California’s recent atmospheric currents are directing precipitation higher into the mountains and onto the state’s crucial snow pack. The rain alone is a problem for low-lying areas already experiencing devastating flooding, but the prospect of rain on the deep mountain snow has become widespread flood warnings.
When rain falls on snow, complex flood risks arise that are difficult to predict. These risks also increase with climate change.
In much of the United States, heavy rains can coincide with seasonal snow cover. When this happens, the resulting water runoff can be much greater than from rain or slush alone. The combination has resulted in some of the country’s most devastating and costly floods, including the 1996 Midwestern floods and the 2017 flood that damaged California’s Oroville Dam.
But rain itself has limited energy to melt snow. Rather, it’s warm temperatures, high winds and high humidity — which can transport significant energy in the form of latent and sensible heat — that primarily drive snowmelt during rain-on-snow events.
The snowpack has air spaces that allow water to move. When the rain falls, the water can flow down through the layers of snow relatively quickly to reach the ground. How streams respond to this runoff depends on how much water is already flowing and how saturated the soil is.
If the ground is not yet saturated, it can dampen or delay a flood response by absorbing rain and melting snow. But when the ground is saturated, melting snow combined with rain can cause rapid and devastating flooding.
Knowledge of weather and hydrological conditions is required to predict whether a flood will occur. This requires knowledge of pre-storm soil moisture and snow conditions, the height at which rain turns to snow, precipitation rate, wind speed, air temperature and humidity, and estimates of how these factors contribute to snowmelt. In addition, each factor varies over time and in complex ways during a storm, particularly in a mountainous landscape.
This is why so little is known about rain-on-snow floods, which are considered combined extreme events despite the great damage they can cause.
Right now, in California’s mountains, the mid-elevations — where significant rain or snow can fall — are being hit the hardest. At lower elevations, it rained instead of snow, allowing less snow to melt. And at the highest elevations, colder temperatures keep deep snow from accumulating and precipitation is less likely.
If all storms were generated equally, there would be clearly defined rain and snow zones and the risk of rain-on-snow flooding would be low. Instead, the height of the snow zone varies widely not only during an event, but also from storm to storm.
The most destructive rain-on-snow events occur when the currents are already high and the soil is saturated, as happened in California after a series of warm atmospheric currents. The order in which these storms occur is especially important when assessing flood risk, as these events are caused in part by rapid alternations between cold periods with snow accumulation and subsequent warm precipitation.
How the flood risk from rain to snow will change as the planet warms is not yet fully understood. Will an increase in extreme precipitation and precipitation in winter lead to more rain-on-snow and flood risks? Or will less snow cover and greater soil moisture deficits reduce rain-on-snow flood risk in a warmer climate?
Some points become clearer. In a warm climate, there is less risk of precipitation falling on snow at lower elevations as snow cover decreases, especially in warmer regions such as the Pacific Northwest. However, rain-on-snow events can be more common at higher elevations. An increased risk of flooding arises from the rain-snow transition zone, which extends higher up into alpine areas that have traditionally experienced snowfall.
Flood protection and reservoir management systems in these mountainous regions must account for these shifts – in addition to changes in precipitation intensity and storm sequence – to fully understand and prepare for future local flood risks.
As the planet warms, the rain-on-snow effect is just one of many costly risks local communities must consider. This week, California is seeing what could become a more common occurrence due to climate change.
Keith Musselman is an assistant professor of geography and mountain hydrology and climate change at the University of Colorado Boulder.
Source: LA Times