Small Craft Advisory 666, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT SUN MAR 13 2011

.SYNOPSIS...A VIGOROUS PACIFIC FRONTAL SYSTEM WILL MOVE THROUGH
WESTERN WASHINGTON TODAY THROUGH MONDAY MORNING...BRINGING WINDY
CONDITIONS AND MORE RAIN AND MOUNTAIN SNOW. ANOTHER SYSTEM WILL
FOLLOW CLOSELY ON ITS HEELS...AFFECTING THE AREA MONDAY NIGHT
THROUGH TUESDAY NIGHT. A COOL SHOWERY PATTERN WILL PREVAIL WEDNESDAY
THROUGH FRIDAY...BUT IT WILL BE A RELATIVE BREAK IN THE ACTION.

&&

.SHORT TERM...A LARGE VIGOROUS PACIFIC FRONTAL SYSTEM IS POISED OVER
THE OFFSHORE WATERS EARLY THIS MORNING. IN THE MEANTIME WESTERN
WASHINGTON IS MOSTLY CLOUDY BUT GENERALLY FREE OF PRECIPITATION...
WITH PATCHY FOG OVER THE SOUTH PUGET SOUND REGION AND IN THE
SOUTHWEST INTERIOR. TEMPERATURES AT 2 AM RANGED FROM THE MID 30S TO
THE MID 40S.

THE SYSTEM JUST OFFSHORE IS ASSOCIATED WITH A DEEP VERTICALLY
STACKED LOW NEAR 46N/140W. THE WARM FRONT...WHICH CURRENTLY EXTENDS
FROM THE WASHINGTON OFFSHORE WATERS EAST-SOUTHEAST TO THE OREGON
COAST...WILL LIFT NORTHWARD ACROSS THE FORECAST AREA THIS MORNING
THROUGH EARLY AFTERNOON. THE COLD FRONT...WHICH WILL PROBABLY BE IN
THE PROCESS OF OCCLUDING...WILL THEN SWEEP ACROSS WESTERN WASHINGTON
FROM THE WEST LATE THIS AFTERNOON AND THIS EVENING. FINALLY A FAIRLY
STRONG POST-FRONTAL TROUGH -- OR IT MIGHT DEVELOP INTO A BENT BACK
OCCLUSION -- WILL MOVE QUICKLY THROUGH LATE TONIGHT.

THIS SYSTEM WILL BRING WIND -- ESPECIALLY AT THE COAST AND IN THE
NORTHWEST INTERIOR...RAIN -- HEAVY AT TIMES TODAY OVER THE COAST AND
OLYMPIC MOUNTAINS...AND MOUNTAIN SNOW.

IT LOOKS LIKE ONLY THE COASTAL ZONES FACE A THREAT OF HIGH WIND...BY
DEFINITION SUSTAINED WINDS 40 MPH OR GUSTS 60 MPH. THE FIRST THREAT
WILL BE WITH THE LANDFALL OF THE COLD OR OCCLUDED FRONT THIS
AFTERNOON...WHICH WILL BE ACCOMPANIED BY A SOMEWHAT NEGATIVELY
TILTED 120 KT JET. ANOTHER THREAT WILL COME WITH THE PASSAGE OF THE
POST-FRONTAL TROUGH OR BENT BACK OCCLUSION LATE TONIGHT. EACH OF
THESE THREATS LOOKS RELATIVELY MARGINAL...BUT I WILL GO WITH A
LOW-END HIGH WIND WARNING FOR THIS AFTERNOON THROUGH TONIGHT. THE
NORTHWEST INTERIOR WILL LIKELY HAVE WIND ADVISORY WINDS...AND THE
REMAINDER OF THE LOWLANDS WILL HAVE MORE GENERALLY WINDY CONDITIONS.

THERE WILL BE LOTS OF PRECIPITATION TODAY THROUGH MONDAY...WITH THE
HEAVIEST OCCURRING THROUGH THIS EVENING AS THE WARM AND COLD FRONTS
MOVE THROUGH. THE OLYMPIC MOUNTAINS WILL GENERALLY RECEIVE 2 TO 6
INCHES DURING THE 36 HOUR PERIOD...WITH 1 TO 3 INCHES OVER THE
CASCADES. FOR THE LOWLANDS THE COAST WILL GET 1 TO 3 INCHES OF
RAIN...AND 0.5 INCH TO 2 INCHES OVER THE INTERIOR LOWLANDS. THE
RAINFALL ALONG THE SOUTH SLOPES OF THE OLYMPICS WILL PROBABLY BE
SUFFICIENT TO DRIVE THE SKOKOMISH RIVER ABOVE FLOOD STAGE...WHICH IS
DISCUSSED FURTHER IN THE HYDROLOGY SECTION BELOW.

HEAVY SNOW...BY DEFINITION 12 INCHES IN 12 HOURS OR 18 INCHES IN 24
HOURS...IS A GOOD BET FOR THE HIGHER SPOTS IN THE CASCADES AND THE
OLYMPICS. THE HEAVY SNOW THREAT IS MAINLY FOR THE HIGHER SPOTS
EXPOSED TO SOUTHERLY OR SOUTHWEST FLOW...AND THAT MEANS PRIMARILY MT
BAKER AND PARADISE AT MT RAINIER. PRECIPITATION WILL BE LESS AT
STEVENS PASS AND SNOQUALMIE PASS...AND THE SNOW LEVEL IN THE
CASCADES WILL BE IN THE 3500 TO 4500 FT RANGE TODAY AS THE HEAVIEST
PRECIPITATION OCCURS. A WINTER STORM WATCH IS ALREADY IN EFFECT...
AND IT WILL BE UPGRADED TO A WARNING WITH THE MORNING PACKAGE.

ANOTHER FAIRLY VIGOROUS FRONTAL SYSTEM WILL FOLLOW RIGHT ON THE
HEELS OF THIS FIRST ONE...BRINGING MORE RAIN AND WIND AND MOUNTAIN
SNOW MONDAY NIGHT THROUGH TUESDAY NIGHT. WITHOUT TOO MUCH CONCERN
FOR THE DETAILS AT THIS POINT...IT WILL LIKE PRESENT ONLY A MARGINAL
THREAT OF HIGH WINDS FOR THE COASTAL ZONES. THE SNOW LEVEL WILL BE
IN THE 3000 TO 4000 FT RANGE AS THE SYSTEM MOVES THROUGH...SO MORE
SNOW WILL PILE UP IN THE MOUNTAINS BUT AGAIN MAINLY AT THE HIGHER
ELEVATIONS.  MCDONNAL

.LONG TERM...MODELS AGREE THAT THE MONDAY NIGHT THROUGH TUESDAY
NIGHT FRONTAL SYSTEM WILL BE THE LAST ONE FOR AT LEAST A COUPLE DAYS
AND POSSIBLY LONGER. ON WEDNESDAY WESTERN WASHINGTON WILL BE IN A
RATHER COOL SHOWERY POST-FRONTAL PATTERN...AND THAT WILL CONTINUE
INTO THURSDAY AS SHOWERS DECREASE AND FLOW ALOFT WEAKENS. THE TREND
IN THE MODELS OVER RECENT RUNS IS TO DIRECT THE NEXT FRONTAL SYSTEM
INLAND TO OUR SOUTH ON FRIDAY.  MCDONNAL

&&

.HYDROLOGY...AS MENTIONED ABOVE THE FRONTAL SYSTEM MOVING THROUGH
WESTERN WASHINGTON TODAY THROUGH MONDAY WILL BRING 2 TO 6 INCHES OF
PRECIPITATION TO THE OLYMPICS...WITH THE SNOW LEVEL 3500 TO 4500 FT
WHEN THE HEAVIEST AMOUNTS OCCUR. OUR QPF HAS MEAN AREAL
PRECIPITATION OF 3.6 INCHES OVER THE SKOKOMISH BASIN...AND MOST OF
THAT WILL BE RAIN RATHER THAN SNOW.

THE LATEST NWRFC GUIDANCE SHOWS THE SKOKOMISH FLOODING BY LATE
TONIGHT. ANOTHER STRONG FRONTAL SYSTEM...WHICH WILL PROBABLY BE JUST
A LITTLE COOLER AND LESS WET... WILL MOVE THROUGH MONDAY NIGHT
THROUGH TUESDAY NIGHT AND DRIVE THE RIVER HIGHER. FOR NOW THE FLOOD
WATCH WILL REMAIN A WATCH...AS THE ONSET OF THE EVENT APPEARS TO BE
MORE THAN 12 HOURS AWAY.

RIGHT NOW IT LOOKS LIKE ONLY THE EXCEPTIONALLY FLOOD-PRONE SKOKOMISH
RIVER WILL FLOOD. FLOODING LOOKS UNLIKELY ON ANY OTHER RIVER IN OUR
FORECAST AREA...BUT SOME COULD BE CLOSE AND WILL BEAR WATCHING.
MAINLY THAT INCLUDES THE CHEHALIS AND SKOOKUMCHUCK RIVERS...WHICH
EXPERIENCED MINOR FLOODING LAST FRIDAY ON THEIR MOST FLOOD-PRONE
REACHES.

THE GREEN RIVER IS NOT EXPECTED TO FACE A FLOOD THREAT IN THE NEXT
TEN DAYS.  MCDONNAL

&&

.AVIATION...A STRONG WARM FRONT OVER CENTRAL OREGON AND A STRONG
COLD FRONT IN THE OFFSHORE WATERS WILL OCCLUDE AS THEY MOVE THROUGH
WESTERN WASHINGTON THIS EVENING. WARM ADVECTION PRECIPITATION HAS
BEGUN ON THE CENTRAL WASHINGTON COAST AND WILL SPREAD NE THROUGH THE
AREA EARLY THIS MORNING. LOW LEVEL PRES GRADIENTS ARE EASTERLY TO
NORTHEASTERLY...SO EXPECT CIGS AND VISIBILITIES TO REMAIN RATHER
GOOD NEAR THE CASCADES AND EAST OF PUGET SOUND. THERE WILL LIKELY BE
DISTINCT RAIN SHADOWING FROM AROUND BELLINGHAM SW THROUGH THE SAN
JUANS TO THE AREA AROUND THE EASTERN STRAIT OF JUAN DE FUCA AS
INCREASING SOUTH TO SOUTHWEST FLOW INTERACTS WITH THE OLYMPICS.
THIS AREA WILL SEE SURFACE WINDS INCREASE TO 25-35 KT DURING THE
MORNING HOURS. THE COAST AND THE SOUTH SLOPES OF THE OLYMPICS WILL
SEE PERIODS OF HEAVY RAINFALL LATER THIS MORNING RESULTING IN MVFR
TO IFR VIS AND CIGS...THAT WILL CONTINUE IN THROUGH TONIGHT. AREAS
OF LOW LEVEL WIND SHEAR WITH SURFACE WINDS E-SE 8-15 KT AS S WIND
35KT AT 020 WILL DEVELOP OVER THE INTERIOR FROM ABOUT EVERETT
SOUTHWARD AT 15Z THIS MORNING AND WILL CONTINUE THROUGH ABOUT 03Z
THIS EVENING.

KSEA...STEADY LIGHT RAIN WITH CIGS REMAINING 040-050 WILL DEVELOP
RAPIDLY AROUND 12Z AND CONTINUE MUCH OF THE DAY SUN. SURFACE WINDS E
5-8 KT WITH S WIND 35 KT AT 020 WILL GIVE LOW LEVEL WIND SHEAR AT
TIMES THROUGH 03Z MON. WINDS AT 050-100 WILL INCREASE TO 50-65 KT
15Z THIS MORNING AND CONTINUE THROUGH THIS EVENING. ALBRECHT

&&

.MARINE...GALES ARE RAPIDLY DEVELOPING ON THE COAST AND WILL SPREAD
THROUGH MOST WATERS EXCEPT PUGET SOUND AND HOOD CANAL BY MIDDAY. THE
CULPRIT IS A STRONG INCOMING FRONTAL SYSTEM THAT IS ASSOCIATED WITH
A 975 MB STORM CENTER NEAR 46N 141W.

DESPITE THE FRONTAL PASSAGE THIS EVENING...THE VERY WINDY PERIOD
WILL CONTINUE BEHIND THE FRONT LATE TONIGHT INTO MONDAY AS A
STRONG BENT BACK OCCLUSION ROTATING AROUND THE OFFSHORE STORM SYSTEM
MOVES THROUGH THE WATERS. AFTER A SHORT LULL LATER MON OR MON NIGHT
THAT WILL BE FOLLOWED BY ANOTHER STRONG FRONT THAT WILL MOVE THROUGH
THE WATERS TUE. THIS NEXT STRONG FRONT WILL ALSO GIVE A ROUND OF
GALES TO ALL WATERS EXCEPT PUGET SOUND AND HOOD CANAL.

THE COMBINATION OF STRONG WINDS SUN INTO SUN NIGHT AND A LARGE SWELL
TRAIN DEVELOPING SOUTH OF THE OFFSHORE STORM SYSTEM WILL RESULT IN
HIGH SEAS IN THE COASTAL WATERS. LATER TODAY COMBINED SEAS WILL
BUILD TO 20 FT...AND MAY APPROACH 30 FT LATER TONIGHT INTO MON.
ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...AVALANCHE WATCH FOR THIS AFTERNOON AND EVENING FOR THE NORTHERN
       WASHINGTON CASCADES...OLYMPICS...AND MT HOOD AREA.
    .WINTER STORM WARNING FOR THE CASCADES AND OLYMPICS THROUGH
       TUESDAY EVENING.
    .HIGH WIND WARNING FOR THE COASTAL ZONES THIS AFTERNOON AND
       TONIGHT.
    .WIND ADVISORY FOR THE NORTHWEST INTERIOR THIS AFTERNOON AND
       TONIGHT.
    .FLOOD WATCH FOR MASON COUNTY.

PZ...GALE WARNING COAST...STRAIT OF JUAN DE FUCA...NORTHERN
       INLAND WATERS...ADMIRALTY INLET.
    .SMALL CRAFT ADVISORY PUGET SOUND AND HOOD CANAL.
    .SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

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