Area Forecast Discussion National Weather Service Portland OR 1007 AM PDT Thu Mar 22 2018 ...Updated aviation and marine sections. .SYNOPSIS...A strong cold front will move through southwest Washington and northwest Oregon this morning, with snow levels lowering into the coastal mountains and Cascade foothills later today and tonight, and possibly as low as around 1000 feet Friday and Saturday. Look for accumulating snow in the mountains especially tonight and Friday in the onshore flow behind the front. Look for some breezy winds today. There will be a chance of thunder at times especially near the coast through Saturday, and possibly inland on Saturday. The pattern should dry out quite a bit next week as an upper ridge builds, though there are some hints that the upper ridge may be a bit "dirty" at times. && .SHORT TERM...Today through Saturday...A strong cold front is just off the coast, pulling up ample moisture ahead of it from a system off the California coast. The models say the front will come onshore along the coast around 5 AM this morning, then move through the inland areas by mid to late morning. Breezy conditions will accompany the front, but the winds should stay below warning or advisory criteria. Snow levels will drop dramatically behind the front, falling into the coastal mountains and the Cascade foothills this afternoon and evening. Onshore flow is forecast to pick up tonight as a lot of the could air cumulus seen out between 130W and 140W spreads in under southwesterly low level flow. The 850 mb winds reach 40 to 45 kt tonight which should aid the snowfall in the coastal mountains, foothills, and higher Cascades. Have issued several winter weather advisories to deal with this. The 850 mb winds feeding all of this onshore don`t let up until later Friday and Friday evening, which is when the snowfall will ease. Road surfaces will start out quite warm, so it will take some time for snow to start to accumulate on roadways through the coastal mountains and the Cascades, but eventually it will. But this will cause the heaviest snow accumulations to be at the elevations of the ski resorts such as Timberline and Mount Hood Meadows. Snow levels will fall to around 1000 feet Friday into Saturday, but accumulations will be light. There is always a risk of a few flakes below this level early Saturday, but this is not certain. It is getting a bit late in the season to get snow down to the valley floor, and the offshore low has taken a circuitous and lengthy route over the water which will further modify how cold the air is. All in all, don`t think we will see any snow to the valley floor. The parent cold upper low will dumbbell around and come onshore late Saturday and Saturday night, keeping showers around. Model lifted indices suggest thunder is possible near the coast the next few days, possibly clipping the Kelso area Friday afternoon, and a greater part of the inland valleys on Saturday as the low moves onshore and through the area. Tolleson .LONG TERM...No Changes. Previous discussion follows... Saturday night through Wednesday. Forecast models in slightly better agreement today concerning the weekend and into early next week. The upper level trough is expected to advance inland into far southern Oregon or northern California late Saturday night before reaching the Great Basin on Sunday. This will enable a cold air mass to move into the Pacific Northwest and maintain lower snow levels (1,500 to 2,000 feet) through Sunday afternoon or evening. Meanwhile, a building upper level ridge over the Pacific will eventually bring rising heights late Sunday through the start of next week. While showers will continue in the northwest flow aloft on Sunday, the flow will turn to a more north or perhaps northeast direction early next week. Thus, restricted mention of precipitation to a slight chance on Monday and removed all mention of PoPs starting Monday night. Depending on the strength and position of the ridge, it is possible some light precipitation may be possible across the northern portions of the area at times given some weak impulses embedded in the flow. Given this pattern, have trended temperatures gradually warmer for the start of next, with temperatures again approaching 60 in the interior lowlands for the middle of next week.Cullen && .AVIATION...A rain band associated with a weak cold front is moving through the interior this morning. Based on radar trends, it should be fully through the lowlands and into the Cascades by 18Z. Post-frontal showers will develop this afternoon, along with some breezy southerly winds. There will be potential for a few thunderstorms as well, but confidence in timing and location not high enough to include in the TAFs at this time. Expect a mix of VFR/MVFR through 18Z, then predominantly VFR conditions this afternoon. However, brief periods of MVFR conditions are likely during as heavier showers pass. A secondary front will bring more organized shower activity to the coast this evening, then into the interior lowlands overnight. Expect conditions to deteriorate to predominantly MVFR as the steadier showers move in. PORTLAND AND APPROACHES...Expect mainly VFR conditions during the late morning as rain tapers to showers. However, an occasional heavy shower may bring a brief period of MVFR There is also potential for an isolated thunderstorm at some point this afternoon. Pyle && .MARINE...A cold front moved through the waters earlier this morning. Southwesterly winds behind the front are now gusting in the 20 to 25 kt range, and will continue through the afternoon. A stronger secondary front will bring southerly winds of 25 to 30 kt this evening and early in the overnight hours. The 12Z NAM and GFS runs suggest that there may be a brief period of low-end gales in PZZ255/PZZ275 in the evening. Decided not to issue a gale warning at this time, as the duration appears to be very short and the area affected rather small. Winds will remain above small craft advisory criteria into Fri morning, but should decrease through the day. An offshore low dropping south towards the CA waters may bring another round of small craft winds Sat. Then more benign conditions are expected late in the weekend and into early next week as higher pres builds toward the waters. Seas will build above 10 feet this afternoon, peaking at 12 to 14 feet late tonight, then subsiding below 10 feet Friday night across the north and Saturday morning south. The seas will be a mix of wind wave, fresh SW swell and distant source NW swell. Pyle && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory from 10 PM this evening to 6 PM PDT Friday for Cascade Foothills in Lane County-Northern Oregon Cascade Foothills. Winter Weather Advisory from 6 PM this evening to 6 PM PDT Friday for Northern Oregon Cascades. Winter Weather Advisory from 11 PM this evening to 11 PM PDT Friday for Cascades in Lane County. Winter Weather Advisory from 6 PM this evening to 5 PM PDT Friday for Central Coast Range of Western Oregon-Coast Range of Northwest Oregon. WA...Winter Weather Advisory from 6 PM this evening to 1 PM PDT Friday for South Washington Cascades. Winter Weather Advisory from 6 PM this evening to 5 PM PDT Friday for Willapa Hills. Winter Weather Advisory from 6 PM this evening to noon PDT Friday for South Washington Cascade Foothills. PZ...Small Craft Advisory for winds until 2 PM PDT Friday for Coastal Waters from Cape Shoalwater WA to Florence OR out 60 NM. Small Craft Advisory for hazardous seas from 5 PM this afternoon to 11 PM PDT Friday for Coastal Waters from Cape Shoalwater WA to Cascade Head OR out 60 NM. Small Craft Advisory for hazardous seas from 5 PM this afternoon to 5 PM PDT Saturday for Coastal Waters from Cascade Head OR to Florence OR out 60 NM. Small Craft Advisory for Rough Columbia River Bar from 8 AM to noon 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 CWA or forecast area.