LARIMER COUNTY, Colorado — The wildfires in Colorado may be out, but first responders are preparing for other challenges now. Larimer County first responders are keeping close tabs on the Alexander Mountain Fire burn scar, which left behind over 9,600 acres of charred ground and soil.
We asked 9NEWS Weather Impact Meteorologist Cory Reppenhagen about how incoming rain could affect the Alexander Mountain fire burn scar.
How much rain is needed to flood a burn scar?
Reppenhagen: Every burn scar is different. People would be surprised to know how very little rain it takes to cause catastrophic and deadly flooding in a burn scar. Yesterday’s flash flooding warning that came out was only for two- to three-tenths of an inch of water and they were talking about catastrophic flooding potential with just that little of rain.
What happens when rain hits a burn scar?
Reppenhagen: The soil, the ground after it's burned, it can’t absorb the rain. All the rain that falls out of that cloud is going to run down that hill where normally some of that rain would be absorbed into that soil. The ground gets hydrophobic. It cannot absorb any rain so it’s all free to run and it all runs downhill until it gets to lowline ground or a stream or river.
What specific challenges make the Alexander Mountain Fire burn scar more susceptible to flooding?
Reppenhagen: The Alexander Mountain burn scar is on very steep terrain in some spots and that reduces the rainfall it takes to cause flash flooding even lower and that’s how we got to that two-tenths of an inch of rain in 20 minutes to cause that flash flooding
How long does the threat of flash flooding on a burn scar last?
Reppenhagen: The first one to two years after the burn, flash flooding potential really doesn’t change. It’s not until that third year you really start to see the good healing. So we’re going to be looking at the Alexander Mountain Fire burn scar for not only this year, but next year and year after that for flooding.
Is there anything first responders can do to prevent flooding?
Reppenhagen: There are things that can be done, but they take time. It is good news that yesterday, we did get a pretty good test on the burn scar. There was a lot of rain, clear of any major problems. But, if you think about it, you have a 9,000-acre burn and if only 500 to 1,000 acres is the bad part, did the rain hit that sweet spot? That's what we don't know so there's going to continue to be concern about that burn scar until all 9,000 acres have been tested with rain.