Area Forecast Discussion
Issued by NWS Portland, OR
Issued by NWS Portland, OR
339 FXUS66 KPQR 150506 AFDPQR Area Forecast Discussion National Weather Service Portland OR 1006 PM PDT Tue May 14 2024 Updated aviation discussion. .SYNOPSIS...High pressure will maintain mostly clear, dry, and warm conditions through tomorrow (Wednesday). Expect the marine layer to return to the coast tonight. On Thursday, the ridge begins to break down as a trough pushes southeast into BC from the Gulf of Alaska. This will return slight chances for precipitation for higher terrain as well as cloud cover. The long term forecast looks mostly dry, however, weak troughing could bring slight chances of precipitation. && .SHORT TERM...Tonight through Thursday...Satellite imagery as of 3 PM PDT depicts mostly clear skies across northwest Oregon and southwest Washington as upper level high pressure builds over the Pacific Northwest. High temperatures this afternoon are on track to reach the mid 70s for interior valleys and upper 50s to low 60s along the coast and higher terrain. As we cool down tonight, we`ll likely see a re-development of the marine layer along the coast. Onshore flow doesn`t look strong enough for the marine stratus to filter into the Willamette Valley via Coast Range gaps and the Lower Columbia River Valley. However, there could be some back-building stratus against the Cascades, particularly in the east Portland Metro Area. Locations in this area could start tomorrow morning with some cloud cover, but will quickly dissipate as daytime heating progresses. Clear and dry conditions prevail tomorrow (Wednesday) as high pressure remains over our area. Tomorrow is expected to be the warmest day of the week as models suggest the upper level ridge building to its maximum amplitude. Afternoon highs tomorrow are forecast in the upper 70s for interior valleys and upper 50s to 60s along the coast and higher terrain. NBM suggests a 30-50% chance of exceeding 80 degrees tomorrow (highest in the Portland Metro Area) within interior valleys. With this summertime pattern in place, we`ll also maintain northerly to northwesterly surface winds. On Thursday, ensemble guidance shows an upper level trough from the Gulf of Alaska progressing southeastward toward British Columbia. This weak trough will begin breaking down the ridge into more zonal upper level flow over the Pacific Northwest. Additional cloud cover will begin filtering in to the area, leading to partly sunny skies. Thus, we`ll see cooler afternoon highs for interior valleys (about 4-8 degrees cooler than Wednesday). By evening time, precipitation chances (15-35%) over higher terrain return as the upper trough quickly brushes the northern most areas of the Pacific Northwest. -Alviz .LONG TERM...Thursday Night through Monday...As the weak upper trough quickly progresses through the Pacific Northwest, the Cascades will maintain a 15-35% chance of precipitation. Snow levels will be above 6500-7500, so any snow shower activity would be well-above the Cascade passes. Friday to Saturday, WPC cluster analyses depict mostly zonal flow or weak troughing over the Pacific Northwest. This suggests that we`ll be mostly dry and maintain relatively cooler high temperatures. On Sunday, the majority of clusters suggest another trough dipping down into the Pacific Northwest from Canada. This scenario could also return a slight (15-24%) chance of precipitation over higher terrain. By early next week (Monday), some uncertainty remains with the progression of the aforementioned trough. About 60% of ensemble members suggest that the trough will continue progressing southward into the Great Basin. In this case, we would maintain more cloud cover and precipitation chances. Meanwhile, 40% of the members suggest a return to either zonal flow or ridging. If this pans out, we would return to clearer and drier conditions. -Alviz && .AVIATION...Predominately VFR conditions with clear skies remain in place for most locations under continuing high pressure. An exception is along the coast where low-level stratus will push in from north to south around 05-10z Wed, bringing 90% chance of MVFR cigs at coastal terminals. Slightly more cloud cover is expected at northern areas (such as KAST) with around a 60-80% chance of IFR cigs with a 40-70% chance of IFR cigs at other coastal terminals. Stratus clouds along the coast dissipate going into Wednesday morning, with 90% confidence in return to VFR conditions by 20z Wed. There is also a 30-40% probability of backbuilding stratus along the Cascade foothills in eastern Clark, central Multnomah, and northern Clackamas counties dropping ceilings below VFR levels around 12-17z Wed. This would mainly affect KTTD. Northerly winds continue through the TAF period. Gusts 20-25 kts along the coast will be decreasing by 08z Wed, then increasing back to 20-25 kts after 18z Wed, mainly for the central Oregon coast. Elsewhere, sustained winds remain less than 10 kts, briefly increasing to 10-15 kts around 21z Wed. -JLiu/HEC PDX AND APPROACHES...VFR with relatively clear skies throughout most of the TAF period, other than some light high cloud cover. North winds with gusts up to 20-25 kts will decrease around 07z Wed, remaining less than 10 kts through the TAF period. Some cloud cover backbuilding from the Cascade foothills could approach KTTD and KPDX around 12-17z Wed, but model guidance points towards it not quite reaching KPDX. Most likely outcome is continued clear skies through the night. -JLiu/HEC && .MARINE...Typical summertime pattern of northerly winds as high pressure builds through mid week. Winds for the rest of this afternoon and evening will see gusts up to 30 kt possible. A few geographically isolated pockets may see one-off gusts to 35 kt, but due to how isolated this would be, a Gale Warning has not been issued. Winds will continue to barely meet Small Craft Advisory criteria each afternoon and evening until at least the weekend, as diurnal driven changes in wind continue to be the main determining factor. In regards to seas, nothing overly remarkable as the primary swell is from the northwest, and the wind wave from the north. Wind waves will range from around 4 to 6 feet. Combined seas will be 6 to 8 ft through Tuesday, though conditions ramp up mid-week. -Muessle/JLiu && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...Small Craft Advisory until 11 AM PDT Wednesday for PZZ251>253- 271>273. && $$ www.weather.gov/portland Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland