It’s been called the “summer of flash flooding” in the US. The worst was the tragic flooding in Texas Hill Country on July 4, which took over 135 lives. But there have also been significant flash floods in other places across the country, from Ruidoso, New Mexico, to West Virginia, to Chicago, to the Washington, DC area, and many other places in between.

Here in Colorado, thankfully we haven’t experienced a lot of flash flooding so far this summer. There have been a handful of flash flood warnings and reports, but no major incidents. However, we are now in the midst of the peak season for flash floods. The last 10 days or so of July and the first week of August are when we’ve historically seen by far the most flash flood activity across the state.

One of the worst disasters in state history, the 1976 Big Thompson flood, happened on July 31. The Fort Collins flood of 1997: July 28. The Saguache Creek flood in the San Luis Valley in 1999: July 25. The heavy rain on the Grizzly Creek burn scar that closed I-70 for weeks in 2021: several rounds of storms in late July, especially on the 30th and 31st. And that’s just a sampling; the list could go on and on!

It’s not the only time of year at which flash flooding happens in Colorado. The graph above shows another peak in early to mid June, which is when some other historic floods have occurred like the 1921 Arkansas River flood and the 1965 flood on the South Platte in Denver. There’s also a big spike in September associated with the Great Colorado Flood of September 2013. Still, it’s remarkable how sharp of a peak there is in late July into early August.

What’s so special about late July and early August?

Meteorologically, the end of July through the beginning of August is when atmospheric moisture is at its highest on average. The North American Monsoon regularly transports moisture into Colorado in late summer, and at both Grand Junction and Denver, the precipitable water—the total amount of water vapor measured throughout the atmosphere—peaks right around August 1.

At this time of year, the winds through the atmosphere tend to be pretty weak, as the jet stream is positioned far to our north. That means that when storms do form, they don’t tend to move very quickly, and in some situations can stay over the same location for hours. And they have plenty of moisture to tap into (at least by Colorado standards), leading to large rainfall accumulations.

Flash flooding isn’t just about the rainfall, however. It also matters *where* that rain falls. When slow-moving, heavily raining storms develop over complex terrain, or over wildfire burn scars, that water can quickly turn into runoff or a debris flow. The combination of extreme rain rates in a steep canyon led to the Big Thompson flood. There were many unfortunate parallels between this month’s tragic flooding in Texas and the Big Thompson flood, including a rapid “wall of water”, people visiting the area on a holiday weekend, and challenges with communicating warnings, among others. Eve Gruntfest’s analysis of what people did during the Big Thompson flood remains relevant and will provide a point of comparison for studies of the 2025 Texas hill country disaster.

What can we expect during flash flood season this year? (Keep a close eye on Wednesday.)

Last week, on July 22-24, there were some storms that produced heavy rainfall, and several flash flood warnings were issued across the state, but no significant flash flooding was reported. Then, the moisture moved out, resulting in very hot and dry conditions for late July. But the outlook for the coming week has some reason for concern, in part because it’s our climatological peak in flash flooding, and also because a significant surge of moisture will move into Colorado.

Following the near-record highs and dry conditions on Sunday and Monday, a cold front is expected to move through Colorado sometime on Tuesday, with winds from the east (i.e., upslope flow) and plentiful moisture behind it. This figure from NOAA’s Global Ensemble Forecast System shows the precipitable water at Denver going from extremely low on Sunday (below the climatological 10th percentile) to extremely high (above the 90th percentile) on Wednesday. Anytime the PW gets above 25 mm (~1 inch) it warrants paying attention to for the potential of heavy rainfall.

For the last several years, my research group has developed tools that use machine learning to identify the probability of excessive rainfall and severe weather. These models have been consistently showing a strong signal for heavy rainfall along the Front Range on Wednesday, July 30th. In fact, for the current version of these models that have been running since 2020, this is the first time that both models (which were trained using slightly different definitions of ‘excessive rainfall’) have had probabilities greater than 20% four days in advance along the northern Front Range. Probabilities are relatively high for Thursday the 31st as well.

Flash flooding remains extremely difficult to forecast, because it requires predicting both the rainfall itself, and what will happen to that water once it hits the ground. So it’s too early to say exactly what will play out this week. But when forecast models are pointing to the potential for heavy rainfall that lines up with the climatological peak in flash flooding (the last week of July), it’s worth keeping a close eye on. If you live in a flood-prone area, or will be traveling through a beautiful Front Range canyon this week, take a moment to think about how you’ll get warnings if they are issued (do you have a NOAA weather radio?), and what you might do in case of a flash flood.