Heavy Snow in Washington, December 17 to 18, 2008

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST TUE DEC 16 2008

.SYNOPSIS...A WEATHER SYSTEM MOVING SOUTH ACROSS BRITISH COLUMBIA
WILL BRING SNOW ON WEDNESDAY INTO WEDNESDAY NIGHT. COLD WEATHER WILL
CONTINUE GRIPPING THE REGION LATER THIS WEEK AS ANOTHER SURFACE HIGH
OVER BRITISH COLUMBIA PUSHES MORE COLD AIR SOUTHWARD INTO WESTERN
WASHINGTON. OVER THE WEEKEND A FRONTAL SYSTEM ARRIVING FROM THE WEST
MAY BRING MORE SNOW.

&&

.SHORT TERM...NORTHWEST FLOW PREVAILS ALOFT OVER WESTERN WASHINGTON
THIS EVENING...WITH A FRONTAL SYSTEM DIGGING SOUTH TOWARD THE AREA
THROUGH BRITISH COLUMBIA. SOUTHERLY OFFSHORE WINDS HAVE DEVELOPED
OVER THE FORECAST AREA...DUE TO THE APPROACH OF THE SURFACE LOW
ASSOCIATED WITH THE FRONTAL SYSTEM. TEMPERATURES ARE STILL BELOW
FREEZING ACROSS WESTERN WASHINGTON...WITH THE EXCEPTION OF THE COAST
...AND CLOUD COVER IS BEGINNING TO INCREASE AHEAD OF THE INCOMING
FRONT.

PRECIPITATION WILL FIRST REACH THE AREA AROUND MIDNIGHT AND SPREAD
SOUTHEAST ACROSS WESTERN WASHINGTON OVERNIGHT. BY MIDDAY WEDNESDAY
THE ENTIRE FORECAST AREA SHOULD BE RECEIVING PRECIPITATION.
PRECIPITATION WILL GRADUALLY TAPER OFF FROM NORTH TO SOUTH WEDNESDAY
NIGHT AS THE SYSTEM MOVES OFF TO THE SOUTHEAST. NORTHERLY WINDS WILL
DEVELOP BEHIND THE SYSTEM WEDNESDAY NIGHT AND THURSDAY AS ANOTHER
STRONG SURFACE HIGH BEGINS ITS MIGRATION SOUTHWARD FROM THE YUKON
INTO BRITISH COLUMBIA...AND A DEEP UPPER LOW -- AROUND 516 MB --
WILL MOVE INTO NORTHERN WASHINGTON ON THURSDAY.

IN THE REALM OF POTENTIAL SNOW EVENTS FOR THE WESTERN WASHINGTON
LOWLANDS...THIS IS A RARE CASE WHERE CONFIDENCE IS RATHER HIGH THAT
PRECIPITATION WILL FALL ALMOST ENTIRELY AS SNOW. THE ONLY PLACE THIS
IS REALLY IN DOUBT IS ALONG THE SOUTH PART OF OUR COASTAL ZONES AND
IN THE LOWER CHEHALIS VALLEY. THE CURRENT TEMPERATURE OF THE LOWER
ATMOSPHERE AND LOW DEW POINTS...AND THE TRACK OF THE SURFACE LOW
SOUTH ALONG THE COAST AND THEN SOUTHEAST ACROSS NEAR THE SOUTHERN
BORDER OF THE STATE FAVOR SNOW OVER RAIN.

THE 00Z WED SUITE OF MODEL SOLUTIONS COMING IN THIS EVENING
GENERALLY REINFORCES OUR THINKING UP TO THIS POINT. THEY ALL SHOW
THE LIGHTEST SNOWFALL AMOUNTS WILL OCCUR JUST EAST OF THE OLYMPIC
MOUNTAINS DUE TO SHADOWING -- IN OTHER WORDS THE CITY OF SEATTLE AND
MOST OF THE KITSAP PENINSULA AND HOOD CANAL AREA. STILL STORM TOTAL
SNOW ACCUMULATION OF 1 TO 5 INCHES IS A REASONABLE FORECAST FOR THIS
AREA...WITH MOST OF THE SNOW OCCURRING PROBABLY BETWEEN NOON AND
MIDNIGHT.

THE MODELS ALSO AGREE THAT THE HEAVIEST SNOW WILL FALL IN A PUGET
SOUND CONVERGENCE ZONE TYPE BAND THAT MORE OR LESS STAYS PUT OVER
SNOHOMISH COUNTY. THE NAM IS ESPECIALLY STRONG WITH THIS IDEA...WITH
SNOW BEGINNING THERE AROUND DAYBREAK WEDNESDAY MORNING AND
CONTINUING THROUGH WEDNESDAY NIGHT. IF IT IS CORRECT THEN PARTS OF
THE SNOHOMISH COUNTY LOWLANDS WILL REALLY GET HAMMERED...BUT 3 TO 8
INCHES IS A REASONABLE ESTIMATE FOR NOW ALL THINGS CONSIDERED. IT IS
ALWAYS POSSIBLE HOWEVER THAT THE CONVERGENCE ZONE WILL SHIFT SOUTH
INTO KING COUNTY WEDNESDAY EVENING ...RESULTING IN A MORE EVEN
DISTRIBUTION OF SNOWFALL IN THE PUGET SOUND AREA. THIS IS PROBABLY
THE TRICKIEST ASPECT OF THE TERRAIN INFLUENCED UNCERTAINTY IN THE
FORECAST.

ONE THING THAT IS VERY HIGH CONFIDENCE HERE IS THAT THE CASCADES AND
WEST SLOPES OF THE OLYMPICS WILL RECEIVE HEAVY SNOW...GENERALLY ONE
TO TWO FEET. ELSEWHERE STORM TOTAL ACCUMULATIONS IN THE RANGE OF 2
TO 8 INCHES ARE LIKELY. THE WINTER STORM WARNING AND WATCH MESSAGE
...WHICH I WILL UPDATE THIS EVENING WITH LITTLE CHANGE...HAS MORE
SPECIFIC RANGES BY REGION OF WESTERN WASHINGTON.

NOTE THAT THE WARNING IS IN EFFECT FOR THE REGIONS OF OUR FORECAST
AREA WHERE THE SNOWFALL WILL START SOONER AND WHERE CONFIDENCE IS
HIGHEST THAT WARNING CRITERIA -- 4 INCHES IN 12 HOURS OR 6 INCHES IN
24 HOURS -- WILL BE MET. THE WATCH WILL REMAIN IN EFFECT WHERE
CONFIDENCE IN ACCUMULATION SEEMS QUESTIONABLE -- I.E. SEATTLE AND
THE KITSAP PENINSULA...AND WHERE THE ACCUMULATING SNOW WILL BEGIN
LATER -- I.E. THE SOUTHWEST INTERIOR.

FRIDAY WILL BE COLD WITH A CLEARING TREND...AS NORTHERLY FLOW ALOFT
TAKES OVER AND THE SURFACE HIGH MOVING SOUTH THROUGH BRITISH
COLUMBIA PUSHES COLD AIR INTO THE REGION. MCDONNAL

.LONG TERM...HERE IS THE LONG TERM SECTION FROM THE AFTERNOON
ISSUANCE OF THE FORECAST DISCUSSION -- HAVE NOT SPENT A LOT OF TIME
ON THE LONG TERM. BOTH THE GFS AND EURO BRING THE NEXT SYSTEM
SATURDAY NIGHT. THEY ARE CONVERGING ON A COLD SOLUTION...IE MORE
SNOW. CONFIDENCE REMAINS LOW ON TIMING AND OTHER DETAILS BUT IT DOES
SEEM LIKE THE COLD PATTERN WILL REMAIN THROUGH THE WEEKEND. AT THIS
POINT IT IS NOT UNTIL DAY SEVEN...TUESDAY...THAT THE PRECIPITATION
TYPE MIGHT BE RAIN. BURKE

&&

.AVIATION...CLOUDS QUICK TO FORM THIS EVENING AS THE FLOW BEGINS TO
GO ONSHORE. THE OVERCAST WILL FILL IN OVER ALL OF WESTERN WA
OVERNIGHT AOB 030. AREAS OF LIGHT SNOW WILL DEVELOP COAST BEFORE
MIDNIGHT AND INLAND AROUND 12Z. THE NAM CONTINUES TO SHOW MUCH LESS
PRECIP OVER THE KITSAP PENINSULA...SHADOWED BY THE OLYMPICS. THE
HIGHEST AMOUNTS ARE OVER SNOHOMISH COUNTY. THIS MEANS A HIGHER VSBY
FOR KSEA/KBFI AND ONLY LIGHT PRECIP AMOUNTS AND LOWER VSBY MORE SNOW
AT KPAE AND CERTAINLY KBLI. KCLM WILL BE A TRICKY SPOT AS ALWAYS
WED. I THINK THERE IS A VERY GOOD CHANCE THAT KHQM SEES A VERY QUICK
SWITCH TO RAIN BY DAYBREAK...NOT LONG AFTER PRECIP DEVELOPS AND THAT
KOLM WILL SWITCH OVER TO RAIN OR A WET MIX MIDDAY. WITH HEAVIEST
PRECIP OVER SNOHOMISH COUNTY KPAE SHUD BE ONE OF THE SNOWIER TAFS ON
WED. 19

KSEA...MOS HAS DEW POINTS LOW ENUF FOR SNOW WED AND NAM 1000-850MB
THICKNESS SUGGESTS PRECIP WILL BE SNOW FOR MOST OF THE DAY. W/
THICKNESS PEAKING AT 1294M ARND 21Z THERE WILL PROBABLY BE ONLY A
FEW HOURS WHERE IT CHANGES TO WET SNOW OR A MIX...THE SNOW LEVEL
PROBABLY PEAKS ARND 500FT IN THE AFTERNOON. NAM TOTAL PRECIP DURING
THE DAY WED ARND KSEA IS QUITE LIGHT...PROBABLY LOOKING AT AN INCH
OF SNOW. THE ERROR BAR ON THAT FCST IS ALSO AN INCH...TRACE TO 2 IS
REASONABLE FCST. FOR WED NITE THE PRECIP SHOULD BECOME SHOWERY AND
1000-850MB THICKNESS DROPS BELOW 1280M BY MIDNITE...WITH COOLER
TEMPS...LESS PREDICTABLE SHOWERS...AND SNOW LEVEL BACK DOWN TO SEA
LEVEL WED NITE. 19

AN ASIDE...THE 500MB TEMP FALLS FROM ARND -25C 12Z WED TO -36C 12Z
THU AND AN AMAZING -40C 00Z FRI ON THE NAM MODEL. SOMEWHERE IN THERE
THE AIR MASS WILL BECOME UNSTABLE AND THE PRECIP IN THE NAM LATE WED
NITE MIGHT VERY WELL HAVE A TSTM AND LCLLY HEAVIER SHOWERS. EVEN
THRU THU MORNING THE MODEL HAS A SFC LOW SOMEWHERE OVER WRN
WA...WITH LIGHT CONVERGENCE AND VERY COLD AIR ALOFT. 19

&&

.MARINE...PRESSURES ARE FALLING RIGHT WHERE THEY SHOULD BE...IN AN
ARC ACROSS SRN B.C. AND ALONG THE B.C. COAST. PRES FALLING 1MB/HR
NRN VRILS AND B.C. COAST NE TO MCINNIS IS. A 1017MB LOW FORMS IN
THAT AREA AND PRES FALLS AS LOW DROPS SE SO THAT BY WED EVENING A
1001MB LOW SHUD BE NEAR THE STRAIT OF JUAN DE FUCA. THE FCST LOOKS
FINE SHOWING AN INCREASE IN WLY FLOW COAST AND STRAIT AND SLY FLOW
INLAND WATERS. I DONT EXPECT TO MAKE ANY SGFNT UPDATES THIS EVENING.
19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING LATE TONIGHT THROUGH THURSDAY MORNING FOR
     WESTERN WHATCOM COUNTY...SAN JUAN COUNTY...NORTHWEST INTERIOR
     ...STRAIT OF JUAN DE FUCA AREA...EVERETT AND VICINITY...EAST
     PUGET SOUND LOWLANDS...NORTH COAST...CASCADES...AND OLYMPICS.

     WINTER STORM WATCH LATE TONIGHT THROUGH THURSDAY FOR THE
     ADMIRALTY INLET AREA...SEATTLE AND VICINITY...TACOMA AND
     VICINITY...THE SOUTHWEST INTERIOR...HOOD CANAL AREA...LOWER
     CHEHALIS VALLEY...AND CENTRAL COAST.

PZ...SMALL CRAFT ADVISORY ALL WATERS EXCEPT PUGET SOUND.

     GALE WATCH FOR THE EAST ENTRANCE AND NORTH INLAND WATERS WED
     NIGHT.

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