Brilliant flash of light followed by a sonic boom

Witnessed at around 1 am on July 28th, 2016, in Yakima, Washington.  As I work nights, I walk my German Shepherd on my nights off, and we usually go for a 5-mile walk.
The night is clear, the stars are out and the moon is just rising.
We were walking on a section of the road that curves up around a church and housing area and is higher than the surrounding area, which gives a good partial view of the surrounding city, of which in the day time also gives a great view of Mount Adams.
My GSD stopped to sniff the air when there was a sudden and very bright flash of light to the South West of us. For a brief second, I could actually see Mount Adams in the distance. Then it was gone.
Right after that, there was a very loud boom sound, not the sound of thunder, but more a sonic boom.
I stood there for a couple of minutes watching the area where the light flash had happened but saw nothing else other than the stars in the sky.
It was most certainly not heat lighting, of which I am very familiar with in that it happens a great deal in the area in the summer time, and I have spent a great many hours outside at night watching it flash along the surrounding mountains.