Hot air rises, creating a low pressure system. Moist air is sucked into the low pressure system from the Pacific, and when it rises in the low pressure system, it condenses to form clouds, which rain.
Winds blowing from the west in Denver have to drop in elevation after going over the Front Range.
El Nino is a periodic change in strength of prevailing winds and ocean currents. This reduces the flow of warm surface water to the western Pacific, leading to cooler than normal surface water temperatures in the western Pacific near Indonesia, Papua New Guinea, and Australia, and warmer than normal ocean surface temperatures in the eastern Pacific near South America. It generally creates wetter than normal weather in South America, and creates droughts in Indonesia.