Heavy Snow in Washington, November 21 to 22, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
300 AM PDT SAT NOV 21 2009

.SYNOPSIS...A WEAK RIDGE WILL MOVE THROUGH WESTERN WASHINGTON THIS
MORNING...FOLLOWED QUICKLY BY A VIGOROUS FRONTAL SYSTEM. THIS SYSTEM
WILL BRING RAIN AND WINDY CONDITIONS TONIGHT AND SUNDAY. HEAVY SNOW
WILL FALL IN THE MOUNTAINS WITH THE SNOW LEVEL REMAINING AT OR BELOW
3000 FEET. A WEAKER FRONT WILL MOVE THROUGH THE AREA MONDAY AND
MONDAY NIGHT...AND ANOTHER WEAK FRONT WILL ARRIVE LATE IN THE WEEK.

&&

.SHORT TERM...WE HAVE NORTHWEST FLOW ALOFT OVER WESTERN WASHINGTON
BETWEEN AN UPPER TROUGH THAT MOVED THROUGH THE AREA OVERNIGHT AND A
WEAK UPPER RIDGE OFFSHORE ALONG 130W. THERE ARE STILL SCATTERED
SHOWERS ACROSS THE FORECAST AREA THIS MORNING...AND A PUGET SOUND
CONVERGENCE ZONE EXTENDING ACROSS ISLAND AND SNOHOMISH COUNTIES. IT
IS STILL BREEZY IN SOME AREAS...BUT THE WIND SHOULD EASE OVER THE
NEXT FEW HOURS AS THE SURFACE HIGH PRESSURE THAT HAS BEEN BUILDING
INTO THE AREA SHIFTS EAST OF THE CASCADES AHEAD OF THE NEXT FRONT.
THE FREEZING LEVEL HAS FALLEN TO AROUND 3000 FT AND TEMPERATURES
ACROSS THE WESTERN WASHINGTON LOWLANDS WERE IN THE UPPER 30S TO MID
40S AT 3 AM.

THE WEAK RIDGE OFFSHORE WILL MOVE QUICKLY ACROSS WESTERN WASHINGTON
THIS MORNING...AND THE CONVERGENCE ZONE WILL LIKELY WEAKEN AND
DISSIPATE. THE SOUTH END OF AN OCCLUDED FRONT SWINGING INTO BRITISH
COLUMBIA WILL QUICKLY FOLLOW THE RIDGE...WITH SIGNIFICANT
PRECIPITATION REACHING THE COAST AND OLYMPICS THIS AFTERNOON. A
FRONTAL WAVE BEGINNING TO DEVELOP NEAR 45N/145W WILL DEEPEN AS IT
MOVES RAPIDLY EASTWARD WITH THE HELP OF A 140 KT JET. IT WILL MOVE
THROUGH THE FORECAST AREA TONIGHT.

HEAVY SNOW IN THE MOUNTAINS LOOKS LIKE A SURE BET. THE SNOW LEVEL
WILL RISE ONLY TO AROUND 3000 FT AS THIS FEATURE MOVES THROUGH...AND
PRECIPITATION AMOUNTS WILL BE IMPRESSIVE. LIQUID AMOUNTS OF 3 TO 4
INCHES IN THE OLYMPICS ARE POSSIBLE THROUGH SUNDAY MORNING --
TRANSLATING ROUGHLY TO 30 TO 40 INCHES OF SNOW. EVEN THE RELATIVELY
DRIER HURRICANE RIDGE AREA SHOULD RECEIVE WINTER STORM WARNING
AMOUNTS...IE 12 INCHES IN 12 HOURS OR 18 INCHES IN 24 HOURS. WATER
EQUIVALENTS IN THE CASCADES WILL LIKELY BE IN THE 1 TO 3 INCH
RANGE...FOR SNOWFALL TOTALS OF 10 INCHES TO 2.5 FT. THE WINTER STORM
WATCH WILL BE UPGRADED TO A WARNING WITH THE EARLY MORNING FORECAST
PACKAGE.

ONE MATTER THAT IS SOMEWHAT UNCERTAIN IS THE TRACK OF THE SURFACE
LOW. THE MOST LIKELY SCENARIO IS THAT IT WILL REACH THE CENTRAL
WASHINGTON COAST AROUND MIDNIGHT TONIGHT...TRACK NORTHEAST ACROSS
THE OLYMPIC PENINSULA AND THE NORTH PART OF THE PUGET SOUND REGION
TO AROUND MT BAKER AT 4 AM...THEN HEAD RAPIDLY INTO BRITISH
COLUMBIA. IT WILL PRODUCE BREEZY TO WINDY CONDITIONS...BUT THERE
APPEARS TO BE LITTLE THREAT OF A WIND STORM.

POST-FRONTAL SHOWERS WILL TAPER OFF SUNDAY AFTERNOON AND EVENING...
AND AN UPPER RIDGE WILL MOVE INLAND SUNDAY NIGHT. THE RIDGE WILL
QUICKLY HEAD EAST OF THE CASCADES MONDAY AND ANOTHER FRONTAL SYSTEM
JUST UPSTREAM OF THE RIDGE WILL ARRIVE. THIS SYSTEM COULD INITIALLY
BRING DECENT PRECIPITATION AMOUNTS TO THE COAST AND OLYMPIC
PENINSULA MONDAY AFTERNOON...BUT IT WILL WEAKEN AS IT MOVES INLAND
MONDAY NIGHT. BY RECENT STANDARDS THIS WILL BE AN UNEVENTFUL SYSTEM
AND IT WILL MARK THE BEGINNING OF A PERIOD OF QUIETER...THOUGH NOT
NECESSARILY DRY...WEATHER. MCDONNAL

.LONG TERM...A WEAK UPPER RIDGE WILL FOLLOW THE FRONT THAT MOVES
THROUGH MONDAY AND MONDAY NIGHT. THERE IS JUST A CHANCE OF RAIN IN
THE FORECAST FOR TUESDAY AND WEDNESDAY...WITH A FAIRLY GOOD CHANCE
FOR SOME DRY WEATHER OVER THOSE TWO DAYS. A WEAK FRONT WILL SLOWLY
MOVE ACROSS WESTERN WASHINGTON WEDNESDAY NIGHT AND THURSDAY. ANOTHER
FRONTAL WAVE WILL PROBABLY KEEP THE AREA WET THURSDAY NIGHT AND
FRIDAY...THOUGH DISAGREEMENT BETWEEN MODELS DROPS OFF CONSIDERABLY
BY LATE IN THE WEEK. MCDONNAL

&&

.HYDROLOGY...THE SYSTEM COMING IN TODAY THROUGH MIDDAY SUNDAY ...
WHILE IT WILL PRODUCE IMPRESSIVE AMOUNTS OF PRECIPITATION...WILL
HAVE MINIMAL EFFECT ON RIVERS EXCEPT POSSIBLY FOR THE TWO RIVERS
THAT ARE ALREADY CLOSE TO OR ABOVE FLOOD STAGE -- THE SKOKOMISH AND
THE LOWER REACH OF THE CHEHALIS IN GRAYS HARBOR COUNTY. WITH THE
SNOW LEVEL ONLY RISING TO AROUND 3000 FT...IT WILL BE A MOUNTAIN
SNOW PRODUCER NOT A FLOODING RAIN PRODUCER.

THE SKOKOMISH RIVER IS FORECAST TO FALL BELOW FLOOD STAGE ABOUT NOON
TODAY. THERE WILL BE A PLANNED RELEASE OF 3000 CFS FROM CUSHMAN DAM
#2 INTO THE NORTH FORK SKOKOMISH RIVER TODAY. THIS RELEASE WILL KEEP
THE SKOKOMISH RIVER RUNNING HIGH. THE FORECAST CALLS FOR THE RIVER
TO REMAIN JUST BELOW FLOOD STAGE TONIGHT AND SUNDAY.

A FLOOD WATCH REMAINS IN EFFECT FOR THE LOWER REACH OF THE CHEHALIS
RIVER IN GRAYS HARBOR COUNTY...WHICH HAS BEEN WITHIN A FOOT OF FLOOD
STAGE FOR THE PAST TWO DAYS. IT IS CONCEIVABLE THAT THE
PRECIPITATION FROM THE INCOMING SYSTEM COULD HELP FINALLY NUDGE THE
LOWER CHEHALIS ABOVE FLOOD STAGE. IF FLOODING DOES OCCUR IT WILL BE
MINOR.

THE NEXT SYSTEM ON MONDAY SHOULD MOVE THROUGH QUICKLY AND BE TOO
WEAK TO CAUSE ANY FLOODING...EVEN WITH RISING SNOW LEVELS. FOR THE
REMAINDER OF THE WEEK...THE AIR MASS OVER THE NORTHWEST WILL BE
WARMER WITH HIGHER SNOW LEVELS...HOWEVER THE SYSTEMS THAT ARE
EXPECTED TO AFFECT THE AREA WILL BE FAIRLY WEAK AND ARE NOT EXPECTED
TO CAUSE ANY ADDITIONAL FLOODING CONCERNS.

NO FLOODING IS EXPECTED AT ANY TIME ON THE GREEN RIVER. MCDONNAL

&&

.AVIATION...DELAYED.

&&

.MARINE...A STRONG FRONTAL SYSTEM WILL APPROACH THE AREA TODAY...
RESULTING IN INCREASED SOUTHERLY FLOW. THE OCCLUDED FRONT WILL THEN
MOVE ACROSS THE AREA THIS EVENING. STRONG ONSHORE OR WESTERLY FLOW
WILL DEVELOP ON SUNDAY IN THE WAKE OF THIS FRONT. EXPECT THE NEXT
FRONTAL SYSTEM TO AFFECT THE WATERS ON MON.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WARNINGS AND/OR FLOOD WATCHES REMAIN IN EFFECT FOR A
     PORTION OF THE CWA.

     A WINTER STORM WARNING IS IN EFFECT FOR THE OLYMPICS AND
     CASCADES FROM THIS AFTERNOON THROUGH SUNDAY MORNING.

PZ...A GALE WARNING REMAINS IN EFFECT FOR THE COASTAL WATERS AND
     WEST ENTRANCE TO THE STRAIT OF JUAN DE FUCA THROUGH THIS
     EVENING.

     A SMALL CRAFT ADVISORY IS IN EFFECT FOR THE REMAINING
     WATERS.

     A SMALL CRAFT ADVISORY FOR ROUGH GRAYS HARBOR BAR CONDITIONS
     IS IN EFFECT TIL NOON MON.

WWW.WEATHER.GOV/SEATTLE

FOR AN ILLUSTRATED VERSION OF THE FORECAST DISCUSSION...PLEASE SEE
WWW.WEATHER.GOV/SEATTLE/GAFD/LATEST_WEBAFD.HTML.