Heavy Snow in Washington, February 21 to 22, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
400 AM PST TUE FEB 21 2012

.SYNOPSIS...RAINFALL WILL INCREASE THIS MORNING FOR MOST OF THE AREA
AS A WARM FRONT MOVES INTO WESTERN WASHINGTON. WITH THE STRONG WEST
TO NORTHWEST FLOW ALOFT ACCOMPANYING THE FRONT THE CENTRAL PUGET
SOUND AREA WILL BE RAINSHADOWED TO SOME EXTENT. RAIN...HEAVY AT
TIMES OVER THE COAST AND MOUNTAINS WILL CONTINUE THROUGH WEDNESDAY
MORNING ALONG WITH MILD TEMPERATURES. SHOWERS WILL SLOWLY DIMINISH
WEDNESDAY NIGHT. ANOTHER FRONT WILL ARRIVE FRIDAY...USHERING IN A
COOLER PATTERN FOR THE WEEKEND.

&&

.SHORT TERM...SATELLITE IMAGERY SHOWS A LARGE FETCH OF MOISTURE FROM
WEST OF THE DATELINE NEAR 30N ALL THE WAY TO WESTERN WASHINGTON THIS
MORNING. STRONG WARM ADVECTION WILL GET STARTED TODAY AND CONTINUE
THROUGH WEDNESDAY MORNING...WITH RAIN HEAVY AT TIMES ALONG THE COAST
AND OVER THE MOUNTAINS. THE SNOW LEVELS THIS MORNING WILL START OUT
AROUND 4000 FEET BUT QUICKLY RISE TO 6000 TO 7000 FEET BY LATER THIS
MORNING. AMSU TOTAL PRECIPITABLE WATER IMAGERY NOT THAT IMPRESSIVE
WITH THE TPW VALUES IN THE CORE OF THE MOISTURE FETCH NEAR 1.5
INCHES. WHAT IS IMPRESSIVE IS THE STRENGTH OF THE 850-700 MB WINDS.
MODELS SHOW THE WINDS IN THE LAYER INCREASING TO 50-70 KNOTS THIS
MORNING. THE WESTPORT PROFILES CONFIRMS THIS TRANSITION IS ALREADY
IN PROGRESS WITH THE WINDS AS LOW AS 3000 FEET ALREADY UP TO 50
KNOTS. WITH THE STRONG OROGRAPHIC FLOW EXPECT HEAVY PRECIPITATION
AMOUNTS IN THE CASCADES...ESPECIALLY THE CENTRAL CASCADES WITH THE
WEST TO NORTHWESTERLY FLOW ALOFT...AND TO A LESSER EXTENT THE WEST
SLOPES OF THE OLYMPICS. SEE HYDRO SECTION FOR A MORE DETAILS. ON THE
FLIP SIDE...WITH THE WEST NORTHWESTERLY FLOW ALOFT THE CENTRAL PUGET
SOUND WILL BE SOMEWHAT RAINSHADOWED DURING THIS EVENT. AT THIS POINT
IT LOOKS LIKE THE SEATTLE AREA WILL HAVE THE LOWEST TOTAL RAIN EVENT
TOTALS IN THE AREA BY WEDNESDAY AFTERNOON. STILL HAVE HIGH POPS FOR
THE SEATTLE AREA BUT EXPECTING NOT MUCH QPF.

MODELS HAVE BEEN FAIRLY CONSISTENT OVER THE LAST FEW RUNS SHOWING
THE COLD FRONT DROPPING SOUTH THROUGH WESTERN WASHINGTON WEDNESDAY
MORNING. THIS WILL END THE HEAVY PRECIPITATION IN THE MOUNTAINS AS
WELL AS CAUSE THE SNOW LEVEL TO DROP BACK DOWN TO NEAR 3000 FEET.
NOT MUCH IN THE WAY OF SHOWER ACTIVITY BEHIND THE THE FRONT. THERE
IS THE POSSIBILITY OF A PUGET SOUND CONVERGENCE ZONE BUT WITH THE
WINDS ALOFT THE CONVERGENCE ZONE WILL PROBABLY END UP GETTING PUSHED
INTO THE CASCADES. THIS WILL KEEP SOME PRECIPITATION GOING BETWEEN
STEVENS AND SNOQUALMIE PASS BUT WITH THE LOWERING SNOW LEVELS DO NOT
EXPECT THIS PRECIPITATION TO HYDROLOGICALLY SIGNIFICANT.

SHOWERS WILL COME TO AN WEDNESDAY NIGHT WITH A LITTLE BREAK IN THE
ACTION FOR THE LOWLANDS ON THURSDAY AS A WEAK UPPER LEVEL RIDGE
BUILDS INTO THE AREA AS WELL AS HIGH PRESSURE AT THE SURFACE.
TEMPERATURES ON THURSDAY WILL ACTUALLY BE COOLER EVEN WITH MORE
SUNSHINE THEN THE TEMPERATURES TODAY AND WEDNESDAY.FELTON

.LONG TERM...EXTENDED MODELS IN GOOD AGREEMENT WITH ANOTHER FRONT
MOVING INTO WESTERN WASHINGTON ON FRIDAY BRINGING MORE RAIN TO THE
AREA. COOL UPPER LEVEL TROUGH SETTLING INTO THE PACIFIC NORTHWEST
BEHIND THE FRONT LATER FRIDAY NIGHT INTO SATURDAY. MODELS AGREE THE
COLDEST AIR WILL REMAIN BOTTLED UP OVER BRITISH COLUMBIA WITH THE
JET WELL SOUTH OF THE AREA. STILL...THE GFS HAS THE 1000-850 MB
THICKNESS VALUES DROPPING BELOW 1300 METERS SATURDAY WITH THE ECMWF
VALUES JUST A TOUCH HIGHER. WILL STAY WITH THE LOW SNOW LEVEL IDEA
SATURDAY INTO SATURDAY NIGHT. AIR MASS CONTINUES TO COOL ON SUNDAY
WITH 1000-850 MB THICKNESS VALUES WELL BELOW 1300 METERS. CURRENT
FORECAST HAS THE SNOW LEVEL NEAR 500 FEET AND WILL STAY WITH THIS
FORECAST GIVEN THE CURRENT MODEL RUNS. AIR MASS STARTS TO MODERATE A
LITTLE ON MONDAY WITH DRIER NORTHERLY FLOW ALOFT AS THE UPPER LEVEL
RIDGE WELL OFFSHORE EARLY IN THE WEEKEND APPROACHES THE COAST. FELTON

&&

.HYDROLOGY...WITH THE WEST NORTHWESTERLY FLOW ALOFT THE CENTRAL
CASCADES LOOK TO BE THE PRECIPITATION BULLSEYE FOR THIS EVENT. SNOW
LEVELS ARE STILL AROUND 4000 FEET EARLY THIS MORNING BUT WITH THE
WESTPORT PROFILER SHOWING THE INCREASE IN THE WINDS ALOFT THE WARM
ADVECTION SHOULD START KICKING IN SOON WITH THE SNOW LEVELS RISING
INTO THE 6000-7000 FOOT RANGE. PRECIPITATION RATES OVER THE
MOUNTAINS WILL INCREASE DRASTICALLY THIS MORNING. THERE ARE SOME
SMALL DIFFERENCES IN THE MODELS WITH THE NAM MOVING THE
PRECIPITATION BULLSEYE OVER THE NORTH CASCADES LATER TONIGHT WHILE
THE GFS KEEPS THE HEAVIEST PRECIPITATION FOCUSED ON THE CENTRAL
CASCADES. THE MODELS ARE IN GOOD AGREEMENT THAT THE COLD FRONT WILL
MOVE THROUGH WEDNESDAY MORNING BRINGING ABOUT AN END TO THE HEAVIER
PRECIPITATION. HAVE GONE WITH THE GFS SOLUTION IN THE QPF GRIDS THIS
MORNING WITH STORM TOTAL PRECIPITATION...FROM 12Z TODAY TO 18Z
WEDNESDAY...BETWEEN STEVENS AND SNOQUALMIE PASS IN THE 4 TO 6 INCH
RANGE WITH HIGHER AMOUNTS POSSIBLE. LESSER AMOUNTS TO THE NORTH AND
SOUTH OF THIS AREA WITH THE EXCEPT OF AROUND MOUNT RAINIER. EVEN
OUTSIDE THE BULLSEYE 2 TO 4 INCHES OF RAIN IS FORECASTED.

THEREFORE THE RIVERS OF GREATEST RISK OF MAJOR FLOODING ARE THE ONES
FLOWING OUT OF KING AND SNOHOMISH COUNTIES...THE STILLAGUAMISH...
THE SKYKOMISH...THE TOLT...THE SNOHOMISH...AND THE SNOQUALMIE. OTHER
RIVERS WITH AT LEAST PART OF THEIR DRAINAGE IN KING AND SNOHOMISH
COUNTIES INCLUDE THE SKAGIT AND CEDAR RIVERS...AND THESE ALSO HAVE A
HIGH RISK OF FLOODING.

THERE STILL IS ENOUGH UNCERTAINTY THAT ALL RIVERS IN WESTERN
WASHINGTON HAVE SOME RISK OF FLOODING. A FLOOD WATCH HAS BEEN ISSUED
THAT COVERS ALL MAIN STEM RIVERS...INCLUDING THOSE ON THE OLYMPIC
PENINSULA AND ALSO THE ENTIRE CHEHALIS RIVER DRAINAGE AND IT WILL BE
CONTINUED WITH THIS FORECAST PACKAGE. EXPERIENCE HAS SHOWN THAT
DURING HEAVY RAINFALL EVENTS THE CONVEYOR BELT OF WARM MOIST AIR CAN
MOVE AROUND IN UNEXPECTED WAYS. STRONG WEST NORTHWESTERLY FLOW INTO
THE OLYMPICS FAVORS BIG RISES ON THE BOGACHIEL AND SATSOP
RIVERS...AS WELL AS ALL RIVERS ON THE WESTERN SLOPES OF THE OLYMPICS.

ONE FEATURE WORTH NOTING IS THAT STRONG FLOW ALOFT WILL ALSO INHIBIT
HEAVY RAIN DOWNSTREAM OF THE OLYMPICS...OVER THE LOWLANDS AROUND
PUGET SOUND INCLUDING SEATTLE AND TACOMA.

ANOTHER MODERATELY WET WEATHER SYSTEM IS EXPECTED TO MOVE THROUGH
THE AREA FRIDAY. HOWEVER...THIS WILL BE A MUCH COLDER SYSTEM
AND LIKELY MORE OF A MOUNTAIN SNOW PRODUCER VERSUS A RIVER FLOOD
THREAT. FELTON

&&

.AVIATION...STRONG MOIST NW FLOW ALOFT WILL REMAIN OVER W WA THROUGH
WEDNESDAY MORNING. CONTINUING WARM AIR ADVECTION OVER THE AREA TODAY
FOLLOWED BY AN APPROACHING COLD FRONT TONIGHT WILL KEEP THE AIR MASS
MOIST WITH AREAS OF RAIN. CIGS WILL REMAIN BELOW BKN-OVC020 WITH
PREDOMINANT IFR CIGS THROUGH TONIGHT. THERE WILL BE AREAS OF
VISIBILITY 2-5SM IN RAIN AND MIST. STRONG LOW LEVEL ONSHORE FLOW
WILL CAUSE AREAS OF GUSTY SURFACE WINDS. STRONG W FLOW ALOFT OVER
THE CASCADES THROUGH TONIGHT 35-65 KT FROM FL050 TO FL100 WILL
LIKELY CREATE HEAVY ICING CONDITIONS.

KSEA...A WARM FRONT FOLLOWED BY AN APPROACHING COLD FRONT WILL CAUSE
CIGS TO REMAIN BELOW BKN-OVC020 WITH PREDOMINANT IFR CIGS THROUGH
TONIGHT. VISIBILITIES COULD POSSIBLY GET DOWN TO 2SM IN RA BR AT
TIMES. SW SURFACE WINDS WILL BE IN THE 15G25 KT RANGE TODAY...AND
COULD GUST TO 30 KT. KAM

&&

.MARINE...ONSHORE FLOW WILL INCREASE TODAY AND TONIGHT AS A WARM
FRONT CROSSES THE AREA AND A COLD FRONT APPROACHES FROM THE NW. THE
COLD FRONT WILL MOVE ACROSS THE AREA WEDNESDAY MORNING. ONSHORE FLOW
WILL BE STRONG ENOUGH TO PRODUCE GALES IN THE STRAIT OF JUAN DE
FUCA...PROBABLY LEAKING OVER TO ADMIRALTY INLET LATE
TONIGHT/WEDNESDAY. GALES ARE UP FOR THE STRAIT...AND A GALE WATCH
FOR ADMIRALTY INLET. STRONG SW PRESSURE GRADIENTS WILL PRODUCE SCA
WINDS OVER PUGET SOUND...HOWEVER THE GFS GRADIENT COULD BE STRONG
ENOUGH TO PRODUCE A GALE...EXCEPT FOR THE WESTERLIES IN THE STRAIT
BLOCKING THE FLOW TO THE NORTH. ADMIRALTY INLET FORECAST IS A
PROBLEM SINCE IT WILL BE STUCK BETWEEN THE W FLOW THROUGH THE STRAIT
AND SW FLOW COMING UP PUGET SOUND. MODELS HAVE BEEN CONSISTENT WITH
SCA WINDS OVER THE COASTAL WATERS...BUT NW GALES ARE POSSIBLE FOR A
SHORT TIME MAINLY OVER THE N PART BEHIND THE COLD FRONT LATER
WEDNESDAY. KAM

&&

.AVALANCHE...PREVIOUS DISCUSSION FOLLOWS...ABOUT 2-3 FEET OF
SNOWFALL WAS SEEN AT MOST SITES ON FRIDAY AND SATURDAY. THIS NEW
SNOW ACCUMULATED A VARIETY OF LAYERS INCLUDING CRUSTS AND LOCAL HOAR
FROST LAYERS FROM EARLIER IN FEBRUARY. FATALITY ACCIDENTS OCCURRED
AT STEVENS AND SNOQUALMIE ALONG WITH OTHER CLOSE CALLS ON SATURDAY
AND SUNDAY.

VERY STRONG WEST FLOW OVER THE FLAT OFFSHORE RIDGE SHOULD CARRY
MAJOR WARM FRONT MOISTURE TO THE OLYMPICS AND CASCADES ON TUESDAY
AND TUESDAY NIGHT. SEVERAL INCHES OF RAIN SHOULD BE SEEN AT MOST
SITES NEAR AND WEST OF THE CREST WITH SNOW LEVELS RISING TO THE
6-7000 FOOT RANGE FROM NORTH TO SOUTH IN THE CASCADES. THIS SHOULD
CAUSE AVALANCHES AT LEAST INVOLVING MOST OR ALL THE RECENT SNOW WITH
POTENTIAL WEAK LAYERS AND BED SURFACES PROVIDED BY THE HOAR FROST
AND CRUSTS. IN ANY CASE AVALANCHES COULD EASILY ENTRAIN MORE SNOW
RESULTING IN WIDESPREAD POTENTIAL LARGE AVALANCHES.

AN AVALANCHE WARNING HAS BEEN ISSUED FOR EXTREME AVALANCHE DANGER ON
TUESDAY NEAR AND WEST OF THE CREST AND A HIGH DANGER ELSEWHERE. THE
NWAC RECOMMENDS AVOIDING ALL AVALANCHE TERRAIN ON TUESDAY IN THE
OLYMPICS AND NEAR AND WEST OF THE CREST WITH TRAVEL IN AVALANCHE
TERRAIN NOT RECOMMENDED ELSEWHERE. PLEASE VISIT NWAC.US FOR DETAILS.
FERBER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCH IN EFFECT FOR MOST OF WESTERN WASHINGTON
     TUESDAY AFTERNOON THROUGH THURSDAY AFTERNOON.
    .EXTREME AVALANCHE DANGER ABOVE 4000 FEET TUESDAY OLYMPIC AND
     CASCADE MOUNTAINS.
PZ...GALE WARNING CENTRAL/EAST STRAIT OF JUAN DE FUCA.
     GALE WARNING WEST STRAIT THIS EVENING TO WEDNESDAY
      MORNING.
     GALE WATCH ADMIRALTY INLET LATE TONIGHT/WEDNESDAY.
     SMALL CRAFT COAST COAST...WEST ENTRANCE...NORTH INLAND
      WATERS...PUGET SOUND.
     SMALL CRAFT ADVISORY 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.