A complex confluence of factors combined this week to drench Boulder and much of the Front Range with days of rain that claimed lives, destroyed homes and ruined personal property -- and, as it pounded on for days, shattered local records for moisture.
Longstanding rainfall records were swept away in what an official with the United States Geological Survey late Thursday confirmed was for Boulder a 100-year flood -- although that agency no longer uses that term.
"We're getting away from using that terminology, because people tend to think that's the storm that happens every 100 years," said Robert Kimbrough, associate director of the USGS Colorado Water Science Center. "But really, it's the chance of it happening in any given year is one in 100."
Asked if that was synonymous with what many refer to as a 100-year flood, Kimbrough said, "Correct. That's a term the USGS no longer uses, but it could be referred to as a 100-year event.
"It's a large event, certainly."
Kimbrough said that evaluation was based on a stream flow at Boulder Creek at North 75th Street on Thursday morning of 4,500 cubic feet per second. The previous peak flow measured at that point was 2,050 cfs in May 2003. Flow levels continued to increase throughout the day, with Kimbrough reporting at 7:45 p.m. that the flow was measured at 5,370 cfs.
National Weather Service meteorologist in charge Nezette Rydell said Boulder was sandwiched between two systems. To the west of Colorado, a low pressure system stalled out, pulling late-season monsoonal flow from the eastern Pacific and Gulf of California. Meanwhile, a high-pressure system to our northeast helped lock it in place long enough to rain, rain and rain some more.
Also, at the same time, a cold front moved through Boulder on Monday that produced moisture and an upslope flow.
The product of those divergent atmospheric influences was disaster -- and history.
"We have never had anything this big," said Boulder meteorologist Matt Kelsch.
He reported totals that washed away records for a any single 24-hour period, any three-day period, and any single month.
Between 6 p.m. Wednesday and 6 p.m. Thursday, Boulder recorded 9.08 inches of rain, Kelsch said, by far the wettest day in the city's history since weather records were first officially kept in 1897.
Since the beginning of the storm on Monday, Boulder had received 12.27 inches of rain as of 6 p.m. Thursday, Kelsch said, enough to make September 2013 the wettest month in city history.
Lyons meanwhile received an impressive 6.06 inches of rain between Wednesday and Thursday evening, Kelsch said.
Prior to this weather event, Boulder's wettest single day saw 4.80 inches of rain come down on July 31, 1919. All remaining top-10 runners-up were totals of 3.6 inches or less.
Looking back, Rydell said, forecasters expected the middle of this week would see rain. They did not predict history would be made.
"I don't like to use the word 'surprise', but things have to come together exactly correctly, to have it be this heavy," said Rydell. "Looking at Wednesday, we were saying it would be rainy. To have forecast an 8-inch amount in Boulder County is not something we would have been able to do."
The weather system's reluctance -- or inability -- to move through the area, is what triggered Boulder's deluge.
"Weather systems that don't move, spell trouble one way or the other," Rydell said. "If it doesn't move, it compounds itself."
Kelsch said, "If you had had asked me (late last week), I would have told you it looked like a wetter, cooler period. But there is no way I would have told you we were going to have 10 inches of rain. This kind of thing is so localized."
Kelsch added, "Most of the big rain events in Colorado tend to be in the summer months, like July and August. September, this late in the year, is unusual, but not unheard of."
Kimbrough, at the USGS, said his office was not caught entirely by surprise by the week's storms.
"We really rely on the National Weather Service forecasts, and we have been well aware of the strong monsoonal flow for the past week, so we were prepared for the high flow -- but we were a little caught off-guard by the magnitude of this event," Kimbrough said.
"But we approached this storm like we do any flood response," he said. "... (Thursday) morning we met about 6:30, and we coordinated our field response. We probably have about 10 crews out right now who are equipped to make take measurements and to repair stream gauges as needed.
"So, we weren't caught off guard as far as having to respond. But we have to admit, this is a large event."
Balaji Rajagopalan is a professor of civil, environmental and architectural engineering at the University of Colorado and a fellow at the Cooperative Institute for Research in Environmental Sciences, who specializes in hydrology and hydroclimatology.
Now a resident of Superior, he said he has seen this before -- during tropical monsoons in his native India.
"A one-day event here can give you flash floods, but if it has been dry, most of it ends up going into the ground," said Rajagopalan. "But if it is this setting up for three or four days in a row, this reminds me of India. It reminds me of a tropical cyclone."
Rajagopalan added, "As engineers we always think we can tame nature, we can build this and build that for a 100-year storm, or a 20-year storm. But at some point we should realize that the best course is to stay out of nature's way and not put yourself at risk.
"You just can't tame nature."
Contact Camera Staff Writer Charlie Brennan at 303-473-1327 or firstname.lastname@example.org.