Take eastern Washington. Here are three visible satellite images over Washington State and northern Oregon today at 9 AM, 11 AM, and 4 PM. At 9 AM it was virtually clear east of the Cascades crest and down into the Columbia basin This is not unusual because that area is the zone of downslope flow during normal westerly flow aloft. By 11 AM, some shallow cumulus was forming on the hills of the eastern slopes and a few big cumulonimbus clouds were firing off over the Okanogan. But by 4 PM, major convection was firing off over the normal dry eastern slopes and the Columbia basin.
Here are the radar images for roughly 9 AM and 4 PM. Big difference, with heavy showers over SW Washington and over the Columbia Basin during the afternoon. Radar coverage is poor over the eastern slopes, so the radar imagery is underplaying it.
So what was going on? Aloft, we had an upper trough over us that was associated with cool temperatures. To show this, here is the 500 hPa (about 18,000 ft) forecast from the UW WRF model for 2 PM.. Colors are temperature and solid lines are heights (similar to pressure). The blue colors are cold temperatures.
So the sun had a good shot over the clear zone of eastern Washington this morning, leading to warming and substantial instability. But there was something else.
Convection is substantially aided by a little lift to push parcels of air upward. Sort of priming the pump of instability. As the upper trough moved eastward today, it created lift that encouraged the release of convection. Lightning detection networks indicated dozens of strikes over eastern Washington; perhaps this is the origin of a substantial fire that begin near Wenatchee around 4 PM (see image courtesy of the Wenatchee World newspaper)