Rain-On-Snow: A Closer Look at December’s Unprecedented Flooding
By Charlie Peachey

The December 18-19 storm that produced unprecedented flooding across most of New England will be remembered by most as one of the most impactful storms in recorded history. Most stream gauges along rivers in and around the White Mountains measured their highest or second-highest flood totals. The last time that flooding of this magnitude was observed was during Hurricane Irene. Even days after the rain stopped, many rivers in the area were still considered to be at a moderate flood stage. Significant damage was also done to local infrastructure like roads and bridges, resulting in the National Guard assisting with local rescues of people trapped by the floods.

Ariel view of the flooding that occurred on 12/18 at Plymouth State University in Plymouth, NH

Aside from the impacts on the public, the economic toll the storm took on many local communities was also significant. Specifically, many ski resorts across New Hampshire, such as Loon Mountain, Attitash, Cranmore, and Wildcat, were forced to close due to all the snow melt and flooding. So, towns relying on the revenue provided by ski tourism in the winter are likely to see some of the worst economic repercussions from this storm. Especially because the week of Christmas tends to be the busiest time of the ski season in New England, and many ski resorts won’t be able to open fully again by then. This was also true for several ski resorts in Maine. Sunday River debatably experienced the worst impacts out of any ski area in New England, with the primary road leading to the mountain getting washed out. So, a significant effort by the ski resort and town was required to open the road for skiers again.

The washed-out road leading to Sunday River after the storm on 12/18

Six inches of rainfall in under twenty-four hours will often be enough to produce floods for most areas of New Hampshire, but the additional influence of a melting snowpack only exacerbates the issue. This was the case for areas around the White Mountains. Specifically, Jackson, NH, made the national news for the videos of the torrential flooding at Jackson Falls. Runoff from the melting snowpack on the summit of Mount Washington combined with over six inches of rainfall produced extreme flooding at the base. It caused major impacts to areas along the Wildcat and Ellis Rivers, including most of Jackson’s town center and main street.

Washed out bridge on Lancaster Road in Gorham during the storm on 12/18

Despite the havoc caused by this major flooding event, there is a lack of research on storms that produce these types of widespread hydrologic impacts in and around the White Mountains. However, several published studies have investigated these types of storms concerning the mountains in the Pacific Northwest. They have even been able to define these types of storms roughly as rain-on-snow events.

So, what is defined as a rain-on-snow event, and why has it only been studied out west? Unfortunately, there is no widely accepted definition of a rain-on-snow, but it can generally be defined as an event where liquid precipitation (rain) falls on a snowpack of a specific volume. Because rain-on-snow events are often associated with floods out west, the definition of a rain-on-snow event often relates to the total rainfall accumulation and snowpack volume required to produce floods. So, the definitions often change to consider how the local climate affects floods in the area selected to study. Also, obtaining high-quality data about the precipitation type (liquid vs. solid) and the total amount of water held in the snowpack during potential rain-on-snow events can be a significant challenge for researchers.

However, the Mount Washington Observatory is unique in the fact that there is an abundance of high-quality precipitation and snowpack data going back for decades. It enables us to be one of the first organizations to investigate rain-on-snow events east of the Rocky Mountains.

Table of the various definitions of rain-on-snow used in previous studies

Preliminary background research on rain-on-snow events at the summit began late last year, but I started working on the project in August. Since then, our fall intern, Amy Cotter, and I have continued the work begun by previous observers and interns and have just recently started to obtain our first round of results.

What we have found so far is that rain-on-snow events are prevalent at the summit and seem to be increasing in recent years, with anywhere from 20 to 40 different events happening each winter. These initial findings have motivated us to take a closer look at the seasonilty and magnitude of the events, so plenty of research still needs to be done before any significant conclusions can be made. Furthermore, experiencing rain-on-snow events and widespread flooding firsthand has been a considerable factor guiding our future work on the project.

Graph of the annual amount of rain-on-snow events that have been observed at the summit since 1990

Our work ahead will include producing case studies that take a closer look at last week’s storm and other rain-on-snow events, with the goal of better understanding the local implications of these events for nearby rivers and the snowpack. The extreme flooding from last week’s storm showed how high the nearby rivers can rise and the damage that can occur when a significant rainfall event is combined with a major snowpack melting event. So, if we know the characteristics of rain-on-snow events on the summit, nearby towns will be able to be more prepared to deal with future events.

Additionally, rain-on-snow events can destabilize the snowpack, which increases the chances of an avalanche. Therefore, organizations such as the Mount Washington Avalanche Center will benefit from this project by gaining a much deeper understanding of the weather patterns that can lead to avalanches and using that info to inform the public of the best ways to stay safe while traversing the Presidential Mountain Range.

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