Small Craft Advisory 633, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
230 AM PST FRI JAN 28 2011

.SYNOPSIS...A BAND OF PRECIPITATION WILL SET UP OVER WESTERN
WASHINGTON TODAY AND CONTINUE THROUGH EARLY SUNDAY AS MOISTURE WRAPS
NORTH FROM LOW MOVING INTO NORTHERN CALIFORNIA. A MODIFIED ARCTIC
COLD FRONT WILL MOVE INTO THE NORTH INTERIOR LATE TONIGHT AND SLIDE
SOUTH THROUGH WESTERN WASHINGTON ON SUNDAY. MOST OF THE MOISTURE
WILL BE SOUTH OF THE AREA AS THE COLD AIR SPREADS INTO THE AREA. A
RIDGE OF HIGH PRESSURE WILL BUILD OVER THE AREA ON MONDAY AND
PERSIST THROUGH THE UPCOMING WEEK...GIVING A PERIOD OF COLD
NIGHTS...COOL DAYS...AND DRY WEATHER.

&&

.SHORT TERM...AN UPPER LEVEL RIDGE SITS ALONG ABOUT 138W OVER THE SE
GULF OF ALASKA. IN THE MEAN TIME...A MATURE LOW IS UNDERCUTTING THE
OFFSHORE RIDGE AND IS MOVING TOWARD THE EXTREME NORTHERN CALIFORNIA
COAST. MOISTURE WITH A DEFORMATION ZONE EXTENDING EAST FROM THE
NORTHERN END OF THIS LOW IS APPROACHING THE AREA EARLY THIS MORNING.
SATELLITE IMAGERY AND MODELS ALL SHOW THIS AREA OF MOISTURE MOVING
RIGHT INTO WESTERN WASHINGTON THIS MORNING AND PERSISTING THROUGH
TONIGHT. TO THE EAST OF THE OFFSHORE RIDGE...A DEVELOPING ARCTIC
HIGH FROM YUKON EASTWARD THROUGH THE NORTHWEST TERRITORIES OF CANADA
IS SPREADING RAPIDLY SOUTH.

BY 12Z SUNDAY A MODIFIED ARCTIC BOUNDARY SHOULD BE APPROACHING THE
BELLINGHAM AREA THEN WILL BE MOVING SOUTH THROUGH WESTERN WASHINGTON
SUN INTO EARLY SUN EVENING. WESTERN WASHINGTON JUST GETS BRIEFLY
NICKED BY THIS ARCTIC BOUNDARY AS THE VERY COLD AIR SLIDES SE
THROUGH THE CANADIAN PRAIRIES AND US GREAT PLAINS. BUT IT APPEARS
THAT ENOUGH COLD AIR SPILLS INTO CENTRAL BRITISH COLUMBIA THAT
WILLIAMS LAKE BC TO BELLINGHAM PRES GRADIENTS REACH -14 TO -16 MB
SUNDAY MORNING. THESE GRADIENTS ARE NOW SHOWN BY ALL MODELS. THE
WIND IN THE NORTH INTERIOR...PARTICULARLY IN THE SAN JUANS AND IN
THE FRASER OUTFLOW AREA OF WESTERN WHATCOM COUNTY...WILL LIKELY RISE
TO N/NE 25 TO 35 WITH GUSTS TO 50 MPH SUN MORNING. THE NEXT SHIFT
WILL LIKELY NEED TO ISSUE A WIND ADVISORY FOR THE OUTFLOW AREA
BEGINNING EARLY SUN MORNING.

ELSEWHERE IN THE INTERIOR OF WESTERN WASHINGTON...THE FORECAST
BEGINS TO GET TRICKY SUN MORNING AND CONFIDENCE IN THE DETAILS IS
NOT HIGH. WHILE THE DEFORMATION ZONE LINGERS OVER THE AREA EARLY
SUNDAY AS THE COLD FRONT MOVES SOUTHWARD THROUGH THE AREA...THE GFS
IS THE FASTEST IN DRYING CONDITIONS OUT ACROSS THE AREA...NEVER
SHOWING MUCH OF A POTENTIAL FOR SNOWFALL ACROSS THE AREA. THE NAM12
SHOWS A POTENTIAL OF AN INCH OR SO OF SNOW SUN MORNING AS
PRECIPITATION COMES TO AN END AND COLD AIR FILTERS IN. THE 00Z ECMWF
IS SLOWEST WITH MOISTURE AND HAS RATHER HIGH QPF AMOUNTS
SUN...THOUGH LOW LEVEL THICKNESSES ARE JUST MARGINAL FOR SNOW SUN.
IF THE ECMWF WERE TO VERIFY...THERE WOULD BE A POTENTIAL FOR AT
LEAST A FEW INCHES OF WET SNOW OVER WESTERN SNOHOMISH/N KING CO AREA
AND FOR THE CASCADE FOOTHILLS ABOVE 300-500 FEET ELEVATION. WILL
REFRESH THE SPECIAL WEATHER STATEMENT FOR NOW.

CONDITIONS CLEAR AND DRY FROM THE NORTH LATER SUN THROUGH SUN NIGHT.
TEMPERATURES ON MON MORNING WILL FALL TO AROUND 20 IN THE FRASER
OUTFLOW AREA WITH OTHER LOCATIONS IN W WA FALLING INTO THE MID 20S
TO AROUND 30.

BY MONDAY...THE HEART OF THE ARCTIC AIR IS DROPPING INTO THE
NORTHERN PLAINS AS UPPER RIDGING FLOPS EAST INTO BRITISH COLUMBIA.
DAYTIME TEMPERATURES WILL RISE TO AROUND 40 MON WITH PLENTY OF
SUNSHINE AS THE AIR MASS RAPIDLY MODERATES WITH WARMING ALOFT.
ALBRECHT

.LONG TERM...A PERIOD OF DRY WEATHER WILL PERSIST THROUGH MUCH OF
THE UPCOMING WEEK AS A STRONG RIDGE ALOFT EXTENDS FROM JUST OFFSHORE
SW TO AROUND 35N 135W. IT COULD GET BREEZY TO LOCALLY WINDY NEAR THE
CASCADES TUE AND WED IN EASTERLY FLOW AS COLD AIR REMAINS TRAPPED E
OF THE CASCADES. ALBRECHT

&&

.HYDROLOGY...NO HYDROLOGICALLY SIGNIFICANT PRECIPITATION IS EXPECTED
FOR THE NEXT 10 DAYS. NO FLOODING IS EXPECTED...INCLUDING ON THE
GREEN RIVER. ALBRECHT

&&

.AVIATION...NORTHWEST FLOW ALOFT OVER THE PACIFIC NORTHWEST WILL
CONTINUE TODAY AND TONIGHT. AN UPPER TROUGH OVER BRITISH COLUMBIA
WILL MOVE SOUTHEAST TOWARD THE NORTHERN ROCKIES DURING THE PERIOD...
AND AND A FRONTAL SYSTEM OVER THE OREGON OFFSHORE WATERS WILL MAKE
LANDFALL IN NORTHERN CALIFORNIA. BETWEEN THESE TWO FEATURES...A
DEFORMATION ZONE WILL BRING PRECIPITATION TO WESTERN WASHINGTON.

AT THE SURFACE PRESSURE GRADIENTS WILL BE WEAK TODAY WITH A TROUGH
OVER WASHINGTON. HIGH PRESSURE MOVING SOUTH THROUGH BRITISH COLUMBIA
WILL PRODUCE NORTHERLY OFFSHORE FLOW BEGINNING TONIGHT...AND PUSH
COLDER AIR INTO THE FORECAST AREA ESPECIALLY NEAR THE OUTFLOW ZONE
OF THE FRASER RIVER. THE AIR MASS WILL REMAIN MOIST AND STABLE.

AS PREVIOUS DISCUSSIONS HAVE MENTIONED...THE AIR MASS WILL PROBABLY
BECOME COLD ENOUGH FOR SNOW ON SUNDAY AND THERE COULD BE A LITTLE
LOWLAND SNOW.

THERE WAS RELATIVELY LITTLE CHANGE OVERNIGHT WITH MVFR CEILINGS
ACROSS WESTERN WASHINGTON AT 2 AM...SCATTERED SHOWERS MAINLY ALONG
THE WEST SLOPES OF THE CASCADES...AND PATCHY FOG ALONG THE COAST.
AS RAIN DEVELOPS TODAY WITH THE DEFORMATION BAND...CONDITIONS WILL
PROBABLY DETERIORATE SOMEWHAT FURTHER FOR A MIX OF LOW-END MVFR AND
LOCAL IFR.

KSEA...SOUTH WIND 5-10 KT...SHIFTING TO NORTHEAST LATE THIS EVENING.
MCDONNAL

&&

.MARINE...WEAK PRESSURE GRADIENTS WILL PREVAIL TODAY WITH A WEAK
TROUGH OF LOW PRESSURE ACROSS THE REGION. HIGH PRESSURE MOVING SOUTH
THROUGH BRITISH COLUMBIA WILL PRODUCE NORTHERLY OFFSHORE FLOW ACROSS
THE FORECAST AREA BEGINNING LATE TODAY AND STRENGTHENING TONIGHT AND
SUNDAY.

AS USUAL IN THIS SITUATION...THE STRONGEST NORTHEASTERLIES WILL BE
IN THE NORTHERN INLAND WATERS AS A RESULT OF OUTFLOW FROM THE FRASER
RIVER. HIGH END SMALL CRAFT ADVISORY WINDS ARE LIKELY THERE TONIGHT
AND SUNDAY...AND LOW-END GALES ARE POSSIBLE. AT THIS POINT IT LOOKS
LIKE THE PERIOD OF STRONGEST WINDS IN THE NORTHERN INLAND WATERS
WILL OCCUR LATE TONIGHT AND SUNDAY MORNING.

SMALL CRAFT ADVISORY WINDS ARE ALSO LIKELY OVER THE STRAIT OF JUAN
DE FUCA...ESPECIALLY AT THE WEST ENTRANCE...AND ACROSS THE NORTHERN
PORTION OF THE COASTAL WATERS.

A MORE DUE EASTERLY OFFSHORE FLOW WILL PERSIST EARLY NEXT WEEK.
MCDONNAL

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE
PZ...SMALL CRAFT ADVISORY TONIGHT AND SUNDAY FOR COASTAL
      WATERS...WEST ENTRANCE STRAIT OF JUAN DE FUCA...NORTHERN
      INLAND WATERS.

     SMALL CRAFT ADVISORY GRAYS HARBOR FOR ROUGH BAR CONDITIONS.

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