Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
676 FXUS66 KPQR 240512 AFDPQR Area Forecast Discussion...UPDATE National Weather Service Portland OR 1011 PM PDT Thu May 23 2024 Updated aviation discussion .SYNOPSIS...Weak ridging gives way to yet another trough of low pressure shifts inland on Friday. Rain returns on Friday through Saturday but will be showery. non-impactful weather through the weekend. High pressure builds on Monday through Tuesday which will cause temperatures to rise. && .SHORT TERM...Now through Sunday...Shortwave trough will slowly eat away at the weak ridge that is currently stationed over the Pacific Northwest. This incoming ridge will bring another round of showers and breezier conditions. Starting synoptically, the pattern is fairly nondescript as the jet stream is more zonal until Saturday morning. Shifting to the mid-levels, there is a bit more energy with a vorticity maximum and shortwave trough which digs further south over the northern California border. With this support, will see orographic lift over the Coast Range and Cascades and thus, enhancing precipitation there. Snow levels may briefly lower to around 4000 feet Friday night into Saturday morning, allowing for a dusting of snow as low as the Cascade passes. Conditions will begin to improve by later Saturday as the trough departs to the east and makes way for the next period of shortwave ridging. -Muessle/CB .LONG TERM...Sunday night through Thursday...A bit more variability in the long term forecast as we encounter a pattern shift. Sunday night will transition from zonal flow to general ridging. On Monday expect clearer skies and warmer temperatures. As it stands, there is fairly good agreement that we will see temperatures in the mid to upper 70s through the Willamette Valley and seasonable along the coast. Ensemble models are all in good agreement of this ridging pattern with the axis sitting right over Idaho and Nevada. We will be on the divergent side of the ridge so there will not be strong winds necessarily, but with thermally induced low pressure along the coast we cannot rule out an easterly wind reversal. High pressure intensifies Tuesday as the ridge shifts eastward.Most of the models are showing a developing area of low pressure over the Alaska Panhandle which will slowly shift southward. The ECMWF is the outlier with this pattern though and keeps that low well to the northwest. As it stands, light precipitation is possible along the northern portions of the forecast area. If the ECMWF does manifest then dry conditions will persist. By Wednesday the pattern becomes much more convoluted with widespread disagreement in the overall pattern. Aloft there is a mix of zonal and much stronger ridging. The most recent ensembles (as of 12Z) are showing more models with a strengthening digging trough on Wednesday, while others maintain a more northerly track. Ultimately it will come down to the strength of the ridge to determine just how long the spring time temperatures and clear skies will persist. -Muessle && .AVIATION...VFR conditions prevailing overnight as an upper ridge shift east of the area. Weakening front near the coast midday Friday and over the interior valley mid to late afternoon. HREF indicating 40-60% chance for MVFR cigs below 3000 ft at the coast, increasing to 60-80% chance by 18Z. There is also a 60-70% chance for IFR at the coast 20-23Z Fri. Inland there is a 20-40% chance for cigs 12-17Z Fri over the valley, but better chance towards 00Z Sat. This may be overdone a bit so will leave VFR for most locations and let next model runs/forecaster delve into the details. PDX AND APPROACHES...VFR conditions expected through Friday evening. But there is about a 40% chance for cigs near 3000 ft 12-17Z Fri, but near 70% chance near 00Z Sat. This may be overdone considering last night HREF guidance had high chances with low turnout of MVFR conditions. /mh && .MARINE...Shortwave ridge of high pressure will build over the area resulting in seas and winds remaining relatively calm through at least Friday morning. However, a slightly more active pattern is expected as the next system drops down from the northeastern Pacific. This system will bring relatively stronger NW winds with gusts up to 20 kt and seas will also build in response towards 7 to 9 ft with a dominant period of 8 to 10 seconds. The Friday/Saturday system could bring periods of steep and choppy seas along with isolated gusts up to 25 kt. However, there is still a fair amount of uncertainty in the timing and magnitude. Therefore will be holding off on issuing any Small Craft Advisories at this time. These elevated conditions will persist through Saturday, with another weak ridge of high pressure that will bring benign conditions. /42 && .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