Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 222350 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 350 PM PST Thu Feb 22 2018 .SYNOPSIS...Cold and sometimes unsettled northwest flow aloft will continue into the middle of next week. The next system will move southeast across Western Washington on Friday and Friday night, with lowland snow transitioning to rain. Moderate to heavy mountain snow is expected into Saturday morning. Another cold wet system will dive in late Saturday night and Sunday, with another round of moderate to heavy mountain snow, along with some localized areas of lowland snow. A dry lull is possible late Monday, before conditions become unsettled again from late Tuesday through next Thursday. && .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. With clear skies for most of tonight, will be cold and frosty with another hard freeze. Many of the colder spots in Southwest Washington will drop into the teens tonight. 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. With another enhancement of precip late Friday night, this could really cause the snow to pile up both in the mountains and in some of the Cascade foothill communities. While it will be raining along most of I-5, foothill communities like North Bend, Gold Bar and Sedro Woolley could pick up a few more inches of snow late Friday night. Once the shortwave exits later Saturday morning, 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. A short lull in precip is expected late Saturday, then a colder and more vigorous trough will dive southeast along the B.C. coast, spreading precip into Western Washington on Sat night. Moderate to heavy snow is expected in the mountains, given cold onshore flow. After the attendant cold front passes Sunday morning, snow level will drop back into the 500-1000 foot range with an abundance of convective showers in cold northwest onshore flow. Mesoscale models really point to inland portions of the North Coast (such as Forks) getting a shot of heavy snow, though Cascade foothill communities could also get more snow. Will have to watch for any persistent Puget Sound Convergence Zone, which could locally drop snow level further and bring accumulations on hills north of Seattle. Haner .LONG TERM...A longwave trough over the western U.S. will continue to bring cold northwest flow until Wed. A shortwave ridge within this flow around Mon night or Tue could bring a dry period. The official forecast simply reflects this with a decrease to low chance PoPs for now. The longwave pattern should retrograde next Wed and Thu, pushing the longwave trough axis either to the U.S. West Coast or just offshore. With the trough axis either overhead or just offshore on Wed and Thu, the theme of low snow levels and showery weather will re-emerge. Haner && .AVIATION...Northerly flow aloft over Western Washington this afternoon will increase tonight, then back to northwest late tonight and continue Friday. The air mass will be generally dry tonight, with moisture increasing Friday as the next frontal system moves into the region. At the surface moderate northerly flow will continue today, ease this evening, and turn southerly late tonight. Western Washington had VFR conditions with no ceiling at 3 pm, and there should be no significant cloud cover below 12,000 ft through tonight. Conditions will deteriorate Friday as the next frontal system approaches and moves into the forecast area. Widespread low-end MVFR to local IFR conditions are likely Friday afternoon or evening with snow or snow/rain across the area. KSEA...North wind 7-13 kt easing to 4-8 kt this evening and switching to southerly around midnight; southerly wind rising to 10-16 kt in the afternoon. There should be no significant cloud cover below 12,000 ft tonight. Conditions will lower Friday, eventually becoming low-end MVFR or possibly IFR at times. KSEA is likely to receive about 1 inch of snow, probably between 2 pm and 8 pm Friday. McDonnal && .MARINE...Moderate northerly flow will ease this evening and turn southerly tonight, as a weak ridge moves southeast across the waters. A frontal system will move southeast across the region Friday afternoon and night. This system should bring solid small craft advisory winds to all marine zones, both with the southerlies ahead of the system and the west to northwest flow following it. Brief gales in the post-frontal westerlies in the central and eastern Strait of Juan de Fuca are not out of the question. A more vigorous system will move across the area Saturday night and Sunday. There is a better possibility of gales with this system, especially over the coastal waters and in the central and eastern Strait of Juan de Fuca in the post-frontal westerly flow. A weaker front will arrive Tuesday. McDonnal && .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 6 AM Friday to noon PST Saturday 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 from 6 AM Friday to 6 PM PST Saturday 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 6 PM PST this evening for Northern Inland Waters Including The San Juan Islands. && $$ www.weather.gov/seattle

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