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.  
 

 
 

 
 
WWW.WEATHER.GOV/SEATTLE  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab WA Page Main Text Page