For the past several days, unusually cold air, associated with an upper level trough has been positioned over our region, resulting in cold nighttime temperatures east of the Cascades and puffy cumulus clouds over much of the region. Below is a nice example from North Seattle Saturday morning.
An upper level chart (500 hPa, about 18,000 ft above sea level) for Friday morning shows the upper level trough (solid lines) and the cold temperatures (blue colors) over us.
With cold air aloft and relatively clear nighttime skies, some locations in western Oregon/Washington dropped into the mid-40s, while some locations in eastern Oregon plummeted to freezing Friday and Saturday morning (Saturday mornings lows shown below).
With cold air aloft and strong heating from the powerful near-solstice sun, there has been a large change in temperature with height (a large lapse rate). This leads to instability--up and down motions in the lower atmosphere--as the atmosphere start to convect. Such convection leads to the puffy white cumulus clouds, which form in the upward motion.
The convection is enhanced over high terrain (which acts an elevated heat source). A visible satellite image shows the cumulus clouds from space--you can see how high terrain is favored.
As a result, precipitation is not over for us--with showers quite possible on Tuesday. The accumulated precipitation through 5 AM July 4th, show substantial amounts over NE Washington and BC, with some extending down to Portland.