The latest weather that passed through Colorado over the last seven days gave way to a 1 in 1000 weather event. Which actually means that there is a .1% chance of a storm event like that happening yearly, so its very rare. This type of storm event is actually called Return Period Event. This does not mean that a 1000 year storm happens regularly every 1000 years. It means the probability of a 1000 year storm has a .1% chance of happening yearly. These storm events are calculated by NOAA with historical records and statistical models. Most weather records only go back to the 1870's. So a statistical model or unobserved event is used to predict the severity of that event so NOAA can label such large historic storms.
Below is a link from NOAA on the 7 day rain totals.