My primary fermentation and bottle conditioning temperatures are well controlled in a brewing fridge. Once bottles are conditioned / carbonation cycle complete I move them to crates which I store in my garage. The temperature here can vary from 15C / 58F to 30C / 86F in one day. The beer at least has some buffer and does in itself not reach these temperatures, but it will fluctuate. Will these ‘storage’ temperatures adversely affect my beer?
Beer should be stored cool. Around 7°C (45°F), never colder than 3°C (37°F) [#1].
Charles Bamforth says that every extra 10°C (50°F) of temperature doubles the rate of beer aging. So when your beer is sitting at 30°C it's aging very quickly!
Limiting oxygen in the packaged beer will really help with beer preservation. But if your weather gets really hot in summer, there's not a lot you can do without getting a second (or 3rd, 4th) refrigerator.
I briefly toyed with the idea of burying bottles deep in the back yard.