Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
516 FXUS66 KPQR 292225 AFDPQR Area Forecast Discussion National Weather Service Portland OR 313 PM PDT Sat Jun 29 2024 .SYNOPSIS...Aside from some light showers across the northern portions of the CWA tonight into Sunday, onshore flow will maintain near average temperatures into early next week. There is 50-60% chance that stronger high pressure offshore will shift over the region towards the end of next week and bring the hottest temperatures of the Summer so far. && .SHORT TERM...Tonight through Monday...Water vapor satellite imagery reveals a shortwave trough over the eastern Pacific that`s knocking on the door of the Pacific Northwest. Some light rain has begun to develop just offshore with some light rain reaching the north coast near Seaside in just the last 1-2 hours. This precipitation appears to be forming in response to mid level lift ahead of an embedded vorticity maxima that is lifting northeastward towards southwest Washington. In addition, KRTX doppler radar indicates a few surface based showers have begun to pop over western Washington County and across the south Washington Cascades despite limited SBCAPE in SPC mesoanalysis. These showers and instabilty are supported by a number of HREF members, though. While statistically the chance of any given location receiving measurable rain this evening is probably near the NBM`s 5-10% PoPs even if several of the HREF members were to pan out exactly, opted to increase PoPs to 15-30% so that there is at least some mention of rain showers in the forecast for the next several hours. In addition, cooling temperatures aloft coupled with surface heating will put us in the running for a short lived thunderstorm or two, but the weak low to mid level flow will promote most showers being rather short lived and keep thunderstorm chances low. Additional low level moisture and conditional instability will promote rain chances continuing into Sunday across mainly the north coast and Cascades from Lincoln and Clackamas Counties northward, respectively. Otherwise, temperatures look to remain near average into early next week. /Neuman .LONG TERM...Tuesday through Saturday...WPC 500 mb cluster analysis remains in very good agreement of upper level ridging building over the Eastern Pacific early next week with generally zonal flow over NW Oregon into SW Washington, leading to little change in our sensible weather Tuesday into Wednesday. Ensembles are in good agreement that inland areas will warm up on Thursday, but there still remains some uncertainty in the magnitude of the warm up. Uncertainty in the forecast grows further Friday into Saturday as ensembles struggle to handle whether or not a shortwave trough impinges on the region, the ridge builds and persists over the Pacific Northwest or a shortwave trough near the Rockies persists close enough to the region to keep temperatures in check. The most likely scenario at this point is that high pressure aloft will remain close enough to the region to keep hot temperatures in place. This is supported by the NBM, which gives a 50-60% chance for the warmest temperatures of the Summer so far developing next Friday and/or Saturday. There`s at least a 30% chance that inland valleys even hit 100F one of those two days. The least likely scenario (around a 10-15% chance) is that we return to near or below average temperatures Friday or Saturday as the aforementioned shortwave trough drops southward towards the region and places the region under stronger onshore flow. /Neuman && .AVIATION...VFR prevailing ahead of upper level trough and weak frontal system approaching the coast. The front is expected to bring MVFR conditions to the coast around 22z Sat to 03Z Sun. Behind the front, marine stratus will settle on to the coast with a high likelihood (80-90%) of IFR conditions from about 03Z-12Z Sun with a few hours of 40-60% chance before and after. Conditions gradually improve through the day with MVFR conditions prevailing. Inland VFR condition prevail with some high clouds, but expected to see west southwest flow at low and mid levels to spread marine clouds with MVFR cigs inland roughly after 10-13Z Sun, then gradually improving to VFR after 18Z Sun. HIRES models show some discrete showers developing in the north coast range and possibly moving into the north Willamette Valley through early this evening. PDX AND APPROACHES...VFR prevailing through about 10Z Sun then increasing chances for MVFR cigs 2000-3000 ft to develop. Chances for MVFR peak at around 60-70% 14-18Z Sun, then rapidly decrease by 21Z Sun for high chances of VFR. HIRES models show some discrete showers developing in the north coast range and possibly affecting KHIO, KPDX/KTTD area 23 Sat-02Z Sun. /mh && .MARINE...Weak high pressure over the northeast Pacific with lower surface pressure over California and the Great Basin will more or less maintain itself into next week. A weak front will temporarily clip the waters today and turn winds more out of the west. Tonight into Otherwise, expect north to northwest winds across the waters with the strongest winds generally off the central coast of Oregon and lighter winds farther north. Winds will generally peak in the afternoon and evening hours each day. Pressure gradients appear increasingly likely to increase by early to mid next week so that there is a >90% chance that northerly wind gusts of 20-30 kt spread northward across the waters. This will in return develop steep wind driven seas of 5-8 ft with a dominant period of 7-8 seconds by late Tuesday or Wednesday. /Batz && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland