Weather scientists recommend new criteria for red flag fire warnings after Marshall fire

Weather forecasters did not issue a red flag warning for wildfire danger ahead of the deadly, wind-driven Marshall fire in 2021 because humidity levels were not low enough to trigger such an alert.

Now, though, National Oceanic and Atmospheric Administration scientists working in Boulder recommend relaxing the humidity threshold and changing the standards for a red flag warning to account for the overall potential rate of a wildfire’s spread.

They also recommend using the Federal Emergency Management Agency’s alert system to warn people in fire zones of the risks.

Those recommendations were included in a new report and published last month in the journal Weather and Forecasting, which is a publication of the American Meteorological Association. The team presented its findings Tuesday to other scientists, emergency management specialists and the media.

The windstorm that drove flames across part of Boulder County on Dec. 30, 2021, was unique, the researchers said, with hurricane-force winds blowing nonstop for 11 hours. That strong, sustained wind was the primary reason the fire spread the way it did, said Stan Benjamin, the report’s lead author and a scientist at the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Studies.

“Even without the fire, the wind that day would have been a major story,” said Eric James, a scientist with NOAA’s Global Systems Laboratory in Boulder and one of the report’s authors.

However, strong wind days such as Dec. 30, 2021, are becoming less frequent and less severe in Boulder County, according to historical weather data.

Since 1996, records of wind gusts are more accurate, so the scientists studied that data to find out how many days wind speeds topped 70 knots, or 80.5 mph, in Boulder and Larimer counties and northern Jefferson counties.

The data shows that there now are two fewer days per year where wind speeds exceed 70 knots than in 1996, said Paul Schlatter, science and operations officer for the National Weather Service in Boulder. As for days when winds exceed 90 knots, or 103.5 mph, they now only happen once every five years as compared to the first 10 years of data, when 100-mph winds occurred an average of two days per year.

The scientists are working on an explanation as to why there are fewer strong wind days.

An investigation by the Boulder County sheriff’s and district attorney’s offices concluded the Marshall fire had two origin points: embers from a week-old fire on property owned by the Twelve Tribes religious cult and an Xcel Energy power line that became unmoored and caused electrical arcing.

Those locations were about 2,000 feet apart, near the intersection of Colorado 72 and Colorado 93, which is one of the windiest points in Boulder County, Schlatter said.

“We knew that was going to be bad.” he said. “Don’t ever start a fire there — and that’s what we got. We got two fires there.”

National Weather Service meteorologists issued a high wind warning at 3 a.m. on the day of the fire and began posting about it on social media at 4 a.m. About 400 other agencies are notified when the weather service has a severe weather watch, he said.

In Boulder County, high wind notices are sent when winds are forecast at 75 mph or higher. In most parts of the United States, those warnings are issued when winds exceed 58 mph.

“It’s a windy place here and we would be putting out high wind warnings all the time if we had a lower threshold,” Schlatter said.

But the humidity that day did not drop below 20%, meaning it was never low enough to trigger the weather service to issue a red flag warning. Such warnings are issued when there is an increased risk of fire danger because of low humidity, high temperatures and strong winds.

The scientists recommended changing the humidity standard, although they did not offer a specific recommendation for a new threshold or whether it should be eliminated entirely.

The red flag guidelines were written decades ago and were intended to advise fire departments, forestry workers and others whose jobs centered around fire prevention. But over the years it has become a public tool, Schlatter said.

The Marshall fire is Colorado’s most destructive, destroying more than $2 billion worth of property in Superior, Louisville and unincorporated Boulder County. Two people died in the fire, and on Tuesday, scientists said that low number was amazing considering how fast the flames spread and how chaotic public evacuation notices were.