During the week starting September 9, a slow moving cold front stalled over Colorado, clashing with warm humid monsoonal air from the south. This resulted in rain and flooding along Colorado's Front Range from Colorado Springs north to Ft. Collins. The situation intensified on September 11 and 12. Worst hit was Boulder County, with up to 8 inches of rain recorded. At least 4 deaths along the Front Range were attributed to the flooding, and evacuations took place in many low lying areas. The town of Lyons in Boulder County was isolated by the flooding of the St. Vrain River, and several earth dams along the Front Range burst or were overtopped. As of late September 13, according to the Office of Emergency Management, there were 172 people unaccounted for and at least 3 dead in flood area of Boulder County.