Small Craft Advisory 654, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
905 AM PST SUN FEB 27 2011

.SYNOPSIS...ONSHORE FLOW WILL ALLOW TEMPERATURES TO SLOWLY MODERATE
TODAY. A COLD FRONT WILL MOVE THROUGH THE AREA TONIGHT WITH HEAVY
MOUNTAIN SNOW. A COOL UPPER TROUGH WILL LINGER OVER THE AREA EARLY
NEXT WEEK FOR A CONTINUED RAIN SNOW SHOWER THREAT FOR THE LOWLANDS.
A STRONGER AND WARMER WEATHER SYSTEM COULD IMPACT THE AREA AROUND
THE MIDDLE OF THE WEEK.

&&

.SHORT TERM...SATELLITE IMAGERY DOES A NICE JOB SHOWING OFF A COLD
FRONT THAT IS NOW MOVING SOUTH DOWN VANCOUVER ISLAND. IN ADVANCE OF
THE FRONT...WESTERLY ONSHORE FLOW CONTINUES TO SLOWLY WARM THE AIR
MASS. TEMPS ALONG THE COAST ARE NOW NEAR 40F...AND INTERIOR TEMPS
GENERALLY RANGE FROM 32F-37F. WITH TEMPS WARMING A FEW MORE DEGREES
TODAY...THE FEW REMAINING POCKETS OF LOWLAND SNOW WILL SWITCH OVER
TO RAIN. SO IT SHOULD BE VERY SAFE TO LET THE CURRENT ROUND OF
LOWLAND ADVISORIES EXPIRE AT 9 AM.

AS THE COLD FRONT MOVES IN FROM THE NORTHWEST LATER TODAY...FRONTAL
LIFT WILL ENHANCE PRECIP...PARTICULARLY OVER THE SRN HALF OF THE
FCST AREA. SNOW LEVELS WILL STILL BE DOWN IN THE 1000-2000 FOOT
RANGE AS THE FRONT MOVES IN...AND W-SW 850 MB FLOW OF 45-55 KT WILL
SUPERIMPOSE STRONG OROGRAPHIC LIFT ONTO THE FRONTAL LIFT. THE SETUP
WILL BRING A MAJOR POUNDING OF SNOW TO THE CASCADES AND CASCADE
PASSES FROM NOW INTO MONDAY MORNING. HEAVIEST SNOW AT SNOQUALMIE AND
WHITE PASSES WILL OCCUR FROM 4-10 PM THIS EVENING. BY MONDAY
MORNING...SNOWFALL SHOULD BE TAPERING OFF WITH A RELATIVE LULL IN
THE SNOWFALL ON MONDAY AFTERNOON.

BEHIND THE FRONT...A SHOWERY PATTERN WILL CONTINUE OVER THE
LOWLANDS. 850 MB TEMPS WILL DROP TO ARND -6C TO -7C BY MON MORNING
AND 1000-850 MB THICKNESS VALUES DROP DOWN TO MARGINAL LEVELS FOR
SNOW. WILL LIKELY BE A SITUATION WHERE HIGHER HILLS OVER THE
LOWLANDS COULD SEE SOME SLUSHY SNOW ACCUMULATIONS ON MON
MORNING...BUT TEMPS ARE NOT EXPECTED TO GET BELOW FREEZING...SO ANY
LOCALIZED ACCUMULATIONS SHOULD NOT STICK AROUND FOR LONG. THE SAME
STORY BASICALLY PERSISTS ON TUE MORNING WITH A SHOWERY PATTERN
CONTINUING AND MARGINALLY FAVORABLE THERMODYNAMICS FOR LOCALIZED
SLUSHY LOWLAND SNOW.        HANER

.LONG TERM...FROM PREVIOUS DISCUSSION...MODELS ARE IN BROAD
AGREEMENT THAT A STORM SYSTEM WILL AFFECT THE AREA AROUND WEDNESDAY.
HOWEVER...THE MODEL TO MODEL DETAILS ARE CURRENTLY ALL OVER THE
PLACE AND NO HIGH CONFIDENCE SOLUTION IS CURRENTLY AVAILABLE. THE
GENERAL TREND OVER THE LAST COUPLE DAYS SEEMS TO SUGGEST A SURFACE
LOW WILL TRACK OFFSHORE AROUND WEDNESDAY BRINGING WIND AND RAIN AND
A BRIEF WARM-UP. WILL HAVE TO WAIT UNTIL THE EVENT GETS CLOSER
BEFORE ANY MORE SPECIFIC DETAILS CAN BE NAILED DOWN. BEYOND
WEDNESDAY...CONFIDENCE IN THE DETAILS IS LOWER THAN NORMAL. FOR
NOW...WILL MAINTAIN A BROADBRUSH FAIRLY WET FORECAST. SCHNEIDER

&&

.HYDROLOGY...A COOL AND UNSETTLED PATTERN IS EXPECTED FOR THE NEXT
WEEK. SNOW LEVELS WILL AVERAGE BETWEEN 1000 AND 2000 FEET. ANY
WARM-UPS OR SPIKES IN THE SNOW LEVEL WILL BE BRIEF. THIS COOL
WEATHER WILL NOT FAVOR FLOODING ON AREA RIVERS. FLOODING IS NOT
EXPECTED ON THE GREEN RIVER FOR THE NEXT 10 DAYS. SCHNEIDER

&&

.AVIATION...MODERATE ONSHORE FLOW ACROSS WESTERN WA WILL RESULT IN
PERIODS OF RAIN TODAY...ESPECIALLY THE COAST AND NORTH PART. THE
CENTRAL PUGET SOUND AREA WILL SEE SHADOWING OFF THE OLYMPICS.
SURFACE TEMPS HAVE RISEN INTO THE MID TO UPPER 30S SO THE THREAT FOR
SNOW SHOWERS HAS PASSED. A COLD FRONT WILL MOVE INLAND TONIGHT
MAINTAINING SHOWERS ACROSS THE REGION. GUSTY SOUTH WINDS AT THE
SURFACE WILL EASE LATE TONIGHT. 33

KSEA...GUSTY SLY WINDS TODAY WITH MODERATE ONSHORE FLOW. WINDS
18015G25KT WITH CIGS IN MVFR RANGE. VC SHOWERS TODAY...WITH PRECIP
INCREASING TONIGHT AS A COLD FRONT MOVES INLAND. TEMPS OVERNIGHT
WILL REMAIN ABOVE FREEZING. 33

&&

.MARINE...SMALL CRAFT ADVISORIES REMAIN IN EFFECT FOR ALL MARINE
ZONES ACROSS W WA TODAY. MODERATE SOUTHERLY FLOW OCCURING ACROSS THE
REGION AS A COLD FRONT APPROACHES. THIS FRONT WILL PASS THROUGH W WA
TONIGHT MAINTAINING BLUSTERY CONDITIONS. THE ACCOMPANYING 987 MB
SURFACE LOW OVER HAIDA GWAII WILL WEAKEN AND SLOWLY DRIFT SOUTH
THROUGH TUE. MEANWHILE...COASTAL WATERS WILL SEE SWELLS OVER 10 FT.

A STRONGER SURFACE LOW WILL TRACK NORTH OFF THE WA COAST TUE NIGHT
OR WED. CONFIDENCE IS LOW AS TIMING OFF THE LATEST MODEL RUNS ARE
QUITE VARYING. THE 00Z ECMWF IS A GOOD 12 HOURS FASTER THAN THE NEW
12Z GFS. HIGH END GALES ARE MOST CERTAINLY POSSIBLE OVER THE COASTAL
WATERS BUT WILL TAKE A LOOK AT THE NEW 12Z EURO AND CANADIAN FOR THE
AFTERNOON PACKAGE. 33

&&

.AVALANCHE...A VERY STRONG FRONTAL SYSTEM SHOULD GRADUALLY SHIFT
OVER THE REGION FROM THE NORTHWEST LATER SUNDAY THROUGH SUNDAY
NIGHT. THIS SYSTEM IS EXPECTED TO CAUSE VERY STRONG CREST LEVEL
WINDS AND HEAVY SNOWFALL INTENSITIES LATER SUNDAY ALONG WITH A MINOR
WARMING TREND THROUGH SUNDAY EVENING. THE CURRENT SNOWPACK STRUCTURE
IS VERY WEAK DUE TO THE RECENT SIGNIFICANT ACCUMULATIONS OF LOW
DENSITY SNOW OVERLYING BURIED HOAR FROST OR FACETED SNOW. NUMEROUS
TRIGGERED AVALANCHE INCIDENTS AND CLOSE CALLS HAVE OCCURRED ALREADY
OVER THE PAST SEVERAL DAYS.

GIVEN THE CURRENT WEAK SNOWPACK STRUCTURE...AND THE ANTICIPATED
WEATHER... AN AVALANCHE WARNING IS NOW IN EFFECT FOR ALL THE
WASHINGTON CASCADES...THE OLYMPICS AND THE MT HOOD AREA. HIGH TO
EXTREME AVALANCHE DANGER IS EXPECTED BY LATER SUNDAY THROUGH SUNDAY
NIGHT. WE ARE RECOMMENDING THAT ALL AVALANCHE TERRAIN BE AVOIDED ON
SUNDAY. PLEASE VISIT WWW.NWAC.US FOR DETAILS. KRAMER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER STORM WARNING CASCADES.
     AVALANCHE WARNING CASCADES AND OLYMPICS.
     WINTER WEATHER ADVISORY FOR THE OLYMPICS.

PZ...SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

WWW.WEATHER.GOV/SEATTLE