High Surf Advisory 13, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 PM PST TUE DEC 23 2008

.SYNOPSIS...A WEAK WEATHER SYSTEM WILL MOVE ONSHORE TONIGHT FOLLOWED
BY A STRONGER SYSTEM ON WEDNESDAY MORNING...BRINGING MORE LOWLAND
SNOW. A LOW PRESSURE SYSTEM MOVING SOUTH ALONG THE COAST ON
CHRISTMAS DAY WILL BRING MORE SNOW SHOWERS INTO THE AREA. A WARM
FRONT WILL MOVE INTO THE AREA ON FRIDAY MARKING THE BEGINNING OF A
WARMER AND WETTER WEATHER PATTERN FOR WESTERN WASHINGTON.

&&

.SHORT TERM...THE NEXT BATCH OF SNOW HAS HELD OFF LONG ENOUGH FOR
FOG TO FORM IN A FEW PLACES THIS EVENING. HOWEVER SNOW IS ON THE
WAY...FLURRIES AT KUIL AT 8 PM ARE THE START. AFTER MIDNIGHT THERE
WILL BE LIGHT SNOW. REAL ACTION IS TOMORROW WHEN A SURFACE TROUGH
MOVES ASHORE. GUIDANCE HIGH TEMPS IN THE AFTERNOON ARE IN THE MID
30S INLAND...40 ON THE COAST. DEW POINTS ARE GOING TO STAY BELOW
FREEZING.

THIS ADDS UP TO MORE SNOW. THERE HAS BEEN A CHANCE THE PRECIPITATION
WOULD CHANGE TO RAIN IN THE AFTERNOON IN SOME AREAS. THAT CHANCE
APPEARS TO BE FADING NOW. WILL NOT CHANGE THE FORECAST YET BUT AM
LEANING TOWARD AN ALL OR MOSTLY-SNOW SOLUTION OVER INLAND AREAS.
THAT DOES NOT MEAN WE WILL GET WARNING AMOUNTS. CURRENT FORECAST OF
2-4 INCHES IN MOST AREAS IS SUPPORTED BY 00Z RUNS OF GFS AND NAM.

MORE INTERESTING WEATHER IS POSSIBLE WEDNESDAY NIGHT. THE GFS
DEVELOPS A SURFACE LOW AND MOVES IT SOUTH ALONG THE COAST. THIS IS
USUALLY A GOOD SNOW PATTERN AND THE HIGHEST AMOUNTS WEDNESDAY NIGHT
COULD BE IN SOUTHWEST WASHINGTON. COLD AIR REMAINS OVER WASHINGTON
THURSDAY AND THURSDAY NIGHT FOR MORE SNOW IN SHOWERS. BURKE

.LONG TERM...PREVIOUS DISCUSSION...A WET ZONAL FLOW PATTERN WILL
PREVAIL FROM SATURDAY THROUGH NEXT TUESDAY. MODELS FOR THE PAST FEW
DAYS HAVE BEEN SHOWING A BAROCLINIC BOUNDARY NEAR OR JUST NORTH OF
WRN WA WHICH WOULD BE A FOCUS FOR PRECIPITATION. SURFACE LOWS AND
FRONTAL WAVES WOULD THEN TRACK EAST ALONG THIS BOUNDARY WITH THE
BOUNDARY OSCILLATING A BIT NORTH AND SOUTH. TIMING AND DETAILS OF
INDIVIDUAL LOW CENTERS HAS BEEN INCONSISTENT IN MODEL RUNS.
REGARDLESS OF THE DETAILS...LOOKS LIKE A RETURN TO SEASONABLE
TEMPERATURES. PRECIPITATION WILL OCCUR EVERY DAY WITH PRECIP TYPE
SOLIDLY IN THE FORM OF RAIN IN THE LOWLANDS WITH PLENTIFUL MOUNTAIN
SNOW. WITH THE TURN TO SEASONABLE AND BREEZY WEATHER...LOWLAND
SNOWPACK COULD MELT OFF SURPRISINGLY FAST OVER THE WEEKEND...
RESULTING IN DRAINAGE ISSUE...PARTICULARLY IN URBAN AREAS. HANER

&&

.AVIATION...WEAK SYSTEM MOVING ACROSS THE REGION THIS EVENING...WITH
STRONGER SYSTEM ON ITS HEELS TO MOVE THROUGH THE REGION WEDNESDAY
MORNING INTO WEDNESDAY NIGHT. CEILINGS WILL GRADUALLY COME DOWN
OVERNIGHT...WITH LIGHT SNOW AND MVFR CEILINGS BEGINNING ON THE COAST
LATE THIS EVENING...THEN SPREADING ACROSS THE INTERIOR BEFORE DAWN.
EXPECTED SNOW AMOUNTS BY 18Z CONTINUE TO BE IN THE 1-3 INCH RANGE.
1000-850MB THICKNESSES APPROACHING 1300 M SOUTH OF SEATTLE AND 5000
FT WINDS APPROACHING SW 40 KNOTS AFTER 18Z SEEM TO SUPPORT A SLOPPY
RAIN/SNOW MIX FOR THE SOUTHERN HALF OF THE WESTERN WASHINGTON
INTERIOR WED AFTERNOON. KBLI/KPAE TERMINALS COULD SEE 2-5 INCHES
TOTAL THROUGH WED AFTERNOON WITH A COMPLETE CHANGE OVER TO RAIN LESS
LIKELY. KHQM SHOULD SEE MOSTLY RAIN BY EARLY WED MORNING.

WITH UPPER LEVEL TROUGH MOVING INTO THE REGION WEDNESDAY
NIGHT...TEMPERATURES ALOFT COOL TO NEAR -38C AT 500MB WITH LIGHTER
WINDS IN THE BOUNDARY LAYER. WITH INSTABILITY SHOWERS MOVING ACROSS
THE AREA...SOME ADDITIONAL ACCUMULATIONS OVER INLAND TERMINALS ARE
POSSIBLE...BUT THE NATURE OF SHOWERY PRECIP MAKES IT TOUGH TO
ESTIMATE POSSIBLE AMOUNTS JUST YET.

KSEA...CIGS LOWER TO MVFR 08Z-11Z WITH LIGHT SNOW DEVELOPING.
ACCUMULATIONS 1-3 INCHES ARE POSSIBLE THROUGH 18Z...THEN CHANGING
OVER TO RAIN OR A RAIN/SNOW MIX FOR THE AFTERNOON HOURS. ADDITIONAL
ACCUMULATION IS POSSIBLE AFTER 06Z THU AS PRECIP TURNS SHOWERY AND
TEMPS COOL ALOFT WITH UPPER LEVEL TROUGH MOVING THROUGH. S TO SE
WIND TO 10 KT OVERNIGHT INTO WEDNESDAY.

&&

.MARINE...SMALL CRAFT ADVISORIES HAVE BEEN POSTED FOR ALL WATERS
EXCEPT FOR THE CENTRAL STRAIT AND PUGET SOUND/HOOD CANAL AS SELY
WINDS PICK UP AHEAD OF WED EVENINGS FRONT. MAY ALSO CONSIDER POSTING
A SMALL CRAFT FOR PUGET SOUND WITH THE MORNING FORECAST PACKAGE AS
SOUTHERLY GRADIENTS TIGHTEN WED AFTERNOON. LARGE SWELL TRAIN STILL
ON TRACK TO REACH THE COASTAL WATERS ON CHRISTMAS DAY. 27

&

WA...WINTER STORM WARNING FOR WESTERN WHATCOM AND SKAGIT
COUNTIES...THE EVERETT AREA...HOOD CANAL...THE EAST PUGET
SOUND LOWLANDS...THE CASCADE AND OLYMPIC MOUNTAINS.
WINTER WEATHER ADVISORY FOR THE SEATTLE AND TACOMA AREAS...THE
SOUTHWEST INTERIOR...THE LOWER CHEHALIS VALLEY...THE NORTH
COAST AND WESTERN STRAIT.
PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT CENTRAL STRAIT AND PUGET
     SOUND/HOOD CANAL.

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