Heavy Snow in Washington, November 17 to 18, 2011

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
330 AM PST THU NOV 17 2011...CORRECTED DATE/TIME

.SYNOPSIS...ONSHORE FLOW WILL KEEP SHOWERS GOING ACROSS THE LOWLANDS
TODAY WITH SNOW IN THE MOUNTAINS. A COLD UPPER TROUGH WILL SETTLE
OVER WESTERN WASHINGTON FRIDAY AND FRIDAY NIGHT WITH SHOWERS AND LOW
SNOW LEVELS. DRIER WEATHER IS LIKELY BY SATURDAY AS THE TROUGH MOVES
OUT OF THE AREA BUT TEMPERATURES WILL BE WELL BELOW NORMAL. A WEAK
DISTURBANCE COULD BRING SOME RAIN ON SUNDAY. EARLY NEXT WEEK...A
SLOW-MOVING FRONTAL BOUNDARY COULD BRING A RETURN TO WINDY AND WET
WEATHER...ESPECIALLY ON TUESDAY.

&&

.SHORT TERM...THE FRONT MOVED THROUGH THE CASCADES JUST BEFORE
MIDNIGHT WITH COLDER AIR STARTING TO USHER BACK INTO THE AREA.
WARMER AIR ALOFT MANAGED TO REACH THE CASCADES AND LIMIT SNOW
AMOUNTS IN THE PASSES WITH THE SNOW LEVELS RUNNING AS HIGH AS 6000
FEET DURING THE AFTERNOON AND EVENING HOURS YESTERDAY. SNOW AMOUNTS
AT THE HIGHER ELEVATIONS PROBABLY WILL REACH CLOSE TO TWO FEET BY
DAYBREAK...WITH MUCH LESSER AMOUNTS IN THE PASSES. THE WLY FLOW IS
QUITE STRONG BEHIND THE FRONT WITH SOME CONVERGENCE ALSO NOTED OVER
SNOHOMISH COUNTY WHICH COULD ENHANCE SNOW IN STEVENS PASS EARLY THIS
MORNING. WITH SNOW INCREASING THE PAST COUPLE HOURS...WILL KEEP THE
WARNING GOING THROUGH 6 AM PST WITH MAY A FOOT POSSIBLE IN A COUPLE
OF THE PASSES...ESPECIALLY STEVENS. SNOW LEVELS WILL FALL THROUGH
THE MORNING HOURS TO AT LEAST 2500 FEET. MODELS STILL SHOW SOME
MODERATE 15-20 KT WLY FLOW AT 850 AS COLDER AIR ALOFT MOVES IN. THIS
SHOULD CONTINUE TO PRODUCE SNOW IN THE MOUNTAINS...ESPECIALLY
THROUGH THE MORNING HOURS BEFORE THE FLOW STARTS TO EASE UP. DECIDED
TO ISSUE A WINTER WEATHER ADVISORY FROM 6 AM PST THROUGH 6 PM PST TO
COVER THE ADDITIONAL SNOW. 4 TO 8 INCHES OF ADDITIONAL ACCUMULATION
SEEMS REASONABLE IN THIS PATTERN...WITH THE HEAVIEST FALLING THROUGH
THE MORNING.

WLY WINDS ARE STILL SCREAMING OUT OF THE STRAIT WITH WHIDBEY ISLAND
BEARING THE BRUNT WITH 30-35 SUSTAINED WINDS GUSTING TO OVER 40 MPH
THE LAST COUPLE HOURS. THERE WAS ALSO A MESOWEST OB AT SANDY POINT
SHOWING A SUSTAINED WIND AT 30 MPH SO THE WIND ADVISORY FOR THE WLY
SURGE PORTION OF THE EVENT SEEMS TO HAVE WORKED OUT. WINDS SHOULD BE
EASING OVER THE NEXT COUPLE HOURS AND WILL HAVE TO WAIT UNTIL 4 AM
TO SEE IF THE ADVISORY CAN BE DROPPED OR WILL NEED TO BE EXTENDED
JUST A BIT LONGER. MOST LIKELY WINDS WILL HAVE EASED ENOUGH TO LET
IT GO.

SHOWERS WILL CONTINUE ACROSS THE LOWLANDS THROUGH TONIGHT AS A COLD
UPPER LEVEL TROUGH ALONG THE B.C. COAST MOVES SWD TOWARD WA. BY
EARLY FRIDAY MORNING THE UPPER LEVEL LOW CENTER WILL SETTLE OVER
CENTRAL WRN WA WITH THE 500MB -38C COLD POOL OVER THE AREA. WHILE
THE MID AND UPPER LEVELS COOL OFF DURING THE DAY FRIDAY...THE LOW
LEVELS REMAIN MARGINALLY COLD ENOUGH SO THAT RAIN SHOWERS ARE
LIKELY TO JUST MIX WITH SNOW SHOWERS AT TIMES. THE ONE AREA TO WATCH
A BIT CLOSER WILL BE THE N INTERIOR FROM EVERETT N TO BELLINGHAM AND
ALSO WWD ALONG THE CENTRAL AND E STRAIT. AS THE DAY
PROGRESSES...THERE COULD BE SOME COLD AIR LEAKAGE FROM THE FRAZER
AND POSSIBLY SOME NORTHERLIES DEVELOPING. THERE IS A SLIGHT
POSSIBILITY OF A LIGHT SPOTTY ACCUMULATION IF ANY CONVERGENCE
ZONE FORMS DURING THIS PERIOD AND ALSO ALONG THE STRAIT IF SOME
BRIEF UPSLOPE DEVELOPS. AGAIN...TEMPERATURES WILL BE MARGINAL AND
ANY SHOWERS CONTAINING SNOW WILL LIKELY ONLY PRODUCE A DUSTING SNOW
BEFORE MELTING AWAY. ONLY THE GFS MODEL TRIES TO BRING A CONVERGENCE
BAND AS FAR SOUTH AS N SEATTLE BY EARLY FRIDAY EVENING BEFORE
DISSIPATING. WHILE UNLIKELY...IF THIS WERE TO DEVELOP THERE COULD
ALSO BE A DUSTING OF SNOW HERE AS WELL. THE MOST LIKELY OUTCOME IS
THAT MOISTURE WILL MOVE S OF THE AREA BEFORE COLDER AIR ARRIVES.
WILL ISSUE ANOTHER SPECIAL WEATHER STATEMENT TO ADDRESS THIS
CHANGING PATTERN.

AS THE AREA DRIES OUT FRIDAY NIGHT...COLDER TEMPERATURES CAN BE
EXPECTED THROUGH MOST OF THE WEEKEND. WIDESPREAD LOW TEMPERATURES IN
THE 20S TO AROUND 30 DEGREES ARE LIKELY BOTH SATURDAY MORNING AND
SUNDAY MORNING WITH DAYTIME HIGHS IN THE UPPER 30S...POSSIBLY
REACHING TO AROUND 40.

.LONG TERM...MODELS ARE QUICKER WITH A WEAK SYSTEM TO POSSIBLY BRUSH
THE AREA ON SUNDAY. DEPENDING ON THE TIMING OF PRECIP...SOME AREAS
ACROSS THE N COULD START OFF BRIEFLY AS SNOW...ESPECIALLY THE
BELLINGHAM AREA. THE COLD AIR WILL MIX OUT BY SUNDAY NIGHT AS A
STRONGER FRONTAL SYSTEM BEGINS TO INDUCE STRONGER SE WINDS OVER THE
AREA. THE GFS IS FASTER WITH THIS SYSTEM BRINGING RAIN INTO WRN WA
LATE SUNDAY NIGHT AND MONDAY....CONTINUING AT TIMES THROUGH
WEDNESDAY. THE ECMWF IS SLOWER WITH THE ONSET OF RAIN DURING THE DAY
MONDAY WITH THE HEAVIER RAIN DEVELOPING TUESDAY AND PERSISTING
THROUGH AT LEAST MID WEEK AS WELL. SEE THE HYDRO SECTION BELOW FOR
FURTHER DETAILS REGARDING RIVERS. TEMPERATURES WILL BE MORE MILD
NEXT WEEK...LIKELY NEAR OR SLIGHTLY ABOVE AVERAGE.  MERCER

&&

.HYDROLOGY...COLDER TEMPERATURES WILL MAINTAIN LOW SNOW LEVELS
THROUGH THE WEEKEND PRECLUDING ANY THREAT OF RIVER FLOODING. BY
EARLY NEXT WEEK...TEMPERATURES BEGIN TO WARM UP AS A MOIST FRONTAL
SYSTEM APPROACHES.

THE GFS MODEL STILL HANGS UP A WET FRONT OVER THE OLYMPIC PENINSULA
ON MONDAY WITH HEAVIER RAINFALL ALSO SPREADING INTO THE NORTHERN
CASCADES. ACCORDING TO THE ECMWF...THE HEAVIER RAINFALL DOES NOT
REACH THE OLYMPICS UNTIL MONDAY NIGHT AND TUESDAY BUT ALSO SPREADS
HEAVIER RAIN INTO THE NORTH CASCADES. WITH BOTH MODELS IN GENERAL
AGREEMENT EXCEPT SOME TIMING ISSUES...THERE IS A GOOD CHANCE OF
FLOODING EARLY NEXT WEEK ON THE SKOKOMISH RIVER IN THIS PATTERN. IF
THE WETTER MODEL RUNS PAN OUT...FLOODING COULD ALSO BE POSSIBLE ON
OTHER OLYMPIC PENINSULA RIVERS...AND PERHAPS EVEN RIVERS FLOWING OUT
OF THE NORTH CASCADES BY TUESDAY. CONTINUE TO MONITOR FORECASTS.
MERCER

&&

.AVIATION...AN UPPER LEVEL LOW NEAR THE CENTRAL B.C. COAST WILL
MAINTAIN STRONG MOIST WESTERLY FLOW ALOFT ACROSS W WA THROUGH TODAY.
STRONG ONSHORE FLOW WILL MAINTAIN GUSTY SOUTHWEST WINDS THROUGH 18Z
WITH WIND SLOWLY EASING AFTER THAT. WITH THE MOIST LOW LEVELS AND
AREAS OF SHOWERS...CIGS WILL SHOW CONSIDERABLE VARIABILITY AND RANGE
FROM MVFR...ESPECIALLY AROUND SHOWERS...TO VFR.

KSEA...GUSTY S-SW SURFACE WINDS 15-20KT GUSTING 25-30KT WILL
CONTINUE THROUGH 18Z...GRADUALLY DECREASING THROUGH 00Z. OTHERWISE
CIGS WILL SHOW CONSIDERABLE VARIABILITY TODAY AS SHOWERS MOVE ACROSS
THE AREA...RANGING FROM BKN020 TO BKN070. KAM

&&

.MARINE...ONSHORE FLOW BECAME STRONG BEHIND LAST NIGHTS COLD
FRONT...BUT WILL BE GRADUALLY EASING THROUGH THIS MORNING AND
AFTERNOON. WILL CONTINUE THE GALES FOR THE CENTRAL AND EAST STRAIT
THROUGH 18Z AND MAINTAIN SCA THROUGH 21Z FOR THE REMAINDER OF THE
INLAND WATERS...EXCEPT PUGET SOUND THROUGH 00Z.

COASTAL WINDS WILL REMAIN IN THE SCA RANGE TODAY POSSIBLY EXTENDING
INTO THE EVENING. OTHERWISE W SWELL WILL BUILD TO 19-22 FEET OVER
THE OUTER WATERS AND TO NEAR 19 FEET OVER THE NEARSHORE WATERS TODAY
THROUGH TONIGHT. THE 19 FEET OVER THE NEARSHORE WATERS IS JUST BELOW
HIGH SURF ADVISORY CRITERIA FOR THE COAST SO THIS WILL BE MONITORED.

PRESSURE GRADIENTS BECOME RATHER WEAK AND OFFSHORE ON FRIDAY AS A
SURFACE LOW MOVES SE ACROSS THE COASTAL/OFFSHORE WATERS.

ANOTHER SURFACE LOW MOVES SE ABOUT 300 NM OFFSHORE ON SATURDAY...BUT
WILL HAVE ONLY LIMITED IMPACT ON THE AREA. KAM

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WARNING IN EFFECT UNTIL 6 AM PST THIS MORNING FOR
     THE NORTH AND CENTRAL CASCADES.
    .WINTER WEATHER ADVISORY IN EFFECT FROM 6 AM PST THROUGH 6 PM
     PST TODAY FOR THE NORTH AND CENTRAL CASCADES.
PZ...GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA.
     SMALL CRAFT ADVISORY REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR SEVERE GRAYS HARBOR BAR CONDITIONS.

YOU CAN SEE AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION AT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.