More than 100 people died in early July’s catastrophic flooding in Texas. Many are still missing.
Deadly flash floods are a horrific experience to which Coloradans can relate as flash floods are Colorado’s deadliest natural disaster, accounting for more fatalities than all other natural disasters combined.
And the state is just beginning its deadliest flash floods months.
More than 500 people have died in flash floods in Colorado since the turn of the century, according to Western Water Assessment at the University of Colorado.
The deadliest flash flood in Colorado history took place in Larimer County when, on the evening of July 31, 1976, a thunderstorm with copious amounts of moisture stalled over the narrow Big Thompson Canyon, funneling a wall of rushing water down the Big Thompson River that killed 144 people.
Three of Colorado’s five most significant floods occurred during the state’s flash flood danger zone, which is from mid-July through mid-September, a time frame that coincides with the state’s monsoon season.
The Coloradoan talked with Greg Heavener, warning coordinator meteorologist, and Jennifer Stark, meteorologist in charge at the National Weather Service in Boulder, about flash flooding in Colorado. The office covers from the Wyoming border to Denver, the mountains from I-70 north and much of the Eastern Plains.
The office issues 35 to 50 flash flood warnings annually on average, Heavener said.
How do National Weather Service meteorologists cope when something happens like the Texas flooding that results in deaths?
Heavener: “We have skin in the game. We are not just an agency at a single point issuing warnings thousands of miles away. We have added layers of complexity because the folks in our offices are part of the community and are impacted by these natural disasters and our families, like other families, can also be put in harm’s way.
“I can’t imagine what the New Braunfels, Texas, (NWS) office is going through.”
How does the National Weather Service determine when to issue a flash flood warning?
Stark: “We first diagnose the atmosphere to see how much moisture is available in thunderstorms and how efficient the rainfall will be. Is the moisture falling over eight hours or in 30 to 45 minutes? The Eastern Plains can handle more rain but hot spots like burn scars and urban areas with 1 to 2 inches of rain per hour can overwhelm the infrastructure. If we had several days of moisture, it won’t take much for flooding because of saturated soils.
“Our goal is to give warnings 30 to 60 minutes out but in Colorado that may not always be the case, especially in the mountains where flash flooding on burn scars can take place in minutes. The high country is where thunderstorms are born.”
Why are flash floods so deadly?
Stark: “Flash flooding at night is probably the deadliest weather because visual clues are gone. The normal idea is things will be fine and not impact me. Our job sometimes is to try to tell people this could be the worst thing you could image happening. I don’t think sometimes people understand flash floods because they can’t imagine what that looks like. It doesn’t have to be raining where you are to experience a flash flood because it is coming downstream, and tame creeks and streams can quickly become swollen, and it comes at high rates of speed. You really have to have awareness upstream if recreating in the outdoors and in the mountains.”
Stark said most flash floods in Colorado occur in late afternoon to evening.
Heavener: “With a warning, we’re not focusing on the heaviest pocket of rain on the map but the routing of the water coming down to provide longer lead times for those in the warning area, which you may only be 1 to 15 minutes away from a wall of water.”
How difficult is it to forecast when a flash flood will take place?
Stark: “Flash floods can vary in scale from relatively local floods to hugely significant (areas), so the variability is really hard to determine in the initial stages of an event unless there is widespread copious amounts of moisture produced by a thunderstorm
“We don’t know if it’s 8 inches or 15 inches of rainfall until the storm is producing it. We proactively change the urgency depending on the situation, raising the tone with a damage tag that alerts. We train for these situations. We have expertise with weather pattern recognition, experience and know the terrain and topography, which is why AI hasn’t replaced humans.”
What areas are prone to flash flooding in Colorado?
Stark: “Many of our burn scars are experiencing some recovery in vegetation and a breakdown of hydrophobic soil (soil that repels water from absorbing), so they are not as prone to flash flooding as initially. We raised the threshold for warnings for those areas. Urban areas have a lower tolerance for intense rain, so those areas are prone.”
What tools do you use to determine the amount of rain that might trigger a flash flood warning?
Stark: “The No. 1 line of defense is radar, which gives us time to take action. We also use ground truth verification like gauges and spotters.”
What weather systems alert you about flash flooding potential?
Heavener: “Late July and August is a time period when storms can develop in the foothills and mountains and they ease off the mountains and then sit there and more storms develop in the same location.”
How does the North American monsoon season affect the chances of flash flooding?
Heavener: “This is when there is the most amount of moisture in the air. It has already started in the desert southwest and can spill over into the eastern side of the state. As we get into this time of year, we are pushing messaging out how we could have some higher than usual activity thunderstorms with more moisture and they could produce pockets of moisture that prompts watches and warnings.”
Here are the best ways to be alerted for flash floods, wildfires and other natural disasters in Northern Colorado
Stark said your best defense against flash floods is having multiple options to receive warnings and alerts because of connectivity challenges in more remote areas.
She said NOAA weather radio broadcasts offer weather service warnings, watches, forecasts and other hazard information 24/7. The broadcasts are on a special weather frequency, so you’ll need a special radio to receive the broadcast. Weather radios are available in electronics stores, some hardware stores and via online sites.
Click here for more information about the service.
Stark said most cell phones come with a free Wireless Emergency Alert (FEMA) app that alerts if your phone is in the warning area. She said the phones come with the app on but that you can choose to turn it off.
Here are other options:
Significant flash floods in Larimer County
Cameron Peak Fire burn scar: Flash flooding on the burn scar resulted in four people dying during the Black Hollow flood July 20, 2021, and two people dying July 16, 2022, along Sheep Creek.
2013 flood: The far-reaching flooding Sept. 9-15, 2013, stretched from Fort Collins to Colorado Springs, with nine fatalities, including two in Larimer County.
Lawn Lake flood: The lake’s earthen dam breached July 15, 1982, in Rocky Mountain National Park, killing three people.
This article originally appeared on Fort Collins Coloradoan: Texas flooding is a reminder of Colorado’s flash flood danger