879
FXUS66 KPQR 161757
AFDPQR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PORTLAND OR
956 AM PST SAT NOV 16 2024
SYNOPSIS
BECOMING WET LATE SATURDAY THROUGH MONDAY AS A PACIFIC
FRONTAL SYSTEM BRINGS VALLEY RAIN AND HEAVY CASCADE SNOW. ATTENTION
THEN TURNS TO A PROLONGED ATMOSPHERIC RIVER EVENT SET TO IMPACT THE
AREA LATE TUESDAY THROUGH FRIDAY, HOWEVER FORECAST CONFIDENCE REMAINS
LOW. NEVERTHELESS, THIS SYSTEM WILL BRING CHANCES FOR RIVER FLOODING,
URBAN FLOODING, DEBRIS FLOWS, LANDSLIDES AND STRONG WINDS. THIS
SYSTEM HAS THE POTENTIAL TO BE A HIGH IMPACT FLOODING EVENT IF THE
WETTEST MODEL GUIDANCE VERIFIES, BUT ALSO HAS THE POTENTIAL TO BRING
ONLY MINOR IMPACTS IF THE DRIEST GUIDANCE VERIFIES. DETAILS ARE
DISCUSSED BELOW.
DISCUSSION...SATURDAY THROUGH FRIDAY NIGHT
A VERY ACTIVE WEATHER
PATTERN IS IN STORE WITH A SERIES OF STORM SYSTEMS SET TO IMPACT THE
AREA OVER THE COMING DAYS. THE MAIN HAZARDS AT HAND ARE HEAVY RAIN,
HEAVY CASCADE SNOW, WIND, AND THE POTENTIAL FOR RIVER FLOODING AND
URBAN FLOODING. THERE ARE ALSO VARIOUS BEACH HAZARDS INCLUDING
SNEAKER WAVES AND TIDAL OVERFLOW FLOODING, HOWEVER THESE HAZARDS ARE
DISCUSSED SEPARATELY IN THE "BEACH HAZARDS" SECTION BELOW.
THE FIRST HAZARD TO DEAL WITH IS RAIN IN THE LOWLANDS AND HEAVY SNOW
IN THE CASCADES. MODEL GUIDANCE CONTINUES TO SHOW A WARM FRONT
LIFTING OVER THE AREA ON SATURDAY, BRINGING RISING SNOW LEVELS (ABOVE
PASS LEVEL) AND A SWATH OF MAINLY LIGHT STRATIFORM RAIN. THIS WARM
FRONT AND THE AREA OF LIGHT STRATIFORM RAIN AHEAD OF IT ARE ALREADY
VISIBLE VIA RADAR, SATELLITE AND SURFACE WEATHER OBSERVATIONS, MOVING
INTO THE COASTAL WATERS EARLY SATURDAY MORNING. RAIN WILL BEGIN AT
THE COAST BY 10 AM SATURDAY BEFORE SPREADING INLAND TOWARDS THE
CASCADES SATURDAY AFTERNOON. PERIODS OF MAINLY LIGHT RAIN LOOK TO
CONTINUE SATURDAY NIGHT. GIVEN SNOW LEVELS WILL BE HIGH AND
PRECIPITATION LOOKS TO BE LIGHT, NOT REALLY EXPECTING ANY IMPACTS
WITH THIS WARM FRONT. THAT SAID, A STRONG COLD FRONT WILL BE QUICK TO
FOLLOW, MOVING OVER THE AREA SUNDAY MORNING AND AFTERNOON FROM
NORTHWEST TO SOUTHEAST. THIS WILL BRING ANOTHER ROUND OF STRATIFORM
PRECIPITATION, ALBEIT A BIT HEAVIER, WITH POST-FRONTAL SHOWERS SUNDAY
NIGHT INTO MONDAY. AS THIS COLD FRONT MOVES THROUGH, SNOW LEVELS WILL
RAPIDLY FALL IN THE CASCADES TO AROUND 2500 FEET BY LATE SUNDAY
EVENING. PROBABILITIES FOR 24-HR SNOW AMOUNTS OVER ONE FOOT ARE NOW
NEAR 100% FOR ALL OF THE CASCADE PASSES WITH A 30-40% CHANCE FOR SNOW
AMOUNTS OF 18 INCHES OR MORE. THIS MEANS ANYONE PLANNING ON
TRAVELLING OVER THE CASCADE PASSES SUNDAY AFTERNOON THROUGH MONDAY
NIGHT WILL NEED TO BE WELL PREPARED FOR DIFFICULT TRAVEL CONDITIONS
DUE TO REDUCED VISIBILITIES AND SNOW-COVERED ROADS. THE WINTER STORM
WATCH IN THE CASCADES HAS BEEN UPGRADED TO A WINTER STORM WARNING
GIVEN THE INCREASED CONFIDENCE IN FORECAST SNOW AMOUNTS AND IMPACTS.
FOR THE LOWLANDS, NOT EXPECTING MUCH IN THE WAY OF IMPACTS WITH THIS
SYSTEM ASIDE FROM POTENTIAL MINOR URBAN FLOODING IN LOW-LYING AREAS
WITH POOR DRAINAGE ON SUNDAY AS HEAVIER SHOWERS COULD PRODUCE HOURLY
RAIN RATES UP TO 0.25 IN/HR. THIS SYSTEM WILL ALSO BRING BREEZY SOUTH
WINDS, HOWEVER WIND GUSTS ARE EXPECTED TO STAY BELOW 35 MPH INLAND
AND BELOW 50 MPH AT THE COAST, THUS LIMITING OVERALL IMPACTS. WHILE
SOME MINOR TREE DAMAGE AND ISOLATED POWER OUTAGES CANNOT BE
COMPLETELY RULED OUT, DO NOT EXPECT EXTENSIVE WIND DAMAGE OR
WIDESPREAD OUTAGES.
BEYOND MONDAY, CONFIDENCE IN THE FORECAST IS LOW AND THE DETAILS ARE
STILL UNCLEAR. THAT BEING SAID, THE LONG-TERM FORECAST STILL LOOKS
QUITE CONCERNING DUE TO AN INCOMING ATMOSPHERIC RIVER WITH POTENTIAL
RIVER FLOODING AND URBAN FLOODING FROM NOV. 19-22. MODELS AND THEIR
ENSEMBLES CONTINUE TO TREND WETTER FOR WESTERN WASHINGTON AND WESTERN
OREGON. THIS SYSTEM IS SET TO BEGIN WITH A RAPIDLY DEEPENING LOW
PRESSURE SYSTEM OVER THE NORTHEAST PACIFIC. THE GEFS/EPS/CMC ALL SHOW
A VERY IMPRESSIVE LOW WITH A CENTRAL PRESSURE FALLING TO AROUND 950
MB. WHILE THE CENTER OF THIS LOW AND THE HIGHEST WINDS ARE SET TO
MOVE TOWARDS VANCOUVER ISLAND, THIS LOW WILL SEND AN INTENSE PLUME OF
MOISTURE RACING TOWARDS THE WA/OR COAST IN THE FORM OF A PROLONGED
ATMOSPHERIC RIVER THAT WILL LIKELY MOVE UP AND DOWN THE COAST FOR A
THREE DAY PERIOD OR SO, OR POTENTIALLY STALLING OUT SOMEWHERE IN
WESTERN WA OR WESTERN OR FOR A SOLID 48 HOURS OR LONGER. IVT PLUME
FORECASTS FROM THE GEFS/EPS SUGGEST THE ATMOSPHERIC RIVER WILL MOST
LIKELY BE MODERATE IN STRENGTH WITH IVT VALUES PEAKING BETWEEN
500-750 KG/MS. HOWEVER, A HANDFUL OF ENSEMBLE MEMBERS SUGGEST A
STRONG OR EVEN EXTREME ATMOSPHERIC RIVER WITH IVT VALUES OVER 1000
KG/MS. IF A STRONG TO EXTREME ATMOSPHERIC RIVER MATERIALIZES AND
STALLS OUT OVER THE AREA FOR LONG ENOUGH, WE WOULD BE FACING
WIDESPREAD RIVER FLOODING WITH MAJOR FLOODING POSSIBLE, AS WELL AS
URBAN FLOODING, LANDSLIDES AND DEBRIS FLOWS. THE NBM 90TH PERCENTILE
FOR 72-HR QPF AMOUNTS BACKS THIS CONCERN UP, SHOWING ALARMINGLY HIGH
RAIN AMOUNTS OF 7-12 INCHES IN THE MOUNTAINS, 6-7 INCHES AT THE
COAST, AND 4-5 INCHES FOR THE INTERIOR LOWLANDS. THIS SEEMS TO BE A
REASONABLE WORST CASE SCENARIO. ON THE CONTRARY, THE NBM 10TH
PERCENTILE (E.G. "BEST CASE SCENARIO"), SHOWS ONLY 2-3 INCHES IN THE
MOUNTAINS, 1.5-2.5 INCHES AT THE COAST, AND AROUND 1 INCH FOR THE
INTERIOR LOWLANDS (THUS LEADING TO ONLY MINOR IMPACTS). THE REASON
THERE IS SUCH A LARGE DEGREE OF MODEL SPREAD IS DUE TO UNCERTAINTY
REGARDING THE EXACT STRENGTH AND TRACK OF THE AFOREMENTIONED SURFACE
LOW IN THE NORTHEAST PACIFIC, AS WELL AS THE EXACT STRENGTH AND TRACK
OF THE ATMOSPHERIC RIVER IT WILL SEND TOWARDS THE COAST. ANOTHER
COMPLICATING FACTOR IS UNCERTAINTY REGARDING THE DURATION OF THE
ATMOSPHERIC RIVER EVENT, AS SOME ENSEMBLE GUIDANCE SHOWS IT STALLING
RIGHT OVER NORTHWEST OR AND SOUTHWEST WA FOR THREE DAYS STRAIGHT,
WHILE OTHER GUIDANCE SHOWS THE ATMOSPHERIC RIVER SHIFTING SOUTHWARD
OVER WESTERN WA/OR INTO NORTHERN CA BEFORE LIFTING BACK NORTH INTO
WESTERN WA/OR AGAIN. MOST ENSEMBLE GUIDANCE FAVORS THE LATTER
SOLUTION, WHICH WOULD HELP LIMIT EXTREME IMPACTS FROM OCCURRING,
HOWEVER AT LEAST MINOR TO MODERATE IMPACTS WOULD LIKELY STILL OCCUR.
REGARDLESS OF THE EXACT STRENGTH/DURATION OF THE ATMOSPHERIC RIVER
AND EXACT RAIN AMOUNTS, FLOODING IS BECOMING INCREASINGLY LIKELY AND
THE FORECAST IS WORTH MONITORING CLOSELY OVER THE COMING DAYS. THE
LATEST HEFS GUIDANCE BACKS THIS UP WELL AS PROBABILITIES FOR RIVER
FLOODING HAVE INCREASED FOR ALL RIVERS. PROBABILITIES ARE HIGHEST FOR
THE COASTAL RIVERS, AS MOST OF THESE RIVERS ARE NOW SHOWING A 30-40%
CHANCE OF REACHING MINOR FLOOD STAGE WITHIN THE NEXT TEN DAYS, EXCEPT
FOR THE WILSON RIVER NEAR TILLAMOOK WHICH HAS A 70% CHANCE. THERE IS
ALSO AROUND A 10-25% CHANCE OUR COASTAL RIVERS WILL HIT MAJOR FLOOD
STAGE, EXCEPT A 32% CHANCE FOR THE WILSON RIVER. GIVEN THE LEAD TIME
AND UNCERTAINTY INVOLVED, THESE PROBABILITIES ARE ALARMINGLY HIGH.
WILL NEED TO MONITOR THE FORECAST CLOSELY OVER THE COMING DAYS TO SEE
HOW QPF AMOUNTS AND PROBABILISTIC GUIDANCE TRENDS. WILL ALSO NEED TO
MONITOR THE POTENTIAL FOR STRONG WINDS WITH THIS SYSTEM AS THERE IS A
10-20% CHANCE FOR WIND GUSTS OVER 45 MPH INLAND AND A 70-80% CHANCE
AT THE COAST. -TK
BEACH HAZARDS
THERE IS A HIGH SNEAKER WAVE THREAT SUNDAY MORNING
THROUGH SUNDAY NIGHT DUE TO AN INCREASING WESTERLY SWELL WITH A
DOMINANT WAVE PERIOD AROUND 14 SECONDS. WITH THE PERIGEAN SPRING
TIDES UNDERWAY (COMMONLY REFERRED TO AS "KING TIDES"), IT IS A GOOD
IDEA TO TAKE EXTRA PRECAUTION ON BEACHES. WAVES CAN RUN UP
SIGNIFICANTLY FARTHER ON A BEACH THAN NORMAL, INCLUDING OVER ROCKS
AND JETTIES. SNEAKER WAVES CAN SUDDENLY KNOCK PEOPLE OFF OF THEIR
FEET AND QUICKLY PULL THEM INTO THE FRIGID OCEAN WHICH MAY LEAD TO
SERIOUS INJURY OR DROWNING. THE SNEAKER WAVE THREAT LOOKS TO REMAIN
HIGH THROUGH EARLY NEXT WEEK.
MINOR TIDAL OVERFLOW FLOODING IS LIKELY AROUND HIGH TIDE OVER THE
NEXT COUPLE OF DAYS AS THE PERIGEAN SPRING TIDES CONTINUE. A COASTAL
FLOOD ADVISORY FOR TIDAL OVERFLOW REMAINS IN EFFECT FOR THE SOUTH WA
COAST FROM 11 AM TO 2 PM SATURDAY. IMPACTS INCLUDE FLOODING UP TO ONE
FOOT ABOVE GROUND LEVEL, IMPACTING PORTIONS OF HIGHWAY 101 NEAR
RAYMOND AND LOW LYING AREAS NEAR BAYS, SLOUGHS AND LOWER REACHES OF
THE COASTAL RIVERS. IMPACTS SHOULD BE LIMITED FOR THE OREGON COAST ON
SATURDAY, WITH ANY FLOODING BEING VERY MINOR AND SHORT-LIVED (AROUND
ONE HOUR OR LESS).
ON SUNDAY, COASTAL FLOOD IMPACTS LOOK TO PEAK AND WILL NO LONGER BE
LIMITED TO THE SOUTH WA COAST. IT APPEARS CRITERIA WILL BE MET FOR
THE SOUTH WA COAST, NORTH OR COAST AND CENTRAL OR COAST SUNDAY
AFTERNOON AROUND HIGH TIDE. CURRENTLY PLANNING ON ISSUING A COASTAL
FLOOD ADVISORY FOR ALL THREE OF THESE ZONES AFTER THE ONGOING
ADVISORY FOR THE SOUTH WA COAST ENDS.
AVIATION
CONDITIONS GENERALLY VFR THROUGHOUT THE REGION, WITH
JUST A FEW POCKETS OF FOG LEFT. A WARM FRONT IS IMMINENTLY ABOUT
TO MOVE OVER THE REGION, BRINGING STRONGER SOUTHERLY GUSTS AND
DETERIORATING CIGS. BY AROUND 20-22Z SAT, MOST TERMINALS LIKELY
DROP TO MVFR THROUGHOUT THE REGION, WITH AROUND 25% CHANCE OF IFR
CIGS AT TIMES THROUGH THE REST OF THE DAY. SOUTHERLY WINDS
INCREASE TO SEE GUSTS AROUND 25 KT AT THE COAST, AND 15-20 KT
INLAND. CEILINGS AND VISIBILITY STAY REDUCE THROUGH THE REST OF
THE TAF PERIOD, WITH WINDS LIKELY STRENGTHENING SLIGHTLY SATURDAY
AFTERNOON.
PDX APPROACHES...VFR CONDITIONS CURRENTLY IN PLACE, BUT
DETERIORATION IS EXPECT ONCE A WARM FRONT BEGINS TO MOVE INTO THE
AREA. BETWEEN 20-22Z SAT, CEILINGS LIKELY DROP TO MVFR AS RAIN
BEGINS. VISIBILITY MAY BE REDUCED TO AROUND 4-5SM AS WELL.
SOUTHERLY WINDS BEGIN TO INCREASE AT THIS TIME AS WELL, WITH GUSTS
UP TO 20 KT BEING POSSIBLE THROUGH THE REST OF THE TAF PERIOD.
/JLIU
MARINE
A STORM SYSTEM ARRIVES THIS MORNING, BRINGING GALES
FORCE WINDS TO ALL WATERS. A GALE WARNING WILL REMAIN IN EFFECT
FROM 7AM TO 10PM TODAY IN RESPONSE TO ELEVATED WINDS BEYOND 35 KT.
WINDS WILL BEGIN TO WIND DOWN TONIGHT, DOWN TO AROUND 20-25 KT.
ADDITIONALLY, SEAS WILL REMAIN AROUND 9 TO 11 FEET. AS A RESULT,
A SMALL CRAFT ADVISORY WILL BE IN EFFECT FROM 10PM TONIGHT TO 4PM
SUNDAY.
WITH A WNW CONTINUING TODAY INTO TOMORROW, WAVE HEIGHTS
ARE EXPECTED TO REACH 14-16 FT AT 13 SECONDS BY SUNDAY NIGHT.
SEAS REMAIN ABOUT THIS HIGH THROUGH AT LEAST WEDNESDAY. MODELS ARE
COMING INTO BETTER AGREEMENT REGARDING A MUCH STRONGER SYSTEM
ARRIVING NEXT TUESDAY, THOUGH THE EXACT TIMING AND STRENGTH OF
THIS SYSTEM ARE STILL SOMEWHAT UNCERTAIN. HOWEVER, INDICATIONS ARE
THAT WE ARE LOOKING AT AROUND A 60% PROBABILITY OF STORM FORCE
WINDS FOR ALL WATERS, WITH VERY STRONG SOUTHERLY WINDS THROUGH
TUESDAY. COULD BE LOOKING AT WAVES IN THE HIGH TEENS TO LOW 20S,
AND WIND GUSTS UP TO 50-55 KT DURING THAT TIME. /JLIU/HALL
PQR WATCHES/WARNINGS/ADVISORIES
OR...WINTER STORM WARNING FROM 10 AM SUNDAY TO 10 PM PST MONDAY FOR
ORZ126>128.
WA...WINTER STORM WARNING FROM 10 AM SUNDAY TO 10 PM PST MONDAY FOR
WAZ211.
COASTAL FLOOD ADVISORY UNTIL 2 PM PST THIS AFTERNOON FOR WAZ201.
PZ...GALE WARNING UNTIL 10 PM PST THIS EVENING FOR PZZ210-251>253-
271>273.
SMALL CRAFT ADVISORY FROM 10 PM THIS EVENING TO 4 PM PST SUNDAY
FOR PZZ210-251>253-271>273.
WWW.WEATHER.GOV/PORTLAND
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