These air masses are relatively unstable quickly forming cumulonimbus clouds when surface heating is intense. The secondary maximums found in Colorado, Arizona, and New Mexico are due to another climatic factor. All of these areas are on the leeward side of the Rocky Mountains. Mountain slopes in these areas that face the sun absorb more direct solar radiation and become relatively warmer creating strong updrafts that form into cumulus clouds. If the differential heating is also supplemented by winds from the east, the cumulus clouds are further enhanced to become thunderstorms.
Few thunderstorms occur along the west coast of the United States.
This region is dominated by cool maritime polar air masses which suppress convectional uplift over land. Average number of thunderstorm days per year in the United States. Severe Thunderstorms Most thunderstorms are of the variety described above. However, some can develop into more severe storms if the conditions exist to enhance and prolong the mature stage of development. Severe thunderstorms are defined as convective storms with frequent lighting, accompanied by local wind gusts of 97 kilometers per hour, or hail that is 2 centimeters in diameter or larger.