Dangerous, disruptive rain-on-snow events are expected to increase in northern Alaska

A rapidly warming North will see more instances of rain falling on snow — a potentially dangerous combination.

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Caribou migrating in Gates of the Arctic National Park, April 2012. Caribou food supplies are especially vulnerable to rain-on-snow events. (Zak Richter / NPS)

Winter rains falling atop snow in the far North are disruptive and dangerous for people and sometimes deadly for wildlife. In January, a deluge of midwinter rain destabilized a rock cliff and loosening a nearly fatal rock slide on the highway leading south of Anchorage. Rainfall damaged trail conditions enough to force cancellation of two important Alaska sled-dog races. And while the crust of ice created by rain on snow is treacherous for human travel, it can kill animals like caribou, which have evolved to dig through soft snow to find the tundra plants that are their food.

Expect more such winter rains in the future in northern Alaska, scientists warn.

A University of Alaska Fairbanks-led study projects that as the climate continues to warm, rain-on-snow events they will become far more common in northern Alaska. The biggest increases will be on the North Slope, where rain-on-snow events will become five to 20 times more frequent than they are now, according to the study, published in the Journal of Applied Meteorology and Climatology.

The prediction of more rain on snow correlates with an expected increase in precipitation in general.

“Everywhere, it’s going to be more rainy,” said Peter Bieniek of UAF’s International Arctic Research Center, the lead author of the study. “Down the road, it’ll get more impactful as you go north because it’ll just be even warmer.”

Alaska has already gotten wetter. Records from 18 weather stations around the state show an average 17 percent increase in precipitation from 1949 to 2016, according to a study led by Gerd Wendler of UAF’s Alaska Climate Research Center.

Up to now, records of Alaska rain-on-snow events have been spotty. In Canada, there are some records — there is an inventory of more than 600 rain-on-snow events since 1985 in the Canadian Arctic Archipelago, for example. In Alaska, the records of past weather conditions tend to note temperature and precipitation but not the type of precipitation.  That means the past must be reconstructed from several sources.

“You have to go back to the hourly observations to try to figure out when it was snowing and when it was raining. It’s doable, but it’s a very laborious process,” said climatologist Rick Thoman of the Alaska Center for Climate Assessment and Policy, a co-author of the study projecting future rain-on-snow events.

What Bieniek, Thoman and the other authors found was that there was a trend toward increased rain-on-snow events in Alaska from 1979 to 2013, but it was weak and not uniform. Rain-on-snow events increased in frequency in northern parts of Alaska over the period, but they declined in frequency in southwestern Alaska. However, there are signs that extreme events are increasing in recent years; in Nome, for example, seven of the most severe rain-on-snow episodes recorded between 1979 and 2013 occured in in the last decade of that study period.

As for the future, the expected increase in rain-on-snow events is tied to the expected increase in precipitation.

A warmer Arctic means wetter Arctic because a warmer atmosphere holds more water vapor. In the Arctic, that future precipitation is more likely to come in the form of rain than snow, several scientists say. One 2017 study, by scientists at the Royal Netherlands Meteorological Institute, projects total Arctic precipitation about Latitude 70 will increase by 50 percent to 60 percent by the end of the 21st century, with almost all of that coming in the form of rain. The title of their study, published in the journal Nature Climate Change, summarizes the future: “Towards a rain-dominated Arctic.”

Rain that falls on snow is notorious for the conditions it creates. In rural areas, rain atop snow makes travel over stretches of tundra dangerous. In urban areas, the roads become iced-over and treacherous for motorists. If rain comes early in the winter in a place like Fairbanks, it can form a long-lasting hazard, Thoman said. “If it rains at the beginning of the season, you’ve got ice on the roads for months,” he said. “It’s the gift that keeps on giving.”

The most serious rain-related event this winter in Alaska, so far, was the near-fatal rock slide linked to a midwinter warm-up. Torrents of rain and high winds destabilized the rocky cliff along the Seward Highway, the sole road south out of town. In the early morning darkness of Jan. 25, a mass of rocks equivalent to a dump truck load crashed down onto the road and onto a vehicle driven by Jason Carter of Anchorage. He was critically injured and hospitalized with brain trauma.

Such slides have become recurring problems along the highway, said Shannon McCarthy, a spokeswoman for the Alaska Department of Transportation and Public Facilities.

“Basically, this is the kind of weather where we see rock slides coming down,” said Shannon McCarthy,

Alaska’s transportation department was already planning to make safety improvements at high-risk sites along the highway because of concern about the crumbling rock cliffs, McCarthy said. A magnitude 7 earthquake that struck Anchorage loosened slides in some of those places, but even before then, the state has won federal support for the projects, which will include rock removal, mesh draping and culvert construction, she said.

For wildlife, rain falling atop snow can be deadly. “Anything that needs to dig through the snow to get food, if there’s an ice layer, there’s impacts,” Bieniek said.

Caribou and reindeer are particularly vulnerable. The annual Arctic Report Card released in December cites freezing rain as one of the factors behind the last two decades’ 56 percent decline in global caribou and reindeer populations.

In one well-known Alaska case, rain falling on snow in the winter of 2003-04 caused the Teshekpuk Lake caribou herd of the central North Slope to move to better winter browsing 200 miles east in the Arctic National Wildlife Refuge. About third of collared animals died that winter, making it the herd’s the biggest winter mortality on record, according to the Bieniek-led study.

In Russia, there have been mass die-offs of Yamal reindeer, with tens of thousands lost to starvation that was tied to separate rain-on-snow events in 2006 and 2013. Those winter rains, in turn, have been linked to reduced Arctic sea ice.

One of the most dramatic rain-on-snow cases occurred in late 2003, when about 20,000 muskox died on Canada’s Banks Island. Inuit hunters found surviving animals months later, wandering and disoriented, according to reports. The death toll was about a quarter of the island’s muskox population.

Even mild rain-on-snow events can cause far-reaching damages to muskox populations, one scientist has found. Research by Joel Berger of Colorado State University found that among muskox populations in western Alaska and on Russia’s Wrangel Island, difficulties encountered by pregnant females were manifested in their offspring, which three years after rain events had smaller-than-normal skulls and skeletons. Results are described in a study published a year ago in the journal Scientific Reports.

Rain does not have to be severe to make browsing conditions poor, said Berger, who has extensively studies muskox and described those studies in a 2018 book. “Muskox really like it cold and clear,” he said.

For those who hate the icy conditions created when rain falls on snow, the projections hold potentially good news for the southern parts of Alaska.

Rain-on-snow events will become less frequent in Anchorage, the state’s largest city, in the southwestern hub of Bethel and in the capital city of Juneau on the state’s southeast panhandle. The change will be most dramatic in Juneau, where rain-on-snow frequency will is expected to be cut by more than 80 percent from what is considered the current norm.

The reason for the decrease in events? Less snow.

“Increasingly, what happens will be it’ll be just rain,” Thoman said.