175 FXUS66 KPQR 210436 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 936 PM PDT Fri Aug 20 2021 Updated aviation and short term discussions. .SYNOPSIS...Strong high pressure will remain well offshore the next several days, allowing a series of upper level disturbances to approach the region from the Gulf of Alaska. This setup will bring relatively cooler temperatures through this weekend, along with a chance for light rain or drizzle this evening into tomorrow. Slightly warmer weather should develop early next week, but onshore flow will help keep temperatures near seasonal normals. && .SHORT TERM... Tonight through Sunday night...The shortwave trough and associated cold front that brought rain showers and marine stratus to the area has since pushed east leaving patchy mostly cloudy skies in it's wake. Radar imagery continues to show light showers along the north coast although little to no rain is reaching inland locations. Appears most of the dynamic forcing needed as subsided and more of a stable environment remains, but the ample marine onshore flow will support the showers. The same will hold true into tomorrow morning as the aforementioned shortwave trough evolves into a closed upper low over the area before gradually pushing east of the Cascade crest by late tomorrow morning. How strong will onshore flow get tonight and tomorrow? Well, the observed pressure gradient between North Bend, OR and Spokane, WA was +6.3 mb at 21Z Friday, which is nearly identical to the forecast pressure gradient from the 12Z iteration of the GFS. This increases confidence that the GFS is on track in regards to the strength of the low-level onshore flow ongoing across the region. If the pattern continues to ring true, it would suggest that the pressure gradient will strengthen slightly more overnight into Saturday morning. With the aid of the strong onshore flow, the HREF ensemble mean for clouds cover shows high probabilities for a widespread marine stratus through tomorrow morning, before the stratus deck attempts to slowly break up during the mid to late afternoon and evening hours. This will help keep temperatures relatively cool tomorrow, with highs most likely topping out in the low to mid 70s for the interior lowlands. Not much change is expected on Sunday as the onshore flow pattern continues, with morning low clouds giving way to some breaks of sun during the afternoon hours. People with recreation plans in the high Cascades this weekend should keep the cooler temperatures in mind, especially since overnight lows are expected to be in the upper 30s to mid 40s for elevations above 4000 feet Saturday night and Sunday night. -Muessle/TK && .LONG TERM...Monday through Thursday night...Models and their ensembles continue to suggest a prolonged period of zonal to near-zonal 500 mb flow will occur over the Pacific Northwest next week. The WPC's cluster analysis for 500 mb heights shows very little variation between each individual cluster solution, which means confidence is high that the aforementioned synoptic-scale pattern will verify. At the surface, thermally-induced low pressure looks to remain anchored over northern CA and far southwest OR, hinting at a typical summer-time north to northwest flow regime for northwest OR and southwest WA. It appears onshore flow will be a bit weaker next week compared to this weekend, which should result in less widespread morning cloud cover and warmer temperatures. The latest NBM guidance helps back up this statement by producing mean values for high temperatures in the low to mid 80s Tuesday through Friday for the interior lowlands. That said, highs in the upper 80s to lower 90s cannot be completely ruled out. As of right now, the probability for temperatures reaching 90 degrees or warmer on any given day next week are generally 10-15 percent or less. Regardless of the outcome, extreme heat is not expected. Even if high temperatures do rise into the upper 80s to lower 90s, overnight temperatures would provide significant relief as forecast lows are in the upper 40s to upper 50s. Aside from the warmer temperatures next week, expect mainly dry conditions to continue. The exception will be if the marine layer is able to deepen enough to support chances for drizzle. The GFS output for 0-1 km relative humidity does hint at potential drizzle across the north OR/south WA coast and the Willapa Hills Monday morning, Wednesday morning, and Friday morning. That said, the EURO does not show as strong of a signal and overall forecast confidence is low in regards to the potential for drizzle next week. -TK && .AVIATION...An upper level trough will move through the area tonight, followed by dry northerly flow aloft by Sat afternoon. Generally lo MVFR to IFR stratus along the coast and VFR inland. Deep onshore flow continues tonight with occasional LIFR with light pcpn overnight at the coast, then low MVFR likely for the remainder of Sat. Inland, marine stratus should begin filling around 08-10Z Sat with MVFR cigs possible into Sat morning before lifting to VFR with cloud bases around 040-050 remaining through late afternoon, then followed by gradual clearing Sat night. For detailed regional Pac NW aviation weather information, go online to: https://weather.gov/zse KPDX AND APPROACHES...Mainly VFR broken stratus today. A slight chance of precipitation will continue through to night, but little to no visibility impacts expected. MVFR stratus likely to return by 10Z Saturday with cigs 025-030 range through 16-19Z. -MH/DDH && .MARINE...Not much change in the overall weather pattern. High pressure will remain offshore along with weak thermally induced low pressure across northern CA and far southwest OR. This will continue to drive north to northwest winds across the coastal waters during the next several days. Winds will generally reach as high as 10 to 15 kt with gusts up to 20 kt through the weekend. Latest models show pressure gradients increasing somewhat Monday, with wind gusts possibly reaching 25 kt by Mon afternoon, though should be confined to the central waters. Seas, predominately driven by a northwest swell, of around 7 to 9 feet with a dominant period around 8 to 10 seconds, are expected to continue through Fri afternoon. Will maintain the Small Craft Advisory through 5 pm. Seas will be steep and choppy at times. Seas are then expected to gradually subside through Sat evening and then remain around 5 to 7 feet through Sunday night. Seas are expected to become mostly wind driven by the middle of next week as the typical summertime wind pattern persists through the week. /DDH && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$