Heavy Snow in Washington, December 15, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST SAT DEC 15 2007

.SYNOPSIS...A VIGOROUS COLD FRONT OVER WESTERN WASHINGTON EARLY THIS
MORNING WILL MOVE TO THE CASCADES BY DAYBREAK. COOL MOIST ONSHORE
FLOW WILL KEEP SHOWERS AND MOUNTAIN SNOW IN THE FORECAST THROUGH
TONIGHT. ANOTHER STRONG FRONTAL SYSTEM WILL REACH THE AREA SUNDAY.
RATHER UNSETTLED CONDITIONS WILL PERSIST THROUGH NEXT WEEK.

&&

.SHORT TERM...RAIN WILL TURN TO SHOWERS BY DAYBREAK OVER MOST OF
WESTERN WA WITH THE PASSAGE OF A VIGOROUS COLD FRONT. GALE FORCE
WINDS IN THE NORMAL SPOTS EARLY THIS MORNING...WESTERLY FLOW SFC AND
ALF IN THE WAKE OF THE FRONT. THE PRECIP OVER THE LOWLANDS WILL
BECOME FAIRLY SCATTERED...BUT OVER THE MOUNTAINS MORE OR LESS
CONTINUOUS SNOW SHOWERS THRU THE DAY. WILL EXTEND THE SNOW ADVISORY
THRU THE DAY. PLENTY OF SHOWERS INDICATED BY THE MODELS THRU SAT
NIGHT...AND THEN RAIN WITH THE NEXT STRONGER FRONTAL SYSTEM DUE IN
ON SUNDAY. THE FCST SEEMS TO HAVE THINGS TIMED CORRECTLY THRU
MONDAY. SNOW LEVEL WILL AVERAGE 1500 FEET AND THE CASCADES WILL
FINALLY BEGIN TO ADD TO THE SNOWPACK SUBSTANTIALLY. THRU NEXT WEEK
THE 5 WAVE 500MB CHART TELLS THE TALE...WET AND COOL WITH A VERY
PROGRESSIVE ZONAL PATTERN AND RAIN OR SHOWERS EACH DAY. COMPARING
THE GFS WITH THE ECMWF ON THE DETAILS: THE FRONTAL SYSTEM FOR SUNDAY
IS VERY SIMILAR...BOTH MODELS HAVE A DEEP PARENT LOW PRESSURE CENTER
TRACKING THRU THE CHARLOTTES AND FROPA FOR WRN WA SUNDAY EVENING.

.LONG TERM...THE NEXT SYSTEM IN THE SERIES MOVES EAST ALONG 42N
WHILE DEEPENING ON MONDAY...CROSSES 130W BY DAYBREAK TUE...THEN
TRACKS NE TO JUST OFF ASTORIA TUE AFTERNOON (MM5GFS) AND INTO PUGET
SOUND BY EVENING. IF THAT PANS OUT WE WONT HAVE GALE SOUTHERLIES BUT
WE WILL SEE STRONG SW TO WESTERLY WINDS FOR AWHILE TUESDAY NIGHT.

THE THIRD SYSTEM IS THE LEAD FRONTAL SYSTEM AT THE EXIT REGION OF A
STRONG WESTERLY JET FCST TO DEVELOP AND PROGRESS ACROSS THE PACIFIC
MORE OR LESS ALONG 42N FROM BEYOND 160W TO THE CALIF/ORE BORDER BY
12Z WED. THE DETAILS WILL CERTAINLY WAFFLE AROUND...BUT FOR NOW THE
MM5 EXTENSION IS TAKING THE DEEP LOW THRU WRN ORE WED EVENING THEN
INTO THE COLUMBIA BASIN EARLY THU. IF THE JET BUCKLES MORE THAN FCST
BY THE MODELS THAT SYSTEM TOO WILL BE WORTH WATCHING. MM

&&

.AVIATION...MVFR WILL BE THE PREVAILING CONDITION ACROSS WRN WA
TODAY...MIXED IN WITH AREAS OF VFR AND LOCAL IFR. THE FRONT OVER THE
INTERIOR OF WRN WA EARLY THIS MORNING WILL MOVE QUICKLY EAST OF THE
CASCADES...BUT SHOWERS WILL CONTINUE OFF AND ON AS A WEAK TROUGH
MOVES THROUGH THE AREA. BREEZY CONDITIONS WILL CONTINUE AS WELL.
ANOTHER VIGOROUS FRONT WILL MOVE INTO THE AREA TONIGHT...FOR ANOTHER
ROUND OF RAIN...WIND...AND MVFR TO IFR CONDITIONS.

.KSEA...SOUTH WIND 14-20 KT GUSTING 28 KT WILL VEER TO SOUTHWEST
OVER NEXT FEW HOURS...THEN EASE TO 10-16 KT GUSTING 20 KT AROUND
NOON...THEN BECOME SOUTH AROUND 6 PM AS THE NEXT FRONT APPROACHES.
MCDONNAL

&&

.MARINE...GALES WILL CONTINUE EARLY THIS MORNING FOR THE
COAST...EAST ENTRANCE STRAIT...AND NORTHERN INLAND WATERS. HIGH END
SMALL CRAFT WILL BE THE FORECAST FOR THE REMAINING WATERS...BUT THE
CENTRAL STRAIT AND PUGET SOUND HAVE SOMETHING LIKE A 20 PCT CHANCE
OF LOW END GALES THIS MORNING. WIND WILL EASE SOMEWHAT TODAY...BUT
THE NEXT APPROACHING FRONT WILL CAUSE ANOTHER ROUND OF GALES FOR THE
COASTAL WATERS...ENTRANCES TO THE STRAIT...ADMIRALTY INLET...AND
NORTHERN INLAND WATERS BEGINNING LATE TONIGHT AND CONTINUING THROUGH
SUN. A MINOR RIDGE WILL CROSS THE AREA MON...FOLLOWED BY ADDITIONAL
FRONTAL SYSTEMS ON TUE AND WED NIGHT. MCDONNAL

&&

.AVALANCHE...DESPITE STILL RELATIVELY SHALLOW SNOW COVER IN MOST
MOUNTAIN LOCATIONS AVALANCHE DANGER DEVELOPING ALMOST ON PAR WITH
EARLY DECEMBER SITUATION...BUT WITHOUT THE RAIN TO CAP THINGS OFF.
WE DO HAVE AN OLD CRUST AS THE BED SURFACE AND PLENTY OF WEAK LAYERS
OVER THE CRUST...AND NOW LOADING THIS WITH PLENTY OF INCREASINGLY
DENSE WIND SLAB OVER THE NEXT 36-48 HOURS OR MORE. SHOULD BE PLENTY
OF DANGER TO KEEP AVALANCHE CONTROL PROGRAMS AND BACK COUNTRY
ENTHUSIASTS OCCUPIED AND HOPEFULLY CAREFUL. HIGH DANGER CERTAINLY
LIKELY FOR MOST OF SUNDAY IN AVALANCHE TERRAIN...AND HAVE ISSUED
WATCHES AND FORECASTS TO COVER EVENT. HOPE BC TRAVELERS EXERCISE
MORE CAUTION THAN EARLY DECEMBER. MOORE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
.SNOW ADVISORY CASCADES AND OLYMPICS TODAY.
.AVALANCHE WATCH CASCADES AND OLYMPICS FOR SUNDAY.
.GALE WARNING ALL WATERS EXCEPT SMALL CRAFT ADVISORY FOR THE CENTRAL
STRAIT OF JUAN DE FUCA AND PUGET SOUND/HOOD CANAL.
.SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR CONDITIONS.

WEATHER.GOV/SEATTLE