The rain has been produced by a low pressure system that has been stationed over Nevada since late Sunday, said National Weather Service meteorologist Todd Dankers in Boulder.
The low has drawn subtropical moisture from the Mexican mainland over New Mexico and into the Rockies’ foothills in Colorado — and it’s been trapped by a stationary upper level ridge over the Great Plains and another system over the Great Lakes, Dankers said. The moisture becomes rain when it hits the mountains, the end result of a system he described as “a monsoon conveyer belt.”