In the afternoon a dreadful hail storm came on, which lasted half an hour. Some of the lumps of ice that fell weighed 3 ounces, and measured 7 inches in circumference. The ground was covered with them, as white as snow.
Sergeant Patrick Gass, Lewis and Clark Expedition- June 27, 1805 near the great falls of the Missouri
It is not uncommon for hailstorms to form during a warm summer thunderstorm. Hail is produced when falling water droplets are carried back upward into colder air, freeze and fall back toward earth. Hail grows larger as this updraft process is repeated, each time adding layers to the forming ball of ice.
Hailstorms are common along the eastern side of the Continental Divide where winds are forced upward and updrafts are intensified during thunderstorms.
Hailstorms are usually brief, but can be very intense. Protecting yourself from hail is a matter of taking cover quickly. Seek shelter and keep your head protected. Get creative fast and use a cooler top or backpack or whatever is handy to cover your head if you are in an open exposed area. Make sure to seek shelter for your pets, too.