Why Rain on Snow in the California Mountains Worries Scientists

California is getting another atmospheric river, with more rain on snow expected. How does the rain-on-snow effect differ by elevation in the mountains there?
In the California mountains right now, it’s the middle elevations that people need to pay attention to.

The lower elevations have primarily seen rainfall rather than snow, so there is less snowpack to melt. And in the highest elevations, colder temperatures promote the continued accumulation of deep snowpack and rainfall is less likely.

In the middle transition zone – where either substantial rainfall or snowfall can occur – rain-on-snow events are most common, causing both melting and risk of roof collapses.

If all storms were created equal, there would be well-defined rain zones and snow zones, and the rain-on-snow flood risk would be low. But that isn’t what happens. Instead, not only does the snow zone elevation vary during an event, but it also varies substantially from one storm to the next.

The most destructive rain-on-snow events occur when rivers are already running high and soils are saturated, which can occur in response to a series of warm atmospheric rivers interacting with a deep snowpack – like California’s mountains have right now. The order in which these storms occur – or the storm sequencing – is especially important for assessing flood risk because these events are, in part, caused by rapid shifts between cold periods of snow accumulation followed by warm rainfall events.

What does research show about the future risk of rain-on-snow events in a warming climate?
Even less is known about how rain-on-snow flood risk may respond as the planet warms.

In a warmer climate, there will be less risk of rain falling on snow in the lower elevations as the snowpack declines, particularly in warmer regions such as the Pacific Northwest.

But at higher elevations, more frequent rain-on-snow events are expected. While warmer temperatures are expected to increase rainfall intensity, research shows that’s not the most important driver of this risk. Much of the expected increase in rain-on-snow flood risk is a result of the rain-snow transition zone expanding higher in elevation to include alpine areas that historically received predominantly snowfall.

Flood control and reservoir management systems in these mountainous regions will have to consider these future changes in rain-on-snow events – in addition to changes in rainfall intensity and storm sequencing – to fully understand and prepare for the local flood risk as the planet warms.

So, will projected increases in precipitation extremes and winter rainfall increase rain-on-snow occurrence and the associated flood risk? Or will less snow cover and larger soil moisture deficits reduce rain-on-snow flood risk in a warmer climate?

In a future climate, the response of rain-on-snow flood risk is expected to change in complex and often contradictory ways. The projected changes are likely to vary by region, season, climate model, emissions scenario and future time horizon. It’s a costly risk that requires more research.

Keith Musselman is Assistant Professor in Geography, Mountain Hydrology, and Climate Change, University of Colorado Boulder. This article is published courtesy of The Conversation.