Hi Jim -
Depending on where you start the Sierra Cascades and how long you need to ride it, you should be fine. August 15 would give you a little more leeway. Three months to do the TransAm is more than ample. Not sure how you plan to connect them - riding northbound along the coast is on the wrong side of the highway for views and against the wind.
Most importantly, late summer/early fall is the major fire season in the West as seen by the current Yosemite fire. In the Pacific region most rain comes in the winter and spring with very little summer rain. In California, there is almost no rain at all in the summer months. There are almost always fires in late summer - it's natural.
That said, the coast is truly spectacular and much less subject to wildfires. You might consider some combination of the two - either planned beforehand or flexible as weather and fire season permit.
BTW - If you are not truly set on the TransAm there are other more northerly options - because a July crossing of the Great Plains can be truly brutal. Who was it that said, "Mad dogs and Englishmen"? Riding in 40C temps with no shade can be a challenge. Perhaps starting with the TransAm - shifting to the Lewis & Clark from St. Louis into South Dakota - then riding to Yellowstone and up to Glacier before heading west on the Northern Tier to Washington.
Best - J