From KOAA.com (Sam Schreier):
There are many ways a storm is formed. It can be a cold front moving through a region, pushing warm and moist air up into the sky, or something as simple as warm and moist air running up a mountain slope and getting lifted into the atmosphere (orographic lift). The latter is a pretty common occurrence for southern Colorado, especially in Colorado Springs and Denver.
Hail formation relies very heavily on an important piece of a thunderstorm, the updraft. An updraft provides the fuel and power to a thunderstorm by lifting warm and moist air into the heart of a storm…
As water droplets move through an updraft, they collect more water and ice around them, growing larger as they travel up into the storm. Some of these ice droplets fall out of a storm as small hail or melt back to rain, but some fall back into the storm updraft. Ice droplets that fall back into an updraft will collect more water and ice their surface, growing larger as they travel up through the storm.
Strong storms come with strong updrafts and can send hail through the center of a storm multiple times, eventually turning these ice droplets into damaging hail stones. Eventually, hailstones will get so big they can no longer be held in a storm by its updraft and fall out of the storm with the rain.
What makes the Front Range so susceptible to large hail?
The easy answer is probably pretty obvious, we live next to some big mountains. One of the ways storms are formed is warm or moist air being pushed up against a mountain range. This process is called orographic lift and it’s probably the most common way storms develop in Colorado.
Storms that form over the mountains are typically not drawing in very warm or moist air, meaning they usually stay pretty weak or non-severe. On the other side of the mountains near Colorado Springs or Denver, there is usually warmer and moist air, both of which are fuel for thunderstorms. The air in and over the mountains is also less dense, meaning there’s a larger volume of air in Colorado Springs compared to Cripple Creek.
Once storms form over the mountains, the jet stream will usually push them east into a warmer and moister airmass. This allows thunderstorms to grow explosively which in turn creates a stronger updraft and allows larger hail to form within a storm.
This is the basic reason why Colorado Springs, Denver, and the surrounding area tend to see so many hail events through the Spring and Summer seasons. Storms that form over the mountains grow rapidly over the towns right next to the mountains and can drop small to large hail as they travel east to the plains. As Colorado Springs and Denver grow larger in area, that leaves more and more of both cities exposed to falling hail from thunderstorms.