As the planet warms, scientists expect that mountain snowpack should melt progressively earlier in the year. However, observations in the U.S. show that as temperatures have risen, snowpack melt is relatively unaffected in some regions while others can experience snowpack melt a month earlier in the year.
The winter storms that dumped heavy snow and rain across California early in 2021 are likely not enough to negate what will be a critically dry year, state water officials believe.
California’s Department of Water Resources on Tuesday recorded a snow depth of 56 inches and water content of 21 inches at Phillips Station in the Sierra Nevada. The water content of the overall snowpack was 61% of the average for March 2 and 54% of the average for April 1, when it is historically at its maximum.
The Department of Water Resources today conducted the third manual snow survey of the season at Phillips Station. The manual survey recorded 56 inches of snow depth and a snow water equivalent (SWE) of 21 inches, which is 86% of average for this location. The SWE measures the amount of water contained in the snowpack and is a key component of DWR’s water supply forecast.
“As California closes out the fifth consecutive dry month of our water year, absent a series of strong storms in March or April we are going to end with a critically dry year on the heels of last year’s dry conditions,” said DWR Director Karla Nemeth. “With back-to-back dry years, water efficiency and drought preparedness are more important than ever for communities, agriculture and the environment.”
As the planet warms, scientists expect that mountain snowpack should melt progressively earlier in the year. However, observations in the U.S. show that as temperatures have risen, snowpack melt is relatively unaffected in some regions while others can experience snowpack melt a month earlier in the year.
This discrepancy in the timing of snowpack disappearance—the date in the spring when all the winter snow has melted—is the focus of new research by scientists at Scripps Institution of Oceanography at the University of California San Diego.
In a new study published March 1 in the journal Nature Climate Change, Scripps Oceanography climate scientists Amato Evan and Ian Eisenman identify regional variations in snowpack melt as temperatures increase, and they present a theory that explains which mountain snowpacks worldwide are most “at-risk” from climate change. The study was funded by NOAA’s Climate Program Office.
Looking at nearly four decades of observations in the Western U.S., the researchers found that as temperatures rise, the timing of snowpack disappearance is changing most rapidly in coastal regions and the south, with smaller changes in the northern interior of the country. This means that snowpack in the Sierra Nevada, the Cascades, and the mountains of southern Arizona is much more vulnerable to rising temperatures than snowpack found in places like the Rockies or the mountains of Utah.
The scientists used these historical observations to create a new model for understanding why the timing of snowpack disappearance differs widely across mountain regions. They theorize that changes in the amount of time that snow can accumulate and the amount of time the surface is covered with snow during the year are the critical reasons why some regions are more vulnerable to snowpack melt than others.
Using a new model, the Scripps researchers theorize that snowpack in coastal regions, the Arctic, and the Western U.S. may be among the most at-risk for premature melt from rising temperatures. Graphic: Courtesy Scripps Institution of Oceanography
“Global warming isn’t affecting everywhere the same. As you get closer to the ocean or further south in the U.S., the snowpack is more vulnerable, or more at-risk, due to increasing temperature, whereas in the interior of the continent, the snowpack seems much more impervious, or resilient to rising temperatures,” said Evan, lead author of the study. “Our theory tells us why that’s happening, and it’s basically showing that spring is coming a lot earlier in the year if you’re in Oregon, California, Washington, and down south, but not if you’re in Colorado or Utah.”
Applying this theory globally, the researchers found that increasing temperatures would affect the timing of snowpack melt most prominently in the Arctic, the Alps of Europe, and the southern region of South America, with much smaller changes in the northern interiors of Europe and Asia, including the central region of Russia.
To devise the model that led to these findings, Evan and Eisenman analyzed daily snowpack measurements from nearly 400 sites across the Western U.S managed by the Natural Resources Conservation Service Snowpack Telemetry (SNOTEL) network. They looked at SNOTEL data each year from 1982 to 2018 and focused on changes in the date of snowpack disappearance in the spring. They also examined data from the North American Regional Reanalysis (NARR) showing the daily mean surface air temperature and precipitation over the same years for each of these stations.
Using an approach based on physics and mathematics, the model simulates the timing of snowpack accumulation and snowpack melting as a function of temperature. The scientists could then use the model to solve for the key factor that was causing the differences in snowpack warming: time. Specifically, they looked at the amount of time snow can accumulate and the amount of time the surface is covered with snow.
“I was excited by the simplicity of the explanation that we ultimately arrived at,” said Eisenman. “Our theoretical model provides a mechanism to explain why the observed snowmelt dates change so much more at some locations than at others, and it also predicts how snowmelt dates will change in the future under further warming.”
The model shows that regions with very large swings in temperature between the winter and summer are less susceptible to warming than those where the change in temperature from winter to summer is smaller. The model also shows that regions where the annual mean temperature is closest to 0˚C are less susceptible to early melt. The most susceptible regions are ones where the differences between wintertime and summertime temperatures are small, and where the average temperature is either far above, or even far below 0˚C.
For example, in an interior mountain region of the U.S. like the Colorado Rockies, where the temperature dips below 0°C for about half the year, an increase of 1°C can lead to a quicker melt by a couple of days—not a huge difference.
However, in a coastal region like the Pacific Northwest, the influence of the ocean and thermal regulation helps keep the winter temperatures a bit warmer, meaning there are fewer days below 0°C in which snow can accumulate. The researchers hypothesize that in the region’s Cascade Mountains, a 1°C increase in temperature could result in the snow melting about a month earlier in the season—a dramatic difference.
One of the most “at-risk” regions is the Arctic, where snow accumulates for nine months each year and takes about three months to melt. The model suggests that 1°C warming there would result in a faster melt by about a week—a significant period of time for one of the fastest warming places on Earth.
This study builds upon previous work done by Scripps scientists since the mid-1990s to map out changes in snowmelt timing and snowpacks across the Western U.S. The authors said that a “shrinking” winter—one that is shorter, warmer, and with less overall precipitation—has adverse societal effects because it contributes to a longer fire season. This could have devastating impacts on already fire-prone regions. In California, faster snowpack melt rates have already made forest management more difficult and provided prime conditions for invasive species like the bark beetle to thrive.
Funding for this work was provided by a NOAA/CPO grant to the University of California.
Sierra snowpack is so vital to California as it provides one third of the state’s water supply and it seems more and more lately we are seeing this dwindle. You can see from 2002 to 2011 60% of the time the Sierra snowpack was 100% or better, a pretty good trend.
How is California’s snowpack measured? Why is it important? And how is our snowpack stacking up this winter, so far? NBC 7 meteorologist Crystal Egger breaks it all down.
Precipitation is below average in California for the current water year. Despite recent storms that increased the statewide Sierra Nevada snowpack to 70% of average to date, the state is experiencing its second consecutive below average year for rain and snow.
Recent storms have boosted California’s vital Sierra Nevada snowpack but not enough to fully compensate for a dry start to winter and residents should use water wisely, a state official said Wednesday after the season’s latest measurements.
Precipitation is below average in California for the current water year. Despite recent storms that increased the statewide Sierra Nevada snowpack to 70% of average to date, the state is experiencing its second consecutive below average year for rain and snow. The water year starts on October 1 and ends September 30.
The Department of Water Resources Feb. 3 conducted the second manual snow survey of the season at Phillips Station. The manual survey recorded 63 inches of snow depth and a snow water equivalent, or SWE, of 17 inches, which is 93% of average for that location, according to the DWR. The SWE measures the amount of water contained in the snowpack and is a key component of DWR’s water supply forecast.
“The recent blast of winter weather was a welcome sight, but it was not enough to offset this winter’s dry start,” said DWR Director Karla Nemeth. “While there is still a chance we will see additional storms in the coming weeks, the Department and other state agencies are preparing for the potential for a second consecutive year of dry conditions.”
Below normal precipitation is favored throughout most of the southern tier of the United States in late February to early March, according to the National Weather Service Climate Prediction Center. La Niña conditions remain over the Pacific Ocean.
Statewide snow survey measurements reflect those dry conditions. Measurements from DWR’s electronic snow survey stations indicate that statewide the snowpack’s SWE is 12.5 inches, or 70% of the February 3 average, and 45% of the April 1 average. April 1 is typically when California’s snowpack is the deepest and has the highest SWE.
“The recent atmospheric storms have brought rain and snow to the northern Sierra Nevada, but conditions are still well below normal,” said Goldy Herbon, San Diego County Water Authority senior resources specialist.
Following last year’s below average water year, northern California is now experiencing its second straight water year of below average precipitation.
How it started.
How it's going.Season-to-date snowfall through January 23 vs February 4.#CAwx #CAsnow ❄️☃️🏂🌨️ pic.twitter.com/UOvjEmxa5k
— NWS Sacramento (@NWSSacramento) February 4, 2021
Another source of the state’s water supply, the Colorado River Basin, isn’t faring much better. The water year 2021 precipitation at the end of January for the Colorado River Basin is 66% of normal, according to the U.S. Bureau of Reclamation.
Precipitation totals to date are behind historical averages for two of the main sources of San Diego County’s supply, with two months to go in the winter season but, a couple of atmospheric rivers can change that quickly.
“The San Diego region will have a plentiful and reliable source of water due to actions taken by the Water Authority and its 24 member agencies to diversify the water supply,” said Herbon. “Continuing efforts to expand supply sources, including desalination, water reuse, and recycling, will ensure that the water needs are met for the region’s 3.3 million people and its $245 billion economy.”
Fall 2020 was extremely dry, especially in the Sierra Nevada, and follows last year’s below-average snow and precipitation, according to the DWR. With only a couple months remaining in California’s traditional wet season, DWR officials say Californians should look at ways to reduce water use at home.
Statewide Sierra Nevada snowpack is 71% of average for February 4, 2021. Graphic: California Department of Water Resources
California’s water picture is heading in two different directions.
A major storm last week and a more modest system Tuesday continued to boost the Sierra Nevada snowpack, the source of one-third of the state’s water supply, in promising ways. But the Bay Area and most cities across Northern California remain stuck in one of the worst two-year rainfall deficits seen since the 1849 Gold Rush, increasing the risk of water restrictions and dry wildfire conditions locally next summer.
The Water News Network is San Diego County’s source for water news and information.
