Among the most dramatic weather features in eastern Colorado are the giant thunderstorms that develop from May through September. On a typical day in late spring or summer, cumulus clouds develop by early afternoon. They are driven by the heating of low-level air and resulting instability. These fleecy clouds can build into towering cumulus and sometimes into powerful cumulonimbus (thunderheads).

Often the storms will develop over the mountains near Boulder, then move eastward onto the plains by late afternoon, growing stronger with time. Most storms last from one to two hours, although a few (called supercells) can survive for more than six hours. Tornadoes and large hail sometimes occur with these storms. Large hail is rare in Boulder, but tornadoes east of town can occasionally be seen from the NCAR mesa.

Lightning is always a danger, even from the weakest thunderstorm. The best protection from lightning is shelter-inside a building or automobile, but not under a tree.