California’s mountain snow cover is vanishing a month early, in a worrying setup for fire season

Mountain snow has rapidly diminished in parts of the Western United States this spring, and that steep decline continues during this week’s intense heat.

Bryan Henry, a meteorologist with the National Interagency Fire Center in Boise, Idaho, said that, with climate change, it has become routine to see above-average melting rates during springtime. “But the magnitude we’re seeing right now is pretty startling,” he said.

In some parts of California, for example, snow cover could be completely gone more than a month earlier than average; it typically lingers into July.

Gradual runoff from melting snow replenishes soil and plant moisture into summer, even after the rainy season ends. But rapid, early snowmelt means a longer dry season, drier vegetation and larger fires in mid- and high-elevation forests. Warmer springs and summers, and the resulting shifts in snowmelt timing, have helped to drive marked increases in burned area in Western forests since the mid-1980s.

Scientists gauge the amount of water available in snowpack with a metric called snow water equivalent. Although April typically marks the beginning of snowmelt season, this year’s sharp snow water drop-off between mid-April and early May is striking and is due to both above-average temperatures and a lack of precipitation in many areas, including California, the Pacific Northwest, Nevada and southern Colorado.

It’s one of many signs that, unlike last year, this won’t be a quiet summer for Western wildfires.

“Some of these areas we’re talking about were incredibly dry entering the winter and are still very dry,” Henry said.

Henry is particularly troubled by the trends he is seeing in Northern California, Oregon and central Washington.

Concerns, again, for Northern California

Snow water equivalent percentage of normal across the West on April 15, left, and May 5, showing the rapid decline in snow cover in parts of the area. (USDA/NRCS)

Northern California seemed destined for a long 2020 fire season when it accrued huge rain and snowfall deficits during a record-dry February. But after a few lucky storms in March and April, snowpack in the Northern Sierra had rebounded, climbing back to 69 percent of normal by April 9.

However, the second half of April was warm and dry, and snowpack has plummeted to 19 percent of normal for this time of year, and will disappear completely by early June, several weeks ahead of schedule.

The melting will get a big boost from this week’s strong ridge of high pressure that stretches from the Baja to the Canadian Arctic, bringing intense heat to much of the West. “The hottest part of that ridge will be right over Northern California and the Cascades,” Henry said, “which is precisely where we don’t want it to be.”

While snow melt is one concern, other elements are coming together to set up a potentially worrisome and early summer fire season in Northern California, where drought is intensifying and soil moisture is extremely low in some areas.

“In general, fuels at all elevations will reach critically dry levels about a month earlier than usual, and up to two months earlier than in 2019,” reads the latest wildfire outlook for northern third of the state.

In addition, spring storms that temporarily boosted both snowpack and soil moisture came with a cost: more fuel to burn later in the season. “We originally were expecting a below-normal [fine fuel crop] due to inadequate soil moisture values, but the timely spring rains likely brought upon a more robust grass crop,” said Brent Wachter, a fire meteorologist with the National Interagency Fire Center’s predictive services division in Redding, Calif.

With mostly warm and dry weather predicted in May, those grasses will probably be dried out and able to spread fire in June.

There is a final possible curveball in the forecast: early-summer wind events.

“I think the jet stream will be active over the region during the month of June,” Wachter said. “So there will be some heightened wind periods for all elevations.” Those winds could lead to problematic fire weather east of the Sierras, in the greater Bay Area and within the Sacramento Valley, he said. Given recent heat and continued drying, there will be a real risk of large fires if these winds materialize.

In some good news, that potentially problematic jet stream could bring welcome showers next week as a trough dips into the West Coast. But both Henry and Wachter said rain from that system will most likely only buy some time and temporarily interrupt recent drying.

It won’t alter the trajectory toward an active fire season in June and beyond.