Lookout mountain isn't that much higher than Denver. So you might see as much as a 10°F difference from lower elevation to higher. However just find the nearest weather reporting stations to your start, finish and middle. Keep track of their current temps and what their 10 day forecasts are.
https://www.weatherwx.com/hourly/co/...+mountain.html
https://www.mountain-forecast.com/pe...forecasts/2311
That's what I did for a month or more before I climbed Mount Evans, CO back in early August.
Take a jacket with you and put it on when you stop. If it's really cool, like 40'ish, you'll think you are fine when you stop, but 20 minutes later you'll need some heat. So either get to pedaling or get someplace warm.