Why, how and when do rain-on-snow events produce exceptional runoff?

When warm rain falls on a deep snowpack, the result is sometimes a modest rise in river flow and sometimes a disastrous flood. Predicting which, and how warming will shift these events, is the aim of this question.

Question

Why, how and when do rain-on-snow events produce exceptional runoff? Which combinations of weather, snowpack state (depth, liquid water content, structure) and catchment conditions (soil moisture, frozen ground) turn rain on snow into an extreme flood?

Background

The authors of the twenty-three unsolved problems in hydrology note that this question is more specific than some others on their list. They kept it because rain-on-snow events often produce bigger floods than expected, a clearly defined yet vexing phenomenon, and because this flood mechanism matters in many parts of the world [1].

What is known

• At 4,318 sites in the western USA (1949–2003), rain-on-snow events were most common from October to May and declined at lower elevations, with fewer snowfall days and fewer days with snow on the ground [2].
• Simulations at 4 km resolution for western North America project fewer rain-on-snow events at low elevations but more at high elevations, where snowfall turns to rain; water available for runoff increases in 55% of river basins, with flood-risk increases of 20–200% [3].
• Snowpack simulations of more than 1,000 historical events at 116 sites in the Swiss Alps show that initial liquid water content and snow depth control the timing and amount of runoff; the snowpack damps short, intense rain but amplifies longer events [4].
• In the February 2017 Oroville flood in California, snowmelt added only about 21% to the water input; rain routed through the snow and soils wetted by earlier storms better explain the extreme runoff [5].

What would count as progress

No single result settles a question like this; contributions like these would move it forward:

• An open, multi-region catalogue of rain-on-snow events with meteorology, snowpack observations and observed runoff, suitable for testing models.
• A model or classification that predicts, before an event, whether runoff will exceed what the rain alone would produce, validated on held-out events.
• Measurements during events (snowpack liquid water, soil moisture, energy balance) at instrumented sites, to settle which processes dominate where.

References

1. Blöschl, G. et al. (2019). Twenty-three unsolved problems in hydrology (UPH) – a community perspective. Hydrological Sciences Journal 64(10), 1141–1158.
2. McCabe, G. J., Clark, M. P. & Hay, L. E. (2007). Rain-on-snow events in the western United States. Bulletin of the American Meteorological Society 88(3), 319–328.
3. Musselman, K. N. et al. (2018). Projected increases and shifts in rain-on-snow flood risk over western North America. Nature Climate Change 8(9), 808–812.
4. Würzer, S., Jonas, T., Wever, N. & Lehning, M. (2016). Influence of initial snowpack properties on runoff formation during rain-on-snow events. Journal of Hydrometeorology 17(6), 1801–1815.
5. Haleakala, K. et al. (2023). Watershed memory amplified the Oroville rain-on-snow flood of February 2017. PNAS Nexus 2(1), pgac295.

Source

Imported from Twenty-three unsolved problems in hydrology, question 11, curated by Blöschl et al. Listed as open in the source (2019) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.