Flash flooding can turn any area dangerous in a matter of minutes, but the conditions are especially ripe in mountainous terrain.

As meteorologist Sara Tonks explains, the speed and intensity of flooding in mountain regions can be quite shocking.

“A creek that is 6 inches can quickly go to 10 feet deep within an hour,” she says. 

So why are mountains such a hotspot for this kind of rapid flooding?

It starts with the landscape itself.

“When you have mountain shapes, you’ve got essentially a funnel,” Tonks explains. “If there’s rain over that funnel, it all funnels down into the valley.”

That means water from a wide area is suddenly rushing into one place, turning small creeks into surging torrents almost instantly.

(MORE: A Meteorologist’s Tips To Prepare For And Survive A Flood)

At the same time, the atmosphere is working overtime through a process called orographic lifting.

“When moist air approaches that hill, it gets forced up, it condenses and then it rains out,” Tonks says. “There’s both a geographic shape that’s funneling the water and then also the orographic lifting is helping produce more water."

In places like California, there’s another dangerous layer: burn scars left behind by wildfires.

“The soil in burn scars is definitely less able to absorb water very quickly,” Tonks explains. Instead of soaking in, rainwater rushes downhill, often carrying debris with it.

Recent disasters show just how dangerous this combination can be.

(MORE: Severe Weather Myth Vs. Fact)

Tonks points to Hurricane Helene as a prime example.

“All those mountain regions were just funneling the water into areas that could not handle that amount of water in that short a period of time,” she says.

Even beyond mountains, the risk doesn’t disappear. Hilly terrain and urban areas with poor drainage can face similar dangers. Heavy rain upstream can funnel into communities downstream, catching people off guard.

And in some regions, there’s another critical issue: detection and communication.

“The radar might not be able to accurately measure the amount of rainfall that has already fallen,” Tonks says, which can delay warnings.

(MORE: Three Things To Know About Flash Flood Emergencies)

Add in limited cell service or rugged terrain, and people may not realize the danger until it’s too late.

Not to mention once flooding begins, escape routes can vanish quickly. 

“There’s a chance that your access to major roadways could already be washed out,” Tonks warns. “That's one of the things we saw a lot with Hurricane Helene. All of these mountain roadways got washed out, and people were left with no way of communicating or traveling to the outside region."

The bottom line: Flash flooding is fast, unpredictable and often underestimated, whether you live in the mountains, near hills or even in a city with aging drainage systems.

That's why, Tonks says, it's important to know what type of weather specifically impacts your region and what makes your community uniquely situated for impact.