Heavy Snow in Washington, December 23 to 26, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
830 AM PST SUN DEC 23 2007

.SYNOPSIS...A STRONG FRONTAL SYSTEM WILL MOVE THROUGH WESTERN
WASHINGTON TODAY. SHOWERS AND A PUGET SOUND CONVERGENCE ZONE WILL
PERSIST TONIGHT. TEMPORARY HIGH PRESSURE BUILDS OVER THE REGION
MONDAY. ANOTHER FRONTAL SYSTEM WILL REACH THE AREA CHRISTMAS DAY. A
COOL AND ACTIVE WEATHER PATTERN WILL PREVAIL THROUGH NEXT WEEK.

&&

.SHORT TERM...ALTHOUGH SATELLITE IMAGERY SUGGESTS A GREAT DEAL OF
THE MOISTURE ASSOCIATED WITH THE INCOME FRONTAL SYSTEM IS UP OVER
B.C. THERE IS STILL A PUNCH TO THE SYSTEM GOING TO PLOWS THRU WRN WA
TODAY: PRESSURE FALLS ARE CENTERED OVER THE WA COAST AT 8AM AND
SATIMG IS ENHANCING NOW ALG THE COAST. IT WILL GET A LITTLE MUSHY UP
IN THE MOUNTAINS TODAY AS THIS IS THE WARMEST IN THE LATEST SERIES
OF FRONTAL SYSTEMS. THE UWS MM5NAM VALID THIS AFTERNOON HAS THE +4C
ISOTHERM INTO PUGET SOUND AND THE 0C LINE INTO THE CENTRAL CASCADES.
THE FORECAST FOR THE CASCADES LOOKS LIKE A GOOD ONE WITH THE SNOW
LEVEL RISING TO 6000 FEET EXCEPT STAYING SNOW IN THE PASSES...IT
WILL BE WET SNOW THO THIS AFTERNOON. THE NAM12 SETS UP THE PSCZ BY
06Z THIS EVENING KEEPS IT OVERNIGHT...SHIFTING IT A TAD SOUTH BY 12Z
AND DISSIPATING IT OVER EASTERN KING COUNTY MONDAY MORNING. POST
FRONTAL TEMPS AT 850MB FALL FROM -2C WHEN THE PSCZ SETS UP TO -5C AT
DAYBREAK...SO AFTER THE HEAVY SNOW WARNING WHICH MAY HAVE SOME
TROUBLE VERIFYING DUE TO THE MUSH FACTOR AND WARMTH OF THIS FRONTAL
SYSTEM WILL GIVE WAY TO A SNOW ADVISORY FOR MAINLY THE CENTRAL
CASCADES THAT SHOULD VERIFY WELL. FOR THE LOWLANDS...JUST A RAINY
BREEZY DAY WITH GALES IN THE TYPICAL MARINE LOCATIONS. MM

.LONG TERM...COLD UPPER TROF OVER PACNW TUE NITE AND WED WITH LOW
SNOW LEVELS AND SHOWER ACTIVITY. WEAK RIDGE WED NITE BUT H500
HEIGHTS ONLY RISE A BIT AND 1000-500MB THICKNESS ON GFS IS BACK DOWN
IN THE 520S THU NITE AND FRI. A WARMER/STRONG FRONTAL SYSTEM
ARRIVES SATURDAY WITH A 976MB LOW INTO VANCOUVER ISLAND AND FILLING
THRU SUNDAY WITH STRONG ONSHORE FLOW INTO WESTERN WA W/ THICKNESS
AGAIN BACK INTO THE 520S. THE FORECASTS HAVE VERY LOW SNOW
LEVELS...PERHAPS A TAD TOO LOW CONSIDERING THE PATTERN AND STRONG
ONSHORE FLOW. ALTHOUGH THE DETAILS WILL CERTAINLY FALL TO THE
MESOSCALE MODELS AS WE GET INTO NEXT WEEK JUST BEING ON THE COLD
SIDE OF THE JET IS NOT USUALLY ENOUGH TO PUT SNOW IN THE LOWLANDS
IN ONSHORE FLOW SO I MAY TRY TO EXCLUDE THE S WORD FROM ZONES THAT
HAVE VERY FEW HILLS HIGH ENOUGH TO CATCH IT AND DO SOME WORK ON THE
FREEZING/SNOW LEVEL GRIDS/FCSTS FOR THE AFTERNOON PACKAGE. MM

&&

.AVIATION...UPPER RIDGE OVER WESTERN WA THIS MORNING TO MOVE E OF
THE CASCADES THIS AFTERNOON. WLY FLOW ALOFT BECOMING SW THIS
AFTERNOON AND EVENING. PACIFIC COLD FRONT 200 NM OFFSHORE AT 15Z TO
REACH COAST NEAR 20Z AND CASCADES 00Z. LOW LEVEL ONSHORE FLOW.
ASSOCIATED UPPER TROF WELL OFF COAST TO MOVE ONSHORE 09Z MON FOR
MORE UNSTABLE AIR MASS AND SHOWERS. PUGET SOUND CONVERGENCE ZONE
LIKELY TO DEVELOP AFTER 03Z MON.

RAIN HAS SPREAD ONSHORE IN ADVANCE OF THE INCOMING COLD FRONT.
EXPECT CEILINGS TO DROP INTO THE MVFR CATEGORY WITH LOWEST
CONDITIONS OF 1000-2000 FT NEAR THE FRONT 20Z-24Z. ENOUGH WIND AND
MIXING TO AVOID IFR CEILINGS MOST LOCATIONS. VIS TO DROP INTO MVFR
CONDITIONS AS WELL. WEST SOUTHWEST WIND ALOFT TO RISE WITH THE
FRONTS APPROACH..REACHING UP TO 50 KT AT 5000 FT AROUND 21Z AND 65
KT AT 10000 FT.

FOLLOWING FRONT..SOME GRADUAL IMPROVEMENT BUT A PUGET SOUND
CONVERGENCE WILL LIKELY DEVELOP AND STAY IN THE VICINITY OF KPAE AND
NOT MOVE SOUTH TO KBFI AT LEAST THROUGH THE NIGHT. IT LOOKS LIKE IT
WILL LAST A WHILE AND HAS THE CHANCE TO MOVE S THRU KBFI AND KSEA ON
MON AFTERNOON.

KSEA...SOUTHEAST WIND 4-8 KT TO RISE AS FRONT APPROACHES TO 8-12 KT
BY 20Z. THE COLD FRONT EXPECTED TO CROSS KSEA AROUND 23Z WITH WIND
VEERING TO SOUTH OR SOUTHWEST 12-16 KT AND GUSTING TO 24 KT AND
SLOWLY EASING TONIGHT. BUEHNER

&&

.MARINE...THE INCOMING FRONT HAS ALREADY PRODUCED LOW END GALES AT
DESTRUCTION ISLAND IN THE OUTER COASTAL WATERS THIS MORNING. IT HAS
THE POTENTIAL TO ALSO PRODUCE MARGINAL GALES IN THE ENTRANCES TO THE
STRAIT..ADMIRALTY INLET AND NORTHERN INLAND WATERS. HAVE GALES UP IN
THE ENTRANCES AND JUST ISSUED FOR ADMIRALTY. EXPECT POST-FRONTAL
WESTERLIES IN THE STRAIT WILL LIKELY BE BELOW GALE AS WELL.

THE NEXT PACIFIC FRONTAL SYSTEM IS EXPECTED TO PRODUCE SIMILAR WIND
CONDITIONS MON NIGHT AND TUE. THIS SYSTEM WILL CONTINUE THE TREND OF
BUILDING WESTERLY SWELL TO 20 FT OR A LITTLE HIGHER ON MON AND A
HIGH SURF ADVISORY MAY BE NEEDED BY THEN. BUEHNER

&&

.AVALANCHE...HEAVY AMOUNTS OF SNOW WITH A WARMING TREND ARE EXPECTED
SUNDAY MIDDAY THROUGH SUNDAY NIGHT. THIS SHOULD MAKE NATURAL OR
TRIGGERED AVALANCHES LIKELY IN MANY AREAS AND AN AVALANCHE WARNING
IS IN EFFECT. PLEASE VISIT WWW.NWAC.US FOR DETAILS. FERBER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
.HEAVY SNOW WARNING FOR THE CASCADES.
.AVALANCHE WARNING FOR THE OLYMPICS AND CASCADES.
.GALE WARNING COASTAL WATERS...WEST ENTRANCE STRAIT...EAST ENTRANCE
STRAIT AND ADMIRALTY INLET.
.SMALL CRAFT ADVISORY CENTRAL STRAIT...NORTHERN INLAND WATERS AND
PUGET SOUND AND HOOD CANAL.
.SMALL CRAFT ADVISORY FOR SEVERE BAR CONDITIONS GRAYS HARBOR BAR.

WEATHER.GOV/SEATTLE