FXUS66 KPQR 230401 AAA AFDPQR Area Forecast Discussion...UPDATED National Weather Service Portland OR 901 PM PDT Tue Oct 22 2024 Updated Aviation Discussion && .SYNOPSIS...Dry through early Wednesday morning with light showers returning to the Cascades Wednesday late morning and afternoon. Dry weather Thursday into Friday before a wet pattern returns this weekend into early next week. Fog chances in low lying valleys most mornings this week with frost possible Wednesday morning, becoming more widespread Thursday and Friday mornings. A Frost Advisory is in effect Wednesday and Thursday mornings. && .SHORT TERM...Tuesday afternoon through Thursday Night...Satellite imagery shows scattered high clouds streaming over the region this afternoon with dry weather and partly sunny skies. Cloud cover is expected to increase late tonight into tomorrow morning, especially for areas south of the Portland metro area, as a weak low pressure system approaches the PacNW tonight and moves through the region during the day tomorrow. There's a 70-95% chance that low lying areas in the northern Coast Range, SW Washington, and Hood River Valley where cloud cover is not expected to be as thick will have low temperatures around or below 36 degrees tomorrow morning, creating a potential for frost. A Frost Advisory has been issued for these areas through 8 AM. These areas and south into the Portland metro could also see low stratus and fog development in the morning hours tomorrow. There's a 15-30% chance of light showers over the N Oregon and SW Washington Cascades and up to a 50% chance for the Central Oregon Cascades tomorrow afternoon as the low passes through, but accumulation in most places is expected to be light, less than 0.05 inch. Ensemble guidance is in good agreement that the low should move into Idaho by late tomorrow night with weak ridging building behind it, leading to dry weather and clearing skies tomorrow night into Thursday morning. The cold air behind the weather system along with clearing skies will allow for more widespread frost Thursday morning for most locations out of populated areas (such as Portland/Vancouver metro, Salem, Eugene, etc) with a 60-95% chance of low temperatures falling to at least 36 degrees. A Frost Advisory has also been issued for most of the CWA except for the coast and high Cascades for midnight through 8 AM Thursday. A few locations have the potential to reach Freeze Warning criteria with anywhere from a 40-70% chance of temperatures falling to 32 degrees in portions of Hood River Valley as well as the southeastern portion of the Willamette Valley south of Corvallis (outside of Salem and Eugene population areas) and adjacent Cascade foothills. Some locations could also see stratus or fog again, though the probability is lower due to the lower temperatures. Daytime temperatures today through Thursday will remain fairly steady with upper 50s to low 60s in the inland valleys low to mid 50s along the coast, and upper 40s to low 50s along the Coast Range. The Cascades will see the only major deviation with 40s to 50s today and Thursday, dropping to the 30s and 40s tomorrow. -HEC .LONG TERM...Friday through Monday...Ensemble guidance remains in good agreement that weak ridging continues through Friday morning before longwave troughing begins approaching the coast Friday afternoon/evening, bringing increasing cloud cover. Dry weather is expected through most of the day Friday with daytime temperatures 5-10 degrees warmer than Thursday. The next period of wet weather begins Friday evening into Saturday as the first of what ensemble guidance suggests will be numerous frontal systems pushes into the PacNW. All of the WPC 500 mb cluster analysis indicates precipitation is possible every day late Friday through at least early next week, though of course uncertainty remains in exact timing and amounts of rain. However, all of the precipitation looks to be beneficial with limited impacts expected at this time. The GFS and Euro ensembles indicate first push on late Friday/Saturday will have a decent IVT push of > 250 kg/ms, producing a good amount of rain across the entire region. Cooler temperatures are expected behind this frontal system, with snow levels falling to the high Cascades. Following frontal systems could produce snow over the high Cascades Sunday into early next week, though amounts are uncertain at this point. -HEC && .AVIATION...Weak high pressure remains across the region at the surface with a trough in the upper levels just off the OR/WA coasts. This is supporting southwesterly flow aloft along with high level clouds as seen within the nighttime microphysics band. Breaks in the upper level clouds reveal areas of stratus, mainly across the OR Cascade foothills and a bit across the southern Willamette Valley, building toward KEUG. Guidance has shifted stratus/fog probabilities down since the afternoon, with generally a 20-30% chance for MVFR CIGs and 10-15% for MVFR VIS for both inland and coastal sites. Based on the latest obs and expected trends, KEUG is most likely to see sub-VFR conditions at some point tonight. Light and variable winds tonight become north/northwest around 5 knots tomorrow. PDX APPROACHES...VFR conditions expected though NBH guidance suggests around a 20% for MVFR CIGs to develop after 12-18Z. Winds light and variable, become northwesterly around 5 knots. -Batz && .MARINE...Surface high pressure will persist across the northeast Pacific and result in relatively light winds and benign seas through early Friday. Northerly winds will strengthen each afternoon and evening across the waters off the central coast of Oregon, but these winds should generally remain 20 kt or less. A surface low pressure is then forecast to develop over the eastern Pacific that is expected to race north-northeastward towards Vancouver Island late Friday into early Saturday. This will result in increasing southerly winds and seas across the waters during this time. Right now there is a 50-60% chance for Gale Force southerly wind gusts during this time, but winds of this magnitude are still not in the official forecast yet because it remains uncertain if they will manifest themselves during the Friday night or Saturday time periods. Either way, there is a high confidence (>90% chance) that seas climb into the teens during this time period. There is only a 10% chance that seas climb above 16 ft during this time period. A large upper level storm system will settle across the region over the weekend into early next week. Additional surface low pressures may develop and produce brief spikes in wind across the waters during this time period. At this point, the overall trend should be for seas and winds to subside between early Saturday and Monday. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for ORZ104-106-121. Frost Advisory from midnight Wednesday night to 8 AM PDT Thursday for ORZ104>107-109-114>118-121-123>125. WA...Frost Advisory from midnight tonight to 8 AM PDT Wednesday for WAZ208. Frost Advisory from midnight Wednesday night to 8 AM PDT Thursday for WAZ203>205-208. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland