Q: What is a “snowmelt flood”?
When melting snow is a major source of the water involved in a flood, it is considered a snowmelt flood. Snowpacks store water. Unlike rainfall, which reaches the soil almost immediately, snow stores the water for some time until it melts, delaying the arrival of water at the soil for days, weeks, or even months. Once it does reach the soil, water from snowmelt behaves much as it would if it had come from rain instead of snow – the water either infiltrates into the soil or it runs off (or both). Flooding can occur whenever the rate of water input exceeds the ability of the soil to absorb it or when the amount of water exceeds natural storage capacities in soil, rivers, lakes and reservoirs.
Q: Are snowmelt floods common? Are they severe?
Some snowmelt flooding occurs every year in the northern U.S. Most events are relatively minor and affect localized areas, but not all. Eight of the most significant floods of the 20thcentury (in terms of area affected, property damage, and deaths) were related to snowmelt. The Northeast and North Central U.S., and some areas of the Western U.S.are particularly susceptible to snowmelt flooding.
Q: What factors lead to snowmelt flooding?
Six factors typically contribute to snowmelt flooding in Winter and Spring:
- High soil moisture conditions prior to snowmelt.
- Rainfall during the late Fall is particularly important because there is less evapotranspiration and less time for the soil to drain and dry before it freezes.
- Ground frost or frozen soil
- Deep, hard ground frost prevents snowmelt from infiltrating into the soil. Cold temperatures prior to heavy snowfall and normal or above normal soil moisture contribute to this.
- Heavy winter snow cover
- Unseasonably heavy snow cover means there is more water stored and available for snowmelt. Also, when heavy snowcover is widespread, it usually keeps air temperatures cooler and delays spring warming, which increases the potential for more rapid snowmelt and for spring rains occurring with snowmelt. In most parts of the country, the heaviest snowfalls usually occur in late February or March.
- Widespread heavy rains during the melt period
- Rain at this time contributes more water for flooding. Also, heavy rain can warm up cold snowpacks, causing them to begin melting earlier than they would otherwise. “Rain-on-snow” events are watched carefully for this reason.
- Rapid snow melt
- Most often, snowmelt is a relatively slow phenomenon. Snowmelt rates are usually comparable to light-moderate rainfall. Important exceptions to this can occur, especially during unusually warm periods with high dewpoint temperatures (humidity), and when nightime temperatures remain above freezing. Snowmelt rates can be much higher than normal under these conditions.
- Ice jams in rivers
- Snowmelt and the breakup of river ice often occur at about the same time. Ice jams sometimes occur, often in response to higher, fluctuating river flows associated with snowmelt. Ice jams can act as dams on the river that result in flooding.
Q: How do I find out if snowmelt flooding is likely? Are snowmelt floods forecasted?
The NWS provides extensive information about snowpack conditions across the country. A wide array of ground, airborne and satellite observations are used to monitor snow conditions. Local weather forecast offices routinely provide flood forecasts, whether or not they’re related to snowmelt.