Small Craft Advisory 1145, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
930 PM PST SAT DEC 22 2012

.SYNOPSIS...LOW PRESSURE ALOFT OFF THE COAST IS EXPECTED TO CONTINUE
TO ROTATE SHOWERS ONSHORE BEFORE IT MOVES INLAND BY LATE SUNDAY.
HIGHER PRESSURE ALOFT SHOULD BRING A BRIEF DRY SPELL MONDAY. THEN
ANOTHER PACIFIC FRONTAL SYSTEM IS FORECAST TO SPREAD MORE
PRECIPITATION ONSHORE WITH THE POSSIBILITY OF SNOW ON SOME LOWLANDS.
A SHOWERY REGIME WILL FOLLOW WEDNESDAY GRADUALLY EASING THROUGH THE
END OF THE WEEK.

&&

.SHORT TERM...SHOWERS CONTINUE TO MOVE NWD THROUGH THE AREA IN
BATCHES UNDER DIFFLUENT SLY FLOW ALOFT. THE 500 MB TROUGH AXIS IS
STILL OFFSHORE ALLOWING WAVES OF SHOWER ACTIVITY TO MOVE NWD THROUGH
THE AREA. THIS PATTERN REMAINS UNCHANGED THROUGH SUNDAY ALTHOUGH
SURFACE LOW DEVELOPMENT WILL OCCUR IN THE SPLIT SRN PORTION OF THE
SPLIT FLOW HEADING INTO CA. THIS MEANS THE HEAVIER RAINFALL WILL
OCCUR DOWN THAT WAY WHILE WRN WA REMAINS UNDER A SHOWERY REGIME WITH
MAINLY LIGHT MOUNTAIN SNOW ACCUMULATIONS.

THE 500 MB TROUGH AXIS SHIFTS ACROSS THE PAC NW SUNDAY EVENING
BRINGING COLDER AIR ALOFT. A WEAK SURFACE LOW PERSISTS NEAR THE WA
COAST THROUGH MONDAY MORNING WITH SHOWERS CONTINUING BUT DECREASING
AS THE NIGHT WEARS ON. ACCUMULATIONS OF 2 TO 4 INCHES CAN BE
EXPECTED IN THE MOUNTAINS ABOVE 2000 FEET TONIGHT AND ANOTHER 3-6
INCHES SUNDAY/SUNDAY EVENING.

THERE WILL BE A LULL OR BREAK IN SHOWERS MONDAY INTO MONDAY NIGHT.
LOW CHANCE POPS ARE INDICATED MONDAY WHICH LOOKS FINE TO COVER
RESIDUAL SHOWERS. DRY WEATHER IS EXPECTED MUCH OF MONDAY NIGHT WHICH
WILL ALLOW FOR COLDER TEMPERATURES WITH SOME LOWER DEW POINT AIR TO
INFILTRATE THE AREA AS ELY SURFACE GRADIENTS INCREASE. THIS WILL
PRIME THE ATMOSPHERE FOR THE POSSIBILITY OF SOME SNOW OR MIXED
PRECIPITATION ON CHRISTMAS DAY.

THE 00Z MODEL RUNS HAVE CONTINUED THE TREND OF PUSHING BACK THE
ONSET OF PRECIPITATION CHRISTMAS DAY. MODELS OFTEN UNDERESTIMATE THE
EXTENT OF SPLITTING SO IT WILL NOT BE A SURPRISE IF THE
PRECIPITATION IS SLOW TO ARRIVE. EVEN WITH THE SLOWER TIMING
INDICATED BY THE 00Z RUNS RAIN AND PERHAPS MIXED PRECIPITATION OR
SNOW SHOULD REACH THE COAST AND BEGIN PUSHING INTO AREAS W OF THE
PUGET SOUND CHRISTMAS MORNING. THE LATER TIMING MAY MEAN A LESS
CHANCE FOR ACCUMULATION FURTHER INLAND FROM TACOMA NORTHWARD IF
PRECIPITATION OCCURS IN THE AFTERNOON. BUT THERE IS STILL A CHANCE
OF SOME ACCUMULATION JUST ABOUT ANYWHERE ON CHRISTMAS DAY WITH THE
DETAILS STILL UNCERTAIN.

MESO MODELS ARE ARE IN GOOD AGREEMENT OF ONE POTENTIALLY BIG PROBLEM
AREA BEING THE HOOD CANAL AREA AND POSSIBLY MASON COUNTY AND THE
CHEHALIS GAP E OF HOQUIAM IN GRAYS HARBOR COUNTY. SEVERAL INCHES OF
SNOW COULD FALL IN THIS AREA...WITH THE LARGEST AMOUNTS OVER 6
INCHES POSSIBLE IN HOOD CANAL. A SPECIAL WEATHER STATEMENT WAS
ISSUED EARLIER TO ADDRESS THE POTENTIAL SNOW CHRISTMAS DAY.

MANY WANT A WHITE CHRISTMAS AND HISTORICALLY MOST LOWLAND WESTERN
WASHINGTON LOCATIONS HAVE A LESS THAN 10 PERCENT CHANCE. DECEMBER
2008 WAS THE LAST TIME WE HAD A WHITE CHRISTMAS.

SIDE NOTE...THE KATE RADAR IS EXPERIENCING TECHNICAL PROBLEMS AND IS
DOWN UNTIL FURTHER NOTICE. A TECHNICIAN WILL BE LOOKING AT THE
PROBLEM SUNDAY MORNING.

MERCER

.LONG TERM...PREVIOUS DISCUSSION FOLLOWS...ANOTHER UPPER TROUGH WILL
FOLLOW TUESDAY NIGHT AND WEDNESDAY FOR MORE SHOWERS. THEN RIDGING
ALOFT WILL BEGIN TO BUILD THURSDAY AND FRIDAY FOR A GRADUAL DRYING
TREND. THE CANADIAN...GFS AND EURO ALL DIFFER A BIT...YET THE BIG
STORY IS A SPLIT FLOW PATTERN BY NEXT WEEKEND WITH THE PRIMARY
ENERGY DIGGING SOUTH INTO CALIFORNIA AND THE PACIFIC NW REMAINING
PRIMARILY DRY. WILL NEED TO KEEP AN EYE ON ANY SHORTWAVES DIGGING
DOWN THE BACK SIDE OF THE UPPER RIDGE LIKE THE GFS HINTS. BUEHNER

&&

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

&&

.AVIATION...AN UPPER TROUGH AND SURFACE LOW CENTERED OFF THE NORTH
WASHINGTON COAST WILL WEAKEN AND MOVE SLOWLY EAST INTO WESTERN
WASHINGTON LATE SUNDAY. FLOW ALOFT IS SOUTHERLY AND THE AIR MASS IS
MOIST AND SOMEWHAT UNSTABLE.

SOUTH WINDS 18G30 KT AROUND BELLINGHAM ARE CAUSING SOME LOW LEVEL
MECHANICAL TURBULENCE AS FLOW INTERACTS WITH THE TERRAIN. EXPECT
CONDITIONS TO IMPROVE BY ABOUT 08Z AS AN IMPULSE ROTATING AROUND THE
OFFSHORE FLOW LIFTS NORTH INTO BRITISH COLUMBIA.

EXPECT MAINLY VFR CEILINGS AND VISIBILITIES THROUGH SUNDAY. LOCAL
MVFR CEILINGS ARE POSSIBLE AT TIMES IN HEAVIER SHOWERS TONIGHT.
ALBRECHT

KSEA...SOUTH WIND 8-11 KNOTS WILL EASE SOMEWHAT TO 6-8 KNOTS AFTER
06Z. ALBRECHT

&&

.MARINE...A SURFACE LOW OVER THE OFFSHORE WATERS WILL SHIFT TO THE
COASTAL WATERS ON SUNDAY AND DISSIPATE SUNDAY NIGHT. GALE CONDITIONS
HAVE BEEN OCCURRING OVER PORTIONS OF THE COASTAL WATERS AND NEAR
GALE CONDITIONS ARE OCCURRING OVER THE NORTH INLAND WATERS AND THE
EAST ENTRANCE TO THE STRAIT. THE GALE WARNINGS WILL EXPIRE AT 10 PM
AND REPLACED WITH SMALL CRAFT ADVISORIES.

WINDS WILL EASE LATER SUNDAY AND SUNDAY NIGHT AS THE GRADIENTS
RELAX.

A FRONTAL SYSTEM WILL MOVE OVER THE COASTAL WATERS TUESDAY. THIS
WILL BRING GALES TO THE COASTAL WATERS AND PERHAPS THE NORTHERN
INLAND WATERS WITH SMALL CRAFT CONDITIONS ELSEWHERE. THE FRONT WILL
WEAKEN AND MOVE INLAND TUESDAY NIGHT AND WEDNESDAY WITH DIMINISHING
WINDS. SCHNEIDER/ALBRECHT

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...GALE WARNING COASTAL WATERS...EAST ENTRANCE STRAIT OF JUAN DE
     FUCA...AND NORTHERN INLAND WATERS THROUGH 10 PM.
    .SMALL CRAFT ADVISORY ALL OTHER WATERS.
    .SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR.

&&

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