Heavy Snow in Washington, December 8 to 10, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 PM PST WED DEC 8 2010

UPDATED HYDROLOGY SECTION TO ADD GREEN RIVER INFORMATION

.SYNOPSIS...WESTERN WASHINGTON WILL HAVE SHOWERY WEATHER THROUGH
THURSDAY NIGHT IN THE WAKE OF A FRONT THAT PASSED THIS MORNING. SNOW
WILL FALL IN THE MOUNTAINS...HEAVY AT TIMES. AFTER A RELATIVE BREAK
ON FRIDAY...RAINY AND MILDER WEATHER WILL DEVELOP OVER THE WEEKEND
AND CONTINUE INTO EARLY NEXT WEEK.

&&

.SHORT TERM...RADAR SHOWS PLENTY OF SHOWERS AROUND AND THERE ARE
STILL A FEW LIGHTNING STRIKES ON THE COAST. WILL KEEP SHOWERS IN THE
FORECAST THROUGH FRIDAY...WHICH IS WHEN THE UPPER TROUGH FINALLY
MOVES THROUGH THE AREA. THESE SHOWERS WILL BRING PERIODS OF HEAVY
SNOW TO THE MOUNTAINS AND THE WINTER STORM WARNING THERE WILL REMAIN
IN EFFECT. HIGH TEMPERATURES WILL BE 45-50 THURSDAY AND PROBABLY
CLOSER TO 45 FRIDAY. OVERNIGHT LOWS WILL BE IN THE 40S...THEN DIP
INTO THE UPPER 30S FRIDAY NIGHT AFTER THE TROUGH PASSES.

THE LONG AWAITED WEEKEND RAIN EVENT BEGINS SATURDAY. PROBABLY ONLY
SOUTHERN AREAS AND THE COAST WILL BE RAINING BY SATURDAY AFTERNOON
AS A BAROCLINIC BAND WORKS ITS WAY NORTH. SATURDAY NIGHT LOOKS WET
EVERYWHERE WITH HEAVY RAIN POSSIBLE IN THE MOUNTAINS. BURKE

.LONG TERM...SUNDAY WILL BE WET AND WARMER THAN AVERAGE WITH HIGHS
IN THE 50S. THE BAROCLINIC BAND IS FORECAST TO MOVE NORTH DURING THE
DAY WHICH COULD MEAN THE RAIN ENDS OVER SOUTHERN AREAS. HEAVY RAIN
WILL PROBABLY CONTINUE IN THE OLYMPICS AND NORTH CASCADES SUNDAY
NIGHT. A COLD FRONT WILL THEN DRIVE THE WHOLE SYSTEM SOUTHEAST AND
OUT OF THE AREA EITHER MONDAY OR MONDAY NIGHT.

AFTER THE COLD FRONT PASSES THERE IS SOME PRETTY COLD AIR OVER
WASHINGTON TUESDAY AND WEDNESDAY. TEMPERATURES WILL BE LOWER. HIGHS
WILL LIKELY NOT GET HIGHER THAN 40S WITH LOWS IN THE 30S. THERE IS
STILL SOME SOUTHERLY FLOW AT THE SURFACE THROUGH WEDNESDAY BUT LATER
PERIODS COULD HAVE SOME OFFSHORE FLOW. BURKE

&&

.HYDROLOGY...DATA FROM THE SKOKOMISH NEAR POTLATCH GAUGE SRPW1
STOPPED ARRIVING HERE FOR AWHILE BUT IT IS BEING MANUALLY UPDATED ON
THE USGS WEBSITE. THE RIVER IS STILL ABOVE FLOOD STAGE. OUR CURRENT
WARNING EXPIRES AROUND 8 PM TONIGHT AND MAY HAVE TO BE EXTENDED ON
THIS SLOW FALLING RIVER. THE SKOKOMISH IS LIKELY TO REMAIN ABOVE THE
ADVISORY STAGE OF 15.5 FEET FOR ANOTHER DAY OR SO.

A WARM AND WET RAIN EVENT IS ON TAP STARTING SATURDAY NIGHT AND
LASTING AT LEAST THROUGH SUNDAY. THE GFS AND EURO ARE IN BROAD
AGREEMENT FOR SATURDAY NIGHT. RAIN WILL DEVELOP OVER THE SOUTHWEST
LATE SATURDAY AND SPREAD NORTH SATURDAY NIGHT. RAIN COULD BE HEAVY
AT TIMES ON THE COAST AND OVER THE MOUNTAINS. THE SNOW LEVEL SOARS
TO 7000 FEET OR HIGHER BY SUNDAY.

BOTH MODELS THEN LIFT THE BAROCLINIC BAND NORTH INTO BRITISH
COLUMBIA ON SUNDAY. DIFFERENCES EMERGE IN THIS PERIOD...WITH THE GFS
KEEPING SOME HEAVY RAIN DRAPED OVER THE OLYMPICS AND NORTH CASCADES
AND THE EURO SENDING VIRTUALLY ALL HEAVY RAIN INTO BRITISH COLUMBIA.
A COLD FRONT THEN SWEEPS SOUTHEAST...ON MONDAY IN THE GFS AND ON
MONDAY NIGHT IN THE EURO. THERE WILL ALSO BE SOME HEAVY RAIN WHEN
THE COLD FRONT PASSES.

RAINFALL AMOUNTS DEPEND TO A CONSIDERABLE EXTENT ON THE DETAILS OF
WHERE THE BAROCLINIC BAND GOES AND HOW LONG IT STAYS THERE. IT DOES
SEEM LIKELY THAT THE WHOLE AREA WILL GET DOUSED SATURDAY NIGHT AND
THAT MOSTLY NORTHERN AREAS WILL GET SOAKED SATURDAY. THERE IS SOME
QUESTION ABOUT SUNDAY AS THE GFS KEEPS POUNDING THE OLYMPICS AND
NORTH CASCADES BUT THE EURO DOES NOT. HOW THIS TURNS OUT WILL
PROBABLY BE THE KEY TO WHETHER ONLY A FEW RIVERS IN THE OLYMPICS
FLOOD OR SEVERAL MORE IN THE NORTH CASCADES ALSO FLOOD.

CENTRAL CASCADES RIVERS DO NOT SEEM TO BE AT AS MUCH OF A RISK...AT
LEAST SOUTH OF THE SNOQUALMIE. HOWEVER THERE IS A REASONABLE AMOUNT
OF SPREAD BETWEEN DIFFERENT MODELS AND BETWEEN DIFFERENT RUNS OF THE
SAME MODELS THAT THIS SCENARIO IS NOT A SURE BET. IN ANY CASE IT IS
TIME TO START MAKING STATEMENTS. WILL PUT OUT A FLOOD POTENTIAL
OUTLOOK FOR BASICALLY EVERYWHERE...HIGHLIGHTING THE HIGHER THREAT
OVER THE OLYMPICS AND NORTH CASCADES.

THERE IS NOT A SIGNIFICANT THREAT OF FLOODING ON THE GREEN RIVER.
HOWEVER THIS STORM HAS THE POTENTIAL TO BE THE WETTEST SYSTEM TO
AFFECT THE REGION IN OVER A YEAR AND THE THREAT OF MINOR FLOODING
ON THE GREEN RIVER WHILE LOW IS NOT ZERO.

BURKE

&&

.AVIATION...SCATTERED SHOWERS THROUGH THURSDAY...ESPECIALLY OVER AND
NEAR THE MOUNTAINS. ONSHORE FLOW WITH SW FLOW ALF. BLUSTERY SURFACE
WINDS...BUT PRESSURE GRADIENTS GRADUALLY EASE THU AND FRI. THE AIR
MASS IS MOIST AND UNSTABLE ENOUGH FOR ISOLATED THUNDERSTORMS...BEST
CHANCE AT THE COAST.

KSEA...NO UPDATES TO THE FCST EXPECTED...SCATTERED SHOWERS THRU
THU...THERE MAY BE A TENDENCY FOR THE CLOUDS TO SPREAD OUT AND CIGS
LOWER TONIGHT WITH MORE SHOWERS...THEN BREAK UP AGAIN THU.

&&

.MARINE...THE AST-UIL GRADIENT PEAKED JUST UNDER +5MB THIS MORNING
BUT HAD EASED TO +3MB AT 2PM...SO WILL CHANGE THE COASTAL GALE
WARNING TO SMALL CRAFT ADVISORY. THE SLY WIND AT TATOOSH ISLAND HAS
BEEN RISING UP ARND 35KT AT TIMES...BUT BUOY41 HAS BEEN RUNNING
CLOSER TO 20KT...SO 20-30KT SEEMS LIKE A GOOD FCST COASTAL WATERS
TONIGHT. BUT STILL BLUSTERY...SMALL CRAFT ADVISORY ALL WATERS XCP
CENTRAL STRAIT. CONDITIONS WILL SLOWLY AND GRADUALLY SUBSIDE THRU
FRI...ALTHOUGH THE NAM GRABS A DISTURBANCE AND GENERATES A LITTLE
STRONGER GRADIENT THU EVENING...AND A 1010MB LOW INTO ASTORIA AT
DAYBREAK FRI...THE GFS IS MORE SUBTLE WITH THESE FEATURES BUT IT IS
POSSIBLE THAT IT WILL STAY BREEZY INTO FRIDAY AND THE SMALL CRAFT
ADVISORIES COULD STAY UP INTO FRI. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNING FOR THE SKOKOMISH RIVER.
    .WINTER STORM WARNING FOR THE NORTH AND CENTRAL WASHINGTON
     CASCADES UNTIL NOON FRIDAY.
    .WINTER WEATHER ADVISORY FOR THE OLYMPIC MOUNTAINS UNTIL
     NOON FRIDAY.
PZ...SMALL CRAFT ADVISORY ALL WATERS XCP CENTRAL STRAIT.
     SMALL CRAFT ADVISORY FOR ROUGH TO SEVERE GRAYS HARBOR BAR
     CONDITIONS.

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