Experts say the historic Western drought is to blame.
The drought has ravaged the region for decades, leaving the soil less like a sponge and more like pavement.
“The moisture isn’t absorbed into the soil as much and all of that water is running off and that is what leads to the roadways and flooding of people’s properties,” National Weather Service Flagstaff meteorologist Tim Steffen said.
“We had widespread heavy rainfall across the Flagstaff area on Wednesday that led to flash flooding and some closures of area roadways. That is our big concern every monsoon season,” Steffen said.
“These types of intense, local rain events happen each summer, but often in unpopulated areas,” said Michael Crimmins, climate science extension specialist for Arizona Cooperative Extension. “The impacts are quite large when they occur in populated areas like Flagstaff.”
Flagstaff been preparing for flash flooding events since the Museum fire north of the city in 2019 left a large burn scar. In the following monsoon seasons, there was little rainfall, creating flash flooding problems in the area.
Drought-driven wildfires worsen flooding
Wildfire burn scars can create a flash flood runway that can last for years.
“You can think of it as concrete where the water is not being absorbed into the soils; it’s all just running off,” Steffen said.
“Downstream of those fire scars all that water is running off, some debris as well from the fire, and that can clog culverts. Those areas are susceptible to flash flooding more so than those areas that haven’t been burned.”
Fires burn off plant matter that normally would hold soils in place during flooding events. Drought underlies the whole problem because there’s not much chance for vegetation to grow back and “keep things in place,” said Daniel Ferguson, director of the Climate Assessment for the Southwest (CLIMAS).
CLIMAS says climate change is affecting monsoon season.
Although storms will be less frequent, they may be more forceful. As the air heats up, it is able to hold more water, leading to heavier downpours and more flash flooding potential than typical monsoon thunderstorms in the past.
Each year more people are killed by flash flooding than lightning, with the average being 88 deaths according to the NWS.
Monsoon rains and the water crisis
But previous years of scant rainfall and recent record-breaking heat have proven detrimental to the Southwest.
“What has contributed to the current drought, especially here in Northern Arizona, is two of the driest monsoons on record in 2019 and 2020. That created a problem,” Steffen said.
Moisture in any form can help the drought by relieving strained greenery and soils. The bulk of Arizona’s yearly rainfall occurs during the monsoon season.
“This is typically when we receive a lot of our precipitation during the year, and winter, so it’s very important that we have a good monsoon season because it does help quell some of the drought problems,” Steffen said.
The 2021 monsoon season has already given some areas of Arizona more rainfall than during the entire 2020 season, according to the NWS. Flagstaff has seen over an inch more rain than it did all last year’s season.
“Monsoon rain however can help rehydrate our soils, which helps get snowmelt runoff into the reservoirs. Also, some portion infiltrates into the ground and replenishes the aquifers.”
But monsoons rains can simply be a short-term fix to a long-term problem.
“In terms of hydrological drought, we really do need to have widespread cool season precipitation, particularly snowfall,” said NWS Phoenix meteorologist Larry Hopper.
“The snow melts into the reservoir which helps improves the water supply, which is a major component of hydrological drought. Monsoon, you don’t usually capture as much rainfall because it’s not as widespread.”
Monsoon season rainfall is typically sporadic in nature, with bursts of downpours in some areas while other areas remain dry. Widespread drought alleviation is difficult unless an active monsoon season drenches the region.
The Southwest will see more monsoonal moisture this weekend bringing heavy rainfall.