The weather in Colorado is very unpredictable this time of year. For example, today (Monday 19 April 1999) it is sunny today and the high will be about 70F in Boulder. I believe that Estes Park is about 5-10F cooler. It is a very nice day! It is quite comfortable outdoors without a jacket. Unfortunately, it snowed twice last week and the forecast is for more snow the end of this week. So, all I can say is `be prepared!'
On a related topic. Estes Park is at altitude. Take precautions! Don't plan to walk any significant distance until you know how your body will respond to the lack of oxygen. In addition, please be aware that altitude can also enhance the effects of alcohol.