I have quite a bit of experience with high altitude hiking (and I'm a weather buff) and my advice would be to not plan on it being either hot or cold, but rather both. The thing about the temperature dropping 5 degrees with every 1,000 ft is not always the case. It is often just as hot (up to 97 easily) in Estes as it is at my home, 3,000 ft. lower. On days like that the biggest difference is it cools off a lot and very quickly in the mountains at night. You can experience the weather of all 4 seasons in a single summer day, including snow and freezing hail above 9,000 ft. That weekend Kathi mentioned where is was cold and rainy, DH and I were on a hike and it was the coldest I've ever been on a hike, including snowshoeing in the winter. The very next weekend we were hiking and it was 96 when we got back to the trailhead at 2 pm at 8,000 ft. Layering is absolutley the key to being prepared. We have a little saying out here: "If you don't like the weather, just wait a minute and it will change."

One last comment about the afternoon thunderstorms, do everything you can to not be above treeline after 1 pm. (This is assuming you have a ways to go to find shelter). You can find dreadful stories of rock climbers and hikers being killed by lightning strikes, particularly on Long's Peak. I can't remember the exact numbers, but I think Colorado came in 3rd place recently for states with the most lightning caused deaths. Some people come to CO all worried about bears and mountain lions, the thing they should worry about is the weather. I don't want to try and scare anyone, but the weather here deserves respect. If you just follow a couple of simple guidelines you'll stay safe and have a great time. And if the weather is too crappy too climb one day, definitely head to New Belgium