Without a 100-year flood since 1894, Boulder officials have said a major flood is "a question of when and not if."
Still, meteorologists are calling the downpour that intensified Wednesday night -- forcing evacuations in the foothills and canyons, collapsing structures, flooding city streets and killing at least one -- somewhere between a 10-year and 25-year flood.
About 6.5 inches of rain fell on central Boulder between 5:30 a.m. Wednesday and 5:30 a.m. Thursday, according to National Weather Service meteorologist Kari Bowen. Most of that was since Wednesday evening. The total since early Tuesday is 8.8 inches, she said, not including Monday night's substantial rainfall.
And meteorologists were calling for more rain Thursday.
Preliminary readings show that Boulder Creek reached 3,100 cubic feet per second at Broadway on Wednesday night, according to Treste Huse, a hydrologist for the National Weather Service. She said the flow would need to be four times that to result in a 100-year-flood.
But with more rain in the forecast Thursday, "We're definitely watching it because the soils will be saturated," Huse said.
"Water is already standing, and we have another risk of flash flooding, so we definitely are still concerned."
She said she thinks Boulder Creek has peaked and won't get higher than 3,100 cfs.
"It could happen, but I doubt it," Huse said.
Weather service officials categorized the flooding late Wednesday and early Thursday as "major" in the Fourmile Fire burn area and "moderate" along Boulder Creek in town.
Boulder has long ranked as one of the state's top flood hazards and made a national list of six "disasters waiting to happen" published by the University of Colorado in 2004, along with a devastating hurricane striking New Orleans.
Meteorologist Jim Kalina described a complex series of weather events that came together to produce this week's storm. An upper-level high-pressure system on the East Coast and low-pressure system over Nevada combined to bring subtropical moisture in through Mexico. That included influx from a tropical storm, he said.
At the same time, a cold front moved through Boulder on Monday that produced moisture and an upslope flow, he said.
"This is pretty rare," he said.
The most moisture Boulder has received on record in the month of September is 5.5 inches, in 1940, according to Camera weather historian Bill Callahan.
A 100-year flood is defined as having a 1 percent chance of occurring any given year.
The Big Thompson flood of 1976 was considered between a 500-year and 1,000-year event. The disaster between Estes Park and Loveland killed 145 people and caused $41 million in damage after dumping 12 inches of rain in three hours.