Don't know how "soon" you'll be here, but the weather is great now, and usually stays nice until about mid-October. The forecasts should be pretty accurate, but now that it's getting cooler (maybe not this weekend), morning fog can happen, but it's a once-in-awhile thing. This is my favorite time for rides up here.
There's LOTS of great riding around here. There's a reason that one of the largest bicycling clubs in the country is here. Good riding on trails (Burke-Gilman is long but crowded on weekends, but not impassable), and it connects with the Mercer Slough Trail from Redmond around the north end of Lake Washington. Plenty of good minor roads. I recommend looking for an Eastside or Bellevue area bike route map, and the books by Bill and Erin Woods are excellent for the entire area.
Flats are not that much of a problem. Mine are mostly the occasional sliver of glass or skinny wire from a tire.