
© Cliff Grassmick APRailroad tracks at 9th Street, East of Airport Road, continue to be flooded in Longmont, Colo, on Sept. 14, 2013.
Boulder picked up almost nine times its average September monthly rainfall in about four days.
Colorado is no stranger to devastating and deadly flash floods, due to a lethal combination of geography and meteorology. When unusually heavy rain falls across the region, narrow canyons and steep mountains help funnel raging torrents of water down into the heavily populated foothills to the west and north of Denver.
One of them, the notorious "Big Thompson" flash flood of July 1976, killed at least 144 people north of Boulder. It caused "the worst natural disaster, in terms of documented lives lost, in Colorado state history," according to Boulder's Flood Safety Education Project website.
Boulder, specifically, is considered to be Colorado's "most at risk" city in terms of potential flood damage, notes Weather Underground weather historian Christopher Burt. This is because it rests against the mouth of a canyon (the Boulder Canyon) from which a creek (the Boulder Creek) bisects the heart of the town.