It has been more than a decade, but the September 2013 flood remains one of the most remarkable weather events in Colorado history.
From September 11–15, 2013, an extraordinary combination of atmospheric moisture, slow-moving storms and terrain produced historic rainfall across much of Colorado’s Front Range. Roads disappeared beneath rushing water, rivers exploded beyond their banks, communities were cut off and rainfall records that had stood for decades were shattered.
And remarkably, much of the most extreme rain fell in roughly 30 hours.
A Perfect Setup for Extreme Rain
The flooding didn’t begin with one giant thunderstorm. Instead, the atmosphere gradually became primed for a major rainfall event.
A very rich, subtropical airmass had settled over Colorado, providing an enormous amount of moisture. Then, on the evening of September 11, a weak disturbance moved into the region and helped trigger widespread showers and thunderstorms.
The key ingredient was how slowly the storms moved.
Rather than quickly passing through the area, heavy rain repeatedly developed over portions of the Front Range. The result was a nearly stationary corridor of torrential rainfall that continued through much of September 12.
Additional showers and thunderstorms followed over the next several days, with another round of locally heavy rain occurring on September 15.
Colorado’s Front Range Was Drenched
The rainfall totals were staggering.
Many locations along the Front Range received 6 to 18 inches of rain during the event. The heaviest rainfall was concentrated across parts of Boulder and Larimer counties, extending from Boulder toward Jamestown, Lyons and the Estes Park area.
In central Boulder County, some locations received between 12 and 18 inches.
To put that into perspective, Boulder recorded 9.08 inches of rain on September 12 alone. That shattered the city’s previous record for its wettest calendar day — 4.80 inches, set on July 31, 1919.
Boulder ultimately received 18.16 inches during September 2013, more than doubling its previous all-time monthly precipitation record of 9.59 inches, set in May 1995.
Even more incredible, the 24-hour Colorado precipitation record was broken at Fort Carson, where 11.85 inches of rain fell on September 12.
This wasn’t simply a wet September. It was an extraordinary precipitation event by virtually any measure.
From Rain to Raging Rivers
The enormous amount of water had nowhere to go.
Flash flooding began quickly as the heaviest rain fell. As runoff accumulated, creeks and rivers transformed into powerful torrents.
Among the waterways that surged well above flood stage were the Big Thompson, Little Thompson, Cache La Poudre, Lefthand Creek, St. Vrain Creek, Boulder Creek, South Boulder Creek, Coal Creek, Sand Creek and Fountain Creek.
Record flood levels were observed along the St. Vrain near Lyons, while the South Platte River also reached record levels at several locations.
The geography of the Front Range made the situation even more dangerous. Water falling across the mountains and foothills rapidly drained into narrow canyons and valleys before reaching communities downstream.
The result was catastrophic flooding in areas that simply couldn’t handle that volume of water.
A Historic Colorado Weather Event
The September 2013 flood eventually affected a huge portion of northern and central Colorado. Thousands of homes were damaged or destroyed, major highways were washed out and numerous communities were temporarily isolated.
But from a meteorological perspective, the event is also a fascinating example of how several ingredients can come together to produce extreme flooding.
Moisture + lift + slow-moving storms + terrain = an extraordinary flood threat.
The atmosphere doesn’t always need a major hurricane or powerful winter storm to produce historic precipitation. In Colorado, a deep supply of moisture combined with a favorable disturbance and storms that refuse to move can be enough.
September 2013 demonstrated that dramatically.
More Than a Decade Later
For many Coloradans, the September 2013 flood is still remembered through the roads that disappeared, rivers that suddenly became raging torrents and landscapes that were permanently changed.
Meteorologically, however, the event remains an important reminder of just how extreme Colorado’s weather can become.
In the span of only a few days, a moisture-rich atmosphere produced rainfall totals that shattered century-old records and triggered one of the most significant floods in modern Colorado history.
The September 2013 flood wasn’t just a historic flood. It was a historic Colorado weather event — one that remains difficult to comprehend more than a decade later.