Tropical Cyclone Inland Flooding
When it comes to tropical cyclones (a generic term for a hurricane, typhoon, or tropical
storm), wind speeds (e.g. severity categories 1-5) do not tell the whole story. Intense rainfall,
not directly related to the wind speed of a tropical cyclone, often causes more damage. Since
the 1970s, inland flooding has been responsible for more than half of the deaths associated
with tropical cyclones in the United States. Typically, greater rainfall amounts and flooding are
associated with tropical cyclones that have a slow forward speed or stall over an area.
Q: What do I need to know about inland flooding from tropical cyclones?
Inland freshwater floods accounted for more than half (59%) of U.S. tropical cyclone deaths
over the past 30 years. Rainfall is typically heavier with slower moving storms as slower
moving tropical cyclones allow heavy rain to persist over a location.
Q: What types of inland flooding are caused by tropical cyclones?
Flash Flooding occurs in creeks, streams, and urban areas within a few minutes or hours of
excessive rainfall. Rapidly rising water can reach heights of 30 feet or more. Streets can
become swift moving rivers and underpasses can become death traps.
River Flooding occurs from heavy rains associated with decaying hurricanes or tropical
storms, and in extreme cases, river floods can last a week or more.
Q: How do I know how severe a flood will be?
Within flood warning products, the NWS conveys the magnitude of observed or forecast
flooding using flood severity categories. These flood severity categories include minor
flooding, moderate flooding, and major flooding. Each category has a definition based on
property damage and public threat.
stage associated with each of the NWS flood severity categories are established in
cooperation with local public officials. Increasing river levels above flood stage constitute
minor, moderate, and major flooding. Impacts vary from one river location to another
because a certain river stage (height) in one location may have an entirely different impact
than the same level above flood stage at another location.
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:
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.
When it comes to tropical cyclones (a generic term for a hurricane, typhoon, or tropical
storm), wind speeds (e.g. severity categories 1-5) do not tell the whole story. Intense rainfall,
not directly related to the wind speed of a tropical cyclone, often causes more damage. Since
the 1970s, inland flooding has been responsible for more than half of the deaths associated
with tropical cyclones in the United States. Typically, greater rainfall amounts and flooding are
associated with tropical cyclones that have a slow forward speed or stall over an area.
Q: What do I need to know about inland flooding from tropical cyclones?
Inland freshwater floods accounted for more than half (59%) of U.S. tropical cyclone deaths
over the past 30 years. Rainfall is typically heavier with slower moving storms as slower
moving tropical cyclones allow heavy rain to persist over a location.
Q: What types of inland flooding are caused by tropical cyclones?
Flash Flooding occurs in creeks, streams, and urban areas within a few minutes or hours of
excessive rainfall. Rapidly rising water can reach heights of 30 feet or more. Streets can
become swift moving rivers and underpasses can become death traps.
River Flooding occurs from heavy rains associated with decaying hurricanes or tropical
storms, and in extreme cases, river floods can last a week or more.
Q: How do I know how severe a flood will be?
Within flood warning products, the NWS conveys the magnitude of observed or forecast
flooding using flood severity categories. These flood severity categories include minor
flooding, moderate flooding, and major flooding. Each category has a definition based on
property damage and public threat.
- Minor Flooding: minimal or no property damage, but possibly some public threat or
inconvenience - Moderate Flooding: some inundation of structures and roads near streams. Some
evacuations of people and/or transfer of property to higher elevations are necessary. - Major Flooding: extensive inundation of structures and roads. Significant evacuations
of people and/or transfer of property to higher elevations.
stage associated with each of the NWS flood severity categories are established in
cooperation with local public officials. Increasing river levels above flood stage constitute
minor, moderate, and major flooding. Impacts vary from one river location to another
because a certain river stage (height) in one location may have an entirely different impact
than the same level above flood stage at another location.
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.
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.
Flood Safety