Every summer starting around mid-June and ending by the beginning of September, moisture streams northward out of the Gulf of California into Arizona, Nevada, and the eastern portions of the Pacific Northwest.
Known as the Southwest Monsoon or the North American Monsoon, this phenomenon is associated with the river of moist unstable air that brings thunderstorms into the southwest U.S. and occasionally into eastern Oregon and Washington.
This moist current, with an origin over the warm Gulf of Mexico, is the result of high pressure that develops during the summer over the Southwest U.S. When this high shifts to near the Four Corners area of the SW (where AZ, NM, CO, and UT meet), the moisture can surge into the eastern portion of the Pacific Northwest. This should happen later this week.
This was associated with the Southwest Monsoon and the Four Corners high, bringing moisture in from the south and southwest ( see upper level (500 hPa, 18,000 ft) weather map below for mid-day on July 29th.:
A plume of moisture associated with the Southwest Monsoon will move northward later this week out of the desert southwest. To illustrate, this image shows you the atmospheric moisture pattern on Saturday, with blue, white, and red being the highest values (in that order.)