666
FXUS66 KSEW 051659
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
859 AM PST TUE DEC 5 2023
UPDATE
NO SIGNIFICANT CHANGES TO THE OVERALL FORECAST. THE
ATMOSPHERIC RIVER DRAPED OVER THE THE AREA WILL CONTINUE TO DRIVE
MODERATE TO HEAVY RAINFALL THROUGH TONIGHT - ESPECIALLY OVER THE
CASCADE MOUNTAINS. THIS WILL CONTINUE TO CAUSE WIDESPREAD
HYDROLOGIC IMPACTS THROUGHOUT THE REGION. PLEASE SEE THE AVIATION
AND MARINE SECTIONS BELOW FOR UPDATES TO THOSE FORECASTS.
PREV DISCUSSION
/ISSUED 322 AM PST TUE DEC 5 2023/
SYNOPSIS
AN ATMOSPHERIC RIVER WILL IMPACT THE REGION TODAY
WITH PERIODS OF HEAVY RAIN AND RIVER FLOODING. AN UPPER LEVEL
TROUGH WILL CROSS THE REGION LATE IN THE WEEK FOR A SHIFT TOWARD
MORE SHOWERY PRECIPITATION, COOLER TEMPERATURES, AND LOWERING SNOW
LEVELS. A BRIEF BREAK IN THE ACTIVE PATTERN IS POSSIBLE ON
FRIDAY BEFORE ANOTHER FRONT ARRIVES OVER THE WEEKEND. DRIER
CONDITIONS ARE POSSIBLE BY EARLY NEXT WEEK.
SHORT TERM /TODAY THROUGH THURSDAY/
WE'VE GOT A PRETTY
EXTRAORDINARY AIR MASS OVER WESTERN WASHINGTON THIS MORNING AS AN
ATMOSPHERIC RIVER CONTINUES TO HAVE THE REGION FIRMLY IN ITS
CROSSHAIRS. TO GAIN A SENSE OF HOW MOISTURE LADEN IT IS, THE
MONDAY AFTERNOON SOUNDING OUT OF KUIL HAD PRECIPITABLE WATER OF
JUST OVER 1.4 INCHES. NO SOUNDING DATA FOR THE MONTH OF DECEMBER
DATING BACK 75 YEARS HAS YIELDED A NUMBER THAT HIGH. SNOW LEVELS
REMAIN AS HIGH AS 9000 TO 10000 FEET ACROSS PORTIONS OF THE AREA
EARLY THIS MORNING WITH RAINFALL RATES OF 0.25 TO 0.50 INCH PER
HOUR ACROSS MUCH OF THE LOWLANDS AND HIGHER IN THE MOUNTAINS.
WE'LL NEED TO CONTINUE TO MONITOR THIS AS SOME RATES ARE NEARING
FLASH FLOOD THRESHOLDS NEAR THE SOURDOUGH AND BOLT CREEK BURN
SCARS. NUMEROUS RIVERS ARE OR WILL BE REACHING MODERATE TO MAJOR
FLOOD LEVELS. MORE ON THIS CAN BE FOUND IN THE HYDROLOGY PORTION
OF THE DISCUSSION.
A COLD FRONT WILL GRADUALLY DRAG ONSHORE LATER THIS MORNING THEN
STALL AROUND PUGET SOUND MUCH OF THE DAY BEFORE EVENTUALLY
SHIFTING EAST OF THE CASCADES TONIGHT. THIS WILL ALLOW RAINFALL
RATES TO GRADUALLY DECREASE AS WELL AS BEGIN TO LOWER SNOW
LEVELS. THE AXIS OF HEAVIER PRECIP WILL SHIFT INTO OREGON BY
WEDNESDAY MORNING. HOWEVER, A FRONTAL WAVE LIFTING NORTHWARD
ACROSS SOUTHERN PORTIONS OF THE CWA COULD ENHANCE PRECIP RATES FOR
A FEW HOURS ACROSS THE CENTRAL CASCADES WEDNESDAY MORNING INTO
MID AFTERNOON. SNOW LEVELS WILL FALL BACK TO PASS LEVELS BY EARLY
THURSDAY AS AN UPPER TROUGH AXIS MOVES ONSHORE. MODELS REMAIN
RELATIVELY CONSISTENT WITH THE FOCUS OF MUCH OF THE PRECIP WITH
THE UPPER TROUGH REMAINING SOUTH OF THE FORECAST AREA. WITH THE
SHOWERY NATURE OF THE PRECIP AND AN ABSENCE OF ANY SIGNIFICANT
ONSHORE FLOW, SNOWFALL FOR THE MOUNTAINS (INCLUDING THE PASSES)
SHOULD BE MODEST AT BEST.
LONG TERM /FRIDAY THROUGH MONDAY/
LOW AMPLITUDE SHORTWAVE
RIDGING SLIDING ACROSS THE REGION WILL PROVIDE A BRIEF BREAK IN
THE WEATHER LATER FRIDAY INTO EARLY SATURDAY BEFORE ANOTHER FRONT
ARRIVES FOR THE LATER HALF OF SATURDAY INTO EARLY SUNDAY.
ENSEMBLES GENERALLY SUPPORT THE IDEA OF THIS SYSTEM BEING RATHER
PROGRESSIVE. LOOKING INTO EARLY PART OF NEXT WEEK, THERE IS RATHER
BROAD SUPPORT IN THE ENSEMBLES OF POSITIVE HEIGHT ANOMALIES JUST
OFFSHORE AS BROAD UPPER TROUGHING DEVELOPS DOWNSTREAM EAST OF THE
ROCKIES. ALL OF THIS POINTS TOWARD A RELATIVELY (AND PROBABLY MUCH
NEEDED) BENIGN START TO THE WEEK WITH NEAR SEASONAL TEMPERATURES
AND A MULTI-DAY DRYING TREND. 27
AVIATION
A MIXED BAG OF CONDITIONS EARLY THIS TUESDAY MORNING AS
A COLD FRONT CONTINUES TO MOVE ONSHORE. THIS FRONT WILL STALL OVER
WESTERN WASHINGTON TODAY WITH ADDITIONAL WAVES OF LOW PRESSURE
RIDING ALONG IT AS A SHORTWAVE TROUGH MOVES NORTHEAST OVER THE
PACIFIC NORTHWEST KEEPING DEEP SOUTHWESTERLY FLOW ALOFT. RAIN WILL
CONTINUE FOR MOST OF THE TERMINALS TODAY WITH PRIMARILY MVFR CIGS.
THE BEST CHANCES FOR BREAKS AND POTENTIAL BRIEF PERIODS OF VFR WILL
BE FURTHEST NORTH OF THE STALLED FRONT AT BLI, KHQM, AND KCLM.
SOUTH-SOUTHWEST WINDS FROM 5 TO 10 KT.
KSEA...PRIMARILY MVFR CIGS TODAY AS A COLD FRONT STALLS OVERHEAD
WITH ADDITIONAL WAVES OF LOW PRESSURE AND STRONG SOUTHWESTERLY FLOW
ALOFT PUMPING AMPLE MOISTURE OVER WESTERN WASHINGTON. THIS WILL
CONTINUE MODERATE TO HEAVY RAIN AT TIMES THROUGH THE DAY, WHICH MAY
LEAD TO BRIEF PERIODS OF IFR OR EVEN LIFR CONDITIONS. RAIN GRADUALLY
TRANSITIONS TO MORE SHOWERS OVERNIGHT TONIGHT, WITH MORE WIDESPREAD
COVERAGE SOUTH OF THE TERMINAL. WINDS MOSTLY OUT OF THE SOUTH-
SOUTHWEST THEN BACK TO THE SOUTHEAST LATE THIS AFTERNOON THEN TURN
LIGHT AND VARIABLE OUT WITH A PREDOMINATELY NORTHEASTERLY
DIRECTION TONIGHT.
MARINE
SOUTHERLY WINDS ARE EASING OFF OVER THE COASTAL WATERS AND
EVEN INTO PORTIONS OF THE INTERIOR WATERS AS WELL AS A COLD FRONT
PUSHES ONSHORE. WINDS WILL CONTINUE TO TURN MORE ONSHORE THIS
MORNING BEFORE TURNING BACK TO THE SOUTH-SOUTHWEST AS ADDITIONAL
WAVES OF LOW PRESSURE MOVE ALONG THE STALLING COLD FRONT. COMBINED
SEAS REMAIN ON THE HIGH SIDE FROM 10 TO 14 FT THIS MORNING, THEN
RISE BACK TO 15 TO 20 FT THIS AFTERNOON OVER THE COASTAL WATERS.
SEAS FINALLY LOOK TO DROP BELOW 10 FT INTO THURSDAY BEFORE ANOTHER
FRONTAL SYSTEM MOVES EASTWARD OVER THE AREA WATERS, BRINGING SEAS
BACK ABOVE 10 FT ALONG WITH THE POTENTIAL FOR GALE-FORCE WINDS.
HYDROLOGY
HEAVY RAIN CONTINUING OVER MOST OF THE AREA THROUGH
THIS AFTERNOON. RAINFALL AMOUNTS IN THE 12 HOUR PERIOD ENDING AT 3
AM FOR THE OLYMPICS RANGE FROM 2.5 TO NEARLY 6 INCHES ( QUINAULT
5.58 INCHES ), NORTH AND CENTRAL CASCADES 1.5 TO ALMOST 4 INCHES,
1.5 TO 2.5 INCHES IN THE SOUTH CASCADES, 1.5 TO 3 INCHES ALONG
THE COAST AND AND 0.75 INCHES TO 1.5 INCHES IN THE INTERIOR
LOWLANDS. SNOW LEVELS UP AROUND 9000 FEET WILL ONLY LOWER SLIGHTLY
TODAY.
HEAVY RAIN WILL CONTINUE IN THE NORTH AND CENTRAL CASCADES THROUGH
THIS AFTERNOON WITH ANOTHER 2 TO 4 INCHES OF RAIN ON TOP OF WHAT
HAS ALREADY FALLEN. RAINFALL AMOUNTS IN OTHER LOCATIONS WILL BE
LESS WITH THE ATMOSPHERIC RIVER SHIFTING SLIGHTLY INLAND LATER
TODAY.
FLOOD WARNINGS ARE ALREADY OUT FOR MOST OF THE RIVERS FLOWING OFF
THE NORTH CENTRAL CASCADES INCLUDING BUT NOT LIMITED TO THE
SKAGIT, STILLAGUAMISH, SNOQUALMIE, SKYKOMISH AND TOLT. WARNINGS
FOR THE SNOHOMISH RIVER WILL BE ADDED THIS MORNING. THE SKAGIT
NEAR CONCRETE WILL REACH MAJOR FLOOD STAGE LATER THIS
MORNING/EARLY THIS AFTERNOON. THERE IS A CHANCE THAT THE SNOQUALMIE
NEAR CARNATION WILL REACH MAJOR FLOOD STAGE TONIGHT.
RIVERS FLOWING OFF THE SOUTH CENTRAL CASCADES ARE WELL BELOW FLOOD
STAGE THIS MORNING BUT WITH THE RAIN CONTINUING THROUGH TODAY
WARNINGS WILL MOST LIKELY BE ISSUED FOR THE COWLITZ, NISQUALLY,
NEWAUKUM AND PORTIONS OF THE CHEHALIS RIVER LATER THIS MORNING OR
THIS AFTERNOON.
FOR RIVERS FLOWING OFF THE OLYMPICS, THE SKOKOMISH RIVER COULD
REACH MAJOR FLOOD STAGE THIS MORNING. THE RIVER CAN'T GO MORE THAN
A COUPLE INCHES ABOVE MAJOR FLOOD STAGE BEFORE IT STARTS FILLING
UP THE VALLEY. FLOOD WARNINGS ARE ALREADY UP FOR THE BOGACHIEL
RIVER. WITH THE ATMOSPHERIC RIVER MOVING INLAND, THE BOGACHIEL
WILL CREST LATER THIS MORNING AND BEGIN RECEDING THIS AFTERNOON.
ON THE EAST SIDE OF THE OLYMPICS LOCALIZED FLOODING WILL OCCUR
NEAR THE DUCKABUSH AND DOSEWALLIPS RIVERS IN EASTERN JEFFERSON
COUNTY THROUGH THIS AFTERNOON.
HEAVY RAIN COMING TO AN END TONIGHT INTO WEDNESDAY MORNING. THIS
WILL ALLOW THE UPPER REACHES OF THE RIVER TO START RECEDING WHILE
THE FLOOD CREST MAKES ITS WAY DOWNSTREAM TO THE LOWER REACHES OF
THE RIVERS. FELTON
SEW WATCHES/WARNINGS/ADVISORIES
WA...FLOOD WATCH THROUGH LATE WEDNESDAY NIGHT FOR ADMIRALTY INLET
AREA-BELLEVUE AND VICINITY-BREMERTON AND VICINITY-CENTRAL
COAST-EAST PUGET SOUND LOWLANDS-EASTERN STRAIT OF JUAN DE
FUCA-EVERETT AND VICINITY-HOOD CANAL AREA-LOWER CHEHALIS
VALLEY AREA-NORTH COAST-OLYMPICS-SAN JUAN COUNTY-SEATTLE
AND VICINITY-SOUTHWEST INTERIOR-TACOMA AREA-WEST SLOPES
NORTH CASCADES AND PASSES-WEST SLOPES NORTH CENTRAL
CASCADES AND PASSES-WEST SLOPES SOUTH CENTRAL CASCADES AND
PASSES-WESTERN SKAGIT COUNTY-WESTERN STRAIT OF JUAN DE FUCA-
WESTERN WHATCOM COUNTY.
PZ...SMALL CRAFT ADVISORY UNTIL 4 AM PST WEDNESDAY FOR GRAYS HARBOR
BAR-WEST ENTRANCE U.S. WATERS STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY UNTIL 4 PM PST WEDNESDAY 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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