Winter Storm in Washington, November 29 to 30, 2010

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 PM PST MON NOV 29 2010

.SYNOPSIS...A STRONG FRONTAL SYSTEM WILL BRING RAIN...WIND...AND
HEAVY MOUNTAIN SNOW THIS AFTERNOON THROUGH TUESDAY. A TROUGH WILL
LINGER OVER THE REGION FOR THE REMAINDER OF THE WEEK FOR A CHANCE OF
SHOWERS.

&&

.SHORT TERM...RADAR AND OBS INDICATE LIGHT RAIN ALONG THE COAST IS
VERY SLOWLY SPREADING INLAND THIS AFTERNOON AS EXPECTED. DRIER AIR
INLAND AT THE LOW LEVELS WILL TAKE A FEW MORE HOURS TO MOISTEN UP
BUT RAIN WILL EVENTUALLY DEVELOP ACROSS ALL OF WRN WA BY EARLY THIS
EVENING. SEE THE HYDROLOGY SECTION FOR DETAILS ON QPF.

SNOW LEVELS ARE CURRENTLY HOVERING AROUND 2000-2500 FEET BUT WILL
RISE TO 5000 FEET BEHIND A WARM FRONT AS STRONG SW FLOW DEVELOPS.
MODELS DELAY WARMING OVER THE CASCADES UNTIL LATE TONIGHT OR EARLY
TUE MORNING WITH ELY GRADIENTS KEEPING COLD AIR TRAPPED IN THE
PASSES. THIS STILL BODES WELL FOR A PERIOD OF FREEZING RAIN TUE
MORNING. SNOWFALL AMOUNTS MAY BE CONSIDERABLY LESS IN THE PASSES
WHICH MAY RECEIVE SNOW TONIGHT GOING OVER TO FREEZING RAIN OR A MIX
THEN BACK TO SNOW TUESDAY AFTERNOON. ABOVE 5000 FEET PRECIP SHOULD
REMAIN ALL SNOW WITH QPF SUPPORTING UP TO 2 FEET ALONG SLOPES
FAVORED BY SLY FLOW SUCH AS BAKER OR PARADISE. ELSEWHERE...PERHAPS 6
TO 12 INCHES AS IT WILL TAKE LONGER FOR SNOW TO BEGIN AND ALSO THE
FREEZING RAIN POTENTIAL IN THE PASSES. MODEL BUFFER SOUNDINGS
SINGLED OUT SNOQUALMIE AND STAMPEDE PASSES AS THE BEST CANDIDATES
FOR FREEZING RAIN TUE MORNING...MAYBE AROUND ONE QUARTER INCH ICE ON
TOP OF THE SNOW...BEFORE GOING BACK OVER TO SNOW BEHIND THE FRONT
TUE AFTERNOON. THE WINTER STORM STATEMENT WAS UPDATED EARLIER TO
ACCOUNT FOR THIS POSSIBILITY.

THIS SYSTEM WILL ALSO PACK SOME STRONG SLY/SE WINDS. MOST MODELS ARE
SHOWING 925 MB SLY WINDS UP TO 50-60 KT ALONG THE COAST AND N
INTERIOR. AS THE FRONT APPROACHES LATE TONIGHT THERE MAY BE ENOUGH
MIXING TO SURFACE SOME STRONGER GUSTS TO THE SURFACE. ALONG THE
COAST...E/SE GRADIENTS WILL LIMIT THE SUSTAINED WINDS BUT GUSTS IN
THE 55 MPH RANGE SEEM REASONABLE GIVEN THE VERY STRONG FLOW JUST OFF
THE SURFACE. THE MM5 MODELS APPEAR TO AGREE WITH THE AREA OF
SUSTAINED 35+ KT WINDS WELL OFFSHORE OVER THE COASTAL WATERS. SE
GRADIENTS AND THE FORMATION OF A MESO LOW IN THE LEE OF THE OLYMPICS
ARE FAVORABLE FOR STRONGER WINDS FROM THE ADMIRALTY INLET/WHIDBEY
ISLAND AREAS NWD. MESO MODELS ALSO SHOW MAX WINDS OF 35 KT OVER THE
WATERS IN THIS AREA WHICH SUPPORT AROUND 20-30 MPH SUSTAINED WITH
HIGHER GUSTS OVER LAND. THE WIND ADVISORY REMAINS IN EFFECT FOR THE
N INTERIOR UNTIL THE FRONT PASSES TUESDAY MORNING. THE STRONGEST
WINDS ACROSS THE PUGET SOUND REGION WILL OCCUR LATER...JUST ALONG
AND BEHIND THE FRONT EARLY TUESDAY MORNING. LOOKS JUST BELOW
ADVISORY CRITERIA BUT WILL INCLUDE SOME GUSTS TO AROUND 40 MPH.
WINDS WILL EASE ACROSS THE ENTIRE ARE TUESDAY AFTERNOON AS THE FRONT
MOVES EAST.

POST FRONTAL WLY FLOW WILL WEAKEN TUESDAY NIGHT ALLOWING SNOW TO
TAPER OFF IN THE MOUNTAINS. A FEW MORE INCHES OF SNOW ARE POSSIBLE
ABOVE 3000 FEET AS SNOW LEVELS COME BACK DOWN BEHIND THE FRONT. JUST
RAIN SHOWERS FOR THE LOWLANDS.

MODELS INDICATE THAT THE UPPER FLOW WILL BECOME SOMEWHAT SPLIT
ACROSS THE WEST COAST ON WEDNESDAY WITH AN UPPER LOW OVER THE
CENTRAL B.C. COAST AND A WAVE DEVELOPING OFF THE CA COAST. WRN WA IS
STILL UNDER WLY ONSHORE FLOW AND SOMEWHAT MOIST FLOW SO THE CHANCE
OF RAIN WILL REMAIN IN THE FORECAST...THOUGH A MOSTLY DRY DAY IS NOT
OUT OF THE QUESTION.

.LONG TERM...BOTH THE GFS/ECMWF SEEM TO BE HAVING DIFFICULTY
RESOLVING DETAILS DUE TO THE MESSY SPLIT FLOW OVER THE AREA. MODELS
SEEM TO BE TRYING TO CONSOLIDATE THE FLOW SOMEWHAT WITH SOME RUNS
LIFTING SOME OF THE MOISTURE ASSOCIATED WITH THE WAVE OVER CA NWD
TOWARD OUR AREA. A TROUGH WILL BE MOVING THROUGH ON THURSDAY AS
WELL. EITHER WAY...RAIN FROM THE WAVE OR JUST SHOWERS UNDER A
TROUGH...KEPT LIKELY POPS IN FOR THE AREA AS SOME PRECIP IS
EXPECTED. POPS WERE TRENDED BACK DOWN TO CHANCE CATEGORY FOR FRIDAY
THROUGH THE WEEKEND AS THE GFS/ECMWF HAVE BOTH TRENDED TOWARD MORE
RIDGING OVER THE REGION. NOT READY TO TRY PULLING POPS OUT ALL
TOGETHER GIVEN CLIMO AND BEING THE EXTENDED PORTION OF THE FORECAST.
WILL NEED TO WATCH HOW MODELS TREND AND CONSIDER DRYING THINGS OUT
IF MODELS CONTINUE TO SHOW THIS SOLUTION. THIS RIDGING IS IN
RESPONSE TO A BROAD DEEP TROUGH DEVELOPING OVER THE ERN PAC NEAR 150
W. LONG RANGE MODELS INDICATE THIS WILL MIGRATE EWD TO RENEW PRECIP
OVER THE AREA SOMETIME EARLY NEXT WEEK.

MERCER

&&

.HYDROLOGY...VERY LITTLE CHANGE IN THE QPF EXPECTED WITH THE FRONTAL
SYSTEM TO AFFECT THE REGION TONIGHT THROUGH TUESDAY. FOR MOST OF WRN
WA THE LOW SNOW LEVELS DURING PART OF THIS EVENING WILL PRECLUDE
FLOODING ON AREA RIVERS. THE ONE EXCEPTION PER USUAL IS THE
SKOKOMISH RIVER WHERE SNOW LEVELS WILL RISE MOST THE BASIN AHEAD OF
THE HEAVIEST PRECIP TONIGHT AND EARLY TUESDAY. IN ADDITION...HEAVIER
RAINFALL IN THIS BASIN IS ALSO HEAVILY FAVORED BY STRONG S/SW FLOW
ALOFT. WHILE THE FRONT IS PROGRESSIVE IN NATURE...MODELS STILL PUT
DOWN A MAX OF ABOUT 2 INCHES IN 12 HOURS IN THE BASIN. LOCAL STUDIES
AS WELL AND THE LATEST HYDRO MODEL RUNS STILL INDICATE THE SKOKOMISH
RIVER WILL PEAK BELOW FLOOD STAGE BUT WITHIN ADVISORY CRITERIA. THIS
SEEMS REASONABLE BUT WILL NEED TO MONITOR THE RIVER CLOSELY TONIGHT
IN CASE THE FRONT SLOWS OR RAINFALL IS HEAVIER THAN EXPECTED. SEE
THE LATEST HYDRO STATEMENT FOR DETAILS. NO FLOODING IS EXPECTED ON
THE GREEN RIVER WITH THIS SYSTEM.

MODELS INDICATE GENERALLY WEAK WEATHER SYSTEMS LATER THIS WEEK AND
THROUGH THE WEEKEND. PERHAPS EVEN A DAY OR TWO OF DRY WEATHER.
EXTENDED MODEL RUNS SUGGEST A BROAD TOUGH OVER THE ERN PAC SHOULD
BEGIN TO EDGE EWD BY EARLY TO THE MIDDLE OF NEXT WEEK. THIS COULD
OPEN THE DOOR TO SOME WETTER FRONTS. FLOODING IS NOT EXPECTED ON THE
GREEN RIVER OVER THE NEXT 7 DAYS.

&&

.AVIATION...AN UPPER TROF AND VIGOROUS FRONTAL SYSTEM ARE JUST
OFFSHORE THE PACNW. THE WARM FRONT IS RIGHT OFF THE COAST AT 2PM AND
THE COLD FRONT WAS OUT ARND 130W BUT FOR ALL INTENTS AND PURPOSES
THIS ROLLS THRU WRN WA AS STRONG OCCLUSION AND I WOULDN'T EXPECT
CONDITIONS TO DO ANYTHING BUT DETERIORATE TIL FROPA LATER TUE
MORNING. SO...LOWERING CIGS TO MVFR...LCL IFR CIGS POSSIBLE LATE
TONIGHT TIL FROPA...AREAS OF LOWER VSBY IN THE PCPN...WHICH IS SNOW
NOT FAR OFF THE SURFACE...AMDAR SOUNDINGS SHOW THE FREEZING LEVEL
EARLY THIS AFTERNOON WAS STILL 2500FT...FOR ABOUT A 1500FT SNOW
LEVEL BUT THE AIR MASS WILL WARM OVERNIGHT AND 850MB TEMPS SHOULD
PEAK ABOVE FREEZING WITH HEAVY WET SNOW OR RAIN UP AT THAT LEVEL AT
DAYBREAK. MTNS AND HILLS WILL BE OBSCURED TNGT INTO TUE. NO BIG
CHANGES TO THE TAFS. 19

KSEA...TAF STILL LOOKS GOOD...THE SLY WIND IN THE LOWER TO MID
LEVELS WILL BE FAIRLY STRONG LATER TNGT TIL FROPA ARND TUE
MORNING...2000FT WIND SHOULD BE S 40KT JUST TUE MORNING UP TIL THE
FROPA ARND 19Z.

&&

.MARINE...GALE WARNINGS CONTINUE ALL WATERS XCP CENTRAL STRAIT AND
PUGET SOUND/HOOD CANAL WITH ADVISORIES. FCST SHOULD BE ABOUT THE
SAME AS THE MORNING PACKAGE AND I HAVE NO REASON TO DOUBT THE
MESOSCALE MODELS WHICH BRING A FULL GALE INTO ALL THE REGULAR SPOTS
THIS EVENING. WINDS WILL SETTLE DOWN AFTER THE FROPA QUICKLY TUE. I
FOLLOWED THE PREVIOUS SHIFT AND KEPT COMBINED SEAS TONIGHT SINCE WE
WERE STARTING OUT WITH SUCH LITTLE SWELL...BUT LATER TUE OR EARLY
WED A WLY SWELL SHOULD BUILD AND WORK INTO THE COASTAL WATERS. THE
GRAYS HARBOR BAR FORECAST WILL NEED A SMALL CRAFT AS THE SWELL
BUILDS...BUT WITH A GALE BLOWING TIMING THE SEAS COMING UP IS NOT
YET A PRIORITY...BUT CERTAINLY BY TUE NITE THE SWELL OUGHT TO BE
CLIMBING TO ARND 14FT. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD PRODUCTS IN EFFECT FOR MASON COUNTY.
    .WINTER STORM WARNING FOR THE OLYMPICS AND CASCADES.
    .WIND ADVISORY FOR THE NORTHWEST INTERIOR.
PZ...GALE WARNING COASTAL WATERS..ENTRANCES TO THE STRAIT OF JUAN DE
     FUCA...ADMIRALTY INLET...AND NORTHERN INLAND WATERS.
    .SMALL CRAFT ADVISORY FOR THE REMAINING WATERS.

&&

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.