT SHOWERS WILL AMOUNT TO LESS. THE AIR MASS WILL
REMAIN COOL WITH THE SNOW LEVEL IN THE MOUNTAINS AROUND 1000 TO
2000 FT.   MCDONNAL

.LONG TERM...THE LONGER RANGE MODELS GENERALLY AGREE THAT THE
OFFSHORE UPPER TROUGH WILL SLOWLY PROGRESS INTO THE PACIFIC
NORTHWEST THIS WEEKEND. WE SHOULD REMAIN IN A COOL AND SOMEWHAT
SHOWERY PATTERN WITH THE MAIN STORM WELL TO OUR SOUTH. THE MODELS
DISAGREE MORE FOR EARLY NEXT WEEK...AND THERE HAS NOT BEEN GREAT RUN
TO RUN CONSISTENCY. THE CURRENT FORECAST FOLLOWS A NEAR-CLIMATOLOGY
COMPROMISE BETWEEN MODEL SOLUTIONS...AND IT LOOKS OKAY FOR NOW.
MCDONNAL

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED ON AREA RIVERS THROUGH THE
NEXT 7 DAYS.   MCDONNAL

&&

.AVIATION...WESTERLY FLOW ALOFT WILL BECOME SOUTHWESTERLY TODAY AS A
FRONT REACHES THE COAST AND MOVES INLAND. AT THE SURFACE STRONG
SOUTHEASTERLY PRESSURE GRADIENTS WILL BRING MARGINAL HIGH WIND TO
THE COAST AND NORTH INTERIOR. AIR MASS IS MOIST AND COOL. LIGHT
PRECIPITATION...MOSTLY SNOW...HAS MOVED ONTO THE COAST AND WILL
REACH PUGET SOUND AROUND 11Z. UP TO AN INCH OF SNOW ACCUMULATION
DURING THE PERIOD 11Z-17Z IS LIKELY BEFORE THE SNOW CHANGES TO RAIN
MID MORNING. A FEW INCHES ARE POSSIBLE WHERE THE PRECIPITATION IS
HEAVIER...MAINLY THE COAST AND SOUTHWEST INTERIOR.

THERE ARE STILL VFR CEILINGS OVER PUGET SOUND BUT CEILINGS AND
VISIBILITIES ARE MVFR IN PRECIPITATION AND ANY VFR CONDITIONS WILL
RAPIDLY DETERIORATE ONCE SNOW STARTS. IN SNOW THERE WILL BE LOCAL
IFR CEILINGS AND ESPECIALLY VISIBILITIES BUT THESE WILL MOSTLY
RETURN TO MVFR AS RAIN TAKES OVER. CONDITIONS SHOULD IMPROVE AFTER
00Z THURSDAY.

KSEA...DISCUSSION ABOVE APPLIES. TAF BEGINS PRECIPITATION AS SNOW AT
12Z...CHANGES IT TO MIXED PRECIPITATION AT 16Z...THEN TO RAIN AT
18Z. COULD GET A WET INCH OF SNOW ON THE GROUND BEFORE THE CHANGE TO
RAIN. TEMPERATURE IS NOT FORECAST TO GET TO FREEZING SO ACCUMULATION
ON PAVED AREAS COULD BE LESS. WIND WILL BE SOUTHEAST AROUND 10 KT
THIS MORNING...INCREASING TO 12G22 KT THIS AFTERNOON. CHB

&&

.MARINE...A STRONG FRONT IS NOW REACHING THE COAST WITH GALE FORCE
WINDS NOTED THERE...THE ENTRANCES...AND THE NORTHERN INLAND WATERS.
GALES WILL END BY 00Z ON THE COAST BUT COULD CONTINUE THROUGH 03Z
OVER THE INLAND WATERS WITH THE APPROACH OF THE TRAILING COLD FRONT.
THERE IS A LOW END GALE FOR ADMIRALTY INLET BUT HAVE HELD PUGET
SOUND TO 20-30 KT. WINDS WILL DIMINISH TONIGHT BEHIND THE FRONT BUT
WILL PROBABLY  REMAIN AT SMALL CRAFT ADVISORY LEVELS.

A LARGE LOW WILL SETTLE IN ABOUT 500 NM OFFSHORE AROUND 45N/135W
LATE WEDNESDAY NIGHT AND CONTINUE THROUGH ABOUT SATURDAY. GRADIENTS
WILL BE MAINLY EASTERLY BUT SHORT WAVES MOVING THROUGH THE FLOW
COULD OCCASIONALLY SWING THE GRADIENTS AROUND TO SOUTHEASTERLY.
WINDS WILL GENERALLY STAY IN THE SMALL CRAFT ADVISORY CATEGORY OR
JUST BELOW. CHB

&&

.AVALANCHE...A STRONG WEATHER SYSTEM INCREASED THE AVALANCHE DANGER
SUNDAY THRU EARLY TUESDAY AS IT DEPOSITED COPIOUS AMOUNTS OF NEW
SNOW AND FORMED LARGE WIND SLABS ON RIDGES AND LEE SLOPES. A BRIEF
BREAK IN THE WEATHER TUESDAY ALLOWED FOR A SLIGHT DECREASE IN SHOWER
ACTIVITY AND IN THE OVERALL AVALANCHE DANGER.

HOWEVER THE STRONG FRONTAL SYSTEM CURRENTLY MOVING THROUGH THE
REGION WILL PRODUCE ADDITIONAL HEAVY SNOWFALL AND STRONG WINDS...
FUELING ANOTHER ROUND OF GENERALLY ELEVATED AVALANCHE DANGER
INCLUDING AVALANCHE WARNING CONDITIONS. TRAVEL IN AVALANCHE TERRAIN
SHOULD BE AVOIDED AT THIS TIME. PLEASE VISIT WWW.NWAC.US FOR FURTHER
DETAILS. DAMICO

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING NORTH AND CENTRAL CASCADES THROUGH
      THURSDAY MORNING.
    .HIGH WIND WARNING NORTH AND CENTRAL COAST TODAY.
    .WINTER WEATHER ADVISORY NORTH COAST...CENTRAL COAST...LOWER
      CHEHALIS VALLEY...HOOD CANAL AREA...SOUTHWEST INTERIOR UNTIL
      10 AM TODAY.

PZ...GALE WARNING ALL WATERS EXCEPT PUGET SOUND.
     SMALL CRAFT ADVISORY PUGET SOUND.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

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