Area Forecast Discussion National Weather Service Portland OR 303 PM PDT Thu Oct 19 2017 .SYNOPSIS...A series of storm systems will bring periods of valley rain that may be heavy at times, mountain snow and coastal winds through the weekend. && .SHORT TERM...Today through Sunday...A modest atmospheric river remains pointed at northwest Oregon and southwest Washington this afternoon. Precipitation should gradually turn more showery tonight as a colder and more unstable airmass spreads into the region. The best chance for thunderstorms will be over the waters tonight and early Friday morning, but cannot rule out a clap or two of thunder Friday afternoon, mainly east of Interstate 5 in the Willamette Valley. Snow levels will likely lower to between 4-5kft late tonight and Friday as the cool and more unstable airmass spreads across the region. There should be some decent snow accumulations above 5000 ft, but temperatures appear marginal enough below that elevation that impacts and criteria probably won`t warrant a Winter Wx Advisory for the Cascade passes. Still would not be surprised to see between 5-10" of snow around 5000-6000 ft through Saturday morning. Expect showers to decrease Friday evening with a tendency for some brief clearing. Model soundings indicate this may result in snow levels temporarily lowering closer to 2500-3000 ft across the Cascades, particularly near the east side of Mt Hood and the lower terrain surrounding Mt Adams. This could matter for a few hours towards daybreak on Saturday as the next round of widespread precipitation arrives. Any snow accumulations at these lower elevations should be brief as a warm front results in snow levels rising to at least 7000-8000 ft by late Saturday. The strong westerly flow that will follow behind the warm front will likely enhance orographic precipitation across the coast range and Cascades late Saturday. A distinct rain shadow should then develop Saturday afternoon and evening and result in rainfall rates dropping off dramatically from the first half of the day in the Willamette Valley. A decently strong south to north pressure gradient will also set up across the area, and likely produce at least breezy winds along the coast and even in the Willamette Valley. Whether or not, it will be strong enough to produce 58 mph gusts along the coast (warning criteria) and 45 mph gusts in the Willamette Valley (advisory criteria) remains unclear at this point. Models have and continue to suggest an atmospheric river will then take aim at the Pacific Northwest. There is high confidence that a period of more intense rainfall will drop southward across the area Sunday. However, it remains unclear if it will sit over northwest Oregon or western Washington in the 12-18+ hours preceding Sunday afternoon. This has big implications for our river forecasts given it has the potential to produce 1" of rain in the valleys and 2-3" of rain across the higher terrain in a 6 hour period if it remains stationary. So the potential of 12-18 hours of that type of rain would undoubtedly produce hydrologic issues even with it still being early in the wet season, particularly across our burn scars and smaller streams. Given model trends, suspect far northwest Oregon and southwest Washington stand the best chance for having the atmospheric river sit over it for an extended period of time, but it`s far from guaranteed. These intense bands of rain can easily deviate 50-100 miles north or south from current model forecasts given the time between now and then. /Neuman .LONG TERM...Sunday night through Thursday....Models are in agreement, that shortwave ridging will develop over the Pacific NW early next week. This will bring a period of dry weather that will likely be followed, by a weak trough towards the middle of the week. This will provide the chance for more rain Tuesday night into Wednesday. Models are suggesting that another ridge of high pressure could bring a dry period late next week. Bishop && .AVIATION...The persistent band of rain over much of NW Oregon and SW Washington Thu will shift east of the area tonight. MVFR and locally IFR conditions will tend to improve overnight as the main area of rain moves off, and a more showery air mass moves in. Generally expect improvement to VFR conditions between 03z and 08z, although there is likely to be a few areas of MVFR conditions persisting past 08z and well into Fri associated with heavier showers. PORTLAND AND APPROACHES...MVFR conditions are likely to prevail through about 03z this evening, as the persistent rain starts to move out this evening. VFR conditions are expected to dominate after 03z as the air mass turns more showery, but it is likely that there will be occasional temporary MVFR conditions beyond 03z and into Fri associated with showers. && .MARINE...Winds to slowly veer around from the south to the west tonight and Fri, with occasional gusts to around 25 kt. Will allow the small craft advisory in the north to expire this evening as winds there appear likely to temporarily weaken some before increasing again Fri morning. A frontal system moving into the waters late Fri night and Sat will bring the threat of gale force winds back by Sat. Expectations are for this system to track a little further south than originally anticipated, meaning the strongest and more persistent gales are likely across the southern waters Sat and Sat night. A long period westerly swell around 20 ft arrived early Thu afternoon, and will likely continue at that level through most of tonight. The swell is expected to slowly decrease Fri into Fri night, eventually dropping below 15 ft by around midnight Fri night. As winds increase though late Fri night and Sat and then another swell train arrives from the west Sat night, expect seas to remain in the 14 to 19 ft range through Sat night. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...High Surf Advisory until 10 AM PDT Friday for Central Oregon Coast-North Oregon Coast. WA...High Surf Advisory until 10 AM PDT Friday for South Washington Coast. PZ...Small Craft Advisory for winds until 6 PM PDT this evening for Coastal waters from Cape Shoalwater WA to Cascade Head OR out 10 NM-Waters from Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM. Small Craft Advisory for hazardous seas until 11 PM PDT Friday for Coastal waters from Cape Shoalwater WA to Cascade Head OR out 10 NM-Coastal waters from Cascade Head to Florence OR out 10 NM-Waters from Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM-Waters from Cascade Head to Florence OR from 10 to 60 NM. Small Craft Advisory for winds until 1 PM PDT Friday for Coastal waters from Cascade Head to Florence OR out 10 NM- Waters from Cascade Head to Florence OR from 10 to 60 NM. Small Craft Advisory for Rough Columbia River Bar until 6 PM PDT Friday. && $$ This discussion is for Northwest Oregon and Southwest Washington from the Cascade crest to 60 nautical miles offshore. This area is commonly referred to as the forecast area.