Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 281710 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 910 AM PST Wed Feb 28 2024 .UPDATE...
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AS OF 9AM...Low level moisture transport vectors indicate the shift to more southwesterly flow, which is echoed by the most recent radar scans indicating more plentiful rain rates in a line from west of Olympia through the Kitsap Peninsula up towards Granite Falls. Several heavy rain bands will likely pass through the area today as the cold front makes it way westward, with stratiform precipitation transitioning to more showery convection. See the previous discussion for the detailed forecast.
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&& .SYNOPSIS...
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An atmospheric river pattern along the Washington coast will bring in continued heavy mountain snow, heavy lowland rain, and breezy winds through Thursday. Active conditions will continue through the weekend and into next week as a series of fronts cross the region. * HEAVY SNOW: Heavy mountain snowfall will continue through Thursday, spreading 1 to 3 feet over the Olympics and 2 to 5 feet over the Cascades with locally higher amounts over the highest peaks. Gusty winds will make for periods of hazardous travel conditions and reduced visibility, and a Winter Storm Warning remains in effect for mountain locations above 2000 ft. * WIND: South to southwest winds will increase today as the storm system offshore intensifies, with gusts reaching 40 to 45 mph across the lowlands and up to 50 mph through the mountain passes. Wind Advisories are in effect for portions of the Puget Sound and Salish Sea lowlands and along the Pacific coast. * HEAVY RAIN: Heavy rainfall will continue through Tuesday across the lowlands with rain totals of 1 to 3 inches. Significant rises on local rivers are expected, and a Flood Watch is in effect much of southwest Washington. See the HYDROLOGY section for more information.
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&& .SHORT TERM /TODAY THROUGH FRIDAY/...
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A deep trough associated with an atmospheric river will continue to intensify along the Pacific coast today, with continued heavy snowfall over the mountains and moderate to heavy rain across lower elevations. Areas of rain shadowing caused by southwest flow aloft along the Puget Sound and Strait of Juan de Fuca will continue to fill in this morning as ample moisture overcomes the dewpoint depression at the surface. Temperatures will also remain elevated this morning and afternoon behind the passage of a warm front, peaking near 50 degrees across the lowlands with snow levels today hovering above 4000 ft. Heavy precipitation will be enhanced today as the upper level flow shifts southwesterly, allowing for greater moisture fetch ahead of an approaching cold front. The lowlands will see 1 to 3 inches of stratiform rainfall through Thursday morning with the heaviest rain amounts slated for areas along the Pacific Coast. The mountains are on track to see significant snow accumulations above 2000 ft, with 1 to 3 feet over the Olympics and generally 2 to 5 feet over the Cascades through Thursday morning. For higher terrain surrounding Mount Baker and Mount Rainier, the highest peaks may see accumulations surpassing 10 feet of snow. Ensembles maintain good agreement over 1 to 2 feet of snowfall through Snoqualmie and Stevens Pass, with higher probabilities (40 to 50 percent) of over 2 feet through White Pass and along US-20 through the Northern Cascades. Snowfall rates of an inch or more per hour through Stevens Pass this afternoon may cause white-out conditions at times, and conditions over the mountains are favorable for triggering large avalanches. Confidence is high over the significant precipitation totals, with consistently high model guidance over the past several days. Strong south to southwesterly winds will also develop throughout the day today as a cold front slowly drops from the northwest, with the strongest winds along the Pacific coast and through the Salish Sea/Puget Sound/Hood Canal and surrounding lowlands. Winds across western Washington will increase to 20 to 30 mph this morning into the afternoon with gusts reaching 40 to 45 mph. Ensemble guidance highlights a moderate (50 to 60 percent) chance of wind gusts peaking above 50 mph across Whidbey and Camano Islands later this morning and into the afternoon. Elevated winds over the mountains will also cause localized areas of blowing snow that will produce hazardous travel conditions through the Cascade passes. Conditions through White Pass will be particularly hazardous, with model guidance highlighting the high potential (70 to 80 percent chance) for gap winds surpassing 50 mph. A frontal passage late this evening and into Thursday will cause both winds and precipitation to lessen, with snow levels lowering to 1000-1500 ft by Thursday morning. While lowland temperatures will likely stay above freezing, a brief period of snow or a rain/snow mix is possible early Thursday morning with little to no accumulation. The bulk of the moisture in this atmospheric river pattern will be pushed southward towards Oregon and California behind the cold front, but wet conditions across western Washington will persist as the core of a low pressure system continues to meander southward. The post- frontal environment on Thursday will become more unstable as convective precipitation spreads across the region. With modest instability up to 200 to 350 J/kg along the Washington coast, thunderstorms may develop along the shore and into the Chehalis Valley. Locally heavy precipitation, graupel, and lightning are possible in heavier showers that develop. Cooler temperatures on Thursday will allow snow levels to drop to 500 ft Thursday night into Friday morning, but any wintry precipitation that falls overnight is not favored to accumulate. Shower activity and breezy winds will continue on Friday as an upper level trough continues to deepen offshore, lifting a front northward across the region. Between Thursday and Friday, an additional 1 to 2 feet of snow is forecast over the Olympics with a foot or less over the Cascades and generally light rainfall amounts across the lowlands. Light accumulating snow is possible along the higher Pacific coast near Forks and the highest elevations of US-101 Friday morning as temperatures may cool enough and QPF is still high enough to potentially see accumulations. Probabilities indicate a 30-60% chance of seeing snow below 1000 to 500 ft in elevation Friday morning. .LONG TERM /SATURDAY THROUGH TUESDAY/...Western Washington will see little relief from wet conditions through the long term, with active weather continuing through the forecast period. The upper level trough over the Pacific Northwest will continue to generate cool and wet conditions through the weekend. Low temperatures near or below freezing across the lowlands on Saturday and Sunday mornings will allow for overnight periods of a rain/snow mix or lowland snowfall, but accumulations are unlikely. Snow levels will remain below the mountain passes, with light snowfall accumulations through the passes over the weekend. Monday will bring another round of potentially heavy precipitation as another trough develops along the western US, but confidence is low. Ensembles continue to favor below-normal temperatures and a chance of precipitation into next week. Lindeman/LH
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&& .AVIATION...
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Westerly flow aloft becoming more southwesterly as the day progresses as an upper level trough over the Pacific continues to deepen. Strong, gusty southerly surface winds in place over much of W WA this morning, the exception being CLM, where the Olympic mtns proving to be an effective barrier. These winds will remain up throughout the day before starting to ease around 10Z tonight. All cigs this morning will be between MVFR and IFR as heavy rain bands move in throughout the rest of the morning hours. RA to +RA possible in the heaviest convective bands. Winds will remain elevated out of the SSW through most of the evening hours, with most speeds dropping off now after 10Z. Some wind shear and turbulence has been noted in area PIREPs this morning and will likely continue throughout the day today. KSEA...MVFR cigs likely to continue throughout the day today as heavy rain bands continue to move across the terminal. Periods of reduced visibilities are possible, down to 4SM or less, in the heaviest rain. Southerly winds generally ranging 10-15 kts with gusts up to 30 kts possible also expected to remain in place for bulk of the TAF period, although likely to see speeds diminish to 8- 12 kts after 10Z tonight and remain that way into Thursday morning. 18/Kristell
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&& .MARINE...
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A strong cold front will pass through the area waters today, resulting in strong, gusty winds over the entire area. Only change to this forecast has been to add follow-up SCAs for locations currently under Gale Warnings. Appears to be a brief lull in winds tonight into early Thursday morning, but it appears could see winds ramp up again by Thursday afternoon for a new round of headlines. Will not act on any of that now for AM forecast package, so as not to throw too many headlines out at once leading to potential confusion. Active conditions expected into the weekend and as such continued potential for additional headlines. Seas are generally hovering between 9-11 feet this morning, increasing a little to 11-14 feet this afternoon before returning to 9-11 ft tonight. Seas expected to rise again with aforementioned Thursday system, potentially reaching up to 16 ft in the outer coastal waters. These heightened seas will remain in place til the end of the week. 18/Kristell
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&& .HYDROLOGY...Rising snow levels, moderate to heavy rain, and to a lesser degree low to mid elevation snowmelt will potentially produce enough runoff to cause minor flooding of some streams and rivers. The Olympics and Cascades could see 3 to 5 inches of rain. Based on the degree of snow level changes, its relation to precipitation, and the amount of precipitation and snowmelt makes for a fair amount of uncertainty on what, if any, rivers will flood. The exception is the Skokomish River which is highly likely to exceed flood stage and possibly Moderate Flood. Continue to monitor river forecasts through the next few days due to the rising rivers. JD && .SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Wind Advisory until 7 PM PST this evening for Admiralty Inlet Area-Bremerton and Vicinity-Everett and Vicinity-Hood Canal Area-Seattle and Vicinity-Tacoma Area-Western Skagit County- Western Whatcom County. Flood Watch through Thursday afternoon for Bellevue and Vicinity- Central Coast-East Puget Sound Lowlands-Hood Canal Area- Lower Chehalis Valley Area-Olympics-Seattle and Vicinity- Southwest Interior-Tacoma Area-West Slopes North Central Cascades and Passes-West Slopes South Central Cascades and Passes. Winter Storm Warning until 4 PM PST Thursday for Olympics-West Slopes North Cascades and Passes-West Slopes North Central Cascades and Passes-West Slopes South Central Cascades and Passes. Wind Advisory until 4 PM PST this afternoon for Central Coast- North Coast. PZ...Small Craft Advisory from 1 PM this afternoon to 4 AM PST Thursday for Grays Harbor Bar. Small Craft Advisory until 4 PM PST this afternoon for West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory until midnight PST tonight for Central U.S. Waters Strait Of Juan De Fuca. Gale Warning until 4 PM PST this afternoon 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-East Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 4 PM this afternoon to midnight PST tonight for East Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until 7 PM PST this evening for Admiralty Inlet- Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal. Small Craft Advisory from 7 PM this evening to midnight PST tonight for Admiralty Inlet-Northern Inland Waters Including The San Juan Islands-Puget Sound and Hood Canal. Small Craft Advisory from 4 PM this afternoon to 4 AM PST 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.
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