597
FXUS66 KSEW 161637
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
837 AM PST MON DEC 16 2024
SYNOPSIS
A SERIES OF FRONTAL SYSTEMS WILL MOVE ACROSS WESTERN
WASHINGTON THIS WEEK WITH A CONTINUED ACTIVE WEATHER PATTERN. A
FRONT TODAY WILL BRING WIDESPREAD RAIN FOR THE LOWLANDS THIS
MORNING AND MOUNTAIN SNOW. A STRONGER SYSTEM TUESDAY INTO
WEDNESDAY WILL BRING HEAVIER PRECIPITATION, BREEZY WINDS, AND
MOUNTAIN SNOW. WARMER, AND UNSETTLED CONDITIONS, LIKELY TO
CONTINUE LATE WEEK.
UPDATE
WET AND RAINY MORNING ACROSS WESTERN WASHINGTON AS A
FRONTAL SYSTEM MOVES ACROSS THE REGION. STRATIFORM RAIN IS MAKING
ITS WAY INTO BC AT THIS TIME, WITH SHOWERS MOVING ONSHORE ALONG
THE COAST. EXPECT HIT/MISS SHOWER ACTIVITY TODAY. TIDES THROUGH
THE NORTH INTERIOR HAVE PEAKED AND ARE ON THEIR WAY DOWN WHILE
TIDES ALONG THE OUTER COAST WILL PEAK EARLY THIS AFTERNOON, MAINLY
IN THE MINOR FLOOD CATEGORY. THE CURRENT FORECAST IS IN GOOD
SHAPE AND NO SIGNIFICANT UPDATES ARE PLANNED THIS MORNING.
-WOLCOTT-
SHORT TERM /TODAY THROUGH WEDNESDAY/
A FRONT CONTINUES TO BRING
PRECIPITATION TO WESTERN WASHINGTON EARLY THIS MORNING. THE
MAJORITY OF THE PRECIPITATION IS EXPECTED TO LIFT NORTHWARDS INTO
BRITISH COLUMBIA BY THE MIDDAY TIMEFRAME, ALTHOUGH SHOWERS WILL
LINGER AT TIMES INTO THE AFTERNOON. SNOW WILL CONTINUE OVER THE
MOUNTAINS DURING THIS PERIOD, WITH A WINTER WEATHER ADVISORY
REMAINING IN EFFECT FOR THE SOUTHERN AND CENTRAL CASCADES, THE
ACCUMULATING SNOWFALL MAINLY ABOVE 3000 FEET. AN ATMOSPHERIC RIVER
WILL THEN BE FOCUSED ON WESTERN WASHINGTON TUESDAY INTO WEDNESDAY.
DURING THIS PERIOD, AN ASSOCIATED WARM FRONT WILL LIFT NORTHWARD
TUESDAY BEFORE A COLD FRONT MOVES ACROSS THE AREA ON WEDNESDAY.
MORE INFORMATION ON THIS SYSTEM BELOW:
RAINFALL: PRECIPITATION ASSOCIATED WITH THE AFOREMENTIONED WARM
FRONT WILL SLIDE NORTHWARD TUESDAY MORNING, WITH STEADIER RAINFALL
FOR THE LOWLANDS IN THE AFTERNOON AND OVERNIGHT. A COLD FRONT WILL
FOLLOW RATHER QUICKLY WEDNESDAY MORNING, ALLOWING FOR CONTINUED
HEAVIER RAIN AT TIMES. DURING THIS PERIOD, SNOW LEVELS WILL RISE
RATHER QUICKLY, PEAKING AROUND 7500 TO 8000 FEET TUESDAY NIGHT.
UNCERTAINTY EXISTS ON THE EXACT QPF VALUES, ESPECIALLY FOR THE
OLYMPICS AND CASCADES, WHICH WILL IMPACT THE POTENTIAL FOR ANY
RIVER FLOODING. REFER TO THE .HYDROLOGY SECTION BELOW FOR MORE
INFORMATION. WITH THAT SAID, THE 50TH PERCENTILE NBM (24 HOUR
PERIOD THROUGH 18Z WEDNESDAY) SUGGESTS WIDESPREAD 1-2 INCHES OF
QPF FOR PUGET SOUND, WITH AMOUNTS AROUND 3 INCHES FOR THE COAST.
WIDESPREAD 3 TO 5 INCHES OF QPF EXPECTED FOR THE NORTH CASCADES
AND OLYMPICS DURING THIS PERIOD AS WELL, WITH NEAR 8 INCHES OF QPF
FOR THE OLYMPICS ON THE 90TH PERCENTILE NBM, HIGHLIGHTING THE
CONTINUED UNCERTAINTY IN EXACT AMOUNTS.
MOUNTAIN PASSES: ANOTHER IMPACTFUL ASPECT TO THIS SYSTEM MAY BE
THE PRECIPITATION TYPE FOR THE CASCADE PASSES, PARTICULARLY
STEVENS AND SNOQUALMIE PASS, TUESDAY NIGHT INTO EARLY WEDNESDAY.
ACCUMULATING SNOWFALL IS INITIALLY EXPECTED ON TUESDAY BEFORE
SNOW LEVELS RISE QUICKLY TUESDAY NIGHT. THE PRIMARY CONCERN IS
TUESDAY NIGHT FOR THE PASSES WITH CONTINUED EASTERLY WINDS AND
WARMING TEMPERATURES ALOFT. MODEL SOUNDINGS AT PASS LEVEL SUGGEST
A FREEZING RAIN CONCERN DURING THIS PERIOD, ALSO OBSERVED AT THE
END OF THE 00Z HREF LATER TUESDAY. THE EXACT DETAILS WILL NEED TO
BE FINE TUNED IN REGARDS TO THE TIMING AND ANY POTENTIAL ICE
ACCUMULATION, BUT IF YOU'RE TRAVELING OVER THE PASSES TUESDAY INTO
WEDNESDAY MORNING, CONTINUE TO MONITOR THE WEATHER FORECAST.
WINDS: SOUTHERLY WINDS WILL INCREASE IN THE WARM SECTOR TUESDAY
NIGHT BEFORE THE FRONTAL PASSAGE ON WEDNESDAY. NBM PROBABILITIES
GENERALLY RANGE 30 TO 60 PERCENT FOR GUSTS GREATER THAN 45 MPH
DURING THIS PERIOD FOR THE COAST, AS WELL AS EVERETT NORTHWARD
ALONG THE WATER. PROBABILITIES ELSEWHERE AROUND PUGET SOUND ARE
GENERALLY 15 TO 30 PERCENT OF REACHING THAT THRESHOLD. WILL HAVE
TO MONITOR FOR ANY WIND HEADLINES DURING THIS PERIOD, ESPECIALLY
FOR THOSE AFOREMENTIONED AREAS THAT ARE LIKELY TO BE BREEZY WITH
THE INCREASED SOUTHERLY WINDS.
POST-FRONTAL SHOWERS MAY BE AROUND WEDNESDAY AFTERNOON, HOWEVER A
BRIEF DRY PERIOD IS EXPECTED WEDNESDAY EVENING. OTHERWISE, HIGHS
WILL SLOWLY WARM EACH DAY THROUGH MIDWEEK, WITH HIGHS REACHING THE
LOW TO MID 50S BY WEDNESDAY.
LONG TERM /THURSDAY THROUGH SUNDAY/
ENSEMBLES ARE IN AGREEMENT
THAT RIDGING WILL BUILD INTO THE INTERIOR WEST THURSDAY INTO
FRIDAY. HOWEVER, WESTERN WASHINGTON WILL REMAIN ON THE WESTERN
FRINGE OF THE RIDGING, WHICH MAY ALLOW FOR FRONTAL SYSTEMS TO
BRING PRECIPITATION AT TIMES THROUGH SATURDAY. WPC CLUSTER
ANALYSIS SHOWS THE ECMWF ENSEMBLES AS A MORE DRIER SOLUTION
COMPARED TO THE MULTI-MODEL ENSEMBLE MEAN (GFS, CMC, ECMWF ENS).
HAVE KEPT THE NBM PROBABILITIES IN THE FORECAST, WHICH SHOWS THE
HIGHER PROBABILITIES FOR PRECIPITATION LATE WEEK ALONG THE
OLYMPIC PENINSULA AND NORTHERN CASCADES. TROUGHING NEARS THE AREA
OFFSHORE LATER IN THE WEEKEND, RESULTING IN ANOTHER ROUND OF
INCREASED PRECIPITATION FOR THE AREA. TEMPERATURES WILL BE ABOVE
NORMAL DURING THIS PERIOD, WITH NBM HIGHS IN THE 50S. JD
AVIATION
SOUTH TO SOUTHWESTERLY FLOW ALOFT OVER W WA TODAY WITH
AN UPPER LEVEL TROUGH JUST OFF THE COAST AND GRADUALLY CREEPING
EASTWARD. SOUTH TO SOUTHEASTERLY SURFACE WINDS GENERALLY 5-10 KTS.
PAE IS RUNNING A LITTLE FASTER, MAINLY 10-15 KTS WITH GUSTS UP TO 25
WHILE HQM DIFFERS IN BOTH DIRECTION AND SPEED, EASTERLY AT 8-12 KTS.
MIX OF MVFR TO VFR CIGS OVER THE AREA THIS MORNING WITH MOST
TERMINALS REPORTING EITHER HIGH-END MVFR OR LOW-END VFR, DEPENDING
ON PRECIP AND RAINFALL RATES. THIS MAY ALSO CREATE BRIEF VIS
RESTRICTIONS. CONDITIONS EXPECTED TO IMPROVE BY AFTERNOON AS RAIN
SCATTERS OUT AND CIGS ALLOWED TO IMPROVE. THIS SHOULD BE ENOUGH OF A
BUMP TO BRING WIDESPREAD VFR CONDITIONS BUT THIS RESPITE WILL BE
SHORT LIVED. AN INCOMING WEATHER SYSTEM COMBINED WITH PLENTY OF LOW
LEVEL MOISTURE FROM EARLIER RAINS WILL BRING CIGS BACK DOWN INTO AT
LEAST MVFR TONIGHT, WITH PRECIP TUESDAY KEEPING THEM THERE FOR THE
REMAINDER OF THE TAF PERIOD.
KSEA...VFR CONDITIONS IN PLACE AND WHILE THERE DOES APPEAR TO BE A
SECOND BAND OF PRECIP TRYING TO FOR AND PUSH TOWARD THE TERMINAL,
CURRENT RADAR TRENDS SUGGEST MINIMAL, IF ANY, IMPACT. WHILE SOME
VERY LOW-END POPS POSSIBLE FOR THE REMAINDER OF THE MORNING AND THIS
AFTERNOON, NOT ENOUGH CONFIDENCE FOR TAF INCLUSION. CIGS ERODING
OVERNIGHT DOWN INTO MVFR AFTER 06Z AS NEXT SYSTEM BEGINS TO IMPACT
THE AREA, LOWERING FURTHER TO LOW-END MVFR AS RAIN BECOMES MORE
CATEGORICAL BY LATE TUESDAY MORNING. SURFACE WINDS REMAINING LARGELY
SOUTHEASTERLY FOR THE TAF PERIOD WITH SPEEDS 5-10 KTS, BUT IT IS
WORTH NOTING THAT THERE IS A 5-HOUR WINDOW OF EASTERLIES AFTER 01Z
THIS EVENING.
18
MARINE
NO WIND-DRIVEN HEADLINES IN PLACE AT THIS TIME, HOWEVER
SEAS OVER THE COASTAL WATERS, ESPECIALLY THE OUTER COASTAL WATERS,
REMAIN RIGHT AROUND THE THRESHOLD, OR JUST A LITTLE OVER, FOR SCA
FOR WAVES...AND AS SUCH, THAT HEADLINE WILL REMAIN IN PLACE.
INCOMING WEATHER SYSTEM FOR TUESDAY WILL PROVIDE A BOOST FOR BOTH
WINDS AND WAVES OVER THE COASTAL WATERS, SO WILL NEED TO CONSIDER
EXTENSIONS FOR THE AFTERNOON FORECAST PACKAGE. INCREASING
SOUTHERLIES TUESDAY NIGHT MAY ALLOW FOR AN AREAL EXPANSION TO
INCLUDE AT LEAST THE EAST STRAIT. LATEST MODEL GUIDANCE ALSO SHOWING
GREATER WIND SPEEDS OVER THE COASTAL WATERS DURING THIS TIME FRAME,
ENOUGH SO THAT MAY HAVE TO CONSIDER A GALE WATCH FOR TUESDAY NIGHT
WITH THE AFTERNOON PACKAGE.
MULTIPLE SYSTEMS ARE EXPECTED TO MOVE OVER THE WATERS THROUGHOUT THE
WEEK KEEPING WAVES ABOVE 10 FT AND MORE WINDS FALLING UNDER SMALL
CRAFT CRITERIA. MODEL GUIDANCE DOES HINT AT ANOTHER STRONG SYSTEM
COMING AT THE END OF THE WEEK WITH WAVES POTENTIALLY BUILDING ABOVE
20 FT AND ANOTHER ROUND OF GALE FORCE WINDS.
18/MGF
HYDROLOGY
AREA RIVERS WILL RISE WITH ADDITIONAL PRECIPITATION
THROUGH TUESDAY MORNING AHEAD OF AN INCOMING ATMOSPHERIC RIVER
EVENT THAT WILL CAUSE SIGNIFICANT RISES TO AREA RIVERS. LATEST
DETERMINISTIC FORECASTS SHOW INCOMING LIQUID AMOUNTS OF 4 TO 7
INCHES OVER THE OLYMPIC PENINSULA BETWEEN TUESDAY AND WEDNESDAY,
WITH UP TO 5 INCHES OVER THE CASCADES. SNOW LEVELS WILL ALSO BE
ELEVATED DURING THIS EVENT, LIFTING AS HIGH AS 7000-8000 FT
TUESDAY NIGHT. THE 90TH PERCENTILE NBM SHOWS POTENTIAL FOR UP TO
8 INCHES OF LIQUID OVER THE OLYMPIC MOUNTAINS MID-WEEK (24 HOUR
PERIOD), WHICH WOULD CONTRIBUTE TO A SIGNIFICANT RISE IN RIVERS.
THE EXACT DETAILS OF THIS MID-WEEK SYSTEM REMAIN UNCERTAIN, BUT AT
THIS TIME THE SKOKOMISH RIVER IS FORECAST TO ENTER MODERATE FLOOD
STAGE ON WEDNESDAY WITH MULTIPLE AREA RIVERS REACHING ACTION
STAGE.
ADDITIONAL INCOMING PRECIPITATION TOWARDS THE END OF THE WEEK
MAINTAINS THE THREAT OF RIVER FLOODING ACROSS WESTERN WASHINGTON,
BUT MODELS CONTINUE TO SHOW LARGE DIFFERENCES AND THE FORECAST IS
UNCERTAIN.
JD/15
SEW WATCHES/WARNINGS/ADVISORIES
WA...COASTAL FLOOD ADVISORY UNTIL 10 AM PST THIS MORNING FOR SAN JUAN
COUNTY-WESTERN SKAGIT COUNTY-WESTERN WHATCOM COUNTY.
COASTAL FLOOD ADVISORY UNTIL 4 PM PST THIS AFTERNOON FOR CENTRAL
COAST.
WINTER WEATHER ADVISORY UNTIL 1 PM PST THIS AFTERNOON FOR WEST
SLOPES NORTH CENTRAL CASCADES AND PASSES-WEST SLOPES SOUTH
CENTRAL CASCADES AND PASSES.
PZ...SMALL CRAFT ADVISORY FROM 10 PM THIS EVENING TO 4 PM PST TUESDAY
FOR GRAYS HARBOR BAR.
SMALL CRAFT ADVISORY UNTIL 10 AM PST THIS MORNING FOR CENTRAL
U.S. WATERS STRAIT OF JUAN DE FUCA-EAST ENTRANCE U.S.
WATERS STRAIT OF JUAN DE FUCA-NORTHERN INLAND WATERS
INCLUDING THE SAN JUAN ISLANDS-WEST ENTRANCE U.S. WATERS
STRAIT OF JUAN DE FUCA.
SMALL CRAFT ADVISORY UNTIL 4 AM PST TUESDAY 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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