NWAC Th-Sat forecast:

The upper level ridge that has dominated the recent Pacific Northwest weather is expected to flatten and weaken by Thursday. This shoudl allow warm frontal moisture from a disturbance moving across the eastern north Pacific to spread over the region. Initially across the north part including the Olympics and north Washington Cascades Thursday morning and gradually shifting southward late Thursday. However freezing levels should remain relatively high as the moisture moves in. Northwest flow aloft is expected to increase across the region from Thursday through Saturday. Warm frontal moisture should continue to cause periods of light to moderate precipitation through the extended period. The heaviest expected precipitation should remain over the north part of the Cascades and Olympics with very little precipitation reaching the Mt Hood area or south Washington Cascades. Pass level flow should quickly turn onshore with the onset of precipitation Thursday and persist through the Saturday period. This should limit any lowering of freezing levels in the passes where precipitation is mostly expected to be of the rain variety.