FXUS66 KSEW 112350
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
345 PM PST FRI DEC 11 2009
.SYNOPSIS...CLOUDS WILL GRADUALLY INCREASE ACROSS THE AREA OVERNIGHT
THROUGH EARLY SATURDAY AS THE FLOW ALOFT BECOMES MORE WESTERLY. THE
NORTHERN PORTION OF A LOW PRESSURE SYSTEM MOVING INTO CALIFORNIA
WILL BRING A THREAT OF MEASURABLE SNOW TO THE FAR SOUTHERN PART OF
THE AREA WHEREAS ELSEWHERE ONLY FLURRIES ARE EXPECTED. A LOW
PRESSURE SYSTEM OVER WESTERN CANADA WILL BRING A BETTER CHANCE OF
MEASURABLE PRECIPITATION ON SUNDAY...WITH POSSIBLY SIGNIFICANT
SNOWFALL OCCURRING OVER PARTS OF THE NORTH INTERIOR LOWLANDS. ON
MONDAY...A STRONGER STORM WILL AFFECT THE REGION. THIS SYSTEM WILL
SERVE TO SCOUR OUT THE COLD AIR THAT HAS BEEN IN PLACE FOR ABOUT A
WEEK.
&&
.SHORT TERM...
A DENSE CLOUD COVER PERSISTED OVER THE FAR NW ZONES LATE THIS
AFTERNOON WHEREAS THE REST OF THE CWA PLENTY OF SUNSHINE. EXPECT THE
CLOUDS TO GRADUALLY INCREASE ACROSS THE REST OF THE AREA OVERNIGHT
THROUGH SATURDAY MORNING AS THE FLOW ALOFT DEVELOPS A MORE WLY
COMPONENT.
SATURDAY...DESPITE THE INCREASED CLOUD COVER...WILL BE A GENERALLY
DRY DAY. THERE WILL BE A CHANCE OF FLURRIES OVER MUCH OF THE AREA.
THE BEST CHANCE OF MEASURABLE PRECIP WILL BE THE FAR SE ZONES WHERE
THE NRN PORTION OF A LOW PRES SYSTEM MOVING INTO NRN CA WILL CLIP
THE AREA.
AS STATED IN THE MORNING DISCUSSION...THERE WILL BE SOME
RETROGRESSION OF THE LONG WAVE FEATURES THIS WEEKEND. THIS WILL
ALLOW AN UPPER LOW TO DEVELOP OVER BRITISH COLUMBIA OR WRN ALBERTA
LATE SATURDAY NIGHT OR SUNDAY. A SFC LOW IS EXPECTED TO DEVELOP OFF
VANCOUVER ISLAND IN RESPONSE. THIS WILL SERVE TO KEEP A
REINFORCEMENT OF COLD...MODIFIED ARCTIC AIR BOTTLED UP TO OUR N
WHILE AT THE SAME ALLOW SOME MILDER AIR TO MOVE IN FROM THE S ACROSS
THE COAST AND SRN ZONES. THERE WILL ALSO BE SUFFICIENT LIFT ON
SUNDAY FOR A DECENT CHANCE OF SHOWERS. SINCE MUCH OF THE CWA WILL
REMAIN UNDER A COLD AIR MASS...EXPECT THE PRECIP TO FALL AS SNOW. IN
FACT...THERE IS A POTENTIAL FOR SIGNIFICANT SNOWFALL...LOCALIZED
AMOUNTS UP TO 4 OR 5 INCHES...OVER PARTS OF THE N INTERIOR.
THE PRECIP THREAT WILL DECREASE SOME SUNDAY NIGHT AS THE COLD AIR
MASS OVER BRITISH COLUMBIA PLUNGES SWD. THEREFORE EXPECT ANY PRECIP
THAT FALLS ACROSS THE LOWLANDS SUNDAY NIGHT TO FALL AS SNOW. THE NLY
PRES GRADIENT WILL ALSO TIGHTEN SUNDAY NIGHT FOR GUSTY OUTFLOW
WINDS...FROM THE FRASER VALLEY...OVER THE N INTERIOR...WRN WHATCOM
COUNTY AND THE NRN SAN JUANS IN PARTICULAR.
A STRONGER STORM SYSTEM WILL APPROACH THE AREA FROM THE SW DURING
THE DAY MON. ANTICIPATE THE LOW LEVEL FLOW TO BECOME MORE ELY IN
RESPONSE TO FALLING PRESSURES OVER THE ERN PAC. THE WMFNT ASSOCIATED
WITH THIS FEATURE IS EXPECTED TO MOVE ACROSS THE SW PORTION OF THE
CWA MON AFTERNOON. THIS WILL BE A TRICKY FORECAST IN REGARDS TO
WHETHER THE PRECIP WILL FALL AS RAIN OR SNOW. IT IS POSSIBLE THAT
THE SIGNIFICANT PRECIP MAY MORE OR LESS COINCIDE WITH THE ARRIVAL OF
THE WMFNT...WHICH WOULD MEAN THAT THE PRECIP WOULD FALL AS RAIN OVER
THE SW PORTION OF THE CWA. THE COLD AIR WILL BE A BIT MORE STUBBORN
TO VACATE THE NRN ZONES...SO THIS AREA WILL LIKELY SEE SNOW LATE MON
BEFORE THE PRECIP CHANGES OVER TO RAIN. IN THIS TYPE OF PATTERN...AS
FAR AS THE LOWLANDS ARE CONCERNED...THE HOOD CANAL AREA WILL BE THE
PLACE TO WATCH FOR THE POTENTIAL FOR SIGNIFICANT SNOWFALL. LOW LEVEL
ELY FLOW HERE WILL ONLY SERVE TO KEEP THE COLD AIR TRAPPED AGAINST
THE E SLOPES OF THE OLYMPIC RANGE.
.LONG TERM...
THIS PERIOD WILL BE CHARACTERIZED BY A SERIES OF PAC STORMS
AFFECTING THE REGION. ANTICIPATE A MILDER AND WETTER PATTERN TUE
THROUGH THE END OF NEXT WEEK...WITH SIGNIFICANT RAINFALL POSSIBLE
OVER THE SW FACING SLOPES OF THE OLYMPICS IN PARTICULAR. BY THE END
OF THE PERIOD...SNOW LEVELS ARE EXPECTED TO BE NEAR 8K FT.
&&
.HYDROLOGY...
FLOODING WILL NOT BE A CONCERN FOR THE WRN WASHINGTON RIVERS THROUGH
MONDAY. MONDAY...HOWEVER...WILL BE A TRANSITION DAY FROM THE CURRENT
COLD SPELL TO A MILDER AND WETTER PATTERN.
RISING SNOW LEVELS AND INCREASED MOIST...ONSHORE FLOW TUE THROUGH
NEXT WEEKEND WILL RESULT IN RISING RIVER LEVELS DURING THIS PERIOD.
AT THIS TIME...IT APPEARED THAT THE RIVERS FLOWING OFF THE OLYMPICS
WILL HAVE THE POTENTIAL TO EXPERIENCE MINOR FLOODING...POSSIBLY AS
EARLY AS ABOUT THE MIDDLE OF NEXT WEEK.
NO FLOODING IS EXPECTED ON THE GREEN RIVER.
&&
.AVIATION...LIGHT WEST TO NORTHWEST FLOW WILL BE MAINTAINED ALOFT
THROUGH SATURDAY. A LOW PRESSURE SYSTEM WILL MOVE ONSHORE WELL
SOUTH OF THE REGION LATE SATURDAY WHILE HIGHER PRESSURE WILL REMAIN
TO THE NORTH AND EAST. THIS WILL CONTINUE ALLOWING FOR VERY LIGHT
WINDS. A THIN LAYER OF STRATUS ACROSS THE NORTHERN HALF OF THE
REGION WILL LIKELY SLOWLY BUILD SOUTH OVERNIGHT TONIGHT. A FEW
FLURRIES ARE POSSIBLE UNDER IT OVER THE NORTHWEST INTERIOR.
KSEA...AGAIN...THE ONLY POTENTIAL ISSUE WOULD BE THE LAYER OF
STRATUS CURRENTLY NORTH OF KPAE REDEVELOPING FAR ENOUGH SOUTH
OVERNIGHT TO AFFECT THE TERMINAL. THIS COULD HAPPEN EARLY SATURDAY
AND WILL INCLUDE A MENTION IN THE TAF...BUT NOT EXPECTING CIGS
BELOW 3000 FT. OTHER THAN THAT NO ISSUES THROUGH TONIGHT. WINDS
REMAINING VERY LIGHT NORTH-EAST. CERNIGLIA
&&
.MARINE...A LOW PRES SYSTEM WILL BYPASS WASHINGTON WELL TO THE SOUTH
LATE SATURDAY...HAVING A VERY LIMITED AFFECT ON OUR WEATHER. HIGH
PRES MOVING SOUTH THROUGH BRITISH COLUMBIA WILL INDUCE SOME
NORTHEAST FRASER OUTFLOW LATE SUNDAY INTO MONDAY WHICH WILL ALSO
PICK UP THE WINDS IN PORTIONS OF THE STRAIT OF JUAN DE FUCA.
AN APPROACHING...MORE TYPICAL PACIFIC FRONT WILL REACH THE WATERS ON
MONDAY...WITH MODERATELY STRONG SOUTHERLY WINDS. THE REMAINDER OF
THE WEEK WILL BECOME ACTIVE ONCE AGAIN AS A SERIES OF FRONTS ARE
EXPECTED TO MOVE ACROSS THE WATERS FOR PERIODS OF SMALL CRAFT TYPE
WINDS AND SEAS. CERNIGLIA
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...NONE.
PZ...NONE.
$$
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