Small Craft Advisory 1147, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
1004 AM PST MON DEC 24 2012

.SYNOPSIS...AN UPPER LEVEL RIDGE WILL MOVE OVER WESTERN WASHINGTON
TODAY FOR DRIER BUT STILL MOSTLY CLOUDY WEATHER.  A PACIFIC FRONTAL
SYSTEM WILL SPREAD RAIN AND SNOW ACROSS THE AREA ON CHRISTMAS DAY
FOR THE POSSIBILITY OF SIGNIFICANT SNOW OVER PARTS OF THE
LOWLANDS...ESPECIALLY THE HOOD CANAL AREA. THE FRONT WILL MOVE
NORTHEAST OF THE AREA TUESDAY NIGHT...AND THE ASSOCIATED UPPER LEVEL
TROUGH WILL BRING SHOWERS TO THE AREA WEDNESDAY. AN UPPER LEVEL
RIDGE WILL MOVE OVER THE AREA THURSDAY BUT A WARM FRONT EMBEDDED IN
THE RIDGE COULD BRING A LITTLE LIGHT RAIN. ANOTHER WEAK FRONTAL
SYSTEM WILL ARRIVE FRIDAY.

&&

.SHORT TERM...DRIER NW FLOW ALOFT WILL PREVAIL OVER W WA TODAY AS A
WEAK UPPER LEVEL RIDGE JUST OFFSHORE MOVES INLAND. HOWEVER LOTS OF
LOW LEVEL CLOUDS AROUND W WA WILL CLOG UP THE SKIES TODAY AND
OVER-RUNNING HIGH CLOUDS FROM AN APPROACHING FRONTAL SYSTEM WILL
SPREAD OVER THE AREA THIS AFTERNOON. THERE COULD BE SOME SUNBREAKS
IN SPOTS TODAY...BUT OVERALL IT WILL BE CLOUDY. RADAR SHOWS SOME
SCATTERED RESIDUAL SHOWERS AROUND MAINLY OVER THE COAST AND NW
INTERIOR...BUT THESE WILL TAPER OFF THIS AFTERNOON.

THE MAIN FORECAST PROBLEM IS STILL HOW THE POTENTIAL CHRISTMAS STORM
WILL UNFOLD. OVERALL THE MODELS ARE IN FAIRLY GOOD AGREEMENT WITH
THE ONSET OF PRECIPITATION AND TRACK OF THE FRONTAL SYSTEM. HOWEVER
CRITICAL DETAILS REGARDING PRECIP TYPE AND AMOUNT ARE STILL
UNRESOLVED FOR SOME AREAS AND MAY REMAIN UNRESOLVED UNTIL THE STORM
BEGINS TUESDAY MORNING.

THE EVOLUTION OF APPROACHING FRONTAL SYSTEM OVER THE NEXT 36 HOURS
WILL ACTUALLY BE A LITTLE COMPLEX. THE SYSTEM IS CURRENTLY OFFSHORE
BETWEEN 130W AND 150W. THE OCCLUDED FRONT CURRENTLY APPROACHING 130W
THIS MORNING WILL MOVE OVER THE WA OFFSHORE WATERS TONIGHT THEN
BRIEFLY STALL...WHILE THE TRAILING SHORTWAVE TROUGH CURRENTLY NEAR
150W CATCHES UP WITH IT. THAT TRAILING SHORTWAVE WITH UPPER LEVEL
SUPPORT FROM A 70 KT JET STREAK WILL DRIVE THE FRONT INLAND
CHRISTMAS MORNING. A CRUCIAL FACTOR WILL THE THE DRIER E-SE LOW
LEVEL FLOW BELOW 5000 FEET OVER W WA...AHEAD OF THE FRONT LATE
TONIGHT AND EARLY CHRISTMAS MORNING. THE DRIER AIR WILL SET UP A
CLASSIC WET-BULB COOLING EFFECT SNOW EVENT. THE AIR MASS IS WILL BE
COOL AND MARGINALLY TOO WARM FOR SNOW...BUT THE EVAPORATION OF
PRECIP AS IT FALLS THROUGH THE DRY LOW LEVEL AIR MASS WILL COOL IT
ENOUGH TO PUSH THE FREEZING LEVEL DOWN TO NEAR 1000 FEET AND THE
SNOW LEVEL CLOSE TO THE SURFACE. THIS EFFECT WILL TAKE PLACE ACROSS
JUST ABOUT ALL OF W WA.

THE AREA MOST LIKELY TO RECEIVE SNOW WILL BE THE HOOD CANAL AREA ON
THE EAST SIDE OF THE OLYMPICS. THE ABOVE MENTIONED E-SE FLOW WILL
CAUSE THE COOL DRY AIR TO DAM UP AGAINST THE OLYMPICS WHICH WILL
ALLOW SNOW TO DUMP THERE ON CHRISTMAS. MODEL QPF FROM THE 12Z NAM12
AND 12Z WRF-GFS ARE BOTH ABOVE AN INCH OF PRECIP FOR THE AREAS W OF
HOOD CANAL...SO WEST-SIDERS COULD GET A FOOT OF SNOW OR MORE. AREA
ON THE EAST SIDE OF THE CANAL COULD SEE 4 TO 8 INCHES...WITH AMOUNTS
DECREASING RAPIDLY FARTHER AWAY FROM HOOD CANAL. A WINTER STORM
WARNING WILL LIKELY BE ISSUED THIS AFTERNOON FOR HOOD CANAL.

THE REALLY UNCERTAIN PART OF THE FORECAST IS THE REST OF THE
INTERIOR LOWLANDS FROM PUGET SOUND SOUTHWARD. THE NAM AND WRF-GFS
ARE AT OPPOSITE ENDS OF THE SPECTRUM...THE NAM SHOWS SEVERAL HOURS
OF SNOW WITH UP TO 3 INCHES ACCUMULATION FOR THE PUGET SOUND
REGION...PERHAPS A LITTLE MORE FOR THE SW INTERIOR. THE WRF-GFS
SHOWS A FREEZING LEVEL AROUND 1300 FEET AND JUST RAIN FOR THE LOWER
ELEVATIONS...AND MAYBE AN INCH OR SO FOR THE HIGHER HILLS. THE NAM
BUFR SOUNDING SHOWS THE LOW LEVEL TEMPERATURE PROFILE TO BE
ISOTHERMAL BELOW 5000 FEET OVER SEATTLE...SO JUST A TINY BIT OF
WARMING EVEN IN THE SNOWY NAM COULD CHANGE EVERYTHING TO RAIN.

SNOW WILL ALSO FALL OVER THE MOUNTAINS...BUT AMOUNTS WILL NOT BE ALL
THAT SIGNIFICANT. PERHAPS SNOW ADVISORY LEVELS FOR OLYMPICS AND
CENTRAL CASCADES.

PRECIP FROM THE MAIN FRONT WILL END BY 00Z OVER THE AREA...BUT A
TRAILING SHORTWAVE WILL BRING ANOTHER BAND OF PRECIP TO THE AREA
TUESDAY NIGHT. WITH S FLOW OVER THE AREA THIS SHOULD BE ALL RAIN.
THE TRAILING UPPER LEVEL TROUGH WILL MOVE ACROSS THE AREA WEDNESDAY.
KAM

.LONG TERM...PREVIOUS DISCUSSION FROM THE 345 AM AFD...AN UPPER
RIDGE WILL MOVE OVER THE REGION ON THU FOR DRIER WEATHER. THE MEDIUM
RANGE SOLUTIONS WERE IN MORE OR LESS AGREEMENT WITH THE NEXT LOW
PRESSURE SYSTEM SPLITTING AS IT APPROACHES THE COAST THE END OF THIS
WEEK. IT LOOKS LIKE THE BULK OF THE ENERGY WILL HEAD TOWARD OFFSHORE
CA...LEAVING THE FORECAST AREA MAINLY HIGH AND DRY. INTRODUCED FOG
IN THE FORECAST TOWARD THE END OF THE PERIOD DUE TO RIDGE ALOFT AND
STRENGTHENING NIGHTTIME INVERSION.

&&

.HYDROLOGY...
FLOODING IS NOT EXPECTED ON AREA RIVERS THROUGH EARLY EARLY NEXT
WEEK.

&&

.AVIATION...NORTHWESTERLY FLOW ALOFT TODAY AND THIS EVENING WILL
BECOME SOUTHWESTERLY LATE TONIGHT. AT THE SURFACE LIGHT GRADIENTS
WILL BECOME INCREASINGLY EASTERLY AS A FRONT APPROACHES THE COAST
LATE TONIGHT. AIR MASS MOIST AND STABILIZING.

CEILINGS ARE MAINLY MVFR WITH A PATCH OF IFR AROUND PUGET SOUND. ON
THE COAST CEILINGS ARE JUST ABOVE 3K FT. TREND TODAY WILL BE TOWARD
IMPROVEMENT WITH THE IFR CLOUD DECK THE FIRST TO GO...AND CEILINGS
RISING TO 3-4K FT LATE IN THE AFTERNOON. LATE TONIGHT PRECIPITATION
WILL BEGIN ON THE COAST AND SPREAD INLAND TUESDAY MORNING.

KSEA...DISCUSSION ABOVE APPLIES. EXPECT SOUTHERLY WINDS 4-7 KT. CHB

&&

.MARINE...WEAK GRADIENTS TODAY WILL BECOME INCREASINGLY EASTERLY
TONIGHT. A FRONT WILL REACH THE COAST LATE TONIGHT AND MOVE INLAND
TUESDAY. A SURFACE LOW WILL MOVE ONTO THE COAST TUESDAY EVENING AND
DISSIPATE OVERNIGHT. GALE FORCE WINDS ARE FORECAST ON THE COAST AND
AT THE WEST ENTRANCE TUESDAY NIGHT WITH SMALL CRAFT ADVISORY WINDS
ELSEWHERE.

WEAK SYSTEMS WILL IMPACT THE AREA ABOUT THURSDAY AND AGAIN ABOUT
FRIDAY. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WATCH IN EFFECT FOR THE HOOD CANAL AREA AND
     SOUTHWEST INTERIOR FROM LATE TONIGHT THROUGH TUESDAY
     AFTERNOON.

PZ...GALE WARNING COAST AND WEST ENTRANCE LATE TONIGHT AND TUESDAY.

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