How Warming Climates Are Reshaping Flood Risks Across America
A new study reveals that climate change is creating wildly divergent and potentially dangerous rain-on-snow flood scenarios across the U.S.
Breaking the Ice:
In an illuminating new study, researchers explore how climate change is transforming one of the most dangerous and overlooked weather hazards in the contiguous United States: rain-on-snow (ROS) flood events.
By applying a high-resolution, kilometer-scale land surface model to four historically devastating ROS floods, the team developed “storyline” simulations to examine how these floods might behave under warmer conditions. Their findings reveal a startling truth: not all ROS floods are created equal, and their responses to warming vary dramatically depending on geography, elevation, and local snowpack conditions.
The report’s key takeaway? As the climate warms, some flood events will intensify, while others may weaken or shift in timing and location—posing new challenges for flood risk management and infrastructure planning.
Quick Melt:
Climate change doesn’t just amplify existing flood risks—it reshuffles the entire deck, so to speak. In California, for instance, the January 2017 flood would likely intensify significantly with each additional degree of warming, driven by concurrent increases in rainfall and snowmelt. By contrast, the Mid-Atlantic’s 1996 flood, despite its deadly effects, would weaken in the face of warming in response to reduced snowpack and snowmelt potential in the relatively low-elevation region.
Perhaps most revealing is the way elevation shapes these potential outcomes. At higher elevations, increased temperatures can still leave snowpack intact, allowing rainfall and accelerated snowmelt to combine into dangerous floods. But at lower elevations, snow increasingly disappears before storms hit—shifting the flood mechanism from ROS to rainfall-only events. In some cases, this transition may reduce flood intensity; in others, it introduces entirely new flood dynamics.
These nuances complicate risk assessments. A one-size-fits-all approach to flood planning will no longer suffice. Instead, policymakers must adopt region- and elevation-specific strategies that account for how ROS events are evolving. This includes updating infrastructure standards, reevaluating reservoir management strategies, and revising emergency evacuation protocols—all with an eye toward how localized climate conditions affect snowpack, rainfall, and runoff.
The Thaw:
What are the Hydrometeorological Processes Behind Rain-on-Snow Floods? AccumulationZone Explains.
ROS events occur when warm rainfall lands on existing snowpack, initiating rapid melting. This dual water input—from both rain and snowmelt—can cause abrupt surges in runoff that overwhelm watersheds and downstream infrastructure. From a thermodynamic standpoint, warming influences ROS dynamics across multiple categories.
One of the clearest effects of warming is a shift in the phase of precipitation. As air temperatures rise, snowfall becomes less frequent, especially at mid-elevations. Instead, more precipitation falls as rain, which can directly contribute to runoff or interact with snowpack in destabilizing ways. This phase transition is controlled by the atmospheric freezing line, which climbs upward with warming, pushing the zone of ROS activity higher into mountainous regions.
The thermal condition of snowpack also plays a crucial role. Snow that is already near its melting point—what scientists call “ripe” snow—can rapidly absorb heat from rainfall or warmer air, accelerating meltwater production. These ripe conditions become more common at higher elevations in a warming climate, even as lower elevations lose their seasonal snow cover entirely. As a result, runoff from ROS events may shift upward in elevation and increase in intensity.
Soil and subsurface dynamics also respond to warming. When soils are frozen, they limit water infiltration and encourage surface runoff. But as temperatures rise and soils thaw, infiltration capacity increases—at least initially—absorbing some of the incoming water and potentially mitigating flood peaks. However, this benefit is contingent on antecedent moisture. In soils that are already saturated or in areas with diminished snow accumulation, infiltration may be limited, and runoff could spike.
Complicating matters further is the concept of runoff efficiency, which measures how much of the total water input during an event ends up as actual streamflow. The study found that this efficiency changes with temperature due to shifts in snowpack structure and soil absorption. Warmer temperatures may enhance infiltration in some areas but also reduce snow retention, leading to more immediate runoff elsewhere.
Finally, elevation plays a defining role. Kilometer-scale simulations revealed that while low elevations may see a decline in ROS-related flooding due to snowpack loss, high-elevation areas could face more extreme ROS floods. Here, deeper snowpack can survive even under warming, and when combined with rain, produce dangerous surges in runoff.
Final Thoughts
ROS flood risk is a moving target under climate change, shifting with the intersection of temperature, terrain, and snow dynamics. Understanding the scientific intricacies of these mechanisms is vital to forecasting future risks, guiding infrastructure adaptation, and ultimately enhancing climate resilience in flood-prone regions.