Just a few months ago, Boulder, Colorado was in the grip of yet another drought, and the state itself experienced its worst wildfire on record earlier this year. But after days of heavy rainfall that the National Weather Service called “biblical,” drought and fire is the last thing that Boulder and the rest of the northern Front Range of Colorado has to worry about.
On average Boulder receives about 1.7 inches of rain during the month of September. As of 7 AM on September 16, Boulder had received 17.17 inches of rain so far in the month, smashing the all-time record of 9.59 inches set in May of 1995. 9.08 inches fell on Sept. 12, nearly doubling the previous daily record of 4.80 inches set on July 31, 1919. In fact, Boulder has already broken its yearly record for precipitation—with more than three months left in the year, and the rain still falling.
Parts of Boulder are experiencing a 1-in-1,000 year flood. That doesn’t literally mean that the kind of rainfall seen over the past week only occurs once in a millennium. Rather, it means that a flood of this magnitude only has a 0.1% chance of happening in a given year. This is historically bad luck, due in part to the combination of an active, drenching Southwest monsoon and a low pressure area that trapped over the region. A tropical air mass—unusual in the dry Rocky Mountains—is slowly being hauled across the Front Range by weak southwesterly winds. This is known as an orographic lift, which is converting the incredibly moist air into sheets and sheets of rainfall.