Heavy Snow in Washington, December 1 to 3, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
915 AM PST SAT DEC 1 2007

.SYNOPSIS...A WEATHER SYSTEM WILL BRING LIGHT SNOW TO WESTERN
WASHINGTON TODAY AND TONIGHT. A VERY MOIST PACIFIC FRONTAL SYSTEM
WILL BRING RAIN ON SUNDAY. A THIRD VERY WET SYSTEM WILL BRING HEAVY
RAIN AT TIMES SUNDAY NIGHT AND MONDAY...ALONG WITH WINDY CONDITIONS
TO PARTS OF THE AREA. MORE TYPICAL SHOWERY WEATHER WILL RETURN
TUESDAY THROUGH FRIDAY.

&&

.SHORT TERM...SO FAR THIS MORNING JUST FLURRIES AS FORECAST. MAIN
EXCEPTION HAS BEEN EAST OF LAKE WASHINGTON WHERE A BAND OF SHOWERS
BROUGHT UP TO AN INCH OF SNOW. CAMERAS INDICATE THE SNOW STUCK TO
THE GRASS BUT MOSTLY MELTED ON THE ROADS. THIS BAND HAS DRIFTED OFF
TO THE CASCADES. ANOTHER BAND ALONG THE COLUMBIA RIVER COULD BRING
ANOTHER INCH OR SO TO THE SOUTHWEST INTERIOR THIS MORNING.

GFS STILL KEEPS THINGS MOSTLY DRY THROUGH 18Z...THEN SNOW BEGINS A
LITTLE MORE EARNESTLY. THE ONE-TENTH ACCUMULATED PCPN LINE IS RIGHT
OVER PUGET SOUND AT 00Z...WHICH TRANSLATES TO AROUND AN INCH OF SNOW
THIS AFTERNOON. EVENT LOOKS PRETTY WIDESPREAD. THIS EVENING BRINGS
ANOTHER TWO TENTHS OF MOISTURE OR SO...ANOTHER INCH OR TWO. THEN A
BLOB OF MOISTURE MOVES UP FROM THE SOUTH. THIS COULD BE SNOW OR
MIXED PCPN OR JUST RAIN. GFS MOS HAS SOME WARMING IN TEMPS AND
DEWPOINTS BUT POSSIBLY NOT ENOUGH TO ENSURE CHANGE FROM SNOW TO
RAIN. ON THE OTHER HAND SOUTHERLY WINDS ARE SIGNIFICANT WHICH MAY BE
ENOUGH TO CAUSE THE CHANGE. FORECAST SEEMS IN GOOD SHAPE FOR NOW.
WILL REFRESH THE SNOW ADVISORY LATER THIS MORNING.

HEAVY SNOW STILL FORECAST FOR THE MOUNTAINS. SNOWFALL WILL REMAIN
LIGHT UNTIL WARM ADVECTION KICKS IN LATER TONIGHT. SNOW WILL BE
HEAVY AT TIMES TONIGHT THROUGH SUNDAY NIGHT. DESPITE SOME WARMING
SUNDAY...EASTERLY FLOW WILL KEEP PCPN IN THE FORM OF SNOW IN THE
PASSES THROUGH MONDAY MORNING. NOTE SPECIAL AVALANCHE SECTION OF THE
DISCUSSION BELOW.

JUST A GLANCE AT POSSIBLE FLOODING. MONDAY SYSTEM REMAINS WET AND
WARM AND PROBABLY WILL PUT A WATCH OUT THIS AFTERNOON OR THIS
EVENING.

HIGH WIND...THE SYSTEM ON MONDAY COULD PRODUCE HIGH WINDS ALONG THE
COAST AND OVER THE WESTERN STRAIT. STRONG WINDS OVER THE N INTERIOR
ARE ALSO POSSIBLE...BUT THE SURFACE GRADIENT NOT QUITE AS STRONG AS
ALONG THE COAST. THIS IS STILL MORE THAN 2 DAYS OUT SO WITH ALL THE
OTHER WEATHER...WILL JUST MONITOR FOR NOW. BURKE

.LONG TERM...MOSTLY CLOUDY WITH SHOWERS FOR NOW. BURKE

&&

.AVIATION...SIMILAR THINKING STILL APPLIES TO SNOW FORECAST. LOW
PRESSURE CONTINUES TO DEVELOP OFF VANCOUVER ISLAND...SPREADING
PRECIPITATION TO THE NORTH COAST AND LIGHTER PRECIPITATION OVER THE
INTERIOR.

AT KSEA...WINDS HAVE TURNED SE ACROSS THE SOUTHERN HALF OF PUGET
SOUND THIS MORNING...AND EXPECT THEM TO STAY S-SE TODAY.  FLURRIES
THIS MORNING...WITH STEADIER SNOW LIKELY TO DEVELOP BETWEEN 22-24Z.
THERE WILL STILL BE A CHANCE OF SNOW TO MIX WITH RAIN BUT
ACCUMULATIONS BETWEEN 00-04Z OF A TRACE TO TWO INCHES STILL SOUND
LIKELY. SNOW WILL LIKELY MIX WITH RAIN BEGINNING 04Z-06Z AND
CONTINUE AS A MIX INTO SUNDAY MORNING. DAMICO

&&

.MARINE...WINDS ACROSS ALL WATERS WILL INCREASE AS GRADIENTS TIGHTEN
BETWEEN THE DEEPENING LOW OFF VANCOUVER ISLAND AND HIGH PRESSURE
OVER NORTHEASTERN B.C.

SHORT BREAK IN BETWEEN SYSTEMS OVERNIGHT BEFORE SOUTHEASTERLIES
INCREASE SUNDAY OVER ALL WATERS...WITH AN EVEN MORE POWERFUL SYSTEM
IN STORE FOR MONDAY. STORM FORCE WINDS WITH THE POSSIBILITY OF
HIGHER GUSTS WILL WARRANT HIGH SURF AND COASTAL FLOODING
CONSIDERATIONS EARLY NEXT WEEK. DAMICO

&&

.AVALANCHE...WITH AN ALREADY UNSTABLE SNOWPACK STRUCTURE WELL
ESTABLISHED DUE TO A MULTITUDE OF BOTH BURIED AND NEAR SURFACE WEAK
LAYERS...THE WEATHER FOR THE NEXT 36-48 HOURS PROMISES TO BRING A
DRAMATIC INCREASE IN THE AVALANCHE DANGER. PLENTY OF NEW SLIGHTLY
DENSER SNOW PROGGED TO ARRIVE LATER SATURDAY THROUGH EARLY SUNDAY
SHOULD BEGIN THE DANGER EVOLUTION. THEN LOAD THAT SNOW AND THE
UNDERLYING WEAKNESSES WITH LOTS OF HIGHER DENSITY SNOW OR RAIN
RESULTING FROM GRADUALLY RISING FREEZING LEVELS AND INCREASINGLY
STRONG WINDS SUNDAY INTO SUNDAY NIGHT...AND THE DANGER SHOULD BECOME
HIGH TO EXTREME. HOPE THAT THE RECENT DOWNTURN IN THE AVALANCHE
DANGER DOES NOT LURE FOLKS INTO THINKING THAT ALL IS OKAY WITH OUR
SNOWPACK. IT IS NOT, AND THE NEXT DAY OR TWO SHOULD MAKE THAT PRETTY
CLEAR. BEST FOR BC TRAVELERS TO USE INCREASING CAUTION LATER
TODAY...WITH BACK COUNTRY TRAVEL IN AVALANCHE TERRAIN NOT
RECOMMENDED ON SUNDAY. BEST TO AVOID ANY BC TRAVEL AT ALL LATER
SUNDAY INTO MONDAY. SEE NWAC.US FOR DETAILS. MOORE

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
.GALE WARNING COAST. SMALL CRAFT ADVISORY REST OF WATERS EXCEPT
PUGET SOUND.
.SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.
.WINTER STORM WARNING CASCADES AND OLYMPICS.
.SNOW ADVISORY FOR ALL THE LOWLANDS EXCEPT THE EASTERN STRAIT OF
JUAN DE FUCA.

WEATHER.GOV/SEATTLE