Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 290542
AFDSEW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
940 PM PST FRI NOV 28 2014

.SYNOPSIS...WHILE RAIN CONTINUES TO DIMINISH OVER MUCH OF WESTERN
WASHINGTON...A CONVERGENCE ZONE WILL FORM OVER NORTHERN PORTIONS OF
THE PUGET SOUND AREA TONIGHT WITH SNOW LEVELS LOWERING TO BELOW 300
FEET OVERNIGHT. A MODIFIED ARCTIC COLD FRONT WILL MOVE SOUTH THROUGH
THE AREA LATE TONIGHT INTO SATURDAY MORNING...WITH LIGHT SNOW
SHOWERS EXPECTED WITH THE FRONT. WEAK HIGH PRESSURE ALOFT AND LOW
LEVEL OFFSHORE FLOW WILL BRING MAINLY COOL AND DRY WEATHER SUNDAY
THROUGH EARLY NEXT WEEK.

&&

.SHORT TERM...CURRENT OBSERVATIONS SHOW THAT RAINFALL IS FINISHING
OVER MUCH OF WESTERN WASHINGTON THIS EVENING...WITH THE EXCEPTION
OF SOME SCATTERED ACTIVITY ALONG THE COAST. THE FEATURE ALL EYES ARE
WATCHING TONIGHT IS THE DEVELOPMENT OF A CONVERGENCE ZONE OVER THE
NORTH PUGET SOUND AREA. LOOKING AT TEMPS...OBS...AND WEBCAMS IN THE
AREA...STARTING TO SEE THE TRANSITION OVER TO SNOW. WITH TEMPS IN
THE MID 30S AND TEMPS EXPECTED TO DROP FURTHER...EXPECTING SNOW
CONCERNS IN THE WINTER WEATHER ADVISORY AREA TO BE MORE WIDESPREAD.
ANOTHER SNOW MATTER MAY RESIDE NORTH OF THE ADVISORY AREA...UP NORTH
OF BLI. CURRENT HRRR PUTS A FAIR AMOUNT OF PRECIP UP THERE. SEEING
SOME SNOW AT ORCAS...SO WHILE THIS SOLUTION MIGHT BE A TOUCH
OVERDONE...AS THE MODEL IS CALLING FOR 4 INCHES...THE PROSPECT OF
SNOW IS MATERIALIZING UP THERE. AS SUCH...WILL OPT FOR A WINTER
WEATHER ADVISORY FOR WESTERN WHATCOM...SKAGIT AND SAN JUAN COUNTIES
FOR 1 TO 3 INCHES OF SNOW. TO PRECIP OF A MORE LIQUID VARIETY...WITH
THE RAIN TAPERING OFF...FLOOD CONCERNS WILL DIMINISH
ACCORDINGLY...THOUGH ARE CERTAINLY NOT OFF THE TABLE YET AS RIVERS
STILL REACTING TO EARLIER RAINS. LASTLY...REMAINING FORECAST CONCERN
IS THE DEVELOPMENT OF FRASER OUTFLOW WINDS. AT THIS TIME...WINDS
REALLY HAVE NOT DEVELOPED YET. HOWEVER...MODEL DATA PERTAINING TO
GRADIENTS REMAINS CONSISTENT...SO REALLY ONLY A MATTER OF TIME
BEFORE EXPECTED WINDS DO INDEED DEVELOP.

AS FORECAST ROLLS INTO SATURDAY...THINGS GET CONSIDERABLY
EASIER...ALBEIT COLDER. THE AIR MASS WILL BEGIN DRYING AND COOLING
EVEN MORE FROM THE NORTH. PRECIPITATION WILL PROBABLY SHUT OFF EARLY
SATURDAY AFTERNOON. TEMPERATURES ARE NOT EXPECTED TO MAKE IT OUT OF
THE 30S ON SATURDAY. THE CLEARING AND COLD AIR WILL SET UP LOW
TEMPERATURES IN THE TEENS AND 20S SUNDAY MORNING. THE COLDEST AREAS
WILL BE THE NORTH INTERIOR. HIGHS SUNDAY WILL NOT MAKE IT OUT OF THE
30S.

FOR MONDAY THE MAIN PLAYERS WILL BE THE UPPER LEVEL RIDGE OVER THE
NE PACIFIC...GIVING COLD NW FLOW ALOFT TO W WA....AND A LARGE LOW W
OF CA WHICH WILL BE BRINGING WARM MOIST AIR UP THE COAST FROM CA
INTO OREGON. GFS AND ECMWF HAVE BEEN HAVING DIFFICULTY RESOLVING
THIS BATTLE BETWEEN THE NORTHERN AND SOUTHERN STREAMS THE PAST
COUPLE OF DAYS. MODEL SOLUTIONS HAVE BEEN OSCILLATING BETWEEN WHICH
STREAM WILL IMPACT W WA. THE SOUTHERN STREAM HAS THE UPPER HAND AT
THIS MOMENT AND THE MONDAY FORECAST NOW HAS A CHANCE OF RAIN OVER
THE SW INTERIOR. SMR/KAM

.LONG TERM...FROM 344 PM DISCUSSION...THE MODEL WOBBLES FOR MONDAY
EXTEND THROUGH MID WEEK AND CONFIDENCE IS VERY LOW. THE CURRENT
FORECAST STILL LEANS TOWARD THE MORE CONSISTENT ECMWF...WITH DRY
WEATHER THROUGH TUESDAY. THE ECMWF FINALLY BRINGS SOME MOISTURE FROM
THE OFFSHORE LOW INLAND WEDNESDAY THROUGH FRIDAY FOR A CHANCE OF
RAIN. KAM

&&

.HYDROLOGY...THE FRONT THAT BROUGHT HEAVY RAIN TO THE CASCADES TODAY
HAS MOVED SOUTH AND LOST MUCH OF ITS ENERGY. HEAVY RAIN HAS ENDED IN
WHATCOM AND SKAGIT COUNTIES. SNOHOMISH AND KING COUNTIES ARE STILL
HAVING MODERATE PRECIPITATION BUT IT HAS TAPERED SIGNIFICANTLY.
PERHAPS THE MAIN CHANGE IS THAT MUCH OF THE PRECIPITATION IN THE
MOUNTAINS HAS TURNED TO SNOW. TEMPERATURES HAVE FALLEN INTO THE LOW
30S OVER A WIDE SWATH OF THE CASCADES...FROM THE CANADIAN BORDER
SOUTH TO THE SNOQUALMIE DRAINAGE. WEB CAMERAS SHOW SNOW AT STEVENS
AND SNOQUALMIE PASSES. THE CHANGE TO SNOW GENERALLY MEANS THE
HYDROLOGICALLY SIGNIFICANT PRECIPITATION HAS ENDED AND RIVERS NOW
HAVE ONLY TO DEAL WITH RUNOFF FROM RAIN THAT HAS ALREADY FALLEN.

DISCUSSING THE RIVERS FROM NORTH TO SOUTH...

THE SOUTH FORK OF THE NOOKSACK RIVER HAS CRESTED AND HAS FALLEN
BELOW FLOOD STAGE. THE FLOOD WARNING HAS ENDED THERE. THE MAIN STEM
AT NORTH CEDARVILLE HAS ALSO CRESTED BUT IS STILL JUST ABOVE FLOOD
STAGE. THE WARNING THERE WILL END SOMETIME THIS EVENING. THE
FORECAST FOR THE REACH OF THE RIVER NEAR FERNDALE DOES NOT TAKE THE
RIVER ABOVE FLOOD STAGE.

THE SKAGIT RIVER NEAR CONCRETE IS CRESTING NOW AROUND 32 FEET. THIS
IS WELL ABOVE THE MODERATE FLOOD LEVEL AT CONCRETE AND THIS CREST
WILL PROPAGATE DOWNSTREAM REACH THE MODERATE LEVEL AT MOUNT VERNON
TONIGHT. THE US ARMY CORPS OF ENGINEERS IS OPERATING THE DAMS ON THE
SKAGIT AND BAKER RIVERS. THEIR PLAN IS TO PREVENT MAJOR FLOODING ON
THE SKAGIT AND THAT PLAN APPEARS TO BE WORKING. THE FLOOD WARNING
WILL REMAIN IN EFFECT ON THE SKAGIT THROUGH SATURDAY NIGHT.

THE UPPER REACHES OF THE STILLAGUAMISH RIVER HAVE CRESTED AND ARE
FALLING. IT NOW APPEARS NEITHER FORK OF THE RIVER WILL REACH FLOOD
STAGE ON ITS OWN. HOWEVER THE MAIN STEM AT ARLINGTON HAS REACHED
FLOOD STAGE AND WILL REMAIN NEAR FLOOD STAGE FOR A FEW MORE HOURS.
MINOR FLOODING IS FORECAST ON THE LOWER STILLAGUAMISH.

THE UNCONTROLLED SKYKOMISH RIVER AND THE SNOHOMISH RIVER MAY HAVE
THE BIGGEST FLOOD OF THIS EVENT. AT GOLD BAR THE RIVER APPEARS TO BE
CRESTING NOW LESS THAN A FOOT BELOW THE MAJOR FLOOD STAGE OF 19
FEET. UPSTREAM RIVER GAUGES ARE FALLING. MAJOR FLOODING IS FORECAST
DOWNSTREAM ON THE REACH OF THE SNOHOMISH RIVER NEAR MONROE. THE
RIVER AT MONROE IS ALREADY ABOVE FLOOD STAGE AND WILL CREST AROUND
10 PM TONIGHT JUST ABOVE THE MAJOR FLOOD STAGE OF 17 FEET. FURTHER
DOWNSTREAM NEAR SNOHOMISH THE RIVER IS FORECAST TO CREST JUST BELOW
THE MAJOR FLOOD STAGE OF 29 FEET LATE TONIGHT.

THE SNOQUALMIE RIVER BASIN RECEIVED LESS RAIN AND THE PRECIPITATION
HAS ALREADY CHANGED TO SNOW IN THE MOUNTAINS. THE SUM OF THE THREE
UPPER SNOQUALMIE FORKS INDICATES THE SNOQUALMIE RIVER AT SNOQUALMIE
FALLS WILL CREST AROUND FLOOD STAGE. THE TOLT RIVER TOUCHED FLOOD
STAGE EARLY THIS AFTERNOON AND IS FALLING NOW. THE REACH OF THE
SNOQUALMIE RIVER NEAR CARNATION WILL RISE ABOVE FLOOD STAGE THIS
EVENING AND REMAIN ABOVE FLOOD STAGE THROUGH SATURDAY.

NO FLOODING IS FORECAST ON ANY RIVERS SOUTH OF THE SNOQUALMIE THOUGH
RIVERS WILL RISE SOMEWHAT. A FLOOD WATCH REMAINS IN EFFECT FOR ALL
COUNTIES FROM SKAGIT TO LEWIS BUT NEW FLOODING IS UNLIKELY AND THE
WATCH WILL BE ALLOWED TO EXPIRE AT 6 PM THIS EVENING.

IN THE OLYMPICS THE SKOKOMISH RIVER WILL REMAIN ABOVE FLOOD STAGE
THROUGH SATURDAY. OTHER RIVERS IN THE OLYMPICS CRESTED BELOW FLOOD
STAGE AND ARE FALLING.

COLD AND DRY WEATHER WILL FOLLOW THE FRONT TONIGHT. RIVERS WILL
RECEDE AND THE THREAT OF FLOODING WILL END FOR THE FORESEEABLE
FUTURE. BURKE

&&

.AVIATION...MOIST WESTERLY FLOW ALOFT WILL CONTINUE TONIGHT. MVFR
CIGS ARE LIKELY IN THE INTERIOR. EXPECT GUSTY NE WINDS AT KBLI AS
FRASER RIVER OUTFLOW DEVELOPS. LOWLAND SNOW IS POSSIBLE TONIGHT
INTO SAT MORNING AS A COOL UPPER LEVEL TROUGH CROSSES OVERHEAD. SNOW
WILL TAPER OFF 18-21Z SAT AS THIS TROUGH EXITS AND PRECIP SHIFTS TO
THE CASCADES. THE AIR MASS WILL REMAIN COOL AND DRY SAT NIGHT WITH
NORTHERLY FLOW. 33

KSEA...A COOL AND UNSETTLED AIR MASS WILL BRING A CHANCE OF SNOW
SHOWERS LATE TONIGHT INTO SAT MORNING. BEST TIME FRAME IS 12-18Z AS
WINDS SWITCH TO NLY AND THE UPPER TROUGH AXIS CROSSES THE AREA.
SURFACE TEMPS WILL BE NEAR FREEZING. SNOWFALL UP TO 1 INCH IS
POSSIBLE. DRIER WEATHER IS FORECAST BY 21Z. 33

&&

.MARINE...FRASER RIVER OUTFLOW WILL BRING NORTHERLY GALES TO THE
NORTHERN INLAND WATERS TONIGHT. THERE WERE WESTERLY GALES THROUGH
THE CENTRAL/EAST STRAIT THIS EVENING...BUT WINDS SHOULD EASE AND
TURN N/NE AS THE FRASER OUTFLOW DEVELOPS. NORTH/NORTHEAST FLOW
WILL PREVAIL ACROSS THE REGION THROUGH SAT MORNING...THEN EASE SAT
AFTERNOON. HIGH PRES EAST OF THE CASCADES WILL MAINTAIN OFFSHORE
FLOW SAT NIGHT THROUGH EARLY NEXT WEEK.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...FLOOD WATCHES AND WARNINGS IN EFFECT FOR PORTIONS OF
      WESTERN WASHINGTON.
     WIND ADVISORY WESTERN WHATCOM...AND SAN JUAN COUNTY.
     WINTER WEATHER ADVISORY NORTH PUGET SOUND REGION...STRAIT OF
      JUAN DE FUCA...WESTERN WHATCOM...SKAGIT AND SAN JUAN COUNTIES.

PZ...GALE WARNING NORTHERN INLAND WATERS THROUGH SAT AFTERNOON.
     GALE WARNING CENTRAL AND EAST STRAIT OF JUAN DE FUCA THIS
     EVENING.
     SMALL CRAFT ADVISORY ALL WATERS.
     SMALL CRAFT ADVISORY GRAYS HARBOR BAR FOR ROUGH BAR CONDITIONS.


&&

$$

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