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Assessing the predictability and probability of 21st century rain-on-snow flood risk for the conterminous U.S.

The U.S. faces challenges and bears high risk related to flood prediction and the protection of
life, property and infrastructure. For much of the Nation, the timing of heavy rainfall can coincide with 
seasonal snow-cover. The combined rainfall and melt during so-called rain-on-
snow (ROS) events has historically contributed to some of the Nation’s most destructive and costly
floods. Decision-makers critically lack guidance on ROS flood risk. Assessment requires
accurate estimates of antecedent snowpack, rain-snow height levels, the energy exchange
between the atmosphere and snow surface, rainfall intensity, and soil ice and moisture content. In
a future climate, each of these variables – and the integrated response of ROS flood risk – is
expected to change in complex and often contradictory ways. Notably, will projected increases in
precipitation extremes and winter rainfall increase ROS occurrence and the associated flood risk?
Or will less snow-cover and larger soil moisture deficits reduce ROS flood risk in a warmer
climate? The projected changes are likely to vary by region, season, climate model, emissions
scenario, and future time horizon. We address this grand challenge in hydrology and climate

The goal of our project is to assess national-scale historical (20th century) and future (21st
century) projections of integrated ROS flood risk and the associated confidence / uncertainty as
represented in a suite of CMIP6 climate models.

To achieve this goal, we will use a computationally efficient atmospheric model to dynamically
downscale: 1) historical reanalysis, which provides a baseline against which to compare
projected changes, 2) CMIP6 GCM output (historical and future scenarios), which offers an
assessment of uncertainty due to model error, and 3) a large ensemble from a single GCM
(historical and future), which offers insight into the role of internal climate variability. While
historical and future ROS flood risk assessment is the primary goal of the proposed research, the
intermediary assessment of the climate sensitivity of a full suite of ROS-relevant metrics has
high value and interest spanning environmental disciplines and NOAA Line Offices. We will
address a destructive flood mechanism affecting much of the Nation that intrinsically includes
climate-sensitive snow water resources, soil moisture deficits, and rainfall intensity.
We directly address the NOAA MAPP Program mission to enhance the Nation’s capability to
predict variability and change in Earth’s climate system. We will compare, integrate, and analyze
weather and climate model output to improve scientific understanding of projected changes in a
costly and destructive flood generating mechanism that affects much of the US and the northern
hemisphere. We aim to reveal and represent uncertainties to establish a defensible range of
quantitative storylines of integrated climate change impacts on ROS flood risk. Our assessment
of current and future states of the climate and hydrologic system will serve to identify potential
impacts needed to inform science, service, stewardship, mitigation and adaptation.

Climate Risk Area: Water Resources

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