Xcel Energy shut off power to Boulder twice in a single week as extreme winds collided with unusually dry conditions, leaving tens of thousands of people without electricity, in some cases for days.
The scenes were familiar: downed trees, destroyed fences, darkened neighborhoods. At the National Center for Atmospheric Research’s Mesa Lab, peak wind gusts reached 113 mph on Dec. 19, with humidity dropping below 10%, a combination that sharply elevates wildfire risk.
High winds have long been part of life in Boulder. Being perched at the foothills of the Rockies means the city is prone to powerful downslope windstorms, especially from late fall to early spring. What has changed is the backdrop against which those winds now arrive.
Data from the National Oceanic and Atmospheric Administration show that between 1969 and 2022, Boulder experienced about 260 wind events with gusts exceeding 70 mph. Some were extreme. In January 1969, winds reached 130 mph, tearing roofs from more than two dozen homes across the city. A February 1997 windstorm produced gusts of 103 mph and caused widespread damage. On Dec. 30, 2021, the day of the Marshall Fire, winds reached 115 mph.
Those winds alone did not make the Marshall Fire catastrophic. The problem was what the wind encountered. “The fact that we’re seeing [these downslope windstorms] is by no means out of the ordinary,” said Andrew Winters, an assistant professor of atmospheric sciences at CU Boulder.
But in past decades, winter windstorms were more likely to arrive over snow-covered ground, with vegetation damp and less receptive to fire. A changing climate is eroding that buffer. Warmer temperatures are delaying snowfall, drying out grasses and shrubs, and extending the window for dangerous fire weather deeper into winter.
“When these [wind] events do occur, they’re more likely to occur in a future climate with bare ground that doesn’t have snow on it, with dry fuels that are vulnerable to fire starts,” Winters said.
The broader winter pattern has unsettled even researchers who study climate extremes for a living.
“This December has been ridiculous,” climate scientist Daniel Swain with the University of California Agriculture and Natural Resources, who studies extreme weather events, told Boulder Reporting Lab. He was in South Boulder during Xcel’s shutoffs.
Boulder recorded its latest first snowfall this year, weeks later than normal. Nights have been warmer, and daytime highs have repeatedly reached into the 60s.
“I’ve been getting up early in the morning, but it’s like light jacket weather in a mile-high city in December,” Swain said. “Day after day after day. That’s not normal.”
Windy and dry conditions are a particularly dangerous pairing. Strong gusts accelerate fire spread, while dry vegetation provides abundant fuel, turning small ignitions into fast-moving fires that are difficult or impossible to control. During the Dec. 19 wind event, several small fires sparked in Boulder County and were quickly contained, including a fence fire on Euclid Avenue that was reported by a resident before it could spread to nearby homes. Boulder Fire-Rescue credited fast response and quick calls from the community with preventing a potentially serious fire.
To reduce that risk, utilities including Xcel, which has faced lawsuits alleging its equipment sparked wildfires, have begun using public safety power shutoffs, cutting electricity preemptively during periods of extreme fire danger to prevent sparks. Investigators traced one ignition of the Marshall Fire to near the Marshall Mesa Trailhead, where winds allegedly knocked an Xcel power line loose from its insulator, releasing sparks and igniting dry grass.
Swain said he believes that Xcel’s decision to cut power last week was likely the correct one. “I’m glad [powerlines] weren’t energized when they fell into the house, next to the wood fence, with the wood mulch pile, when winds were gusting to 95 miles an hour.”
The 2023 National Climate Assessment projects that power shutoffs will become more common as climate change increases the number of days with extreme fire danger, particularly in the fall and shoulder seasons.
“The snow historically began here, usually in October,” Swain said. “So you’d have several months of snow before the winds came, and except on very rare occasions, you wouldn’t have crazy fire risk.”
This year, Boulder’s first snowfall did not arrive until Nov. 29, breaking the previous record of Nov. 17.
While last week’s weather resembled the Marshall Fire in terms of wind and low humidity, Swain said one key difference is duration. In late 2021, dry conditions had persisted longer, allowing fuels to become even more volatile. This year’s dry spell has been shorter, he said, but each additional warm, snow-free day increases the risk. “We’re getting closer with every passing day.”
Longer term, Boulder’s winds themselves may change. Early research from CU Boulder and NCAR scientists suggests that wind speeds may be decreasing slightly, with gusts more often topping out in the 90 mph range rather than exceeding 100 mph. Winters, who worked on that research, said the trend could reflect a variety of factors, including changes in measurement equipment, increased tree cover and shifts in atmospheric structure linked to climate change.
Even so, Winters said that even if Boulder sees a modest reduction in peak wind speeds, the wildfire risk remains the same.
“A windstorm with 90 miles an hour is still going to have tremendous fire weather concerns, just like a 100-mile-an-hour windstorm,” Winters said. “It doesn’t change the threat, but maybe it changes the top-end wind speeds.”
Swain is particularly concerned about the added fire risk during prolonged outages. Not just because of the weather conditions, but because residents may turn to candles, fireplaces and lithium-ion batteries, which can catch fire or explode. As of Sunday, there had been only a few small ignitions in Boulder County, with no spread reported.
“Hopefully everyone’s careful,” Swain said. “Hopefully we get lucky again.”