991 FXUS66 KPQR 281741 AAA AFDPQR Area Forecast Discussion Weather Service Portland OR 1040 AM PDT Tue May 28 2019 Updated Aviation section .SYNOPSIS...Low-level onshore flow will prevail for the next several days, keeping temperatures fairly close to normal for late May and the first couple days of June. Extensive low clouds this morning will be slow to break up this afternoon. Otherwise, night and morning low clouds will likely clear to the coast by each afternoon. Weak instability may trigger isolated showers and thunderstorms near the Cascade crest each afternoon through Thursday. && .SHORT TERM...Today through Thursday...GOES-17 fog product shows widespread stratus across much of western WA/OR early this morning, deep enough to bank up against the Cascade crest. With the marine layer this deep and onshore pressure gradients stronger than 24 hours ago, it appears we can expect clouds to linger into the afternoon hours for most of the forecast area. Undercut National Blend of Models guidance considerably for temperatures today, opting instead for more of a persistence-based forecast. With the stabilizing effect of the marine layer, it is difficult to believe any surface-based convection will develop along the Cascade crest this afternoon. However, model soundings suggest reasonably steep mid-level lapse rates of 7-8 deg C/km, and most models show a very weak lobe of vorticity drifting southward from western WA. Thus we opted to leave the slight chance of thunderstorms in the forecast for the Cascade crest, as any showers triggered by this disturbance would have plenty of instability aloft to work with. In fact there is a nonzero chance of showers anywhere in the CWA today into tonight, but the odds for any particular location outside of the Cascades being affected are probably less than 10 percent. Expect little change in the overall weather through Thursday. Very weak mid-level cyclonic flow will linger over Oregon, likely keeping the marine layer rather deep and maintaining robust onshore flow. 06z GFS slowly moves this circulation southward to the OR/CA border Thursday, which could finally lead to a shallower marine layer and sunnier afternoons if it verifies. However, it is very difficult to hang one's hat on this solution due to the weakly-forced nature of this pattern, so it may take until Friday to really feel the effects of an upper level ridge trying to nose into the Pac NW. Maintained a slight chance of afternoon/evening thunder near the Cascade crest through Thursday, as elevations above the marine layer could potentially become warm enough to drive surface-based convection each afternoon. Weagle .LONG TERM...Thursday night through Monday...A broad low pressure system drops down into the Gulf of Alaska midweek this week, trying to bring a change in pattern to the Pacific Northwest. It initially brings zonal flow into BC, but the ridge over the Pacific keeps it just a bit too far north to bring any chance for precipitation to western Washington or Oregon. This means more of the same is in the forecast. Dry conditions with near seasonal normal temperatures through early next week. -McCoy && .AVIATION...Lower MVFR marine stratus continues to lift this morning. Expect improvement to VFR with some scattering across the interior by early afternoon. Cloud deck likely remains in place, but may also lift to low VFR for the coastal areas. Expect a return of MVFR cigs around 1500 ft late this evening and overnight with local IFR fog/low stratus possible. KPDX AND APPROACHES...Gradual improvement to VFR ceilings next several hours with VFR conditions this afternoon. Lower MVFR stratus returns tonight after 09z. Cullen && .MARINE...High pressure offshore and weak trough near southern Oregon coast will maintain relatively relaxed surface pressure gradient across the coastal waters through much of the week. Expect generally northwest winds with winds less than 20 kt through at least Wednesday or Thursday. Late in the week, the thermal trough may strengthen a bit along the southern Oregon coast, resulting in stronger gusts. Meanwhile, seas will generally hold at 4 to 6 feet but will remain somewhat steep as a fresh swell will combine with one or two longer period swells of similar magnitude. Cullen && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland 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.