Small Craft Advisory 1130, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
900 AM PST THU DEC 6 2012

.SYNOPSIS...STRONG WESTERLY FLOW ALOFT IS OVER THE REGION. WEATHER
DISTURBANCES EMBEDDED IN THE WESTERLIES WILL GIVE GUSTY WINDS AT
TIMES...SHOWERS A TIMES...AND SNOW IN THE MOUNTAINS. HIGH PRESSURE
WILL BUILD TOWARD THE AREA OVER THE WEEKEND...BUT WILL NOT BE STRONG
ENOUGH TO KEEP WEATHER SYSTEMS FROM BRUSHING WESTERN WASHINGTON.

&&

.SHORT TERM...THE ISALLOBARIC ANALYSIS...PRESSURE CHANGE
CHART...SHOWS ONE SHORTWAVE OVER WA AND ANOTHER OFFSHORE. THE FIRST
DISTURBANCE IS GIVING SHOWER ACTIVITY OVER THE AREA TODAY AND THE
SECOND SHORTWAVE WILL MOVE THROUGH THE AREA LATER TONIGHT AND
FRIDAY. THERE WILL PROBABLY BE A LITTLE BREAK BETWEEN THE SYSTEMS
THIS EVENING AND THE 12Z NAM SHOWS THE TWO FEATURES PRETTY WELL.
PERIODS OF SNOW WILL FALL IN THE MOUNTAINS AND A WINTER WEATHER
ADVISORY IS IN EFFECT FOR THAT. WITH THIS WESTERLY FLOW ALOFT THERE
WILL BE A BIT OF A RAINSHADOW OVER THE CENTRAL SOUND...NOT STRONG
ENOUGH TO KEEP SHOWERS OUT OF THE FORECAST BUT PRECIP AMOUNTS WILL
BE LIGHTEST ON THE KITSAP PENINSULA AND AROUND HOOD CANAL. AFTER
EACH DISTURBANCE A CONVERGENCE ZONE COULD SET UP WHICH MAY HELP
FOCUS SOME HEAVIER SNOW SHOWERS IN THE CENTRAL CASCADES AT
TIMES...THE 48HR PRECIP TOTAL FROM THE 4KM 12Z UW WRFGFS SUGGESTS A
FEW AREAS IN THE CASCADES AND THE VOLCANOES COULD PICK UP TWO FEET
OF SNOW. AREAS FROM SEQUIM TO DABOB BAY AND OVER TO THE METRO AREA
ARE WELL SHADOWED AND ONLY GET A LITTLE LIGHT SHOWER ACTIVITY AT
TIMES.  THE FLOW ALOFT WILL TURN NORTHWEST FRIDAY NIGHT AS AN UPPER
LEVEL RIDGE BUILDS OFFSHORE. THE FORECAST FOR SATURDAY IS A LITTLE
UP IN THE AIR...A WARM FRONT MAY SNEAK OVER THE BUILDING RIDGE AND
ARRIVE FAST ENOUGH TO HAVE A CHANCE OF RAIN IN THE FCST. THE 12Z NAM
IS HAS A FEW SHOWERS GOING SATURDAY MORNING...MAYBE THE LAST GASP OF
A PSCZ...AND THE WARM FRONT BREAKS UP JUST OFFSHORE SATURDAY NIGHT.
THEN TAIL END OF THE MAIN FRONTAL SYSTEM BREAKS UP OVER WESTERN WA
SUNDAY. 19

.LONG TERM...THERE IS A DIRTY UPPER RIDGE JUST OFFSHORE SUN/MON BUT
IT MAY BREAK DOWN QUICKLY MONDAY NIGHT...12Z GFS HAS A DECENT COLD
FRONT PUSHING THROUGH WESTERN WA TUESDAY AFTERNOON. THE 00Z ECMWF
HAD A WEAKER COLD FRONT PUSHING THROUGH THE AREA TUESDAY
EVENING...WILL HAVE TO SEE IF THE 12Z EURO TRENDS TOWARD THE GFS. 19

&&

.HYDROLOGY...THE LOWER REACHES OF THE CHEHALIS RIVER WILL GRADUALLY
RECEDE BELOW FLOOD STAGE. THE COOL SHOWERY PATTERN WITH SNOW IN THE
MOUNTAINS WILL ALLOW ALL THE RIVERS TO TREND DOWN. THE FREEZING
LEVEL DOES RISE SUN AND MON BUT THERE WILL NOT BE MORE THAN A LITTLE
LIGHT SPOTTY RAIN AT TIMES. THE 12Z GFS HAS A STRONG COLD FRONT
TUESDAY AFTERNOON FOR PERHAPS ANOTHER SHOT OF COOL AIR AND LOW SNOW
LEVELS. HYDROLOGICALLY SIGNIFICANT PRECIPITATION FOR THE RIVERS IS
NOT EXPECTED IN THE NEXT 7 DAYS. 19

&&

.AVIATION...WESTERLY FLOW WILL CONTINUE ACROSS WESTERN WA TODAY AND
TONIGHT. WEAK UPPER LEVEL DISTURBANCES WILL TRIGGER SHOWERS ACROSS
THE REGION. THE AIR MASS REMAINS MOIST AND UNSTABLE. LOW LEVEL
ONSHORE FLOW WILL INCREASE TONIGHT BEHIND A COLD FRONT WITH GUSTY
S/SW WINDS EXPECTED. CEILINGS WILL LIKELY REMAIN IN MVFR RANGE WITH
SHOWERS. 33

KSEA...SHOWERS WILL REMAIN IN THE VICINITY TODAY...BUT THE CENTRAL
SOUND WILL REMAIN MOSTLY RAIN SHADOWED WITH WESTERLY DOWNSLOPE FLOW
OFF THE OLYMPICS. KATX RADAR HAS BEEN SHOWING THIS TREND. MVFR CIGS
EXPECTED. SURFACE WINDS WILL REMAIN S/SW NEAR 10 KT.

&&

.MARINE...ONSHORE FLOW CONTINUES ACROSS WESTERN WA TODAY. HIGHEST
WINDS AND WAVES ARE OVER THE COASTAL WATERS AND WEST ENTRANCE - A
SMALL CRAFT ADVISORY IS IN EFFECT. PUGET SOUND AND HOOD CANAL ARE
ALSO SEEING WINDS WITHIN SMALL CRAFT ADVISORY RANGE...BUT WINDS
SHOULD EASE THIS AFTERNOON /AND MAY BRIEFLY FLIP TO NORTHERLY OVER
THE NORTH SOUND/. SOUTH TO SOUTHWEST WINDS WILL INCREASE AGAIN
TONIGHT AND FRIDAY AS A COLD FRONT MOVES INLAND. WILL LIKELY SEE
SMALL CRAFT ADVISORY WINDS ACROSS ALL ZONES. HIGH PRES WILL BUILD
INLAND ON SAT WITH NWLY FLOW ACROSS THE REGION. A WARM FRONT WILL
CLIP THE REGION ON SUN...FOLLOWED BY A COLD FRONT SUN NIGHT. 33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS REMAIN IN EFFECT FOR THE CHEHALIS RIVER AT
     PORTER.

     WINTER WEATHER ADVISORY IN EFFECT FOR THE OLYMPICS AND CASCADES
     ABOVE 2000 FEET UNTIL 4 AM FRIDAY.

PZ...SMALL CRAFT ADVISORY COASTAL WATERS...WEST ENTRANCE STRAIT OF
     JUAN DE FUCA...PUGET SOUND AND HOOD CANAL.

     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.

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