Small Craft Advisory 942, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
500 PM PDT SUN MAR 18 2012

.SYNOPSIS...A COOL AND SHOWER UPPER TROUGH WILL MOVE OUT OF WESTERN
WASHINGTON LATE TONIGHT AND MONDAY MORNING. A STRONG FRONT WILL
START TO IMPACT WESTERN WASHINGTON LATER ON MONDAY AND MOVE THROUGH
EARLY ON TUESDAY MORNING. A WEAKER SYSTEM WILL PASS THROUGH ON
WEDNESDAY NIGHT AND THURSDAY MORNING. SHOWERS WILL DECREASE LATE IN
THE WEEK AND THIS WEEKEND.

&&

.SHORT TERM...UPDATED AVIATION AND MARINE SECTIONS BELOW. A
SLOW-MOVING UPPER TROUGH AXIS IS LOCATED SQUARELY OVER WRN WA THIS
AFTN...LEADING TO A MOIST AND UNSTABLE AIR MASS. KATX RADAR
INDICATES NUMEROUS SHOWERS OVER THE FCST AREA. SFC WIND FIELDS
INDICATE THE POSSIBILITY OF PSCZ DEVELOPMENT NEAR THE KING/SNO
COUNTY LINE. KLGX RADAR INDICATES DECREASING SHOWER COVERAGE OVER
THE COASTAL WATERS BEHIND THE TROUGH AXIS.

A FLAT UPPER RIDGE WILL BUILD INTO THE PAC NW TONIGHT. WARMING AIR
ALOFT WILL STABILIZE THE AIR MASS AND CAUSE SHOWER ACTIVITY TO
DECREASE TONIGHT.

THE FLAT RIDGE AXIS WILL MOVE ACROSS WRN WA LATE MON AFTN.
HOWEVER...THIS WILL BE A FLAT RIDGE...AND WRN WA WILL LIE UNDER THE
DIRTY PERIPHERY OF THE RIDGE...WHERE WARM ADVECTION AND SOME WEAK
LIFT PREVAIL. SO WILL BE LOOKING AT SOME LIGHT PRECIP DEVELOPING ON
MON...ESPECIALLY FOR THE COAST AND MTNS.

THE NEXT STRONG FRONT WILL SPREAD MODERATE PRECIP ONTO THE COAST
EARLY ON MON EVNG...THEN INTO THE INTERIOR LATE ON MON EVNG. OVER
THE LAST COUPLE OF DAYS...MODELS HAVE GRADUALLY BEEN SPEEDING UP THE
TIMING OF THIS FRONT AND NOW SHOW THE HEAVIEST QPF OVER THE INTERIOR
BTWN 06Z-12Z ON TUE. AHEAD OF THE COLD FRONT ON MON NIGHT...FREEZING
LEVELS RISE TO BTWN 3000 AND 4000 FT...SO NOT WORRIED ABOUT RIVER
FLOODING. COLD FROPA ALONG THE COAST NEAR 09Z TUE...THEN OVER THE
INTERIOR ARND 12Z TUE.

BEHIND THE FRONT ON TUE...SOMEWHAT SHALLOW INSTABILITY AND STRONG
ONSHORE FLOW WILL LEAD TO ABUNDANT SHOWER ACTIVITY. WITH SUCH STRONG
FLOW...SHOWERS WILL MAINLY BE FOCUSED ON WINDWARD MTN SLOPES AND
PERHAPS IN A PUGET SOUND CONVERGENCE ZONE NEAR EVERETT. THE 12Z NAM
SHOWS 925 MB WINDS OF 40-45 KT OVER THE INTERIOR IN THE WELL-MIXED
POST-FRONTAL ENVIRONMENT ON TUE...WHICH COULD TRANSLATE TO SFC WIND
GUSTS OF 40-45 MPH OVER THE PUGET SOUND LOWLANDS. KOLM-KBLI PRES
GRADIENT WILL EASILY REACH OR EXCEED +5 MB. LATER SHIFTS MAY BE
NEEDING TO CONSIDER A WIND ADVISORY OVER THE INTERIOR ON TUE.

LASTLY...THE STRONG FRONT LATE MON NIGHT AND THE OROGRAPHICALLY
ENHANCED SHOWER ACTIVITY ON TUE WILL LEAD TO HEAVY SNOW OVER THE
MTNS. PARADISE AND MOUNT BAKER LIKELY TO GET 18+ INCHES OF SNOW IN
24 HOURS...WHILE THE PASSES SHOULD BE IN THE 6-12 INCH RANGE. THE
FORECAST AMOUNTS ON THE VOLCANOES NECESSITATE A WINTER STORM WATCH
AT THIS TIME.

A 1007 MB FRONTAL WAVE WILL SPREAD INTO NW ORE ON WED. THIS WILL
SPREAD MORE MODERATE RAIN OVER WRN WA ON WED AND WED NIGHT...BUT THE
TRACK OF THE LOW TO OUR SOUTH WILL KEEP THE POTENTIAL FOR STRONGER
WIND TO OUR SOUTH. BY THE SAME TOKEN...THE LOW TRACK TO OUR SOUTH
WILL KEEP SNOW LEVELS IN THE 1500-2500 FOOT RANGE DURING THIS
TIME...WHICH IS UNFAVORABLE FOR HYDRO ISSUES.
HANER

.LONG TERM...THE GFS AND ECMWF HAVE BEEN TRENDING TOWARD A DIGGING
UPPER TROUGH AROUND 130W-140W LATE THIS WEEK AND NEXT WEEKEND. THIS
COULD CONTINUE TO BRING SOME SHOWER ACTIVITY INTO WRN WA THROUGH
THU...BUT AS THE MORE ENERGETIC PORTION OF THE TROUGH DIGS FURTHER
SOUTH OFF THE CALIF COAST NEXT WEEKEND...THIS MAY LEAVE WRN WA IN A
MOSTLY DRY AND MORE SEASONABLY MILD PATTERN. HAVE CONTINUED WITH A
CHANCE OF SHOWERS THROUGHOUT THE EXTENDED PERIOD...BUT HAVE TRENDED
DOWN THE POPS IN A FEW PERIODS. HOWEVER...THE EMPHASIS SHOULD
PROBABLY BE ON THE POTENTIAL FOR DRY AND PARTLY SUNNY WEATHER BY
NEXT WEEKEND. THE 12Z ECMWF IS AN EVEN DRIER SOLN THAN THE RECENT
GFS RUNS FOR NEXT SAT AND SUNDAY...KEEPING THE ACTIVE WEATHER WELL
TO OUR SOUTHWEST OFF THE CALIF COAST.      HANER

&&

.HYDROLOGY...THE CHEHALIS RIVER NEAR PORTER CONTINUES TO FALL
FURTHER BELOW FLOOD STAGE...AND THE FLOOD WARNING FOR THAT RIVER
FORECAST POINT ENDED EARLIER THIS AFTN.      HANER

MODERATE PRECIP AMOUNTS ARE EXPECTED WITH A FAST-MOVING FRONT ON MON
NIGHT. THE FAST MOVEMENT OF THIS FRONT AND ONLY A SHORT 6-HOUR
PERIOD OF SNOW LEVELS IN EXCESS OF 3000 FEET DO NOT FAVOR FLOODING.
MORE MODERATE PRECIP IS EXPECTED ON WED AND WED NIGHT. AGAIN...SNOW
LEVELS WILL BE FAIRLY LOW...IN THE 1500-2500 FOOT RANGE...WHICH DOES
NOT FAVOR FLOODING. LATE THIS WEEK THROUGH NEXT WEEKEND...LITTLE OR
NO PRECIP IS EXPECTED.

THE THREAT FOR FLOODING IN THE NEXT 7 DAYS IS LOW.      HANER

&&

.AVIATION...NO THUNDERSTORMS AS OF 5PM AND THE PUBLIC FORECASTER
TOOK THEM OUT OF THE FCST THIS AFTERNOON...NOTHING TOO IMPRESSIVE ON
RADAR AT 5PM. A COLD UPPER TROUGH AND LIGHT ONSHORE FLOW CONTINUE TO
TOUCH OFF SCATTERED SHOWERS. THE PUGET SOUND BASIN HAS LIGHT
CONVERGING WINDS AND SEEMS TO HAVE THE MOST MOISTURE AND LOWEST
CIGS. OVERALL THE MOUNTAINS ARE MOSTLY OBSCURED IN SNOW SHOWERS AND
CLOUDS. LIGHT SLY FLOW ALF THIS AFTERNOON WILL TURN WLY TONIGHT AS
THE TROUGH SHIFTS INLAND OVERNIGHT. THE AIR MASS IS MOIST AND BY THE
LOOK OF THE SATELLITE IMAGE THIS AFTERNOON IS THE MOST UNSTABLE OVER
SW WA AND THE COAST...WITH A LACK OF SUNBREAKS AROUND PUGET SOUND
TEMPS MIGHT NOT GET HIGH ENOUGH TO TOUCH OFF DECENT CONVECTION. 19

KSEA...MUCH OF THE AVIATION DISCUSSION ABOVE APPLIES TO SEA TAC AS
WELL. FOR KSEA IN PARTICULAR...VRB05KT CONTINUES TO BE A GOOD WIND
FCST THROUGH THE EVENING AS THERE SEEMS TO BE CONVERGENCE RIGHT OVER
PUGET SOUND NEAR SEATTLE. THIS MAY SHIFT NORTH OVERNIGHT WITH A SLY
BREEZE DEVELOPING. WATCH THE RADAR...IF A DECENT SHOWER DEVELOPED
AND CROSSED SEA TAC THAT WOULD GIVE SOME WET SNOW...ALTHOUGH NOTHING
MUCH HAS MATERIALIZED SO FAR TODAY THERE IS A CHANCE OF SNOW SHOWERS
THIS EVENING. 19

&&

.MARINE...STILL NO THUNDERSTORMS AT 5PM...SO REMOVED THEM FROM THE
MARINE FORECAST. LIGHT ONSHORE FLOW CONTINUES. A COLD UPPER TROUGH
WILL CROSS WESTERN WA THIS EVENING. A STRONG FRONT WILL MOVE THROUGH
WESTERN WA LATER MONDAY NIGHT WITH A FROPA BY DAYBREAK TUE. A GALE
WATCH IS UP FOR THE NORMAL WINDIER MARINE AREAS FOR THAT. THE
SURFACE LOW WITH THAT IS FORECAST BY THE MODELS TO DEEPEN TO THE
960S AND TRACK TOWARD HAIDA GWAII TUE MORNING. WITH A BROAD AREA OF
WLY GALES BEHIND THE LOW THERE WILL UNDOUBTEDLY BE PRETTY HIGH SEAS
ARRIVING ON THE WA COAST TUE NITE. 19

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...WINTER STORM WATCH FOR THE WEST SLOPES OF THE NORTH AND CENTRAL
        CASCADES FROM MONDAY EVENING THROUGH TUESDAY EVENING.
PZ...GALE WATCH IN EFFECT FROM MONDAY EVENING THROUGH TUESDAY
        AFTERNOON FOR THE COASTAL WATERS...ENTRANCES TO THE STRAIT OF
        JUAN DE FUCA...AND THE NORTHERN INLAND WATERS.
     SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.