Local officials and weather experts are keeping a close watch on burn scars in Colorado as monsoon rains elevate the risk of debris flows and flash floods.
The National Weather Service in Grand Junction reported several debris flows last weekend as monsoon showers passed over Colorado. Several were in or near burn scars created by major wildfires burning on the Western Slope. The weather service also issued a flash flood watch and warnings for parts of Colorado as the storms swept over the state.
“We are still expecting to see some good rain today (Tuesday), which is why we have that flash flood watch out,” said Braeden Winters, a meteorologist for the National Weather Service in Grand Junction. “I can’t speak to our expectations on the flash flood risk right now, but we’re going to continue looking at the burn scars and typical flash flood-prone locations.”
The National Weather Service issued a broad flood watch on Tuesday for most of the Western Slope, including areas south of Meeker and down to the Colorado-New Mexico border. The watch was set to end Tuesday night.
A flood “watch” means that conditions are favorable for a flash flood to occur, but it doesn’t mean anything is imminent or already happening, Winters said. A flash flood warning means a flood is already happening or expected to occur within the next hour.
Flash floods are particularly concerning in burn scars, the footprint of burned material left by a wildfire. These areas can have water-repellant soil conditions and fewer natural barriers to hold soil in place.
Flash floods can start minutes after light precipitation starts in a severe burn scar with steep terrain. Areas with less severe burn damage and flatter terrain can absorb more rain, according to the National Weather Service. About 300 Coloradans have been killed by flash flooding since the year 1900.
Five major wildfires are burning on Colorado’s Western Slope, spanning nearly 200,000 acres in total. Most are partially or almost fully contained, except for the 5,345-acre Derby fire north of Dotsero in rural Eagle County.
On Sunday, storms quickly dumped a half-inch of rain in the Four Corners area and in Pitkin County. Rio Blanco County, where the Lee Fire is, saw 0.71 inches of rain. The Lee fire topped 100,000 acres Aug. 9 and ranks as the fourth-largest wildfire in Colorado history.
Within minutes of the rainfall Sunday, members of the public reported flash floods in Rio Blanco County, where mini-rivers of “classic black fire debris” were flowing through Shute Gulch, McCarthy Gulch and Half Moon Gulch, according to preliminary local storm reports.
A general guideline is that half an inch of rainfall in less than an hour is sufficient to cause flash flooding in a burn area, depending on factors like the area’s terrain, the severity of the fire and precipitation amount, according to the National Weather Service.
The summer storms also caused debris flows in parts of the Western Slope, according to the National Weather Service preliminary reports.
Law enforcement reported signs of a debris flow — rocks on Colorado 141 — near the Turner Gulch burn scar in Mesa County on Saturday.
On Sunday, a National Weather Service employee reported that a debris flow from the Lee fire burn scar trapped a semitruck on County Road 5 in Rio Blanco County after the driver tried to go through the mud and debris on the road.
“We have had multiple flash flood warnings issued for the Lee fire,” Winters said. “The biggest concern has been on County Road 5.”
The county saw other debris flows: One moved across the lower Powell Park area from the north side of the Lee burn scar, while another crossed County Road 8 near the Elk fire burn scar, according to weather service reports.
It’s difficult to confirm at this time whether the debris flows are a result of the burn scars or if they just occurred in areas already prone to flash floods, Winters said.
“It could be a combination,” Winters said. “Because these are so close to the fire, we are considering it as a heightened concern for us right now.”
The goal right now is to learn how each burn scar reacts to rainfall rates to better understand the risk of flooding and debris flows, Winters said.
In total, the monsoon showers dropped between about 0.5 inches and about 2.5 inches of rain on parts of the Western Slope on Sunday and Monday. Precipitation is still in the forecast for parts of Colorado throughout the week until dry weather returns this weekend, according to weather forecasts.
Summer conditions across Colorado
This has been the first real showing of monsoonal weather patterns in an otherwise weak monsoon season so far this summer.
July was the sixth consecutive month with above-average temperatures. It was also the 29th driest on record statewide, and it was one of the top 10 driest Julys for some parts of Colorado, mainly the Western Slope, Four Corners area and northern Front Range, according to Allie Mazurek, an engagement climatologist at the Colorado Climate Center at Colorado State University.
August precipitation is key for much of southern and western Colorado, Mazurek said during the Water Conditions Monitoring Committee meeting Tuesday. It is typically the month when these areas receive the most precipitation on average.
The northeastern Plains and Colorado Springs area have benefitted the most from summer rainfall, she said.
Drought is worsening in the Western Slope. In October, the region was virtually drought free. Now, it is experiencing some level of drought, including one of the most severe drought categories, “exceptional drought,” according to the U.S. Drought Monitor.
The San Luis Valley in south-central Colorado is also in drought while most of the Eastern Plains have seen some drought relief, Mazurek said.
“Up until a few days ago, the monsoon really hasn’t been very active, and lots of these areas have missed out on precipitation,” she said. “Here’s hoping the next few days will bring some relief.”