Heavy Snow in Washington, December 7 to 8, 2012

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST FRI DEC 7 2012

.SYNOPSIS...MOIST ONSHORE FLOW AND AN ACTIVE PUGET SOUND CONVERGENCE
ZONE WILL RESULT IN SHOWERS TONIGHT WITH LOCALLY HEAVY SNOW IN THE
CENTRAL CASCADES. THE FLOW WILL BECOME MORE NORTH TO NORTHWESTERLY
ON SATURDAY...RESULTING IN SOMEWHAT DRIER CONDITIONS. ANOTHER
WEATHER SYSTEM WILL BRING LIGHT PRECIPITATION TO THE AREA SUNDAY AND
SUNDAY NIGHT. A STRONGER COLD FRONT WILL ARRIVE ON TUESDAY WITH
UNSETTLED WEATHER CONTINUING THROUGH THE END OF NEXT WEEK.

&&

.SHORT TERM...THE SHORT WAVE TROUGH THAT BROUGHT WIDESPREAD SHOWERS
TO WRN WA EARLIER TODAY HAS SHIFTED E OF THE CASCADES. ONSHORE FLOW
BEHIND THE SYSTEM HAS INCREASED WITH MOIST AND UNSTABLE CONDITIONS
PERSISTING. THIS MEANS SHOWERS WILL CONTINUE TONIGHT...THOUGH NOT AS
WIDESPREAD. RADAR IS INDICATING THE PUGET SOUND CONVERGENCE ZONE HAS
BECOME ACTIVE OVER WRN KING COUNTY. SURFACE PRESSURE GRADIENTS WILL
BE FAVORABLE FOR CONVERGENCE ZONE DEVELOP THROUGH TONIGHT. MESO
MODELS GENERALLY AGREE THAT THE CURRENT BAND WILL WEAKEN AND RESIDE
OVER S/CENTRAL KING COUNTY WITH ANOTHER STRONGER BAND DEVELOPING
FROM ADMIRALTY INLET TOWARD SNOHOMISH/N KING COUNTIES. THIS PUTS
BOTH STEVENS PASS AND SNOQUALMIE PASS IN THE CROSS HAIRS OF POSSIBLE
LOCALLY HEAVY SNOW ACCUMULATIONS. DECIDED TO LET THE WARNING EXPIRE
FOR THE N CASCADES BUT WILL MAINTAIN A WINTER WEATHER ADVISORY FOR
THE CENTRAL CASCADES. WHERE CONVERGENCE BANDS STALL...THERE COULD BE
ACCUMULATIONS OF 6 TO 12 INCHES ON TOP OF WHAT HAS ALREADY
ACCUMULATED. SNOW LEVELS WILL FALL TO 1000 FEET TONIGHT WITH SOME
RAIN/SNOW SHOWERS MIXED ON HILLTOPS ABOVE 500 FT. THERE COULD BE A
DUSTING TO 1 OR 2 INCHES ALONG THE FOOTHILLS OR ON HILLTOPS WHERE
ANY CONVERGENCE ZONE STALLS OUT.

A RELATIVE BREAK BETWEEN SYSTEMS IS EXPECTED ON SATURDAY. THERE WILL
STILL BE SOME RESIDUAL MOUNTAIN SNOW SHOWERS...AND RAIN SHOWERS
ACROSS THE S. THE GENERAL TREND WILL BE FOR DRYING FROM THE N. THE
BREAK IN PRECIPITATION WILL CONTINUE INTO SATURDAY NIGHT. AREAS THAT
SEE SOME PARTIAL CLEARING MAY DROP TO NEAR OR JUST BELOW FREEZING.
HOWEVER...MOS GUIDANCE INDICATES A STRATUS DECK TO INSULATE MUCH OF
THE AREA KEEPING MOST AREA JUST ABOVE FREEZING.

NW FLOW ALOFT WILL DOMINATE INTO SUNDAY WITH ANOTHER SYSTEM RIDING
OVER THE ERN PAC RIDGE INTO WRN WA. LIGHT RAIN WILL REACH THE COAST
EARLY...THEN SPREAD INLAND THROUGH THE DAY. A WEAK SURFACE LOW WILL
DIVE SE OFF THE WA COAST BY SUNDAY EVENING WITH RAIN LIKELY ACROSS
THE AREA. THE SYSTEM FIZZLES OUT LATE SUNDAY NIGHT WITH A DECREASING
CHANCE OF RAIN. MODELS SHIFT THE ERN PAC RIDGE EVER SO SLIGHTLY
TOWARD THE AREA ON MONDAY. THIS MEANS LESS CHANCE OF RAIN ALTHOUGH
MODELS STILL SHOW SOME SPOTTY LIGHT QPF AROUND THE AREA. DECIDED TO
KEEP SOME CHANCE POPS FOR NOW AS IT IS NOT CLEAR HOW MUCH DRYING MAY
OCCUR.

.LONG TERM...ANOTHER BREAK BETWEEN SYSTEMS IS EXPECTED MONDAY NIGHT
INTO EARLY TUESDAY. THE GFS/ECMWF AGREE THAT THE NEXT STRONGER
SYSTEM WILL DIG SWD THROUGH THE AREA AS THE UPPER RIDGE OFFSHORE
RETROGRADES WWD. RAIN WITH THIS SYSTEM SHOULD SPREAD ACROSS THE AREA
TUESDAY TRANSITIONING TO SHOWERS TUESDAY NIGHT UNDER THE TRAILING
UPPER TROUGH. BECAUSE THIS SYSTEM APPROACHES FROM THE
N/NW...PRECIPITATION WILL FALL MOSTLY AS SNOW IN THE MOUNTAINS WITH
ACCUMULATION LIKELY DOWN TO PASS LEVELS.

THE TROUGH BROADENS OUT OVER THE WRN U.S. WEDNESDAY THROUGH THE END
OF NEXT WEEK WITH SHOWERS ACROSS THE PAC NW. SNOW LEVELS WILL BE
RELATIVELY LOW DOWN TO THE LOWER PASSES OR PERHAPS THE FOOTHILLS AT
TIMES. THE BULK OF ENERGY WILL WILL BE NEAR THE BASE OF THE TROUGH
WELL S OF THE AREA SO HEAVY PRECIPITATION IS NOT EXPECTED.  MERCER

&&

.HYDROLOGY...FLOODING IS UNLIKELY DURING THE NEXT 7 DAYS. MERCER

&&

.AVIATION...A MOIST AND UNSTABLE NORTHWEST FLOW ALOFT WILL CONTINUE
TONIGHT. WHILE SCATTERED SHOWERS OVER THE OLYMPIC PENINSULA AND OVER
THE COASTAL WATERS WILL DIMINISH OVERNIGHT...THE INTERACTION OF THE
FLOW WITH THE MOUNTAINS OF THE OLYMPICS AND VANCOUVER ISLAND WILL
GIVE TWO CONVERGENCE ZONES THROUGH EARLY SATURDAY MORNING. ONE
LOCATED AROUND SEATAC WILL PERSIST OVERNIGHT BUT WILL GRADUALLY
WEAKEN WITH TIME AND SHIFT EAST. THE OTHER NOW EXTENDING EAST FROM
VICTORIA BC TO THE NORTHERN SAN JUAN ISLANDS WILL DROP SOUTHEAST TO
A POSITION FROM THE EASTERN STRAIT OF JUAN DE FUCA TO NEAR KPAE THEN
ESE INTO THE CASCADES AND PERSIST THROUGH ABOUT 15-18Z SATURDAY.

EXPECT MAINLY VFR CONDITIONS AWAY FROM SHOWERS AND CONVERGENCE
BANDS. LOCAL CIGS WITHIN THE BANDS WILL DROP TO 010-015 TONIGHT AND
EARLY SATURDAY. THE SNOW LEVEL THIS AFTERNOON PER DUAL POLARIZED
DOPPLER RADAR IS 1500-1700 FEET. IT WILL FALL TO 500-1000 FEET LATE
TONIGHT INTO EARLY SATURDAY WITH HEAVIER CONVERGENCE BANDS...MAINLY
ESE OF SEATTLE AND FROM KPAE ESE INTO THE CASCADES. THIS WILL RESULT
IN SOME SHOWERS IN THE FORM OF WET SNOW OR MIXED RAIN AND SNOW
09Z-16Z. ALBRECHT

KSEA...THE SOUTHERN PUGET SOUND CONVERGENCE ZONE SITS NEAR OR JUST
SOUTH OF THE TERMINAL AND WILL REMAIN ABOUT WHERE IT IS THROUGH
EARLY SATURDAY MORNING. NORTH WINDS 4-6 KT WINDS WILL BECOME MORE
VARIABLE THIS EVENING AND MAY OCCASIONALLY TURN SOUTHWEST...BUT
EXPECT THE MEAN POSITION OF THE CONVERGENCE TO SIT SOUTH OF KBFI AND
TO IMPACT TRAFFIC FLOW. PRECIPITATION SHOULD REMAIN IN THE FORM OF
RAIN WITH MIXED PRECIPITATION THREATS REMAINING WELL EAST OF THE
TERMINAL...OVER THE CASCADE FOOTHILLS. CIGS WILL REMAIN 010-030
THROUGH 18Z SATURDAY WITH THE LOWER CLOUDS ERODING ABOUT 18Z.
ALBRECHT

&&

.MARINE...GALE CONDITIONS ARE OCCURRING THROUGH THE STRAIT OF JUAN
DE FUCA THIS AFTERNOON. ONSHORE FLOW IS PRODUCING SMALL CRAFT
ADVISORY CONDITIONS IN THE OTHER WATERS WITH THE EXCEPTION TO
CENTRAL PUGET SOUND AND HOOD CANAL. ONSHORE FLOW WILL GRADUALLY
RELAX THIS EVENING THROUGH TONIGHT AS HIGH PRES OVER THE OFFSHORE
WATERS BUILDS INTO THE INLAND WATERS ON SAT.

SURFACE HIGH PRES WILL SHIFT E OF THE CASCADES SAT NIGHT. A WEAK
FRONTAL SYSTEM WILL MOVE THROUGH THE WATERS SUNDAY AND WILL BE
FOLLOWED BY A ROUND OF POST FRONTAL ONSHORE FLOW MONDAY AND MONDAY
NIGHT. THIS ROUND OF ONSHORE FLOW WILL PRODUCE SMALL CRAFT ADVISORY
CONDITIONS OVER MOST OF THE WATERS.

A STRONGER COLD FRONT MOVING IN FROM THE NORTHWEST ON TUESDAY. SMALL
CRAFT ADVISORY CONDITIONS ARE LIKELY OVER MOST OF THE WATERS WITH
THIS FRONTAL SYSTEM. ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER WEATHER ADVISORY TONIGHT FOR THE CENTRAL CASCADES.

PZ...GALE WARNING STRAIT OF JUAN DE FUCA.

     SMALL CRAFT ADVISORY ADMIRALTY INLET...NORTHERN INLAND
     WATERS...PUGET SOUND AND HOOD CANAL...AND COASTAL WATERS.

     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

&&

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