These seem perfectly understandable to me. The only thing that perhaps sounds "odd' -- not even "wrong" -- about the first is the use of "can". You might use that talking about climate and a continuing capability -- "Temperatures in the islands can go as high as the upper 70s during the summer" -- but not usually for the weather forecast for a specific time and place -- "could" works better in that case, because it's not certain. (If it was certain, you would use "will" as the modal.)
(And to quibble, I might say "as high as 79" is a simpler way of putting "as high as the upper 70s", but it's true that people say things like that.)