Winter Storm Watch 4, NWS Seattle, WA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
913 AM PST MON FEB 21 2011

.SYNOPSIS...AN UPPER LOW OFF THE NORTHERN CALIFORNIA COAST WILL
CONTINUE MOVING SOUTH TODAY. AN UPPER LEVEL TROUGH WILL DEVELOP OVER
THE AREA BEGINNING LATE THIS AFTERNOON AND CONTINUE THROUGH TUESDAY
BRINGING UNSETTLED AND SHOWERY WEATHER. A COLDER AIR MASS WILL MOVE
INTO THE REGION FROM THE NORTH ON WEDNESDAY AND THURSDAY...FOR THE
POSSIBILITY OF SOME LOWLAND SNOW. DRY AND COLD WEATHER COULD RETURN
ON FRIDAY.

&&

.SHORT TERM...MODELS TODAY ARE SHOWING RATHER GOOD CONSISTENCY AND
CONTINUITY...ESPECIALLY CONSIDERING THAT THE CURRENT UPPER LEVEL
CONFIGURATION IS A RIDGE AROUND 150W AND A MEAN TROUGH AROUND
115-120W. IT DOES LOOK INCREASINGLY LIKELY THAT A LATE SEASON ARCTIC
OUTBREAK WILL IMPACT AT LEAST THE NORTHERN HALF OF WESTERN
WASHINGTON BEGINNING LATER WED AND CONTINUING THROUGH EARLY IN THE
UPCOMING WEEKEND.

IN THE SHORT TERM...THE CUTOFF UPPER LOW THAT GAVE SOME SPOTTY LIGHT
SHOWERS TO THE COAST EARLY THIS MORNING IS NOW PULLING SOUTH THROUGH
THE NORTHERN CALIFORNIA OFFSHORE WATERS. A 160 KT NWLY JET STREAK AT
300 MB CENTERED JUST TO THE NW OF JUNEAU IS PLOWING SE. WHILE THIS
FEATURE WILL WEAKEN AS IT MOVES SE...IT WILL BRING QUICK CHANGES TO
THE WEATHER BEGINNING THIS AFTERNOON AS 500 MB HEIGHTS FALL AND
FRONTOGENESIS BEGINS FROM ABOUT THE NW INTERIOR OF WESTERN
WASHINGTON NORTHWESTWARD. THE MAIN IMPACTS OF THIS INCOMING NWLY
FLOW WILL BE TO DEEPEN THE TROUGH OVER THE PACIFIC NW...USHER IN
COOL MARITIME AIR...COOL CONDITIONS ALOFT...AND GIVE INCREASING
SHOWERS BEGINNING LATE THIS AFTERNOON. LATER TONIGHT THE MOUNTAINS
WILL LIKELY SEE INCREASING SNOWFALL...OF A FLUFFY 12 OR 15 TO 1
RATIO...THAT COULD RESULT IN UP TO 18 INCHES OF SNOW BETWEEN THIS
EVENING AND TUE EVENING. THIS WARRANTS THE ISSUANCE OF A WINTER
STORM WATCH LATER THIS MORNING FOR TONIGHT INTO TUE NIGHT FOR THE
CASCADES...THAT MAY NEED TO BE UPGRADED TO A WARNING LATER THIS
AFTERNOON. STRONGER NW WINDS WITH THE INCOMING TROUGH AND JET SHOULD
REMAIN OFF THE COAST...BUT THE RESULTANT W TO NW FLOW THROUGH THE
STRAIT WILL LIKELY RESULT IN THE QUICK FORMATION TONIGHT OF A
CONVERGENCE ZONE THAT HANGS UP SOMEWHERE FROM N KING COUNTY TO
SOMEWHERE OVER SNOHOMISH COUNTY.

WITH ALL THE ONSHORE FLOW...TEMPERATURES IN THE BOUNDARY LAYER WILL
BE MARGINAL FOR WET SNOW WITH THE CONVERGENCE ZONE. BUT THE UW WRF
MODELS DRIVEN BY THE GFS AND THE NAM...AS WELL AS THE OPERATIONAL
RUNS...ALL SHOW SOME POSSIBILITY OF SNOW OR MIXED RAIN AND SNOW WITH
THE CONVERGENCE ZONE VERY LATE TONIGHT AND TUE MORNING. CURRENT
FORECASTS ARE ALREADY WORDED THIS WAY...BUT INDICATE NO
ACCUMULATION. WILL REASSESS THE WORDING THIS AFTERNOON OVER THE
HIGHER TERRAIN OF SNOHOMISH COUNTY AND IN THE EAST PUGET SOUND
LOWLANDS.

THE POTENTIAL FOR BIGGER PROBLEMS BEGIN ON WED AS A 515-520 DAM 500
MB LOW DEVELOPS OVER VANCOUVER ISLAND AND SLIDES SLOWLY S ALONG THE
COAST BY THU MORNING. IN THE MEANTIME A 1050-1055 MB SURFACE HIGH
DEVELOPS OVER THE NORTH CENTRAL PORTION OF BC AND SLIDES S. THE PRES
GRADIENT BETWEEN THE ARCTIC HIGH AND THE LOW PRES DEVELOPING IN THE
COASTAL WATERS WILL SET UP IMPRESSIVE OUTFLOW OVER THE NORTH
INTERIOR STARTING WED AND CONTINUING THROUGH AT LEAST THU.

THE 12Z NAM...WRFGFS...00Z ECMWF AND 12Z GFS ALL HINT AT THE
DEVELOPMENT OF AN ARCTIC BOUNDARY THAT COULD RESULT IN LOCALLY HEAVY
SNOWFALL OVER THE N INTERIOR WED AND POSSIBLY INTO THE CENTRAL AND
SOUTH SOUND AREAS WED NIGHT OR THU. POSSIBLY DAMAGING WIND WITH THE
OUTFLOW OVER THE SAN JUANS AND WESTERN WHATCOM COUNTY CANNOT BE
RULED OUT...AND FREEZING SPRAY AND STORM FORCE CONDITIONS COULD
IMPACT THE WATERS. WILL TAKE A FINAL LOOK AT THE 12Z ECMWF ...BUT IT
IS LIKELY THAT SOME WATCHES WILL BE ISSUED FOR THE NORTH
INTERIOR AREAS THIS AFTERNOON FOR WED AND WED NIGHT. ALBRECHT

.LONG TERM...PREVIOUS LONG TERM DISCUSSION FOLLOWS...A MODIFIED
ARCTIC BOUNDARY IS EXPECTED TO PUSH SOUTHWARD ACROSS WESTERN
WASHINGTON WEDNESDAY NIGHT AND THURSDAY...AS FRASER RIVER OUTFLOW
INCREASES IN RESPONSE TO STRONG SURFACE RIDGING BUILDING SOUTHWARD
OVER INTERIOR BRITISH COLUMBIA. EXACTLY HOW MUCH MOISTURE WILL BE
AVAILABLE BY THIS POINT IN TIME OR HOW THIS SYSTEM WILL EVOLVE IS
STILL IN QUESTION. WITH THAT SAID...WEDNESDAY NIGHT OR EARLY
THURSDAY COULD BE THE BEST PERIOD FOR ACCUMULATING LOWLAND
SNOWFALL...BUT ITS STILL FAR TOO EARLY TO PUT ANY CONFIDENCE IN
DETAILS.

BY THURSDAY NIGHT AND FRIDAY...COLDER DRIER AIR WILL BE FILTERING
INTO THE AREA AS THE UPPER LEVEL TROUGH SHIFTS SOUTH OF THE REGION.
THE 00Z ECMWF STILL DIFFERS FROM THE GFS SOMEWHAT BY THE
WEEKEND...BUT IT IS NOT AS RADICALLY DIFFERENT AS IT WAS 24 HOURS
AGO. THE GFS/CANADIAN/ECMWF ARE IN GENERAL AGREEMENT FOR DRY AND
RATHER COLD WEATHER FRIDAY INTO THE FIRST PART OF WEEKEND.
TEMPERATURES WILL START TO MODERATE BY SUNDAY AS COLD NORTHERLY FLOW
GIVES WAY TO INCREASING ONSHORE FLOW. ANOTHER SYSTEM COULD SLIP DOWN
THE BRITISH COLUMBIA COAST TOWARD THE REGION BY LATE IN THE WEEKEND.
NO CHANGES MADE TO THE EXTENDED FORECAST OVERNIGHT. 27

&&

.HYDROLOGY...THERE IS NO INDICATION THAT A FLOOD PRODUCING PATTERN
WILL DEVELOP DURING THE NEXT TEN DAYS. LOW SNOW LEVELS WILL PRECLUDE
ANY THREAT OF FLOODING ON ANY AREA RIVERS...INCLUDING THE GREEN.

&&

.AVIATION...VFR CONDITIONS EXPECTED THIS MORNING AND AFTERNOON
ACROSS WESTERN WA. A BROAD UPPER LEVEL TROUGH OVER B.C. WILL DROP
SOUTH INTO W WA TONIGHT...AIDED BY A NWLY 120 KT JET. SHOWERS WILL
INCREASE ACROSS THE REGION THIS EVENING AS ONSHORE DEVELOPS. THE
CENTRAL PUGET SOUND AREA MAY STAY SHADOWED BY DOWNSLOPE FLOW OFF THE
OLYMPICS. SNOW SHOWERS ARE POSSIBLE OVERNIGHT...ESPECIALLY IN THE
CONVERGENCE ZONE. THE PATTERN REMAINS ACTIVE ON TUE AS STRONG
ONSHORE FLOW CONTINUES.

KSEA...LIGHT SOUTHEAST WIND THIS MORNING WILL BECOME SOUTHWESTERLY
THIS AFTERNOON AND INCREASE TO 10-15 KT. SHOWERS INCREASING
TONIGHT...MAINLY AFTER 03Z.  33

&&

.MARINE...GRADUALLY INCREASING SOUTHERLY GRADIENTS WILL PREVAIL
TODAY. A FRONTAL SYSTEM WILL MOVE INTO THE WATERS FROM THE NORTHWEST
TONIGHT AND TUESDAY. BRISK ONSHORE FLOW WILL FOLLOW THE SYSTEM WITH
SMALL CRAFT ADVISORY WINDS LIKELY OVER ALL WATERS.

OFFSHORE FLOW WILL DEVELOP BEHIND THE FRONT WEDNESDAY THROUGH
FRIDAY. THE FLOW COULD BE STRONG ENOUGH WED OR THU FOR GALE FORCE
WINDS OVER THE NORTHERN INLAND WATERS...THE WEST ENTRANCE...AND THE
NORTH COAST.  33

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...WINTER STORM WATCH LATER TONIGHT THROUGH TUESDAY NIGHT FOR THE
      WEST SLOPES OF THE NORTH AND CENTRAL CASCADES AND PASSES.

PZ...SMALL CRAFT ADVISORY ALL WATERS.

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