A FloodFlash primer on snowmelt flooding

This week we want to cast a spotlight on snowmelt flooding. As frigid weather continues across the north of the US we can expect an impact on our waterways in the months to come. Let’s dig into snow melt, and why it can pose a big flood problem.

What is snow melt?

A snowmelt flood is simply a flood where melting snow is a major source of the water. Unlike rainfall flooding, which happens in the hours after heavy rain, snow and ice store the water for some time before melting. This frozen water can be stored in huge quantities (last year, water stored in the Sierra snowpack could have covered the whole of California in 5ft of water). When it melts after warmer weather, it unleashes the trapped water.

A snowmelt flood is a flood where melting snow is a major source of the water.

 

Melting snow can contribute significant amounts of water to rivers, as well as making the ground more saturated – these both contribute to a higher likelihood of flooding.

If there’s more water than the ground can take, or the water overwhelms rivers, then flooding occurs. Things can complicate further, with frozen ground, heavy snow cover, unseasonably warm temperatures, and rain during the period of snowmelt all contributing to more severe snowmelt flooding.

A final general point to make about snowmelt is the delay. Depending on winter and spring conditions, snowmelt flooding can occur many months after the snow has fallen. For example, the Sierra snowpack typically reaches its maximum depth around April 1. The month of May might be associated with the cold weather turning, but it’s often when rivers swell most from the snow of the preceding winter.

Where in the US floods from snowmelt?

As you might expect, the northern United States are most likely to experience snowmelt flooding, typically coming round every year. According to the National Weather Training Center, in 2017, 8 of the most significant floods of the 20th century (in terms of area, property damage and deaths) were related to snowmelt.

That doesn’t mean that central and Southern states are completely immune to the impacts of snowmelt. Some of the great American rivers will swell thanks to melting snow, so those downstream should be aware of major events.

As snow melts, particularly from mountainous regions, nearby rivers can swell - increasing the chance of river flooding.

 

Snowmelt can lead to more water in rivers, increasing the chance of these rivers overflowing and causing flooding.

The expert view

We asked Catastrophe Risk Analyst Henry Bellwood for a quick view from the underwriters’ desk. Here’s Henry: “snowmelt is essentially the same mechanism of rainfall flooding, with precipitation causing rivers to overflow, but with a ~3 month delay. That means that extreme winters can lead to extreme springs. This “doubling” of the risk factors leads to additional uncertainty when it comes to predicting the impact of snowmelt.

“As climate change drives unseasonably warm winters, we are witnessing more explosive snowmelt seasons. They are happening at unpredictable times and scales, where abnormal spring conditions lead to flooding that we don’t always have strong historic precedents for. It’s certainly something to be aware of for 2024.”

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Hopefully you feel better equipped to discuss snowmelt with clients. Feel free to get in touch with us to ask about snowmelt risk, or for coverage that pays fast when the weather begins to warm.

 


 

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