You may be able to raise them all summer long. Just be prepared for a way to cool those birds off if you get some temp spikes like the PNW had last summer. I'm in the WA cascades at 2200' and we had 10 days of 105-110 last year. I had Cornish X's at the time but they were still in the brooder. They were 3.5 weeeks old when the heat wave hit so I just kept them inside where it was cooler (Earth berm basement) until it passed. I think I would have to run out and get a little mister setup if they had already been outside.
I plan to get another run of Cornish crosses at the beginning of July. I could possibly run into some terrible days of heat so I'll have a mister setup ready to go. If it gets too hot, I'll just move the tractor within hose range of the spigot and run the misters on a timer or something until the heat passes.
So far this year, we've only had one day where we barely reached 80° and that was just for a few hours. Today it was 50° and they've been calling it "June-uary" here. LOL
Dan