Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
140 FXUS66 KPQR 110923 AFDPQR Area Forecast Discussion National Weather Service Portland OR 223 AM PDT Sun Aug 11 2024 .SYNOPSIS...Westerly onshore flow will continue through this week, bringing a cooling trend along with increasing morning clouds and chances for light rain or drizzle at the coast. Temperatures will fall to near or just below normal by Sunday, lasting through next week. Onshore flow will also bring improving air quality for most locations as smoke will be pushed to the east, aside from areas immediately near and/or downwind of ongoing wildfires. The return of rain chances is possible late this week, though amounts look to be limited at this time. && .SHORT TERM...Sunday through Saturday...Observations early Sunday morning indicate onshore flow continues across NW Oregon and SW Washington with west to northwesterly winds under fairly zonal upper level flow. Satellite imagery shows marine stratus pushing east along the western Coast Range foothills and filtering inland along the Columbia River valley into the Portland metro area and through Coast Range gaps. HREF indicates a 40-60% chance of stratus filling in the Willamette Valley by daybreak and slowly dissipating late morning through early afternoon. A weak trough will begin deepening along the Washington & Oregon coasts today, and this combined with the cloud cover will allow daytime temperatures to fall back to near normal today, low 80s in the Willamette Valley. This upper trough continues deepening along the Western coastline Monday before moving inland Tuesday. Daytime temperatures will continue cooling to below normal Monday and remain fairly steady Tuesday with mid to upper 70s across inland valleys and low to mid 60s across the coast. Additionally, some moisture associated with this trough will allow for chances of drizzle and light rain along the coast late tonight through early Tuesday. Could also see some drizzle over the Cascades as the marine layer deepens under continued onshore flow. NBM notoriously has a difficult time capturing drizzle chances inland, so NBM PoPs remain less than 10%. However, if any drizzle does occur over the Cascades, accumulation would be very light. Wednesday`s weather will be similar to today with temperatures bouncing back to near normal under relatively zonal flow and morning stratus under continued onshore flow. Ensemble guidance is in good agreement of the general upper level pattern late this week with all WPC 500mb clusters indicating longwave troughing developing Thursday and continuing into the weekend with the potential for multiple waves along the trough. Overall, this pattern is likely to bring the return of rain chances for much of the CWA as well as below normal temperatures. However, uncertainty still remains within the ensembles and deterministic models in the specific location and timing details of the main trough and associated shortwaves, which is causing lower NBM PoPs than expected. Latest guidance is indicating an amplified wave moving west into Oregon on Thursday with confidence increasing in showers and an isolated chance of thunderstorms over the central Oregon Cascades. More widespread rain is possible Friday and Saturday depending on the timing of the next waves pushing into the region from the northwest. At this time, rain amounts look to be pretty limited at less than 0.1-0.25 inch for most areas, but we will have to keep an eye on this pattern. -HEC && .AVIATION...Weak westerly flow aloft today while onshore flow persists in the lower levels. Marine stratus again solidly filled in along the coast with CIGs holding around 400-800 ft. The profiler at Astoria also shows the marine layer has deepened to around 3000 ft with some drizzle being reported at KAST. Expect at least IFR conditions to persist at the coast through 18-20Z Sunday, while there remains around a 40% chance of CIGs falling to LIFR early this morning. Northwest winds with gusts up to 20 kt this afternoon for the central OR coast. Inland, again predominately VFR expected through the period, but a stronger marine push up the lower Columbia has caused MVFR stratus to develop at KPDX and KTTD as of 09Z Sunday. CIGs around FL015 will likely persist there through 18Z this morning. Stratus may continue to filter across the Willamette Valley this morning, with probabilities around 60% for KUAO, and 30-40% for KSLE and KEUG. NNW winds up to 10 kt inland this afternoon and evening, except stronger through Coast Range gaps. Smoke from wildfires burning across the region could result in some slant range visibility issues. PDX AND APPROACHES...MVFR stratus with CIGs around FL015 expected to persist through 18-19Z Sunday. Then, VFR expected with increasing high level clouds through this evening. Northwest winds increase up to 8-10 kt this afternoon and evening. /DH && .MARINE...High pressure offshore will maintain north to northwest winds up to 15 kt across the coastal waters Sunday. Seas remain choppy and primarily wind driven around 5 to 6 ft today. Then, the surface high retrogrades west as a weak front drops south of of the Gulf of Alaska on Monday. A broad area of weak low pressure near Vancouver Island causes winds across the coastal waters to shift more westerly on Monday, and veering southerly on Tuesday. Seas expected to subside to around 2 to 4 ft by Tuesday. Fairly benign conditions continue into Wednesday. /DH && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland x.com/NWSPortland