FXUS66 KSEW 220514 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 914 PM PST Wed Feb 21 2024 .UPDATE...Satellite imagery shows the weak upper level low offshore has now moved north to off the north coast. Last main band of showers rotating out of the low along a line from about Hoquiam to Seattle. This feature is moving north slowly and will still be over the Northwest Interior Thursday morning. Isolated showers on either side of the main band. The low will dissipate Thursday with showers decreasing in the afternoon. Drying Thursday night into Friday with a weak upper level ridge building over the area from the south. Current forecast has the trends covered. No update this evening. Previous discussion follows. && .SYNOPSIS...A weakening low will continue to drift northward along the Washington Coast before dissolving on Thursday morning. Upper ridging will build into the region later Thursday with drier weather heading into the weekend. A more active pattern will enter the region on Sunday and continue into next week with a series of systems sliding across the Pacific Northwest. * SNOW: A storm system will bring in heavy snowfall to the mountains towards the end of the weekend with accumulations of a foot or more across the mountain passes through Tuesday morning. Ensembles also hint at the possibility for areas of light lowland snow accumulations early Monday and Tuesday mornings from a rain/snow mix. && .SHORT TERM /TONIGHT THROUGH SATURDAY/...Moisture will continue to be injected into the region by the offshore low, maintaining steady light shower activity through early Thursday morning. Patchy fog and low stratus may develop Thursday morning with residual moisture at the surface. A drier air mass will push northward throughout the day Thursday as a ridge amplifies into the Pacific Northwest, allowing conditions to clear out and warm up several degrees. Fog and low stratus will likely redevelop overnight into Friday and clear out once again by the late morning. Temperatures will remain fairly steady on Thursday and Friday, with most lowlands peaking in the mid 50s. Upper level flow will begin to flatten on Saturday as a trough begins to lower from the Gulf of Alaska. Cooler temperatures will cause snow levels to lower to 2500-3000 ft as precipitation spreads in from the north. The bulk of the moisture with this initial push will be focused over the Northern Cascades, with 2 to 4 inches of snow accumulation above 3500 ft through Saturday night with locally higher amounts over the highest peaks. Stevens Pass will likely see an inch of snowfall on Saturday, with the mountain passes southward favored to see minimal accumulations. .LONG TERM /SUNDAY THROUGH WEDNESDAY/...A potent storm system will push into the Pacific Northwest on Sunday, bringing significant mountain snow to western Washington. The bulk of the moisture with this system will once again be centered over the Cascades, and moisture levels will be enhanced by breezy onshore flow. Widespread precipitation will continue through Tuesday morning with preliminary snow totals of up to a foot over the Olympics and 1 to 2 feet over the Cascades. Significant snowfall will likely cause impacts to travel, with a moderate (60 to 70 percent) chance of over 12 inches of snow through Snoqualmie Pass through Tuesday morning. Snowfall through Stevens Pass is favored to see heavier snowfall amounts, with a high (90 percent) chance of 12 inches of snow accumulation and even a low (20 percent) chance of seeing above 2 feet of snow through the pass. Latest ensemble guidance shows a mean snow accumulation forecast of up to 30 inches for the higher mountains of Snohomish County between Sunday and Tuesday, but snowfall totals remain fairly uncertain this far out. While the primary concern of the extended period is heavy mountain snow, the lowlands are also primed to see a rain/snow mix at times Sunday night into the early part of next week. Precipitation will become more convective Sunday night into Monday, which may allow snow levels to lower towards the surface through evaporative cooling. Any lowland snow accumulations will likely occur on grassy or elevated surfaces and will quickly melt off with daytime temperatures in the 40s. The lowlands will likely see more of the same Tuesday morning before precipitation starts to taper off. Western Washington will see little relief from wet and cool conditions on Tuesday with another storm system on track to cross the region on Wednesday. While this storm system looks to have less moisture than the weekend system, heavy snowfall is once again forecast over the mountains for mid-week. Temperatures will remain below average through much of next week across western Washington, with long-range ensembles favoring cooler and wetter conditions lasting through next week. Lindeman && .AVIATION...Southwesterly flow aloft as weak upper level low off the northern Washington coast dissipates Thursday. Light flow in the lower levels. MVFR ceilings with a band of showers between Hoquiam and Seattle moving north northeasterly with VFR ceilings outside of the band. Isolated IFR ceilings near the Strait of Juan de Fuca and the KItsap Peninsula. More widespread MVFR ceilings after 12z Thursday morning with VFR ceilings all areas Thursday afternoon. KSEA...VFR ceilings with shower activity decreasing after 06z. MVFR ceilings at times 12z-20z with VFR ceilings after 20z. Southerly winds 6 to 10 knots becoming northerly 4 to 8 knots around 00z. Felton && .MARINE...A stacked low/surface trough off the north Washington Coast continues to trigger showers over the waters. Small craft advisory southeast winds continuing in at the East Entrance to the Strait with winds below 20 knots elsewhere. No headlines Thursday through Saturday. Next significant system will approach the area Sunday into next week, with seas building and gusty winds expected across all waters. Seas will remain at 5 to 7 feet and increase above 10 feet Sunday into next week. Felton/HPR && .HYDROLOGY...No river flooding expected in the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 4 AM PST Thursday for East Entrance U.S. Waters Strait Of Juan De Fuca. && $$