BOULDER – Is this week’s flooding in Boulder County a 100-year event? Five hundred? One thousand?
Jim Keeney, weather program manager for the National Weather Service’s central region, said the NWS hasn’t made a determination because flooding is still rampant along Colorado’s Front Range. But he did offer one comparison to lend some perspective on Friday.
The Big Thompson River flood of 1976 has long been a sort of benchmark for flooding in the region. That flood crested at 9.31 feet in the town of Drake. The flooding that slammed the foothills and Front Range this year crested at 10.55 Friday morning.
“So it’s significant from that standpoint,” Keeney said.
More locally, the Boulder Creek crested at 7.78 feet in Boulder, just below the 8-foot mark, which is considered ‘major’ flooding for the area by the NWS.
In Lyons, where the town was completely cut off from the outside by rising waters and the National Guard was called in to rescue residents, Keeney said the NWS meter on the St. Vrain River reported readings of 7 feet before it became inoperable and stopped producing readings. The record in Lyons, he said, was 9.06 feet in 1941, a mark that could well be eclipsed by this flood.
In all, Keeney said more than 3 million people in Colorado – or about 60 percent of the state’s population – was in some sort of flood warning as of Friday morning.
The steady rainfall that caused the flooding was no doubt of record proportions.
Bob Glancy, a warning coordination meteorologist with the NWS in Boulder, said the city had received 13.35 inches over the preceding seven days as of Friday morning. More than 12 inches of that had come in the previous 72 hours, including 4.57 in the last 24 hours.
For comparison, the previous record rainfall for Boulder over the past 100 years in the month of September was 5.5 inches in 1940. In the foothills of Boulder County as well as eastern parts of the county like Longmont, rainfall was comparable, ranging anywhere from 7 to 14 inches, Glancy said.
The flooding in the area left most roads in and out of Boulder impassable. In Longmont, most major north-south streets were closed due to flooding of the St. Vrain River and Left Hand Creek, essentially cutting the town in half. And by Friday morning, the floodwaters had pushed eastward to Interstate 25, causing the closure of the highway from Colorado Highway 7 north to the Wyoming state line.