Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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224 FXUS66 KSEW 221743 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 943 AM PST Thu Feb 22 2018 .SYNOPSIS...A break between systems is occurring today, though cold and dry air continues to filter out of western Canada and into Western Washington. The next system will dive in from the northwest on Friday, with precipitation moving spreading in from the northwest from late Friday morning through Friday afternoon. Low snow levels will continue as this system moves through the area. Another system will follow quickly early Saturday. Cool northwest flow aloft through the middle of next week, with cool wet systems around Sunday and then next Tuesday or Wednesday. && .SHORT TERM...Northwest flow aloft will continue through early next week. For today, a shortwave ridge is crossing the area, and cold dry continental air continues to pour out of the Fraser Valley past Bellingham, reinforcing the cold and dry air mass in place. The next shortwave will dive down the B.C. coast on Friday, with a secondary wave on its heels crossing the area early Saturday morning. Westerly low-level flow in advance of Friday`s shortwave will advect warmer maritime air onshore. The warm advection lift will bring about precip. Precip will spread southeast down the B.C. coast on Friday morning, reaching the northwest Olympic Peninsula by late morning, then spreading across the rest of Western Washington on Friday afternoon. Cold air mass will cause precip to start as snow. Along the coast, precip will quickly change to rain just an hour or two after beginning. Along the I-5 corridor from the Everett area on south, precip will change to rain early Friday evening, after an inch or so of snow has occurred. With onshore flow, orographic lift will enhance precip over the mountains. In the Cascades, this could be an advisory- level snow event. By the time the secondary trailing shortwave arrives early Saturday morning, most of the lowlands will be warm enough for merely cold rain, though snow could linger in the eastern lowlands, i.e. the Cascade foothill communities. Once the shortwave passes, the air mass will cool down some again, with snow level down to 300-500 feet by mid-day Saturday. Post-frontal showers are possible, especially in a Puget Sound Convergence Zone and the mountains. Haner .LONG TERM...From Previous Discussion: The general trend of well below normal temperatures with precipitation will continue through the extended period. GFS and ECMWF in good agreement with another low snow level, 1000 feet or less, system arriving on Sunday with an upper level trough over the area on Monday. Model solutions diverge after Monday with the ECMWF drying things out Tuesday and Wednesday while the GFS brings yet another system down the British Columbia coast Tuesday into early Wednesday with somewhat low snow levels. Current forecast is more in line with the GFS solution and with the inconsistencies in the models will stay with the current forecast for the extended period. Felton && .AVIATION...Northerly flow aloft over Western Washington this morning will continue today, then increase tonight and back to northwest late tonight. The air mass will be generally dry, with moisture increasing late tonight as the next weather system approaches the region. At the surface moderate northerly flow will continue today, ease this evening, and turn southerly late tonight. There is still some lingering low clouds, generally around 1500 ft, especially in the south part of the forecast area; those clouds should dissipate during the next couple hours, with only high clouds above 12,000 ft over the area through tonight. The exception to this is along the north side of the Olympic Peninsula, where northerly low level flow is producing a local deck of stratocumulus clouds around 4000-5000 ft. KSEA...North wind 8-14 kt easing to 4-8 kt this evening and switching to southerly around midnight. The low clouds around 1400 ft will continue to decrease this morning; otherwise there should be no significant cloud cover below 12,000 ft through tonight. The next chance for accumulation snow at KSEA appears to be Friday afternoon. McDonnal && .MARINE...Moderate north to northeast pressure gradients will continue today as a weak low over the far southern Washington coastal waters drops south through Oregon. Small craft advisory winds are expected through this evening over the coastal waters and for today over much of the inland waters. Winds will ease tonight as a weak ridge moves southeast across the area. A strong surface trough will drop southeast into the waters later Friday into Friday night, bringing small craft advisory south to southeast winds to all of the waters. Winds will likely increase further Saturday into Sunday as a couple of strong frontal systems drop southeast through the waters. Small craft advisory conditions are likely over all of the waters, and gales are possible. McDonnal && .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory until 11 PM PST this evening 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. Small Craft Advisory until 3 PM PST this afternoon for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. Small Craft Advisory until 1 PM PST this afternoon for Puget Sound and Hood Canal. && $$ www.weather.gov/seattle

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