918 FXUS66 KPQR 231029 CCA AFDPQR Area Forecast Discussion...CORRECTED National Weather Service Portland OR 329 AM PDT Fri Apr 23 2021 .SYNOPSIS...An upper level ridge centered just offshore tonight will move east across the forecast area Friday. A frontal system is expected to reach the coast Friday evening and then move inland overnight, bringing widespread rain to the region. A closed upper level low will settle over the eastern Pacific on Saturday before drifting inland Sunday morning. Cool and showery conditions should persist Monday, before warmer and drier conditions arrive by mid-week. && .SHORT TERM...Today through Sunday night...At 2 AM PDT earlier this morning, satellite observations showed an area of mid-level and high-level clouds moving into the eastern Pacific, ahead of a north-south oriented frontal boundary located along a line from 40N to 50N at a longitude of 135W. It appears this frontal boundary was a cold front that was beginning to occlude with a warm front. This occluded front will continue pushing eastward towards the Pacific Northwest today, bringing increasing clouds to northwest OR and southwest WA this morning. Expect skies to become mostly cloudy to overcast by the afternoon. Along and near the coast, skies will remain overcast the entire day due to an ongoing marine stratus layer. High temperatures today should be a couple of degrees cooler than yesterday due to the limited solar heating. Models are in fairly good agreement in regards to the timing of the incoming front, which is set to reach the coast Saturday morning. Models continue to produce an area of QPF ahead of the front, bringing widespread stratiform rain into western Oregon and western Washington Friday night into Saturday morning. Areas along and near the coast could see light rain arrive as early as Friday afternoon, but Friday evening seems more likely based on simulated reflectivity guidance from the latest suite of CAM guidance. In addition, forecast GFS/NAM soundings also support light rain holding off until Friday evening along the coast as a fairly impressive layer of dry air between 2500 and 8000 feet takes time to saturate. Light stratiform rain Saturday morning should transition to widespread showers in the post-frontal environment Saturday afternoon. The cloudy and rainy weather on Saturday will also be accompanied by noticeably cooler temperatures with highs in the 50s. Showers should then begin decreasing in coverage Saturday night into Sunday morning, except along the coast and across the Coast Range where a weak closed surface low centered immediately offshore the southern Oregon coast will bring continued showers. This closed surface low is set to move inland on Sunday, bringing showers to the entire forecast area. Sunday will also feature a slight chance of thunderstorms during the afternoon and evening hours, but forecast soundings depict a marginal convective environment with unimpressive mid-level lapse rates and surface-based CAPE values. In addition, vertical wind shear looks to be almost non-existent on Sunday. Overall Sunday looks like the type of day that will feature isolated weak thunderstorms that only wind up producing a flash or two of lightning and possibly some small hail. Expect showers to decrease again Sunday night as an upper level trough that will reside over the western U.S. this weekend moves east of the Cascades. -TK .LONG TERM...Monday through Friday...The deterministic GFS/EURO continue to show a 500 mb shortwave trough diving southward across western Washington and western Oregon Monday morning through Monday afternoon. This deterministic guidance matches GEFS/EPS ensemble mean guidance for 500 mb heights very well, which increases confidence that this shortwave trough will come to fruition. This disturbance will bring another round of showers to the region on Monday before surface high pressure and a weak upper level shortwave ridge bring an end to the showers Monday night. Shortwave ridging will then continue into Tuesday, bringing dry weather. High pressure will also bring less cloud cover and warmer temperatures, with partly sunny skies expected and highs in the mid 60s. The exception to this will be along the coast and over the Cascades, where temperatures should top out in the 50s. The highest peaks of the Cascades should stay in the mid to upper 30s. A weak impulse aloft will bring a chance for light showers Tuesday night into Wednesday, but mainly over southwest Washington and far northwest Oregon. Areas to the south of Salem could wind up staying dry Tuesday night and Wednesday. In fact, Wednesday has the potential to stay dry for the entire forecast area if the upper level ridge advertised by models and their ensembles is amplified enough, since this would act to keep showers further to the north over northwest Washington. The WPC's cluster analysis for 500 mb heights shows nearly half of the total ensemble member space from the Canadian/GEFS/EPS favoring a ridge that is strong enough to keep the forecast area dry on Wednesday. The other half favor a weaker ridge with light showers over northern portions of the forecast area. The NBM was used for the probability of precipitation forecast due to the uncertainty involved with this strength of Wednesday's ridge. Models and their ensembles diverge even more for Thursday through Friday, showing a very wide range of potential outcomes due to uncertainty involved with the upper-level pattern mid to late week. If the model solutions advertising a fairly strong ridge over the region verify, then Thursday and Friday will end up dry and warm with high temperatures most likely well into the 70s due to offshore flow. If the model solutions advertising westerly flow with no ridging at all verify, then a cool and rainy onshore flow pattern would occur. The NBM 1D viewer picks up on the degree of uncertainty well, showing 10th to 90th percentile values ranging from the lower 60s to lower 80s for high temperatures at various points across the Willamette Valley. -TK && .AVIATION...Stratus has begun to fill in the southern Willamette Valley as was expected. This stratus will advect north, mainly staying in the eastern part of the valley and hugging the Cascades. But, as surface high shifts inland tonight, winds will be much lighter, with stratus spread slowing. At this point, suspect will see rather large holes in the stratus in western Willamette Valley from west of KSLE to near KHIO. Stratus will break up bit more quickly Fri am across the interior. But, MVFR to low VFR stratus on the coast will persist through most of the day, with only spotty breaks in clouds during the afternoon. For detailed regional Pac NW aviation weather information, go online to: https://weather.gov/zse KPDX AND APPROACHES...VFR this evening, with scattered mid-level clouds. Stratus has been rather persistent between FL040-050 and may slightly lower this morning. Still expect minimal impact, however. Clouds will gradually break up after 17Z to 18Z, though will see increasing mid/high clouds in the afternoon. JBonk/Rockey && .MARINE...Winds remain relatively light through Friday night as a ridge of high pressure moves across. Steep seas on the order of 8 to 9 ft this evening are expected to subside overnight, then remain around 4 to 6 ft through Friday night. The next chance for small craft advisory level winds is with a frontal system moving east across the waters Saturday and Saturday, when southerly winds are expected to gust into the range of 20 to 30 kt, with strongest winds over the southern waters. Seas are likely to build up by a couple of feet in response to the winds Saturday, dominated by wind waves and a short period swell. More benign wind and sea conditions appear likely to return at the end of the weekend and into early next week, with winds under 20 kt and seas dropping back down under 6 ft. Another low and front may move across the waters Tuesday, however, models continue to weaken this feature and don't necessarily agree on it's timing. Will continue to downplay this event. && .PQR WATCHES/WARNINGS/ADVISORIES... OR...None. WA...None. PZ...None. && $$ Interact with us via social media: www.facebook.com/NWSPortland www.twitter.com/NWSPortland