Mount Washington is known for having the worst weather in the world, so the week before Christmas, most people would expect the summit to be consumed by snowy conditions. But on Dec. 17-19, 2023, precipitation on the mountain fell as rain. The precipitation and snowmelt from this rain-on-snow (ROS) event caused a flood the severity of which is expected only once every 800 years in the Mount Washington Valley, according to the Mount Washington Observatory.

In the aftermath of this storm, weather observer Charlie Peachey recognized that it could serve as a prime example of the devastation ROS events can cause. As a staff member at the Mount Washington Observatory, he has been studying these weather phenomena for the past few years.

ROS events are instances when a measurable amount of rain falls onto an already existing snowpack. They tend to be most common in the late fall and early winter, and also in the early spring. This is because the weather is cold enough to maintain a layer of snow, but also still warm enough for rain to fall. ROS events like the one of December 2023 can cause major floods due to a combination of rain and melting snow.

“These are really impactful rain events or flooding events because no one can really predict the magnitude of them,” Peachey said.

The goal of Peachey’s research is to figure out if ROS events have increased in recent history, and if their seasonality, or when they occur during the year, is changing. This could help predict future flooding and allow residents of the Mount Washington Valley time to prepare.

The Mount Washington Observatory’s weather data set is unique. Their staff stay at the summit with someone there to collect hourly and six-hourly temperature and precipitation data year-round. Consistent human intervention with weather equipment means that the observatory’s data is high quality.

“When we’re going outside to take our observations … we’re also going outside to knock all the ice off of these anemometers (a device that measures wind speed and direction),” Peachey said.

Observatory staff also differentiate between types of precipitation while they’re out collecting other data, while previous studies on ROS events have used temperature as a proxy for precipitation type.

“If it’s below freezing, they’re going to consider it solid,” Peachey said. “If it’s above freezing, they’re going to consider it liquid. But there’s a whole lot of nuances in there.” The observatory’s real-time monitoring improves the accuracy of their data.

The observatory has been in its current location since 1980. Peachey used the data from 1980 to 2020 to create two distinct climatologies, or 30-year periods of average weather conditions (1981 to 2010, and 1991 to 2020). This is the standard in the field of climatology, said Peachey.

“This extensive temporal coverage allows for a comprehensive analysis of rain-on-snow events and facilitates the investigation of trends and patterns over time,” said Dr. Dae Il Jeong, a physical sciences specialist at Environment and Climate Change Canada.

To avoid the data being skewed by months with no likelihood of ROS events and to better study seasonality, Peachey focused on the “winter year” (October through May) instead of the calendar year (January through December). Peachey compared the winter year climatologies of 1981-2010 and 1991-2020 to identify any differences in the frequency and seasonality of ROS days.

ROS days increased 17% between 1981-2010 and 1991-2020, Peachey found. Even more telling is that ROS days in December increased 46% between the two climatologies. In the earlier climatology, ROS days peaked in April with a smaller peak in November. But in the later climatology, the smaller peak occurred in December instead. This increase in ROS events and the change in their seasonality is explained by the increasing temperatures at the summit of Mount Washington, and one of the months that’s warming the fastest on the mountain is December, according to a 2021 study published in Northeastern Naturalist.

The National Oceanic and Atmospheric Administration (NOAA) reports that since the beginning of the 19th century, New Hampshire’s temperature has increased more than 3 degrees Fahrenheit. They also project an increase in the frequency and severity of extreme precipitation events, along with an overall rise in the amount of precipitation in the state. Although Peachey doesn’t want to confirm any results until his study is peer-reviewed, he said, “In general, it most likely is a climate change-related issue, just causing things to become a little more variable and also … making the winter warmer at the beginning … and at the end.”

Ultimately, these results mean that the winter season in New Hampshire is shortening. As New Hampshire Public Radio has reported, the mountains have lost one to two weeks of snow since the 1970s. This has a huge impact on ski resorts, which are the backbone of the state’s tourism industry.

Peachey said that due to warming temperatures, “a lot of ski areas … can’t start building a snowpack until later in the year. … And most of their money honestly is made during the week of Christmas. So, if they’re able to get a lot of snow in by then they’ll probably pretty much be able to survive the year. If they aren’t, then there’s a lot of tourism hits that happen here in the North Country.”

Peachey and intern Laura Wilson’s case study on the December 2023 ROS event illustrates how devastating these storms can be to the ski industry.

Tuckerman Ravine, a popular area for those who want to test their backcountry skiing prowess, is located on the east side of Mount Washington and had a healthy snowpack of up to 83 inches on Dec. 15, 2023. When the Mount Washington Avalanche Center checked the area again on Dec. 21, it was virtually un-skiable due to exposed rocks and rushing water under the remaining snow. The summit’s snowpack also fared poorly, reduced from 5 inches to 1 inch by this ROS event. Unstable snow like this poses an avalanche risk to skiers and hikers alike. The 2021 Northeastern Naturalist study reported a downward trend of snow-covered days both on the summit of Mount Washington and in Pinkham Notch. This storm and others like it could contribute to a continuation of this trend.

Floods like the one of December 2023 don’t just wipe out precious powder, they also cause critical destruction to communities in the Mount Washington Valley.

Gov. Chris Sununu reported more than $4 million in damages in the state from the December 2023 storm. If temperatures continue to rise due to climate change, resulting in an increase in ROS events, the risk for major floods will also grow.

ROS events are difficult to forecast, Peachey said. To address this, he’s currently working on another project to pair with his existing work that he hopes will help “create a usable product for (estimations) here at the summit.” Jeong, the Environment and Climate Change Canada scientist, said that predictive models and informed management practices are “increasingly important in the context of climate change.” Peachey hopes that the data from his studies will help improve this technology. NOAA reports that climate change will continue to alter our winters, and research like that from the Mount Washington Observatory may help adaptation to its consequences.

New Hampshire Bulletin is part of States Newsroom, a nonprofit news network supported by grants and a coalition of donors as a 501c(3) public charity. New Hampshire Bulletin maintains editorial independence. Contact Editor Dana Wormald for questions: [email protected].