For Friday, a high pressure system at the surface will be located in the eastern Pacific with a surface low pressure system directly to the north, off the coast of Canada. The area of high pressure is expected to have a larger influence on the surface winds in the area, with them coming from the north to northwest through much of the weekend. With the systems extending into the low levels, the area between them will act as a funnel for moisture; the cyclonic motion around the low an the anticyclonic motion around the high will cause low level winds from the west. The moist air will condense as it moves up the mountains to the west of Portland, resulting in mostly cloudy conditions in the low levels during Friday morning; drier air from the northwest will cause some slightly clearer conditions in the afternoon. However, an upper level trough centered over the Pacific will bring moisture to the region from the southwest, resulting in mostly cloudy conditions in the upper levels for the remainder of the day. Conditions on Saturday are expected to be mostly the same, though low level winds are anticipated to continue from the west, causing mostly cloudy to overcast conditions in the low levels throughout the day.
Sunday shows much of the same as the rest of the weekend, though it does bring a very minor chance for scattered showers. The GFS is currently the lone model anticipating precipitation as a result of winds throughout the atmosphere from the southwest. Though it is the only one to show showers during the day, I wanted to include the output in case the models starting trending towards the same output before the next update. Regardless, I'm sure that Sunday's conditions will change, hopefully for the better.