Heavy Snow in Washington, December 13 to 14, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST FRI DEC 12 2008

.SYNOPSIS...A STORM SYSTEM WILL SIT OVER EASTERN WASHINGTON TONIGHT.
COLD ARCTIC AIR WILL PUSH FROM SOUTHWEST CANADA ACROSS WESTERN
WASHINGTON SATURDAY AND SATURDAY NIGHT. SNOW SHOWERS TONIGHT THROUGH
SATURDAY NIGHT WILL RESULT IN LOCAL LOWLAND SNOW ACCUMULATIONS.
THERE COULD BE A BETTER CHANCE OF SNOW ON SUNDAY AS AN UPPER LOW
MOVES THROUGH. UNUSUALLY COLD WEATHER WILL PREVAIL NEXT WEEK AS WELL.

&&

.SHORT TERM...THE CENTER OF A DEEP MATURE LOW PRESSURE SYSTEM...
ALONG WITH THE STRONG OCCLUDED FRONTS ASSOCIATED WITH IT...HAS BEEN
MOVING EAST ACROSS WESTERN WASHINGTON THIS AFTERNOON AND EVENING.
THE SURFACE LOW MADE LANDFALL NEAR FORKS ON THE NORTH COAST EARLY
THIS AFTERNOON...TRACKED EAST-NORTHEAST ACROSS THE OLYMPIC PENINSULA
AND THE NORTH PUGET SOUND REGION...AND IS MOVING INTO THE CASCADES
EAST OF ARLINGTON THIS EVENING. AS MENTIONED IN EARLIER DISCUSSIONS
THE TRACK OF THE SURFACE LOW WAS A BIT FARTHER NORTH THAN EARLIER
EXPECTED...ELIMINATING THE THREAT OF LOWLAND SNOW FOR THE SHORT
TERM. REALLY COLD AIR IS STILL WELL NORTH OF THE FORECAST AREA THIS
EVENING.

THE LOW PRESSURE CENTER WILL CONTINUE SLOWLY EASTWARD TONIGHT AND
SATURDAY MORNING...BUT A SURFACE TROUGH EXTENDING WESTWARD FROM THE
SURFACE LOW WILL MOVE SOUTHWARD ACROSS WESTERN WASHINGTON. AS THE
SURFACE TROUGH MOVES ACROSS THE PUGET SOUND REGION...THE
PRECIPITATION ASSOCIATED IT WILL BE ENHANCED BY THE CONVERGENCE
ZONE. COLDER AIR -- BUT NOT YET THE REALLY COLD ARCTIC AIR -- WILL
BE MOVING SOUTHWARD INTO THE AREA OVERNIGHT WITH THE SNOW LEVEL
AROUND 500 FT. SO IT PROBABLY WONT BE COLD ENOUGH FOR GENERAL
LOWLAND SNOW IN THE PUGET SOUND AREA TONIGHT OR SATURDAY MORNING
WHILE THE HEAVIER PRECIPITATION WITH THE TROUGH IS OCCURRING. OF
COURSE THERE IS ALWAYS A CHANCE THE AIR MASS WILL GET COLDER
FASTER...BUT AT THIS TIME I WOULD SAY THE CURRENT FORECAST FOR LOCAL
ACCUMULATIONS OF 1 TO 3 INCHES MAINLY ON HIGHER HILLS AND NORTH OF
SEATTLE LOOKS RIGHT.

ARCTIC AIR WILL BEGIN PUSHING INTO THE FORECAST AREA SATURDAY...AS A
SURFACE HIGH MOVES SOUTH ACROSS BRITISH COLUMBIA AND FORCES THE COLD
AIR THROUGH THE FRASER RIVER VALLEY AND INTO WESTERN WASHINGTON. BY
SATURDAY MIDDAY IT WILL LIKELY BE COLD ENOUGH TO SNOW ACROSS THE
WESTERN WASHINGTON LOWLANDS...BUT THE PRECIPITATION WILL BE LIGHTER
ACROSS THE AREA FOR THE MOST PART. SO AGAIN THE FORECAST FOR LOCAL
ACCUMULATIONS OF 1 TO 3 INCHES WITH THE SHOWERS LOOKS GOOD. THE
NORTH SIDE OF THE OLYMPIC PENINSULA COULD BE AN EXCEPTION TO THIS...
AS THE NORTHEAST WINDS THERE WILL BE UPSLOPING AGAINST THE OLYMPICS
AND THE AIR MASS WILL CARRY MOISTURE IT PICKED UP CROSSING THE
NORTHERN INLAND WATERS. THERE IS A WINTER STORM WATCH IN EFFECT
THERE FOR TONIGHT AND SATURDAY MORNING...WHICH I WILL KEEP IN EFFECT
BUT POSTPONE FOR SATURDAY AND SATURDAY NIGHT.

TEMPERATURES WILL FALL INTO THE 20S SATURDAY NIGHT AS COLD AIR
CONTINUES TO POUR INTO WESTERN WASHINGTON...AND ON SUNDAY THE HIGHS
WILL ONLY CLIMB TO AROUND THE FREEZING MARK. AN UPPER LOW MOVING
NEARLY DUE SOUTH FROM BRITISH COLUMBIA ALONG THE PACIFIC NORTHWEST
COAST WILL BRING ANOTHER CHANCE FOR LOWLAND SNOW. THE AIR MASS WILL
REMAIN JUST AS COLD MONDAY...BUT THE AIR MASS WILL PROBABLY DRY OUT
AS THE UPPER LOW MOVES SOUTH AND DRY NORTHEAST FLOW DEVELOPS ALOFT.

AS FOR THE WIND...I WILL LIKELY END THE HIGH WIND WARNINGS CURRENTLY
IN EFFECT AS THEY HAVE EASED THIS EVENING. THE NORTHEAST ARCTIC
OUTFLOW WILL PRESENT A NEW POTENTIAL FOR HIGH WIND IN WESTERN
WHATCOM COUNTY...WESTERN SKAGIT COUNTY...AND THE SAN JUAN ISLANDS
SATURDAY THROUGH SUNDAY MORNING. WE HAVE A HIGH WIND WATCH ALREADY
IN EFFECT FOR THAT EPISODE...AND I WILL UPGRADE IT TO A WARNING THIS
EVENING. MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
ISSUANCE OF THE FORECAST DISCUSSION -- LOOKS LIKE AN UNUSUALLY LONG
STRETCH OF UNUSUALLY COLD WEATHER NEXT WEEK. BOTH THE GFS AND ECMWF
KEEP A LONGWAVE TROUGH CARVED OUT OVER THE WRN U.S. WITH
NORTH-NORTHEAST FLOW ALOFT BRINGING IN REINFORCING COLD CONTINENTAL
POLAR AIR MASSES. THERE IS FAIRLY GOOD AGREEMENT WITH THE EXTENDED
MODELS IN BRINGING A SHORTWAVE DOWN THE B.C. AND PAC NW COAST ON WED
AND WED NIGHT. GIVEN THE COLD AIR MASS IN PLACE...SNOW WOULD LIKELY
RESULT DOWN TO SEA LEVEL...EXCEPT ALONG THE IMMEDIATE COAST.
HANER

&&

.AVIATION...COLD UPPER LOW AND SFC LOW PRES OVER WA TONIGHT FOR
PLENTY OF ONSHORE FLOW AND SHOWERS AND AN AIR MASS THAT SHOULD
BECOME INCREASINGLY UNSTABLE. 500MB TEMPS SHUD -30C OR EVEN A LITTLE
COLDER...SO AN ISOLD TSTM IS NOT OUT OF THE QUESTION. AT 830PM THE
HEAVIEST SHOWERS ARE MNLY OVER SNOHOMISH COUNTY. 19

KSEA...SHOWERS OVERNIGHT AT TIMES WITH THE SNOW LEVEL ARND 500FT SO
SOME WET SNOW SHUD MIX IN A TIMES. THE WIND SHOULD BE VRB10KT WITH
SOME CONVERGENCE RIGHT AROUND SEATTLE. BY DAYBREAK WE SHOULD BE
GETTING READY TO SWITCH TO A STEADY NLY BREEZE AS GRADIENTS SWING
AROUND TO MORE NLY AS THE FRASER OUTFLOW DEVELOPS AND THE SFC LOW
OVER WA SAGS S. WHEN THE MODIFIED ARCTIC AIR PUSHES INTO THE NORTH
PART OF WRN WA AROUND DAYBREAK SAT THINGS WILL BECOME MORE
INTERESTING WITH REGARDS TO SNOW ACCUM. AS THAT BOUNDARY SINKS SOUTH
OVER THE COURSE OF THE DAY THE CHANCE FOR SNOW TO STICK WILL
INCREASE...BUT CANNOT SAY WHETHER THERE WILL BE ENUF MOISTURE BY
THAT TIME. MODIFIED ARCTIC FRONTS/LEADING EDGE OF CANADIAN AIR IS
OFTEN GOOD FOR 1-2INCHES OF SNOW AS IT PUSHES DOWN TOWARD THE METRO
AREA. BEST GUESS FOR TIMING THAT IS EARLY AFTERNOON...HOPEFULLY
TEMPS WILL BE JUST ABV FREEZING AT THAT TIME AND NOT DROP BLO TIL
AFTER THE PRECIP DRIES UP SAT EVENING. 19

&&

.MARINE....SEAS SHUD REMAIN ARND 25FT OVER THE COASTAL WATERS
THROUGH THE NITE SO ISSUED A HIGH SURF ADVISORY. THERE WAS A MINUS
TIDE THIS EVENING SO NO TIDAL OVERFLOW. A 986MB LOW WAS NEAR
ARLINGTON THIS EVENING AND WILL FILL AND SPREAD OUT OVER WA TONIGHT.
THE NAM SHOWS THE 995MB CONTOUR HOLDING IN MOST OF THE STATE BY 12Z
WITH GRADIENTS RELAXING OVERNIGHT. THE ONE GRADIENT THAT WONT RELAX
OVERNIGHT IS THE NLY GRADIENT OVER B.C...FRASER OUTFLOW SHOULD BE IN
PLACE WITH NE GALES FLOWING OUT OVER WRN WHATCOM COUNTY AND THE SAN
JUANS STARTING UP BY DAYBREAK. NE GALES DEVELOP AROUND THE WRN
STRAIT OF JUAN DE FUCA LATER SAT...HOLDING OFF TIL AROUND 6PM SAT IN
THE WRFGFS. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING FOR THE OLYMPICS AND CASCADES.
    .HIGH WIND WARNING SATURDAY THROUGH SUNDAY MORNING FOR WESTERN
     WHATCOM COUNTY...THE SAN JUAN ISLANDS...AND WESTERN SKAGIT
     COUNTY.
    .WINTER STORM WATCH EASTERN STRAIT OF JUAN DE FUCA FOR SATURDAY
     AND SATURDAY NIGHT.
    .WIND ADVISORY STRAIT OF JUAN DE FUCA AND ADMIRALTY INLET
     ZONES...AND FOR THE LOWLAND INLAND ZONES EVERETT SOUTH.
    .WINTER WEATHER ADVISORY STRAIT OF JUAN DE FUCA ZONES...
     SAN JUAN ISLANDS...NORTHWEST INTERIOR...AND WESTERN
     WHATCOM COUNTY.

PZ...GALE WARNING COASTAL WATERS...STRAIT OF JUAN DE FUCA...
     ADMIRALTY INLET...CAMANO ISLAND TO PT ROBERTS.
    .SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL.
    .SMALL CRAFT ADVISORY IN EFFECT GRAYS HARBOR BAR FOR SEVERE BAR
     CONDITIONS.

WEATHER.GOV/SEATTLE