High Surf Advisory 11, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST SUN DEC 16 2012

.SYNOPSIS...A STRONG PACIFIC STORM WILL RACE TOWARD THE AREA TODAY.
THIS SYSTEM WILL BRING THE POTENTIAL FOR DAMAGING WINDS ACROSS THE
LOWLANDS AND HEAVY SNOW TO THE MOUNTAINS TONIGHT AND MONDAY. LOOK
FOR SHOWERY WEATHER TO PREVAIL IN THE WAKE OF THE OCCLUDED FRONT
THAT WILL MOVE ACROSS THE REGION MONDAY MORNING. THE SNOW LEVELS
WILL ALSO FALL TO NEAR SEA LEVEL MONDAY NIGHT AS A COLDER AIR MASS
SETTLES OVER THE AREA. HIGHER PRESSURE ALOFT WILL SLOWLY BUILD IN
FROM THE WEST DURING THE DAY TUESDAY.

&&

.SHORT TERM...A STRONG PACIFIC STORM SYSTEM IS STILL ON TRACK TO
AFFECT WESTERN WASHINGTON WITH A VARIETY OF WEATHER HAZARDS OVER THE
NEXT COUPLE DAYS. WATER VAPOR SATELLITE IMAGERY THIS MORNING SHOWS A
WELL DEFINED DARK SLOT DEVELOPING WITH THIS SYSTEM OVER THE PAST FEW
HOURS. NORMALLY THIS INDICATES RAPID SURFACE LOW DEEPENING WHICH ALL
MODELS INDICATE SHOULD BE OCCURING THIS MORNING. THE INFERRED
SURFACE LOW IS ALMOST TO 135 W AT 16Z SO THE TIMING APPEARS SOMEWHAT
FASTER THAN WHAT MOST MODELS SHOWED YESTERDAY. THE 12Z RUNS ARE
COMING IN AND HAVE PICKED UP ON THIS SLIGHTLY FASTER TIMING...CLOSER
TO WHAT THE NAM-12 WAS ADVERTISING THE PAST COUPLE DAYS. AHEAD OF
THE LOW...SE WINDS WILL BEGIN TO INCREASE ACROSS PARTS OF THE AREA
LATER THIS AFTERNOON. NOT MENTIONED IN THE STATEMENTS BUT A
POSSIBILITY IS FOR A BURST OF E/SE WINDS ALONG THE CASCADE FOOTHILLS
EARLY THIS EVENING AS ELY CROSS CASCADE GRADIENTS INCREASE. WILL
EVALUATE THIS FURTHER TODAY. THE INITIAL WARM FRONT WILL MOVE ACROSS
THE COAST EARLY THIS EVENING BRINGING A WIND SHIFT FROM E/SE TO SLY
OVER THE COAST AND ALSO THE SW INTERIOR/PUGET SOUND. THERE MAY BE A
BURST OF STRONG WINDS AS THIS OCCURS BUT PROBABLY SUB-ADVISORY.

LATEST MODELS BRING A DEEPENING 970-975 MB SURFACE LOW ONTO THE
COAST NEAR LA PUSH CLOSE TO MIDNIGHT OR SHORTLY THEREAFTER. MOST
MODELS TAKE THE LOW ELY...THEN NELY ACROSS THE N CASCADES AS IT
SLOWLY FILLS. BY 18Z THE LOW SHOULD BE MOVING NE OF THE CASCADES.
THIS TRACK PRESENTS A TRICKY SITUATION SINCE WINDS ARE MOSTLY
TERRAIN DRIVEN OVER WRN WA. THIS LOW TRACK IS NOT A CLASSIC MAJOR
WIND STORM PATTERN...BUT A SURFACE LOW DEEPENING THIS MUCH AS IT
APPROACHES THE COAST WILL ALMOST CERTAINLY BRING DAMAGING HIGH WINDS
TO SOME AREAS. IN THIS CASE MOST THE AREA SHOULD AT LEAST EXPERIENCE
SOME HIGH-END WIND ADVISORY CONDITIONS. LOCATIONS FAVORED BY HIGH
WINDS...SUSTAINED 40+ MPH WITH GUSTS 60+...WILL BE THE CENTRAL/N
COAST...STRAIT OF JUAN DE FUCA...ADMIRALTY INLET...AND POSSIBLY THE
LOWER CHEHALIS AND SW INTERIOR CLOSER TO THE BENT BACK.  BOTH MM5
RUNS INDICATE THE BENT BACK WILL COME IN FURTHER S FROM THE OCEAN
SHORES/WESTPORT AREA INTO THE N OREGON COAST SWEEPING NE AFFECTING
SW WA. IT IS UNCLEAR HOW FAR N THE SW SURGE OF WIND BEHIND THE LOW
WILL GET. AT THIS POINT...WINDS FROM TACOMA NWD TO EVERETT LOOK MORE
IN THE ADVISORY CATEGORY WITH SW WINDS 30-40 MPH GUSTS 55 MPH. NO
CHANGES WILL BE MADE TO THE WIND WATCH/WARNING STATEMENT THIS
MORNING BUT WILL NEED TO MAKE CONVERSIONS TO WARNINGS/ADVISORIES
SOMETIME THIS AFTERNOON. WILL ALSO WANT TO SEE THE 12Z ECMWF BEFORE
MAKING ANY FINAL DECISIONS.

RAIN WILL QUICKLY SPREAD INTO THE AREA THIS AFTERNOON. IR IMAGERY
SHOWS CLOUD TOPS FINALLY SHOWING ENHANCEMENT AS THE LOW STRENGTHENS
OFFSHORE. SNOW LEVELS WILL REMAIN LOW THROUGH THE ENTIRE EVENT SO
ALL THE PASSES WILL GET SNOW ACCUMULATION. SNOW WILL INITIALLY BE
LIGHT OVER THE CASCADES WITH THE FLOW DIRECTION NOT YET FAVORABLE
FOR HEAVIER SNOW. THE OLYMPICS WILL BE THE FIRST TO SEE HEAVY SNOW
DEVELOP LATE TODAY BUT HEAVIER SNOW WILL NOT OCCUR IN THE CASCADES
UNTIL LATER THIS EVENING THROUGH LATE MONDAY NIGHT. WLY WINDS IN THE
PASSES WILL BECOME QUITE STRONG BEHIND THE LOW MONDAY
AFTERNOON/EVENING WITH GUSTS OVER 60 MPH POSSIBLE...CREATING
SIGNIFICANT BLOWING AND DRIFTING SNOW. VISIBILITY WILL BE POOR AS
WELL. 1.5 TO 3 FEET OF SNOW ARE STILL EXPECTED BY LATE MONDAY NIGHT.
WILL UPGRADE THE OLYMPICS FROM A WATCH TO A WARNING THIS MORNING AS
A COUPLE FEET ALONG THE S AND W FACING SLOPES APPEAR LIKELY.

STRONG ONSHORE FLOW WILL BRING SHOWERS TO THE LOWLANDS MONDAY NIGHT
AND TUESDAY. SNOW WILL GRADUALLY TAPER OFF IN THE MOUNTAINS BUT SOME
ADDITIONAL ACCUMULATION IS STILL EXPECTED. SNOW LEVELS WILL PLUMMET
TO 500 TO 1000 FEET. HEAVIER SHOWERS OR ANY CONVERGENCE ZONE
DEVELOPMENT COULD PUT THE SNOW LEVEL DOWN TO THE SURFACE AT TIMES.
LOCALIZED LOWLAND ACCUMULATIONS CANNOT BE RULED OUT BUT IT IS TOO
EARLY TO GUESS WHERE THE CONVERGENCE ZONE WILL SET UP...ALTHOUGH WRN
SNOHOMISH AND FAR N KING COUNTIES ARE OFTEN FAVORED. HIGH
TEMPERATURES ON TUESDAY WILL STRUGGLE OUT OF THE 30S.  MERCER

.LONG TERM...FROM 330 AM DISCUSSION...AN UNSETTLED WEATHER PATTERN
WILL PREVAIL OVER THE REGION DURING THIS PERIOD. CONFIDENCE IN THE
DETAILS WERE NOT VERY HIGH DUE TO SOME DISAGREEMENT IN THE MODELS AS
WELL AS SOME LACK OF CONTINUITY IN THE MODELS.

&&

.HYDROLOGY...FLOODING IS NOT EXPECTED ON AREA RIVERS THROUGH THE END
OF THE WEEK. MERCER

&&

.AVIATION...THE NEXT FRONTAL SYSTEM IS RACING TOWARD THE AREA. RAIN
IS ALREADY SPREADING ACROSS SW WA. THE DEEPENING SURFACE LOW IS JUST
OUTSIDE OF 130W THIS MORNING AND WILL REACH THE NORTH COAST AROUND
MIDNIGHT AND BE INTO THE FRASER RIVER VALLEY OF SOUTHWEST B.C.
BEFORE DAYBREAK. RAIN WILL INCREASE THIS EVENING WITH AREAS OF
SOUTHEAST GALES DEVELOPING IN THE USUAL SPOTS...A SIGNIFICANT PUSH
OF SOUTHWEST WINDS WILL OCCUR AS THE LOW MOVES THROUGH MONDAY
MORNING BUT THE MAIN FEATURE TO BRING STRONG WINDS IS THE BENT BACK
TROUGH/OCCLUSION WHICH SNAPS THE COAST AROUND 2AM IN THE UW WRFGFS.
19

KSEA...SHORT-LIVED IMPROVEMENT AS DEVELOPING OFFSHORE GRADIENTS
REMOVE SOME MOISTURE BUT RAIN REDEVELOPING BY EARLY AFTERNOON AND
CONDITIONS DROPPING THROUGH EVENING AS THE FRONTAL SYSTEM MOVES OVER
THE AREA. STRONG PRESSURE GRADIENTS WILL DEVELOP OVERNIGHT TONIGHT
AS A DEEP LOW TRACKS ACROSS NW WA. 19

&&

.MARINE...A STRONG FRONTAL SYSTEM WILL MOVE OVER WESTERN WA THROUGH
THIS EVENING. SOUTHEAST GALES WILL DEVELOP THIS AFTERNOON COASTAL
WATERS AND ENTRANCES..AND BY EVENING MOST OTHER AREAS. THE LATEST UW
WRFGFS HAS THE WIND SHIFT FROM SE GALES TO SW REACHING GRAYS HARBOR
AROUND 7PM TONIGHT AND THE SHIFT FROM SW TO NW WITH THE BENT BACK
TROUGH AROUND 2AM. THE EASTERLIES IN THE STRAIT PEAK THIS EVENING
AND THE MODEL SHOWS THE WESTERLIES THROUGH THE STRAIT BEFORE
DAYBREAK. A STRONG PUGET SOUND CONVERGENCE ZONE PATTERN WILL IN
PLACE DURING THE DAY MONDAY. 19

&&

.AVALANCHE...A FAST APPROACHING VERY STRONG STORM WILL CAUSE AN
SIGNIFICANTLY INCREASING AVALANCHE DANGER LATE SUNDAY THROUGH EARLY
MONDAY. VERY STRONG WINDS AND HEAVY SNOWFALL AT SLIGHTLY WARMING
TEMPERATURES WILL ADD SIGNIFICANT LOADS OVER THE 1-3 FEET OF LOW
DENSITY SNOW RECEIVED THIS PAST WEEK. A HIGH AVALANCHE DANGER SHOULD
DEVELOP ON MOST STEEP SLOPES BY LATER SUNDAY AND SUNDAY NIGHT
THEREFORE BACKCOUNTRY TRAVEL IN AVALANCHE TERRAIN IS NOT RECOMMENDED
LATER SUNDAY AFTERNOON THROUGH EARLY MONDAY. AN AVALANCHE WATCH IS
CURRENTLY IN EFFECT AND WILL BE UPGRADED TO A WARNING LATER SUNDAY.
PLEASE SEE NWAC.US FOR DETAILS. KRAMER

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER WEATHER ADVISORY IN EFFECT FOR THE OLYMPICS UNTIL 6 PM
     THIS EVENING.

     WINTER STORM WARNING IN EFFECT FOR THE CASCADES FROM 6 PM THIS
     EVENING TO 6 PM MONDAY.

     WINTER STORM WARNING FOR THE OLYMPICS FROM NOON PM PST
TODAY            THROUGH NOON MONDAY.

     HIGH WIND WARNING IN EFFECT FOR THE CENTRAL COAST FROM MIDNIGHT
     TONIGHT UNTIL 10 AM MONDAY.

     WINTER STORM WATCH IN EFFECT FOR THE OLYMPICS FROM THIS EVENING
     THROUGH MONDAY MORNING.

     HIGH WIND WATCH IN EFFECT FOR THE LOWLANDS...EXCEPT THE CENTRAL
     COAST...FROM LATE TONIGHT THROUGH MONDAY MORNING.

     COASTAL FLOOD WATCHES ARE IN EFFECT FOR THE COAST AND INTERIOR
     SHORELINES.

PZ...STORM WARNING COASTAL WATERS AND STRAIT OF JUAN DE FUCA.
     GALE WARNING ALL REMAINING WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH TO SEVERE BAR CONDITIONS GRAYS
     HARBOR BAR.

&&

WWW.WEATHER.GOV/SEATTLE

FOR THE GRAPHICAL AFD...VISIT
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML