FXUS66 KPQR 051046 AFDPQR Area Forecast Discussion National Weather Service Portland OR 345 AM PDT Sun May 5 2024 .SYNOPSIS...Broad low pressure continues to dominate the weather for much of the US West Coast, though precipitation has been dwindling west of the Cascades. Light to moderate snow continues in the Cascades above 4000 feet, largely due to increasing upslope flow as relatively higher pressure builds onshore from the Pacific. Onshore flow will briefly weaken, decreasing the snow in the Cascades this evening. However the next front will bring a resurgence of snow in the Cascades later tonight into Monday morning, with a brief shot of rain in the lowlands followed by scattered showers Monday afternoon. Post-frontal showers will continue into Tuesday, then a significant warming and drying trend is expected later in the week as high pressure builds over the region. && .SHORT TERM...Today through Tuesday...Latest radar imagery, webcams, and surface observations show precipitation has largely decreased across SW Washington and NW Oregon overnight, though light to moderate snow persists in the Cascades above 3500-4000 ft. For a stratiform rain in May, rainfall totals have been impressive over the past 24-36 hours with just about the whole forecast area showing over an inch of rain. The main exception has been the Hood River Valley which has received 0.5-1.0 inch. The heaviest precipitation has been Salem southward, where most locations have received 1.5-2.5 inches since Friday afternoon. The rain has been enough to bring noticeable rises in area drainages, but not enough to threaten flooding. In the Cascades, the primary concern has been snow since snow levels fell below pass level Saturday afternoon. In general, 5-10 inches of snow have accumulated in the Oregon Cascades above 4500 ft, with considerably less in the S WA Cascades and below 4500 ft. While there is high confidence in the evolution of the larger scale features, confidence is lower in the smaller scale, subtler details. For example, while precipitation has generally been decreasing over western WA/OR, there has been an uptick in rain over the past hour across much of Clark, Multnomah, and Clackamas Counties. This enhancement is largely due to deformation banding, which is notoriously difficult to forecast. Most of the latest hi-res model runs do not even have this existing deformation band in their analyses or hour 1-3 forecasts. An educated guess of what is going on is that a lobe of enhanced Q-vector convergence associated with the broader area of deformation precip across NE Oregon is passing over eastern portions of the PDX metro area. If this is indeed what is causing the enhanced precip, it will likely pivot over the north Oregon Cascades over the coming hours while precip decreases over the PDX metro area. For elevations above 3500-4000 ft, this enhanced precip is manifesting as heavier snowfall rates in the Cascades between Santiam Pass and Mount Hood, and should last another few hours before diminishing. Meanwhile, KTTD-KDLS gradients have increased to +4.8 mb, suggesting increased orographic enhancement that will persist throughout the day today. So, while the lift from deformation decreases this morning, it will be replaced by the increasing orographic lift, which should keep light to moderate snow going in the Cascades through this afternoon. This orographic lift should briefly decrease this evening as an occluding cold front approaches the coast, so we will maintain the existing Winter Wx Advisories for the Oregon Cascades until 6 PM this evening for another 3-7 inches of snow. For the lowlands, today looks to be a damp, dreary day with periods of rain and temperatures once again struggling to climb above 50 degrees. The above-mentioned occluding cold front will move onshore tonight, pushed along by an unseasonably strong jet stream that is 120-140 kt at its core. This jet stream will also push the existing broad upper low eastward into the Rockies, yielding a more typical evolution of precipitation across the forecast area beginning with the occluding cold front late tonight/Monday morning. Expect a band of enhanced rain and Cascades snow to sweep across the CWA with the cold front late tonight/early Monday morning, followed by post-frontal showers through Tuesday. Lapse rates steepen behind the front Monday afternoon and may be sufficient for a thunderstorm or two. With low freezing levels for the season, any thunderstorms would be capable of producing small hail. Warming temperatures aloft should lead to shallower convection Tuesday, decreasing the chance of thunder and hail. By then, high pressure will be strengthening over the NE Pacific and building into the Pac NW, bringing an end to our period of unseasonably cool and damp weather. Weagle .LONG TERM...Wednesday through Saturday...Longer range ensemble guidance depicts a much more straightforward forecast beginning midweek than in the short term. All clusters in the 00z WPC cluster analysis depict substantial positive 500 mb height anomalies across the Pac NW by Friday, with at least some degree of offshore flow. This will bring dry and much warmer weather to the region by the end of the week. NBM deterministic guidance continues to show highs in the 60s Wednesday, 70s Thursday, and 80s Friday and Saturday. In fact, NBM probabilistic guidance shows a 50/50 chance of the PDX metro area reaching 90 degrees Saturday, with a 10-30% chance of doing so elsewhere across the interior lowlands. If it seems early for us to be reaching 90 degrees in the interior lowlands, it is early but not unheard of to reach 90 deg F this time of year. In fact, PDX did so on 5 out of 6 days between May 12-17 last year. Spring can be a time of large temperature swings and it appears the upcoming week will be no exception for the Pac NW. Weagle && .AVIATION...Area still under the effect of the upper low, through less than 24 hours ago as the mid level low center is now over southeast Oregon. Wrap around moisture continues into western OR on the back side of the low. Generally seeing VFR conditions across the areas terminals, except MVFR along the coast. HREF guidance does show lower probabilities for MVFR through about 15Z for inland terminals, but then probabilities for MVFR increase, the decrease again later in the afternoon. It's worth noting there are chances(10-20%) for a period of IFR cigs 12-18z Sun, but coverage is not widespread. Wind generally stay less than 10 kts inland but coastal areas appear a little breezier with southwesterly winds 10-20 kt in the afternoon. PDX AND APPROACHES...Currently VFR with light rain, but radar showing developing area of rain heading south from WA. Will probably bring periods of MVFR again to the terminal area. HREF is indicating a MVFR probabilities increasing again after a lull overnight. So will probably see MVFR prevailing again through about 22Z Sun when MVFR probabilities decrease again. This seems reasonable given the low and its effect will be waning. /mh && .MARINE...The active weather pattern continues into early this week before a shift to generally calmer conditions and a more summer-like pattern. The next frontal system arrives today bringing another round of gusts up 30 kt as well as steep seas across most of the waters through Monday morning. Following this front high pressure over the northeast Pacific begins build over the waters and bring breezy west to northwest winds (gusting 20-25 kt) across the waters later Monday into Tuesday. Also expect the arrival of a pounced WNW 9-11 ft swell at near 12 seconds Monday night. The high pressure shifts closer to the waters later next week followed by a larger offshore component to the wind in the Friday/Saturday timeframe. /mh -Schuldt && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Winter Weather Advisory until 6 PM PDT this evening for ORZ126>128. WA...None. PZ...Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this evening for PZZ210. Small Craft Advisory until 8 AM PDT Monday for PZZ251>253- 271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland