Last month was the snowiest May in Aspen since 1999, with 20 more inches added to the already substantial snowpack. Meanwhile, forecasters are predicting a wet June.
Total snowfall for May was four times the average, according to city of Aspen water department figures. It follows the second snowiest March ever recorded — and the records go back to the winter of 1934-35. Only 6 inches fell in April, but with May’s snowfall, the water department has recorded 210 inches thus far, well above the winter average of 155 inches.
The water department also tallied 3.8 inches of rain for the month, which is double the average. One factor behind the heavy winter and wet spring can be found in the Pacific Ocean, said Erin Walter, a meteorologist with the National Weather Service in Grand Junction.
Winter and spring storms were fueled by weak El Niño conditions that shifted atmospheric rivers laden with moisture farther south than in an average year. (El Niño occurs when, among other conditions, sea-surface temperatures are warmer than average.)
Unceasing storm systems that usually blanket the Northwest, Alaska and Canada instead inundated California’s Sierra Nevada mountain range. The San Francisco Chronicle reported in April that Sierra Nevada snowpack is 162 percent of average, more than triple what the number was at the same time last year — and Colorado got the storms’ remnants, Walter said.
“We have had quite a few atmospheric rivers,” she said.
The weak El Niño “definitely influenced the track of storms and the general circulation of our low and high pressure systems,” Walter said. “It’s been a very abnormal winter and spring for us.”
And that may not change anytime soon. Walter said the federal Climate Prediction Center’s one-month outlook for western Colorado, as of May 31, “falls within a 40 percent probability of being above average for precipitation.” The center also is predicting average temperatures for the region.
While the wet, cool spring has meant little snowmelt and allowed for continued skiing, the Colorado Water Conservation Board and local emergency managers are keeping a close watch on river levels. The state board on May 30 said nearly every river basin in Colorado is over 350 percent of average, and that only 35 percent of the measured snowpack in the Colorado River basin had melted by the end of May.
“Widespread, regional snowmelt flooding is not common for Colorado,” the board’s May executive summary says. “However, isolated flooding and extended periods of high water are likely to occur this season.”
CWCB also cited a forecast for June that indicates a “wet month for the entire state,” and adds that areas downstream of recent burn scars, like those on Basalt Mountain and surrounding environs from the Lake Christine Fire, are at heightened susceptibility to flash floods, and mud and debris flows. The board reminded “individuals and business owners [to] consider, be aware of, prepare for, and insure against flood threats.”
“It is also important to note that Colorado’s worst flood events have historically occurred from general spring rainfall and summer thunderstorms, which are completely unrelated to snowmelt flooding resulting from mountain snowpack,” the summary says. “For this reason, even residents in areas with lower snowpack should exercise caution in evaluating flood risk.”
Floods directly related to the melting snowpack are possible but unlikely, and for boaters, “an extended season of high water is a near certainty this year,” the board reported.