21-05-2025
- Climate
- San Francisco Chronicle
Snow is melting rapidly in parts of the West. Here's why
The western U.S. is experiencing a late-season snow drought, according to an update Wednesday by the National Oceanic and Atmospheric Administration's National Integrated Drought Information System. The diminished snowpack could result in water supply issues and increased wildfire risk in the coming months, the authors wrote.
Above-normal temperatures and a lack of precipitation in April and early May caused depletions even in basins where snow had piled up in prior months.
'(These basins) had near normal snowpack for that time of year, but then fell into what we call late- season snow drought due to unseasonably warm conditions and dry conditions,' said Jason Gerlich, regional drought information coordinator at NOAA's Integrated Drought Information System for the Pacific Northwest and Missouri River Basin.
The authors defined a location as being in 'snow drought' if the measured snow water equivalent — the amount of water contained in the snowpack — was below the 20th percentile of historical conditions.
Some automated monitoring sites in the western U.S., including in California and Colorado, experienced record-breaking drops in snow water equivalent in mid-April.
'Snowmelt this time of year is common, but such rapid melt rates are not normal,' the authors wrote.
Quickly melting snow can create floods; it can also lead to problems with the water supply in future months. 'A lot of the basins across the western United States are snow-dominated (and) get most of their flows from the melting of snow,' Gerlich said. Early melt means less water available throughout the summer.
California's major surface reservoirs are generally in good shape, with reservoir storage across the state at 116% of normal as of Wednesday.
'While the rate of the snowmelt has not had a major impact on forecasted April through July seasonal runoff, below average spring precipitation has lowered runoff projections slightly,' said Andy Reising, manager of the California Department of Water Resources' snow surveys and water supply forecasting unit, by email.
That said, there is variation between watersheds across the state. 'Despite a near-average snowpack statewide this season, the seasonal runoff projections for some watersheds are as low as 70 percent of average in some cases,' Reising said.
The statewide snowpack, an average of snow measurements across California, has been steadily declining since April, coming in at 38% of normal for this time of year, as of Wednesday.
The state could face a separate challenge associated with premature snowmelt: increased potential for wildfires. Typically, snow that gradually melts into the warmer months keeps landscapes moistened, dampening the chances for blazes to take off.
'One of our biggest concerns is that due to snow drought … the heightened risk for wildland fire development across the West,' Gerlich said.
National Interagency Fire Center outlooks for May and June call for above-normal wildfire potential in areas experiencing snow drought, including Arizona and New Mexico. Portions of California are also predicted to have increased fire potential during the summer months.
