Heavy Snow in Washington, December 27 to 30, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PST THU DEC 27 2007

.SYNOPSIS...THE NEXT FRONT WILL SPREAD PRECIPITATION ACROSS THE AREA
TODAY WITH SNOW OVER MUCH OF THE WESTERN WASHINGTON LOWLANDS.
SHOWERS AND MILDER AIR WILL FOLLOW THE FRONT TONIGHT AND FRIDAY...
WITH SNOW IN THE MOUNTAINS. ANOTHER FRONT WILL AFFECT THE AREA THIS
WEEKEND. EXPECT COOL WET WEATHER OVER THE LOWLANDS AND HEAVY SNOW IN
THE MOUNTAINS. HIGH PRESSURE WILL BRING DRY WEATHER EARLY NEXT WEEK.

&&

.SHORT TERM...ANOTHER PACIFIC FRONT IS APPROACHING THE AREA THIS
MORNING...WHICH WILL GIVE THE WRN WA LOWLANDS ANOTHER OPPORTUNITY
FOR SOME SNOW. IN THE MEANTIME THE FORECAST AREA IS GENERALLY CLOUDY
WITH TEMPERATURES IN THE LOWER TO MID 30S. A THIN BAND OF SPOTTY
SHOWERS CONTINUES OVER THE NORTHWEST INTERIOR...EXTENDING FROM THE
SAN JUANS TO MOUNT BAKER. THE LEADING EDGE OF PCPN FROM THE INCOMING
SYSTEM IS ALSO STARTING TO MOVE INLAND FROM THE COAST...WITH SHELTON
REPORTING LIGHT SNOW AT 3 AM.

THE FORECASTS AND STATEMENTS IN THE MORNING PACKAGE WONT CHANGE THE
MESSAGE THAT WEVE BEEN ADVERTISING WITH REGARD TO LOWLAND SNOW
TODAY. THE SYSTEM LOOKS SIMILAR TO THE CHRISTMAS DAY FRONT WITH
RESPECT TO TEMPERATURE -- 1000-850 MB THICKNESS AHEAD OF THE FRONT
WILL BE BELOW 1300 M WITH 850 MB TEMPERATURE AROUND -5 C. HOWEVER IT
IS AN OCCLUDED FRONT AND IT WILL HAVE A NEGATIVE TILT AS IT SWINGS
INLAND.

THE NEGATIVE TILT WILL MAKE THE HOOD CANAL/WEST KITSAP PENINSULA THE
FAVORED AREA FOR SNOWFALL. PCPN IS ALREADY STARTING THERE...THE
EASTERLY COMPONENT OF THE FLOW WILL PRODUCE UPSLOPING AGAINST THE
EAST SLOPES OF THE OLYMPICS...AND THE SURFACE PATTERN KEEPS IT
RELATIVELY COLD. A FORECAST SNOW ACCUMULATION OF 2 TO 4 INCHES STILL
LOOKS GOOD THERE. FOR THE REMAINDER OF THE INTERIOR LOWLANDS UP TO 2
INCHES LOOKS RIGHT. WE ISSUED A SNOW ADVISORY FOR THE WRN WA
LOWLANDS ON WED AFTERNOON...AND THAT STILL LOOKS LIKE A GOOD IDEA.

THE FRONT WILL LIFT NORTHEAST THROUGH WRN WA THIS EVENING...AND THE
AIR MASS IN THE WAKE OF THE FRONT WILL GRADUALLY BECOME TOO WARM TO
SUPPORT SNOWFALL AT SEA LEVEL. THE FRONT WILL MOVE THROUGH THE NORTH
INTERIOR OF WRN WA LAST...SO PLACES LIKE WESTERN WHATCOM COUNTY WILL
BE THE MOST LIKELY TO GET ANOTHER COUPLE INCHES OF SNOW TONIGHT.

SOMETHING LIKE 10 TO 20 INCHES OF NEW SNOW CAN BE EXPECTED IN THE
CASCADES BY NOON FRI...AS A RESULT OF PCPN WITH THE FRONT AND THEN
IN THE WESTERLY FLOW BEHIND IT. I WILL CERTAINLY KEEP THE HEAVY SNOW
WARNING IN EFFECT THERE.

FRI WILL BE COOL AND SHOWERY -- BUT TOO MILD FOR LOWLAND SNOW --
THEN ANOTHER VIGOROUS SYSTEM WILL REACH THE AREA SAT. THE MODELS ARE
HAVING TROUBLE PINNING DOWN THE TRACK OF THE SURFACE LOW WITH THIS
SYSTEM...SO THERE IS SOME UNCERTAINTY IN THE FORECAST SPECIFICS --
ESPECIALLY WITH THE WIND. FOR NOW IT WILL HAVE TO BE ENOUGH TO
INDICATE THAT IT WILL BE A RAW WET DAY FOR THE WRN WA LOWLANDS. FOR
THE MOUNTAINS...ESPECIALLY THE CASCADES...THE FRONT ON SAT AND THE
WESTERLY FLOW FOLLOWING IT SAT NIGHT AND SUN WILL MOST LIKELY BE
ANOTHER BIG SNOW PRODUCER. MCDONNAL

.LONG TERM...SUN WILL BE COOL AND SHOWERY OVER THE WRN WA LOWLANDS
...FOLLOWING THE VIGOROUS FRONT ON SAT. AS MENTIONED ABOVE WESTERLY
FLOW BEHIND THE FRONT WILL BRING LOTS OF SNOW TO THE MOUNTAINS
THROUGH SUN. AN UPPER RIDGE WILL BRING DRY WEATHER BEGINNING MON AND
CONTINUING PROBABLY THROUGH TUE NIGHT. MCDONNAL

&&

.AVIATION...CEILINGS MAINLY 2K-4K FT BUT ARE SLOWLY LOWERING. WILL
BE IN THE 1K-2K RANGE WHEN PCPN BEGINS AROUND 15Z IN PUGET SOUND
AREA. WITH TEMPS AND DEWPOINTS BOTH AROUND FREEZING...PCPN LIKELY TO
BE SNOW WITH ONE OR TWO INCHES OF ACCUMULATION THROUGH AFTERNOON.
CURRENT PLAN IS FOR THE SNOW TO CHANGE TO RAIN AFTER 00Z AND WASH
THE SNOW AWAY. WIND AT KSEA S 8-12 KT TODAY. CHB

&&

.MARINE...INCOMING SYSTEM SHOULD PRODUCE GALES ON THE COAST...STRAIT
ENTRANCES...AND NORTHERN INLAND WATERS. WESTERLIES BEHIND FRONT THIS
EVENING SHOULD BE SMALL CRAFT ADVISORY SPEEDS BUT WILL HAVE TO WATCH
ESPECIALLY STRAIT. COULD ALSO HAVE A BURST OF WIND IN PUGET SOUND
AROUND FRONTAL PASSAGE...WHICH WILL BE 03Z OR SO. NEXT SYSTEM DUE
SATURDAY AND MODELS DO NOT INSPIRE CONFIDENCE...HOWEVER WILL
CONTINUE TO GO WITH MORE OR LESS THE GFS SOLUTION WHICH HAS STRONG
GALE FORCE ONSHORE FLOW. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
.GALE WARNING COAST...STRAIT ENTRANCES...NORTHERN INLAND WATERS.
.SMALL CRAFT ADVISORY ADMIRALTY INLET.
.SMALL CRAFT ADVISORY FOR SEVERE BAR CONDITIONS GRAYS HARBOR BAR.
.HEAVY SNOW WARNING CASCADES MIDDAY THURSDAY THROUGH THURSDAY NIGHT.
.SNOW ADVISORY OLYMPICS THURSDAY AND THURSDAY NIGHT.

WEATHER.GOV/SEATTLE