Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
936 FXUS66 KPQR 260021 AFDPQR Area Forecast Discussion...UPDATE National Weather Service Portland OR 521 PM PDT Sat May 25 2024 Updated aviation discussion. .SYNOPSIS...Broad low pressure slowly erodes through Sunday. High pressure still on deck for Monday, though it is quick moving as yet another broad low and longwave trough drops down shifting the ridge eastward. Zonal flow will dominate. Another round of rain expected on Tuesday through the the remainder of the week. && .SHORT TERM...Now through Monday Night...The short term forecast is frankly on the boring side as far as weather is concerned. The ruminates of the cooler front continue to produce very light showers over the region with accumulation being minimal and mainly orographically driven. There are a few weak shortwaves that are moving through the overall zonal onshore flow on Sunday which could prolong these light showers through the afternoon. However, for those looking for a break in the rain and some sun on this Memorial Day, then we have good news for you. While clouds will remain, a transient ridge will form over the state of Oregon. We sit right at the peak of that ridge so, while conditions will be warmer than the last several days, not looking at anything exceptional. Higher resolution models are showing near normal temperatures. The NBM is putting the mean forecast around 73 degrees F for much of the Willamette Valley and in the mid-50s for the coast. If we want to look at what I call the "best case scenario" (aka the warmest solutions), the 90th percentile is producing temperatures of 75-78 degrees F for the inland lowlands including areas within the Columbia River Gorge. The Upper Hood River Valley will again hover in the mid 70s. Looking at the probability spread though it is narrow which means that there is high confidence in this "mean" forecast. This sun will be short-lived though because yet another low pressure system will drop down from the Gulf of Alaska Monday night into Tuesday. -Muessle .LONG TERM...Tuesday through Saturday...Tuesday has seen little change from this morning`s forecast as models have really hung their hat on this trough pattern. Ensemble clusters once showed this pattern as the outlier but now have transitioned it into being the winning outcome. Cooler temperatures and another round of rain is on deck for the entirety of northwest Oregon and southwest Washington on Tuesday through Thursday. Looking at the deterministic long range models, one would suggest a drier pattern. But if you look at the culmination of the probabilities, this will be more of a localized stratiform rain scenario with orographically driven higher amounts. This high probability low accumulation scenario spills over into Wednesday. Thursday will resemble today though in the isolated nature of the rain and accumulation. Most of these systems will track to the north as we sit right at the base of the trough vs being in the thick of it. One challenge in the long term is temperature forecast - specifically the high temperature component. The NBM is showing a wides spread of temperature probabilities. For example, in Portland the high on Tuesday ranges from 62 deg F (10th percentile) to 75 deg F (90th percentile). Thursday shows even more of a spread as the pattern begins to shift between this trough and a ridge forming on Friday. -Muessle && .AVIATION...Onshore continues but is weakening and is forecast to continue to weaken overnight as weak high pressure build over the region. Will see cloud cover slowly thin overnight into Sunday morning. This may be enough for patchy low stratus or fog due to radiational cooling. HREF guidance shows about 10-30% chance for MVFR or lower cigs along the I-5 corridor in the Willamette Valley from KUAO to KEUG. Confidence too low to include in forecast at this time. PDX AND APPROACHES...Expect VFR for the next 24 hours. With cigs in the 5000-6000 ft range tonight, then more scattered Sunday. Evening northwest wind 8-12 kt through about 06Z Sun then becoming light and variable. /mh && .MARINE...Minimal changes to the forecast as conditions have not changed significantly. The front that has brought showers and locally breezy winds is shifting inland which will cause winds to slowly ease. Seas are hovering right around the 6-8 foot mark at 9 to 10 seconds near Astoria Canyon, but further south seas are much lower. Likely due to the influence of the onshore flow with the westerly primary swell. Through the day, showers will come to an end and high pressure builds on Monday. Another area of low pressure moves southeastward from the Gulf of Alaska late Monday night into Tuesday. While this is an area of low pressure, it is not overly deep and is generally staying more north. Thus, we will experience very little impact from this incoming system. Will still see generally onshore flow through the forecast period. Late in the week there is a chance of more of a southerly wind which could bring winds to around 15 to 20 kt with gusts to 25 kt. However, they are not appearing to be widespread. -Muessle && .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