000
FXUS66 KSEW 280525
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
925 PM PST MON DEC 27 2010
.SYNOPSIS...A FRONTAL SYSTEM WILL BRING SHOWERS AND MOUNTAIN SNOW
TONIGHT AND TUESDAY. AN UPPER TROUGH WILL MOVE OVER THE REGION
TUESDAY NIGHT AND WEDNESDAY WITH COOLER AIR AND A THREAT OF A
LOWLAND SNOW SHOWERS. DRY COOL NORTHERLY FLOW WILL PREVAIL FOR THE
REMAINDER OF THE WEEK ALTHOUGH A WEAK WEATHER SYSTEM COULD BRING
SOME RAIN OR SNOW SHOWERS FRIDAY OR FRIDAY NIGHT.
&&
.SHORT TERM...FRONTAL SYSTEM CURRENTLY MOVING THROUGH THE AREA
FASTER AND FARTHER SOUTH THAN EXPECTED. RAIN HAS ALREADY BECOME
SHOWERY AND TAPERED A BIT. AT THIS POINT EXPECTING MAINLY SCATTERED
SHOWERS FOR THE LOWLANDS FOR THE REMAINDER OF THE NIGHT. HAVE
ALREADY UPDATED THE FORECAST TO REFLECT CURRENT DEVELOPMENTS.
THE ONE PLACE IT WILL KEEP PRECIPITATING IS THE MOUNTAINS. EVEN
THOUGH THE BRUNT OF THE FRONTAL DYNAMICS ARE MOVING ONSHORE TO THE
SOUTH TONIGHT...COOL MOIST UNSTABLE ONSHORE FLOW WILL BRING HEALTHY
AMOUNTS OF SNOW TO THE MOUNTAINS TONIGHT AND TUESDAY. THE WINTER
STORM WARNING FOR THE CASCADES LOOKS ON TRACK ALTHOUGH IT MIGHT BE
FAIRLY MARGINAL WITH THE BEST MOISTURE GOING SOUTH. FOR THE
OLYMPICS...IT LOOKS MORE LIKE SNOW ADVISORY AMOUNTS WILL FALL AND
THE WINTER STORM WARNING HAS BEEN DOWNGRADED TO A WINTER WEATHER
ADVISORY.
ONSHORE FLOW WILL PREVAIL ON TUESDAY. LOWLANDS LOOK MORE DRY THAN
WET BUT THE MOUNTAINS WILL CONTINUE TO GET SNOW...GRADUALLY
DECREASING AS THE DAY PROGRESSES. A CONVERGENCE ZONE COULD FORM
NORTH OF SEATTLE FOR A BIT OF ENHANCED SHOWER ACTIVITY IN THAT AREA.
A COLD UPPER TROUGH WILL MOVE OVER THE AREA TUESDAY NIGHT. THIS WILL
BRING COLDER AIR AND AN UPTICK IN THE SHOWER ACTIVITY. IT WILL ALSO
BE COLD ENOUGH BY WEDNESDAY MORNING FOR SNOW DOWN TO NEAR SEA LEVEL.
ITS STILL LOOKING PRETTY UNFOCUSED AND HIT OR MISS BUT A FEW SPOTS
WITH MINOR ACCUMULATIONS CANNOT BE RULED OUT AT THIS POINT.
COOLER DRIER NORTHERLY FLOW WILL DEVELOP WEDNESDAY AS A HIGH
AMPLITUDE UPPER RIDGE BUILD OVER THE EASTERN PACIFIC. SOME LINGERING
SHOWERS WILL CONTINUE...MAINLY DURING THE MORNING AND THESE COULD BE
IN THE FORM OF SNOW DOWN TO NEAR SEA LEVEL.
WEDNESDAY NIGHT AND THURSDAY SHOULD BE DRY AND COOL AS THE UPPER
RIDGE OVER THE EASTERN PACIFIC SHIFTS A BIT EASTWARD. HIGHS ON
THURSDAY WILL STRUGGLE TO REACH 40 MANY AREAS. SCHNEIDER
.LONG TERM...PREVIOUS DISCUSSION...CONFIDENCE IN THIS PERIOD WAS NOT
VERY HIGH DUE TO THE MEDIUM RANGE SOLUTIONS PRESENTING A VARIETY OF
SCENARIOS. EITHER THIS PERIOD WILL BE COMPLETELY DRY OR WE WILL BE
UNDER AN ACTIVE NRN STREAM BY THE END OF THE WEEK. AT ANY
RATE...HAVE COMPROMISED BETWEEN THE EXTREMES AND GENERALLY WENT
BELOW CLIMO AS FAR AS POPS WERE CONCERNED. TEMPS SHOULD SLOWLY
MODERATE WITH TIME. TEMPS SHOULD RUN NEAR OR A LITTLE BELOW NORMAL
FOR THIS TIME OF THE YEAR.
&&
.HYDROLOGY...RAINS IN THE SKOKOMISH RIVER BASIN HAVE BEEN A BIT
LIGHTER THAN PREVIOUSLY EXPECTED WITH THE FRONTAL SYSTEM THAT MOVED
ONSHORE EARLIER TODAY. AT THIS POINT...ONLY EXPECTING SHOWERS
TONIGHT AND THEN DRYING ON TUESDAY. AS A RESULT...THE SKOKOMISH
RIVER HAS REMAINED SLIGHTLY BELOW ADVISORY STAGE THIS EVENING.
RUNOFF FROM RECENT RAINS COULD STILL DRIVE THE RIVER TO FLOOD
ADVISORY STAGE LATER TONIGHT BUT IT IS NOT LOOKING AS LIKELY AS
EARLIER THOUGHT. WILL CONTINUE THE FLOOD ADVISORY THIS EVENING BUT
IT MIGHT BE POSSIBLE TO DROP IT TUESDAY MORNING.
FOR OTHER AREAS...INCLUDING THE GREEN RIVER...FLOODING IS NOT
EXPECTED OVER THE NEXT 10 DAYS. SCHNEIDER
&&
.AVIATION...STRONG WESTERLY FLOW ALOFT WILL CONTINUE TONIGHT AND
TUESDAY. AT THE SURFACE A LOW OFF THE NORTH WA COAST WILL MOVE
NORTHEAST AND WEAKEN TONIGHT. STEADIER RAIN HAS ENDED EXCEPT IN
CASCADES...SO LOOKING FOR A FEW SHOWERS THROUGH TUE MORNING. ONSHORE
SURFACE FLOW WILL BEGIN TUESDAY AND CONTINUE INTO WEDNESDAY. AIR
MASS IS MOIST AND STABLE. LOOK FOR STEADILY IMPROVING CIGS STARTING
ARND 14Z AND CONTINUING THROUGH TUE EVNG.
KSEA...DISCUSSION ABOVE APPLIES. SOUTH FLOW THROUGH TUE MORNING.
LOOK FOR A QUICK SHIFT TO NORTHERLY IN THE 21Z-24Z TIME FRAME ON TUE
AFTN. HANER
&&
.MARINE...1005 MB LOW NOW NEAR THE NORTH WA COAST. IT WILL MOVE
INLAND AND WEAKEN OVERNIGHT. IN THE WAKE OF THE LOW...GRADIENTS WILL
TURN ONSHORE. SMALL CRAFT WESTERLIES EXPECTED TO DEVELOP THROUGHOUT
THE STRAIT WITH A PERIOD OF SMALL CRAFT SOUTHERLIES THROUGH PUGET
SOUND EARLY TUE MORNING. BASICALLY KEPT PREVIOUS ROUND OF SMALL
CRAFT ADVISORIES...EXCEPT FOR EXTENDING THE ADVISORY IN PUGET SOUND
LATER INTO TUE MORNING.
ONSHORE FLOW CONTINUES TUESDAY NIGHT AND BECOMES STRONGER WEDNESDAY.
WOULD NOT BE SURPRISED IF GALE WESTERLIES OCCUR IN THE STRAIT AND
THAT MAY NEED TO BE WATCHED. AFTER WEDNESDAY A DRY NORTHERLY FLOW
ALOFT SETS UP...WITH NORTHWEST WINDS ALONG THE COAST AND NORTHEAST
WINDS OVER INLAND AREAS. HANER/CHB
&&
.AVALANCHE...A STRONG STORM IS EXPECTED TO HIT THE NORTHWEST
MOUNTAINS TONIGHT INTO EARLY TUESDAY HEAVIEST IN THE SOUTH TO
CENTRAL CASCADES. THIS SHOULD DEPOSIT SIGNIFICANT SNOWFALL
ACCOMPANIED BY VERY STRONG WINDS AT RISING FREEZING LEVELS...LOADING
THE RECENT COLD SNOW RECEIVED OVERNIGHT. THIS WEATHER WILL CAUSE A
RAPID AND SIGNIFICANTLY INCREASING AVALANCHE DANGER MONDAY NIGHT
INTO TUESDAY MORNING.
NWAC HAS ISSUED AN AVALANCHE WARNING TO TONIGHT AND TUESDAY MORNING
TO HIGHLIGHT THIS EXPECTED INCREASED DANGER. KRAMER
&&
WA...WINTER STORM WARNING CASCADES.
.WINTER WEATHER ADVISORY OLYMPICS.
.AVALANCHE WARNING CASCADES AND OLYMPICS.
.FLOOD ADVISORY SKOKOMISH RIVER.
PZ...
$$
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