What Caused The 2013 Colorado Flood? What is a 100-Year Flood?

Predicting weather can be a fickle beast, especially in Colorado. We've all seen the sure-fire blizzard fizzle out to produce no snow at all, and we've seen sunny forecasts turn into violent storms. But what on earth could have caused a year's worth of rain to fall on areas in Colorado last week?

We got a pretty detailed explanation from a Denver Meteorologist below.

From Meteorologist Matt Makens that the Denver Channel posted to their Facebook page:

A stalled out frontal boundary (a cold front that moved in from the north and was stopped on top of us by the monsoonal humidity rushing in from the south) provided the energy, or lift, needed to create the rain showers. The monsoon provided the humidity to turn into the intense rainfall. Because the monsoon wanted to rush north, and the frontal boundary wanted to push south, they stopped each other right on top of the Front Range...leading to the "stalled frontal boundary".

Without either weather feature willing to move, the setup was for stationary and slow moving individual storms with plenty of energy and plenty of humidity to drop a lot of rainfall.

Had that cold front been stronger, it would likely have moved farther south before stalling. The opposite is true if the cold front had been weaker – the stall probably would have been farther north.

In any case, the likelihood for historic rain would have hit someone.

Our terrain contributed too; Boulder County probably received more rainfall than any other county, over 20”, due in part to the upslope nature of the storms that got stuck there. Upslope adds lift, or energy, to the storms by forcing the humid air up a mountain side results in very heavy rainfall (just like our upslope snows). In the case of Boulder County, the Flat Irons and mountains were the perfect, in a negative sense, upslope addition. The combination of monsoon, stalled frontal boundary, and upslope "trapped the heaviest rain into a corner" that it couldn't get out of.

That "corner" just happened to be the headwaters for so many rivers and creeks that all conjoin near Greeley, then flow as one river -- Platte River -- through northeastern Colorado. The floodwater is receding now, thankfully, but with trillions of gallons trying to fit down one river it will take a long time to get it out of here; perhaps weeks before all rivers are back to their normal.

So, basically it was just a perfect storm in a perfect spot that caused a lot of damage. That's not very comforting, but it's the best explanation I've heard so far.

All weekend I heard terms thrown around such as 100-year flood, 500-year flood and even 1,000-year flood, which this flood probably was. However this does NOT mean that we will not see another one for 100 years.

Technically a 100-year flood just means that it was a flood event that has a 1% probability of occurring in any given year. Meaning that statistically we could have a 100-year flood every year for the next five-years. Hopefully that is not the case, but I just wanted to clarify what that term means for people. (The 500-year and 1,000-year floods mean the same thing, but with less-than-one percent chances of happening in a given year.)

Wikipedia actually goes further into the statistical probability of 100-year floods and says, "There is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period."