May-be too late for snow

Overview of conditions 

Snow-covered area in May across the western United States was 9 percent above average, at 105,000 square kilometers (41,000 square miles) of snow cover, ranking twelfth in the 25-year satellite record (Table 1). Snow-covered area for May was 67,000 square kilometers (25,000 square miles) below 2011, the highest year, and 53,000 square kilometers (21,000 square miles) above 2015, the lowest May on record.   

Table 1. May 2025 Snow Cover in the Western United States (Relative to the 25-Year Satellite Record)  

Although snow cover was slightly above average in most western states during May (Figure 1), some states showed below average or zero snow cover. In the Southwest, snow cover was below average in New Mexico (56 percent of average) with no snow present in Arizona during May. South Dakota also had zero snow cover. Despite below average snow cover in the Southwest and Great Plains, close to or above average snow cover was observed in all other western states. Snow cover in Colorado was very close to average (99 percent of average) and above average in California (122 percent of average), Nevada (139 percent of average), Utah (107 percent of average), Wyoming (115 percent of average), and Montana (155 percent of average). As in April, the Pacific Northwest states of Oregon (159 percent of average), Washington (110 percent of average), and Idaho (140 percent of average) all showed above average snow cover. 

These geographic patterns were also apparent in major river basins, with zero observed snow cover in the Lower Colorado basin (covering most of Arizona) and 70 percent of average snow cover in the Rio Grande basin (covering southern Colorado and much of New Mexico). In April, the Pacific Northwest was the only hydrologic unit code 2 (HUC2) basin with above average snow cover. By contrast, in May, all HUC2 basins other than those in the southwest showed above average or close to average snow cover. Like in April, the highest above average snow cover was observed in the Pacific Northwest basin (140 percent of average), followed by the Missouri (134 percent of average), California (127 percent of average), and Great Basin (118 percent of average) basins. Snow cover was near average in the Upper Colorado (98 percent of average) and Arkansas-White-Red basins (96 percent of average).   

Snow-covered area in May was near average (Figure 2, upper left). Snow cover in the first two weeks of May was very close to the median of the 25-year satellite record, but storms in the third week brought snow-covered area up to near the seventh-fifth percentile. Snow-covered area then decreased while remaining above average until the last two days of the month when snow cover dipped below average. Large areas of Oregon, Washington, Idaho and Montana had 20 to 40 percent above-average snow cover (Figure 2, upper right). California, Utah, Colorado, and Wyoming had below-average snow cover at higher elevations with above-average snow cover observed at lower elevations, particularly in Wyoming. In Nevada, Arizona, and New Mexico, very little snow cover was observed, but conditions were close to average due to low snow cover in these states in May in the historical record.  

Snow cover duration measured in days summed from October 1 to May 31 continued to increase (Figure 2, lower left). Similar to April, snow cover duration during May remained near the twenty-fifth percentile. In Nevada, Utah, Arizona, and New Mexico, snow cover duration was up to 100 days below average in large areas (Figure 2, lower right). Most of Wyoming also had 20 to 40 fewer snow-covered days than average. The remaining western states showed a mix of below and above average snow-covered days, with Colorado in particular showing well below average conditions in the southern part of the state and well above average snow-covered days in the eastern plains.  

Average snow brightness, known as snow albedo, was close to average at the beginning of May, but steeply decreased during the first two weeks of the month, dropping below the twenty-fifth percentile by May 11th (Figure 3, left). Storms during the third week of May led to an increase in albedo, but albedo then declined for the remainder of the month with values below the twenty-fifth percentile during the final week of May. Albedo varied by state with most snow-covered areas falling near the average. However, below average albedo was observed in high elevation regions of Wyoming while albedo was generally above average in the mountainous regions of Colorado and Utah.  

Conditions in context: Snow water equivalent (SWE)  

Snow water equivalent (SWE) at the beginning of May was mixed with a range of average, below-average, and above-average SWE (Figure 4). Extremely low SWE was recorded across Utah, Colorado, New Mexico, and Arizona. Only 3 stations in New Mexico recorded SWE at the beginning of the month and zero did in Arizona. SWE was also below average in the Washington Cascades. Across the remaining western US states, SWE was mixed with both below and above average measurements recorded in California, Oregon, Nevada, Idaho, Montana, and Wyoming.  

SWE declined rapidly throughout May in all states. Because of above-average temperatures in early May, stations in Nevada, Colorado, Utah, and New Mexico saw record early melt out. By the end of the month, most stations in California and Oregon also recorded zero SWE. Some stations in Washington, Wyoming, and Colorado did record above-average SWE at the end of May, but most measurements were well below average.  

All stations in the West recorded SWE decreases in May, except for a small number in the Sangre de Cristo mountains of Colorado and New Mexico (Figure 5, left). Average change in SWE was also negative across all states, except those with low or zero SWE at the beginning of the month (Arizona, New Mexico, and South Dakota). Some stations in California and Idaho recorded SWE decreases of more than 100 centimeters (40 inches).