613
FXUS66 KSEW 210412
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
812 PM PST SAT DEC 20 2025
UPDATE
OVERALL NO CHANGES TO THE FORECAST TONIGHT. THERE ARE
A DECENT AMOUNT OF MODERATE SHOWERS TRACKING ACROSS THE AREA
AND ARE FORECAST TO CONTINUE TO DO SO THROUGH MIDNIGHT, THUS
ALLOWING THE WINTER STORM WARNING TO CONTINUE. PRECIPITATION
SHOULD EASE UP A BIT TONIGHT BUT WILL CONTINUE TO MONITOR
TRENDS. RIVERS CONTINUE TO TREND DOWNWARDS AS THE DAMS ARE
BEGINNING TO RELEASE LESS WATER. THE FLOOD WARNING WHITE RIVER
BELOW MUD MOUNTAIN DAM HAS BEEN CANCELLED AS WELL. SEE BELOW FOR
AN UPDATE TO THE AVIATION DISCUSSION. 62
SYNOPSIS
PERIODS OF LOWLAND RAIN, MOUNTAIN SNOW, AND BREEZY CONDITIONS
WILL CONTINUE ACROSS WESTERN WASHINGTON OVER THE NEXT WEEK AS A
SERIES OF WEATHER SYSTEMS MOVES ACROSS THE REGION. A TREND
TOWARDS COOLER CONDITIONS LOOKS LIKELY AROUND MIDWEEK.
SHORT TERM /TONIGHT THROUGH MONDAY NIGHT/
ACTIVE CONDITIONS ARE FORECAST THROUGHOUT THE SHORT-TERM
FORECAST AND LIKELY BEYOND. A BROAD UPPER-LOW REMAINS OVER THE
GULF OF ALASKA AND WILL CONTINUE TO EJECT SHORTWAVES INTO THE
REGION ALLOWING LOWLAND RAIN SHOWERS, MOUNTAIN SNOW, AND BREEZY
WINDS AT TIMES. HREF GUIDANCE HAS A MEAGER AMOUNT OF CAPE
PRESENT ALONG THE COAST THROUGH TONIGHT AND SUNDAY SO A SLIGHT
CHANCE OF THUNDERSTORMS MUST BE EXPRESSED AS WELL. OVERALL
RAINFALL AMOUNTS FOR THE LOWLANDS THROUGH MONDAY NIGHT ARE
EXPECTED TO BE WITHIN 0.50-1.50 INCHES AS A WHOLE, THOUGH
LOCATIONS LIKE LIKE PORT TOWNSEND, SEQUIM AND WHIDBEY ISLAND
WILL LIKELY SEE LOWER AMOUNTS DUE TO RAIN SHADOWING. SOUTHERLY
WINDS WILL BE BREEZY AT TIMES THROUGH THE DAY SATURDAY AND AGAIN
ON SUNDAY, WITH WINDS GENERALLY EXPECTED TO PERSIST BETWEEN
25-30 MPH. THE CURRENT WINTER STORM WARNING FOR THE CASCADES HAS
BEEN EXTENDED THROUGH MIDNIGHT TONIGHT.
AN ORGANIZED SYSTEM WILL MOVE INTO THE AREA ON MONDAY WITH MORE
WIDESPREAD - STEADIER RAINFALL, MOUNTAIN SNOW AND GUSTY WINDS.
SOUTHERLY WINDS WILL PICK UP DURING THE MORNING HOURS AND WILL
PEAK IN THE AFTERNOON AT 30-35 MPH, BEFORE DECREASING AGAIN BY
LATE MONDAY. ANOTHER 9-12 INCHES OF SNOW WILL BE POSSIBLE FOR
THE MOUNTAIN PASSES, WITH LOCALLY HIGHER TOTALS EXPECTED FOR
HIGHER ELEVATIONS. RAINFALL AMOUNTS OF 0.25-1.0 INCH ARE
EXPECTED FOR THE LOWLANDS.
MCMILLIAN
LONG TERM /TUESDAY THROUGH SATURDAY/
ENSEMBLES AND DETERMINISTIC GUIDANCE ARE IN LOCK-STEP IN REGARDS
TO THE UPPER-LEVEL PATTERN FOR NEXT WEEK. THE AFOREMENTIONED
UPPER LEVEL TROUGH DIGGING ACROSS THE NORTHEASTERN PACIFIC WILL
CONTINUE TO SWING ADDITIONAL DISTURBANCES INTO THE REGION. AS A
RESULT, PERIODS OF LOWLAND RAIN AND MOUNTAIN SNOW WILL FOLLOW INTO
WESTERN WASHINGTON THROUGH MUCH OF THE NEXT WEEK ALONG WITH
BREEZY WINDS AT TIMES. THE WEATHER PATTERN WILL ALSO ALLOW FOR A
TREND TOWARDS COOLER TEMPERATURES, ESPECIALLY DURING THE
OVERNIGHT PERIOD, WHERE WE MAY START TO SEE MORNING LOWS
APPROACH FREEZING IN SPOTS BY MIDWEEK ONWARD.
MCMILLIAN
AVIATION
TROUGHING OVER THE NORTHEASTERN PACIFIC KEEPS MEAN
WEST-SOUTHWESTERLY FLOW ALOFT ACROSS THE PACIFIC NORTHWEST. SURFACE
WINDS DECOUPLE TONIGHT AND REMAIN SOUTHERLY INTO SUNDAY. THE RESULT
IS STEADY ONSHORE LOW LAND RAIN SHOWERS AND HIGH ELEVATION SNOW
SHOWERS ACROSS WESTERN WASHINGTON. EXPECT LOCALIZED MVFR
VISIBILITIES UNDER THE HEAVIER RAIN SHOWERS AT TERMINALS. BROKEN TO
OVERCAST MVFR CEILINGS ARE LARGELY FORECAST THROUGH THIS TAF PERIOD.
THE EXCEPTION MAY BE A PERIOD FROM 12-18Z SUN WHERE CEILINGS LIFT TO
LOW-END VFR AROUND 3500 KFT AGL. AS THE TROUGH MOVES FURTHER EAST
SUNDAY MORNING, EXPECT ANOTHER ROUND OF INCREASING SHOWER CHANCES
BEGINNING 19Z SUN THAT LAST INTO EARLY MONDAY MORNING, ALONG WITH A
SLIGHT WIND SHIFT TO SOUTHWESTERLY.
KSEA...THE ONGOING RAIN SHOWERS ARE MOST LIKELY INTO THE EARLY
OVERNIGHT HOURS (TAPERING OFF FROM ABOUT 08-10Z) AND LESSER CHANCES
INTO THE SUNDAY MORNING HOURS. WHILE SUSTAINED SOUTHERLY WINDS
REMAIN ELEVATED TONIGHT WIND GUSTS MAY INCREASE AGAIN AROUND 19Z
SUNDAY, UP TO 20 KNOTS, FROM THE SOUTH. MVFR CEILINGS THROUGH
TONIGHT MAY LIFT EARLY SUNDAY MORNING FOR A SHORT TIME TO VFR AHEAD
OF A BAND OF RAIN SHOWERS FORECAST TO MOVE INTO THE SEATTLE METRO
REGION SUNDAY AFTERNOON. THIS BAND OF RAIN SHOWERS MAY PRODUCE A
PERIOD OF MVFR VIS, WITH MVFR CEILINGS ALREADY LIKELY, INTO THE 21-
00Z TIME PERIOD. EXPECT A WIND SHIFT FROM SOUTHERLY TO SOUTHWESTERLY
BEHIND THE RAIN BAND. HIGH RES GUIDANCE SHOWS A PERIOD OF SCATTERED
RAIN SHOWERS LASTING INTO THE 04-07Z MON TIME PERIOD.
MARINE
A 995 MB LOW WILL TRACK NORTH OF HAIDA GWAII ON
SUNDAY WITH GUSTY SMALL CRAFT ADVISORY WINDS OVER MOST WATERS IN
WESTERN WA. A STRONGER PACIFIC STORM SYSTEM, WITH SIMILAR
TRACKING, IS SLATED FOR MONDAY WHICH MAY BRING GALES TO THE
COASTAL WATER - A GALE WATCH IS IN EFFECT. LOW PRESSURE WILL
WEAKEN OFFSHORE ON TUESDAY. THE FLOW WILL TURN OFFSHORE ON
WEDNESDAY AND STRENGTHEN ON THURSDAY DUE TO HIGH PRESSURE OVER
INTERIOR B.C. AND LOW PRESSURE OFF OR/CA COAST. 33
HYDROLOGY
ALTHOUGH SHOWERS ARE EXPECTED OVER W WA TONIGHT AND
SUNDAY, PRECIP AMOUNTS SHOULD NOT INTERFERE WITH THE OVERALL
DOWNWARD TREND BEING SEEN ON MOST AREA RIVERS. RIVERS WITH DAM
REGULATED FLOWS EXPECTED TO REMAIN NEAR OR JUST OVER FLOOD
LEVELS FOR THE TIME BEING AS THE PROCESS OF ELIMINATING EXCESS
WATER FROM PREVIOUS ATMOSPHERIC RIVERS CONTINUE.
AS FOR RIVERS THAT ARE STILL IN FLOOD WITHOUT DAM REGULATION:
* THE CHEHALIS RIVER AT PORTER WILL HOVER NEAR MINOR FLOOD STAGE
FOR THE NEXT FEW DAYS BEFORE RECEDING.
* THE SKOKOMISH RIVER AT POTLATCH SEES RIVER LEVELS GRADUALLY
EASING WHILE REMAINING IN MINOR FLOOD STAGE. HOWEVER,
ADDITIONAL RISES ARE EXPECTED IN THE RIVER FOR THE START OF
NEXT WEEK...KEEPING THE WATERS NEAR MODERATE FLOOD STAGE.
THE LATEST WAVE OF SHOWERS WILL CONTINUE TO FALL ON ALREADY
SATURATED GROUND AND AS SUCH, THE LANDSLIDE THREAT WILL REMAIN HIGH
THROUGH THE WEEKEND.
SEW WATCHES/WARNINGS/ADVISORIES
WA...COASTAL FLOOD ADVISORY FROM 5 AM TO 11 AM PST SUNDAY FOR SAN
JUAN COUNTY.
WINTER STORM WARNING UNTIL MIDNIGHT PST TONIGHT FOR CASCADES
OF PIERCE AND LEWIS COUNTIES-CASCADES OF SNOHOMISH AND
NORTHERN KING COUNTIES-CASCADES OF SOUTHERN KING COUNTY-
CASCADES OF WHATCOM AND SKAGIT COUNTIES.
PZ...SMALL CRAFT ADVISORY UNTIL 4 PM PST SUNDAY FOR ADMIRALTY INLET-
EAST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA-NORTHERN
INLAND WATERS INCLUDING THE SAN JUAN ISLANDS-PUGET SOUND
AND HOOD CANAL.
SMALL CRAFT ADVISORY UNTIL 1 PM PST SUNDAY FOR COASTAL WATERS
FROM CAPE FLATTERY TO JAMES ISLAND 10 TO 60 NM-COASTAL
WATERS FROM CAPE FLATTERY TO JAMES ISLAND OUT 10 NM-
COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE 10 TO
60 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT GRENVILLE
OUT 10 NM-COASTAL WATERS FROM POINT GRENVILLE TO CAPE
SHOALWATER 10 TO 60 NM-COASTAL WATERS FROM POINT
GRENVILLE TO CAPE SHOALWATER OUT 10 NM.
GALE WATCH FROM MONDAY MORNING THROUGH MONDAY EVENING FOR
COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND 10 TO
60 NM-COASTAL WATERS FROM CAPE FLATTERY TO JAMES ISLAND
OUT 10 NM-COASTAL WATERS FROM JAMES ISLAND TO POINT
GRENVILLE 10 TO 60 NM-COASTAL WATERS FROM JAMES ISLAND TO
POINT GRENVILLE OUT 10 NM-COASTAL WATERS FROM POINT
GRENVILLE TO CAPE SHOALWATER 10 TO 60 NM-COASTAL WATERS
FROM POINT GRENVILLE TO CAPE SHOALWATER OUT 10 NM.
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