Here is the latest 13-day forecast total from the National Weather Service GFS model. Along the southeast U.S. coast, precipitation will mainly be the result of Hurricane Matthew this Friday to Sunday. For the the Northwest, a series of strong storms will hit Washington and Oregon, followed by a major atmospheric river that will ram into northern CA next week.
Precipitation with this system will be substantial (2-5 inches) over the mountains of southern BC, the Olympics, and northwest Oregon (see below).
But the big story is what is forecast to happen later next week--a major atmospheric river event in which a plume of large moisture values plows into northern CA and southern Oregon, producing huge amounts of rain. To illustrate, here is a forecast of precipitable water for 11 PM for next Thursday. (Precipitable water is the amount of water produced by condensing all the water in a vertical column of air.) Large values (2-3 inches) are found in a "river" stretching SW to NE into northern CA.
Imagine courtesy of Weatherbell
As this moist air is forced to rise by West Coast terrain, heavy precipitation will result For the 24 h, ending 11 PM on Thursday we are talking about more than 5 inches.
This situation will have to be watched carefully, since it has the potential to cause slope failures and flooding over NW California and SW Oregin if it verifies.