Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
164 FXUS66 KPQR 281813 AFDPQR Area Forecast Discussion National Weather Service Portland OR 1112 AM PDT Tue May 28 2024 .SYNOPSIS...After some light rain today and a few showers on Wednesday, drier weather returns Thursday and Friday. A wetter weather pattern returns over the weekend into early next week. && .SHORT TERM...Today through Friday night...A long wave trough extending from a parent low near the panhandle of Alaska will drag a weak surface front across the region today through Wednesday. This will maintain onshore flow across the region and result in cooler temperatures as well as an increasing probability of light precipitation through today. Therefore, expect daytime high and overnight low temperatures below normal. This pattern is looking to be a textbook example of a high PoP/low QPF event with the highest precipitation totals expected in along the Coast (0.10-0.30 inches), Coast Range (0.10-0.35 inches) and Cascades (0.10-0.50 inches) with light accumulations (0.02-0.10 inches). The time frame in question for this precipitation will be from approximately daybreak through Wednesday afternoon. Wednesday will see minimal changes in temperatures as the region transitions into a post frontal environment with light showers lingering across the CWA. Snow levels will also be around 4000-5000 ft from Tuesday through Wednesday so some light snowfall is possible over the volcanoes with very little accumulation expected. Thursday, the region will undergo a relatively rapid pattern change as a ridging pattern develops. Flow will become more northerly during this time so could see slightly breezier conditions along the beaches and through the Willamette Valley. Friday the ridge will continue to strengthen over the Pacific NW as the axis moves eastward. Temperatures will warm through the latter part of the week and into the weekend. The most recent run of the NBM is still showing about a 10 to 15 degree spread between the 25-75th percentile. However, models are starting to trend towards the cooler solutions at this time. Will continue to monitor as there remains a significant spread within the models and their families. /42 .LONG TERM...Saturday through Monday...Zonal flow takes over on Saturday bringing a relatively benign weather pattern to the forecast area. While the WPC 500 mb clusters show general ridging over the Pac NW through at least Monday, deterministic models are showing broad, surface lows moving into the region. If this scenario manifests, this could result in rainfall to the region, but would not be high amounts. As Sunday approaches, the pattern is a mess of ridging, troughs, cold air intrusion, warming from the south and can best be described as a "mixed bag" of possibilities. This can further be seen in some of the probabilities for temperatures, rainfall totals,etc. In some cases, there is a 15-20 degree spread in the high temperature forecast. So, with that in mind have leaned into the NBM for the latter part of the forecast. /42 && .AVIATION...A weak cold front will push into southwest Washington and northwest Oregon over the next couple of hours. As it does so, light rain and low clouds will keep probabilities of MVFR conditions remaining near or above 50% at any given hour through ~22z Tuesday for most TAF sites across northwest Oregon. Thereafter, daytime heating coupled with low level moisture will create a more showery weather pattern, which should result in most locations turning predominantly VFR. Statistical guidance does suggest a 15% chance for a short lived thunderstorm for KPDX and KTTD between 22z Tuesday and 02z Wednesday, but high resolution statistical model output has probabilities less than 10% during that same time. Given the pattern and high probability of false alarming will continue to keep a mention of thunder out of the official TAF for these sites for now. Nonetheless, any intense shower or thunderstorm would almost certainly bring a brief dip into MVFR thresholds as well. High resolution model guidance does suggest probabilities for MVFR conditions climbs briefly around 17z Wednesday as daytime heating and low level moisture produce an increased likelihood of some MVFR ceilings. PDX AND APPROACHES...Expect a mixture of MVFR and VFR conditions through ~22z Tuesday as a weak front and light rain traverse the region. Conditions should trend to predominantly VFR thereafter. There is a ~10-15% chance for a brief thunderstorm to impact the terminal between 22z Tuesday and 02z Wednesday. Nonetheless, any intense shower or thunderstorm would almost certainly bring a brief dip into MVFR thresholds as well. /Neuman && .MARINE...A weak front is currently crossing the waters today, but few impacts are anticipated. Weak high pressure will build across the northeast Pacific Wednesday into Thursday and lead to increasing northerly winds across the waters. The strongest winds will be off the central coast of Oregon. For example, there is a >90% chance for wind gusts in excess of 20 kt across those waters by Thursday afternoon and evening. Most model guidance suggests winds will back off a bit for Friday, but there is still a high probability (>75% chance) wind gusts will climb above 20 kt across the waters off the central coast of Oregon. There is increasing agreement a more organized and stronger storm system will move into the northeast Pacific over the weekend. This will bring a high probability (>80% chance) for winds to turn southerly and wind gusts to at least climb into Small Craft Advisory thresholds of 25-30 kt. In addition, there is a 30-40% chance for at least a brief period of unusually late season Gale Force southerly wind gusts of 35 kt developing sometime Sunday into Monday across the waters. Probabilities of this happening are highest off the far north Oregon and south Washington coasts than areas farther south towards Lane County. /Neuman && .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