Colorado Travel Guide
Summer in the Rocky Mountains begins in late June or early July. Days are warm, with highs often in the 80s; nighttime temperatures fall to the 40s and 50s. Afternoon thunderstorms are common over the higher peaks. Fall begins in September; winter creeps in during November, and deep snows arrive by December. Temperatures usually hover near freezing by day, thanks to the warm mountain sun, and drop overnight, occasionally as low as -60°F. Winter tapers off in March, though snow lingers into April on valley bottoms and into July on mountain passes.
At lower elevations (Denver, the eastern plains, and the southwest), summertime highs above 100°F are not uncommon, and winters are cold, with highs in the 20s and 30s. The entire state sees snowy winters, even on the plains.