694
FXUS66 KSEW 280353
AFDSEW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SEATTLE WA
853 PM PDT WED OCT 27 2021
UPDATE
NO MAJOR CHANGES TO THE FORECAST THIS EVENING. THE AR
EVENT IS CURRENTLY UNDERWAY ACROSS THE LOCAL AREA AS BROAD
TROUGHING OVER THE PACIFIC PULLS A DEEP, NARROW PLUME OF
SUBTROPICAL PACIFIC MOISTURE TOWARDS THE AREA. A FRONTAL BOUNDARY
RESIDES ALONG THIS DEEP MOISTURE PLUME, THE WARM FRONTAL PORTION
OF WHICH IS AFFECTING THE LOCAL AREA THIS EVENING. DECENT
FRONTOGENESIS ASSOCIATED WITH THE WARM FRONTAL BOUNDARY ITSELF,
COMBINED WITH STRONG WARM AIR ADVECTION IS HELPING CONTRIBUTE TO
WIDESPREAD STRATIFORM RAIN OVER W WA AND THE ADJACENT PACIFIC
TONIGHT. AS THE WARM FRONT LIFT NORTH, THE STRATIFORM RAIN WILL
MOVE NORTH WITH IT, LIKELY RESIDING NORTH OF A ROUGH LINE FROM
THE SOUTHERN OLYMPICS THROUGH THE NORTHERN KING COUNTY. OF COURSE,
SOME UNCERTAINTY EXISTS WITH THE THE EXACT EXTENT OF THE
NORTHWARD TREK OF THE FRONT AS WELL AS THE TIMING. MODERATE TO
HEAVY RAIN WILL RESIDE ALONG AND NEAR THIS AXIS OF MOISTURE
THROUGH THE FIRST OF THURSDAY BEFORE IT VERY SLOWLY SINKS
SOUTHWARD THURSDAY AFTERNOON AND EVENING AS THE COLD FRONT MOVES
ONSHORE. WINDS WILL ALSO BE BREEZY TOMORROW, AND MAY NEED TO BE
ADJUSTED SLIGHTLY UPWARDS IN FUTURE FORECASTS.
PREVIOUS DISCUSSION FOLLOWS, WITH AN UPDATED MARINE AND AVIATION
SECTION. A BRIEF ADJUSTMENT TO THE RAINFALL TOTALS WAS ALSO NOTED
IN THE HYDROLOGY SECTION.
KOVACIK
PREV DISCUSSION
/ISSUED 353 PM PDT WED OCT 27 2021/
SYNOPSIS
SCATTERED SHOWERS WILL GIVE WAY TO STEADIER RAIN THIS
AFTERNOON AS A WARM FRONT LIFTS NORTH INTO THE REGION. EXPECT
HEAVIER RAIN OVERNIGHT AND THROUGH MUCH OF THURSDAY AS AN
ATMOSPHERIC RIVER ARRIVES INTO THE REGION. PERSISTENT RAIN INTO
EARLY FRIDAY WITH PARTICULARLY HEAVY AMOUNTS ACROSS THE OLYMPIC
AND NORTHERN CASCADES. A DRY AND COOLER TREND DEVELOPS FOR THE
WEEKEND, BUT RAIN CHANCES LIKELY RETURN EARLY NEXT WEEK.
SHORT TERM /TONIGHT THROUGH SATURDAY/
SCATTERED SHOWERS
CONTINUE THIS AFTERNOON ACROSS PORTIONS OF WESTERN WASHINGTON WITH
A WARM FRONT AND WIDESPREAD PRECIPITATION REMAINING TO THE SOUTH
OF THE REGION THIS AFTERNOON. SNOW LEVELS REMAIN AROUND 5,000
FEET, BUT WILL BE ON THE INCREASE OVERNIGHT AS HEIGHTS RISE OVER
THE REGION AND LIKELY CLIMB CLOSE TO 8,000 FEET WITH THE BULK OF
THE PRECIPITATION ON THURSDAY. WITH THE STRONG WARM ADVECTION,
EXPECT A RATHER MILD DAY ACROSS THE REGION WITH AFTERNOON HIGH
TEMPERATURES REACHING INTO THE 60S. MEANWHILE, STEADY RAIN OF
INCREASING INTENSITY IS EXPECTED ACROSS THE AREA FROM SOUTH TO
NORTH THIS EVENING AND OVERNIGHT. FORECAST MODELS HAVE REMAINED IN
STRONG AGREEMENT IN HIGHLIGHTING PRECIPITABLE WATER VALUE IN
EXCESS OF ONE INCH THURSDAY AND INTO EARLY FRIDAY AS A DEEP PLUME
OF MOISTURE IS AIMED INTO THE REGION. LITTLE CHANGE TO THE OVERALL
PROBABILITIES IN THE NBM GUIDANCE FOR THE VARIOUS THRESHOLDS, AND
THEREFORE THERE REMAINS CONCERN FOR SOME SHARP RISES ON SEVERAL
AREA RIVERS, AS WELL AS CONTINUED FLOODING ON THE SKOKOMISH. SEE
THE HYDROLOGY SECTION FOR ADDITIONAL DETAILS ON AREAS OF GREATEST
FLOODING CONCERN AS WELL AS SPECIFIC PRECIPITATION FORECASTS. IT'S
ALSO WORTH NOTING THAT SOME GUIDANCE CONTINUES TO SUGGEST THE
POTENTIAL FOR SOME GUSTY WINDS ON THURSDAY AND THURSDAY NIGHT,
WITH GUSTS GENERALLY TO 30 OR 35 MPH THROUGH MUCH OF THE AREA.
THERE ARE A FEW ENSEMBLE MEMBERS THAT ARE SLIGHTLY STRONGER WINDS,
BUT THIS SEEMS TO BE A LOWER PROBABILITY OUTCOME. THAT SAID, GIVEN
THE LEAVES REMAINING ON THE TREES AND RECENT WINDS, THERE MAY
STILL BE SOME LOCALIZED IMPACTS SUCH AS BROKEN BRANCHES AND
LOCALIZED POWER DISRUPTIONS.
GENERAL AGREEMENT REMAINS IN THE ENSEMBLE GUIDANCE FOR A DRYING
TREND THROUGH THE DAY ON FRIDAY AS A TRANSIENT SHORTWAVE RIDGE
SLIDES OVERHEAD. EXPECT A BREAK FROM THE PRECIPITATION ACROSS MOST
OF THE AREA OVER THE WEEKEND, THOUGH WITH THE CLEAR SKIES AND
DRYING AIR MASS EXPECT THE POTENTIAL FOR EFFICIENT RADIATIONAL
COOLING AFTER DARK. THIS MAY LEAD TO WIDESPREAD OVERNIGHT
TEMPERATURES PLUNGING WELL INTO THE 30S SATURDAY MORNING, THOUGH
STILL SOME NOTABLE SPREAD AMONG THE GUIDANCE.
LONG TERM /SUNDAY THROUGH WEDNESDAY/
ENSEMBLE GUIDANCE
CONTINUES TO SUGGEST A CONTINUATION OF THE DRY CONDITIONS THROUGH
MUCH OF SUNDAY, WITH ANOTHER FRONTAL SYSTEM BRINGING A RETURN OF
CLOUDS AND PRECIPITATION CHANCES AT THE END OF THE WEEKEND. SNOW
LEVELS LIKELY LOWER IN THE COOLER AIR MASS THAT ARRIVES WITH THIS
SYSTEM, SO SOME SNOW ACCUMULATIONS MAY AGAIN BE POSSIBLE ON
STEVENS PASS. FOR NOW, FORECAST REFLECTS FAIRLY TYPICAL
TEMPERATURES THROUGH THE FIRST HALF OF NEXT WEEK WITH ROUNDS OF
RAIN AND MOUNTAIN SNOW AT TIMES.
AVIATION
RAIN CONTINUES TO SPREAD INTO THE AREA FROM THE SOUTH
THIS EVENING AND WILL LIKELY CONTINUE THROUGH THURSDAY. CEILINGS
ARE LARGELY VFR AS OF 830PM BUT ALREADY SEEING SOME OBSERVATIONS
OF SCT IFR-MVFR CONDITIONS. EXPECT DETERIORATING CONDITIONS
OVERNIGHT TONIGHT AND THURSDAY WITH WIDESPREAD IFR-MVFR CEILINGS.
VSBYS ARE ALSO LIKELY TO BE REDUCED AT TIMES WHERE HEAVIER RAIN
RESIDES. S WINDS WILL BEGIN TO BE GUSTY TOWARDS DAYBREAK THURSDAY
AND CONTINUE THROUGH MUCH OF THE DAY, SUSTAINED 12-15KTS AND GUSTS
25-30KTS.
KSEA...VFR CEILINGS LOWERING TO MVFR, POSSIBLY IFR, OVERNIGHT
TONIGHT WITHOUT RECOVERY ON THURSDAY. RAIN WILL REMAIN RATHER
PERSISTENT, ALTHOUGH WILL HAVE THE POTENTIAL TO PUSH SLIGHTLY
NORTH THURSDAY MORNING FOR A SHORT PERIOD OF TIME (CONFIDENCE IS
LOW). WINDS BREEZY OUT OF THE S THURSDAY MORNING-EVENING 12 TO
15KTS GUSTING TO 25KTS.
KOVACIK
MARINE
A FRONTAL SYSTEM WILL STALL ACROSS THE REGION THROUGH
THURSDAY NIGHT. SMALL CRAFT ADVISORIES REMAIN IN EFFECT FOR THE
COASTAL WATERS DUE TO BOTH WESTERLY SWELLS AND ELEVATED SOUTHERLY
WINDS. 14-16 FOOT SWELLS WILL GRADUALLY DECREASE TO NEAR 10 FEET
THURSDAY, WITH WINDS EXPECTED TO REMAIN ELEVATED ACROSS PORTIONS
OF THE COASTAL WATERS THROUGH THURSDAY AFTERNOON. ADDITIONAL
ROUNDS OF SCA HEADLINES WILL BE POSSIBLE THROUGH THE WEEK, BUT
WINDS ARE EXPECTED TO TRANSITION FROM SOUTHERLY TO NORTHERLY ON
FRIDAY. WITH EASTERLY GRADIENTS PICKING UP OVERNIGHT, HAVE ISSUED
A SMALL CRAFT ADVISORY FOR THE CENTRAL STRAIT INTO THURSDAY
MORNING.
BUTWIN/BORTH
HYDROLOGY
AN ATMOSPHERIC RIVER IS EXPECTED TO BRING WIDESPREAD
MODERATE TO HEAVY RAIN TO WESTERN WASHINGTON STARTING THIS
EVENING AND PERSISTING THROUGH FRIDAY MORNING. CONFIDENCE IS HIGH
IN THE HEAVIEST ACCUMULATIONS OCCURRING FROM THE SOUTHWESTERN
OLYMPICS TO THE NORTHERN AND CENTRAL CASCADES, GENERALLY NORTH OF
NORTHERN KING COUNTY. THE CURRENT QPF FORECAST CALLS FOR AT LEAST
4 TO 6 INCHES OF RAINFALL IN THESE AREAS, WITH LESSER AMOUNTS OF
2 TO 3 INCHES IN THE SOUTHERN CASCADES AND 1 TO 2 INCHES IN MOST
LOWLAND AREAS, EXCEPT THE NORTHERN OLYMPIC COAST. WIDESPREAD RAPID
RISES IN RIVERS WILL RESULT FROM THESE EXCESSIVE RAINFALL
AMOUNTS. EXPECT THE GREATEST THREAT FOR RIVER FLOODING OUTSIDE OF
THE SKOKOMISH BASIN TO BE IN NORTHERN AND CENTRAL CASCADE BASINS
(E.G. THE NOOKSACK, SKAGIT, STILLAGUAMISH, SKYKOMISH, AND OTHER
BASINS. IN ADDITION, THE SNOW LEVEL WILL RISE TO OVER 9000 FEET
FOR MOST OF THE DURATION OF THE PRECIPITATION RESULTING IN MOST OF
THE PRECIPITATION BECOMING STORM RUNOFF INTO THE RIVERS.
THE SEVERITY OF FLOODING WILL DEPEND ON THE EXACT AMOUNT AND
PLACEMENT OF RAINFALL ACCUMULATION WHICH WILL IN PART BE DETERMINED
BY THE DIRECTION OF FLOW AND THE LOCATION OF THE CORE OF THE MAXIMUM
MOISTURE PLUME IN THE ATMOSPHERIC RIVER. SOME OF THE UNCERTAINTY
STEMS FROM HOW THE MOISTURE GETS AROUND OR OVER THE OLYMPIC
MOUNTAINS. HIGH END TOTALS OF UP TO 5 INCHES ARE NOT OUT OF THE
QUESTION, ESPECIALLY IN THE WESTERN SLOPES OF THE CENTRAL AND
NORTHERN CASCADES. MINOR TO MODERATE FLOODING EXPECTED TO CONTINUE
ON THE SKOKOMISH RIVER AT POTLACH THROUGH AT LEAST FRIDAY NIGHT.
DRIER AIR WILL PUSH INTO THE REGION ON FRIDAY, ENDING HEAVY RAIN BY
AT LEAST FRIDAY AFTERNOON/EVENING. RIVERS WILL CONTINUE TO RUN HIGH
ON FRIDAY, RECEDING OVER THE WEEKEND.
SEW WATCHES/WARNINGS/ADVISORIES
WA...FLOOD WATCH FROM THURSDAY AFTERNOON THROUGH LATE THURSDAY NIGHT
FOR ADMIRALTY INLET AREA-BELLEVUE AND VICINITY-CASCADES OF
SNOHOMISH AND KING COUNTIES-CASCADES OF WHATCOM AND SKAGIT
COUNTIES-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-SEATTLE AND
VICINITY-WESTERN SKAGIT COUNTY-WESTERN STRAIT OF JUAN DE
FUCA-WESTERN WHATCOM COUNTY.
PZ...SMALL CRAFT ADVISORY UNTIL 6 AM PDT THURSDAY FOR GRAYS HARBOR
BAR.
SMALL CRAFT ADVISORY UNTIL 6 PM PDT THURSDAY 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-WEST ENTRANCE U.S. WATERS STRAIT OF
JUAN DE FUCA.
SMALL CRAFT ADVISORY UNTIL 8 AM PDT THURSDAY FOR CENTRAL U.S.
WATERS STRAIT OF JUAN DE FUCA.
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