Snowmelt Flooding
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 20th century (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.
Resources:
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