Snow happens: June storms aren’t uncommon in Colorado, Aspen — for now
Mountain Town News
If the storm that dropped 3 to 6 inches of snow in mountain towns from Steamboat Springs to Crested Butte on June 8 was unexceptional, it provokes a question about the shifting climate.
How can snowstorms occur in June when temperatures in Colorado have been rising significantly in recent decades?
The short answer is that weather remains variable. The climate — the accumulation of weather over longer periods — has been warming, but not so much as to drown out the noise of short-term variability. On any given day, short-term variability will trump broader trends.
June snowstorms are part of that short-term variability.
Records taken at Aspen, Breckenridge and Climax — the mine between Leadville and Copper Mountain — show frequent snow during June for the past 70 to 90 years. In Summit County, this week’s snow was good enough to cause skiers to flock to Loveland Pass for a powder party; the largest June snowfall was 16 inches in 1984.
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June snow is not weird, but in coming decades, it may be.
“I think there’s a strong likelihood we will be measuring some decline in late season snowfalls in the next 30 years,” said Peter Goble, a climatologist with the Colorado Climate Center.
Temperatures have been rising across Colorado for the past 30 years, an average 2 degrees, but more so in some areas — western Colorado and particularly in southwestern Colorado — than others.
It can still get cold — and record-breaking cold at that. But for every new record low temperature in Colorado, there are three record high temperature set, Goble said.
This spring was symptomatic of the gradual shift. The winter produced an “average+ snowpack,” as Eric Kuhn, the former general manager of the Colorado River Water Conservation District, described it in a mid-April tweet. By then, the predicted runoff of the Colorado River into Lake Powell for April-June had declined to 75% of average.
Since then, the spring splish-splash into Powell, the second-biggest reservoir in the Colorado River Basin, has diminished to 57% of average. On the main stem of the Colorado River — including the Roaring Fork, Eagle and Blue rivers — runoff was forecast to be near to slightly below normal. Looking back to mid-winter, there were higher expectations.
Who purloined the precipitation? Likely it was a result of above-average temperatures.
Nearly all of Colorado and Utah had temperatures 2 to 4 degrees above normal, and some places of western and southern Colorado had temperatures as many as 6 degrees above normal, according to a report issued Wednesday by the Western Water Assessment.
Colorado and Utah had average high temperatures in May that ranked among the top 10 highest for the month since 1895.
This is part of a well-defined warming trend in Aspen, Vail and Steamboat Springs, but also Summit County and the Colorado River headwaters in Grand County. The shift is documented on a website sponsored by the Aspen Center for Environmental Studies (ACES). It’s called the Forest Health Index, which has temperature, precipitation and other data for those river basins in Colorado that are at least one-third treed. That’s most of them.
In the Roaring Fork River Basin, for example, the average temperature has bobbed up and down year by year since 1980, but there’s been a general rise.
Less clear are the trends in the average peak streamflow. Precipitation also has giant ups and downs without a remarkable trend. The profiles of the Eagle, Yampa and Blue rivers look similar.
The basins also have charts for frost-free days, which vary as well. In 1980, Aspen had 64, while last year it was 157. Slice a year off each end of that band, comparing 1981 with 2018, and the spread is less impressive, an increase of 29 days.
But no matter how one looks at the numbers, there’s a clear trend toward longer summers, more growing days in Aspen. The same holds true in Summit County, where gardeners can grow more vegetables, but still not as many as (relatively) balmy Aspen.
But from 1980 to 2018 it had grown from 64 to 157. Lop off one year of each end of that, and the frost-free days have increased from 90 to 119 — still about a month longer.
Adam McCurdy, director of forest and climate for ACES, said the numbers come from a combination of satellite, radar and station data to reflect the general state of the river basin. The Roaring Fork data, for example, do not reflect precisely the temperatures and precipitation in downtown Aspen. They’re a more general look at Aspen, Basalt and Carbondale. The same would hold true for the upper Colorado River, which includes Winter Park and Kremmling, almost 50 miles apart.
Taking stock of the Colorado River Basin more broadly, scientists have been producing studies that detect a growing role of warming temperatures in the decreased river flows.
Jonathan Overpeck and Bradley Udall several years ago issued a study that found roughly half of the decreased flows in the Colorado in the 21st century were due to higher temperatures. The water was being taken up by increased evaporation but also transpiration by plants.
In a paper published in May in the Proceedings of the National Academy of Sciences, the two climate scientists — Overpeck from the University of Michigan and Udall from Colorado State University — dissect what is going on.
“It makes sense that longer growing seasons enabled by warming temperatures mean more total evapotranspiration, drier soils and reduced river flows,” they say.
What about increased precipitation? After all, a warming atmosphere can hold more precipitation, about 7% per 1.8-degree increase Celsius.
Outside the American Southwest, some areas have been getting more rain and snow. That unevenness holds true even within Colorado. The Durango area has been getting distinctly drier. Precipitation in the Denver-Greeley area, in contrast hasn’t changed all that much.
Overpeck and Udall would have us think of the Dust Bowl, a time during the 1930s on the Great Plains of both hot temperatures and drought. Recent “flash droughts” on the High Plains in 2012 and 2017 highlight how extreme spring and summer temperatures can speed the onset, and worsen the impact, of dry spells and droughts.
After a huge winter last year, Gunnison County this year found itself in “exceptional drought” in May, while the Aspen area has been in moderate drought, McCury noted.
For water managers in the Roaring Fork Basin as well as those worried about wildfires, there’s worry that the drought will intensify, despite Tuesday’s snow.
Allen Best produces the free e-magazine called Big Pivots, which originally published this story. To subscribe, send an email to firstname.lastname@example.org.
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