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NOUS41 KBTV 170938

Public Information Statement
National Weather Service Burlington VT
538 AM EDT Wed Mar 17 2021

...National Flood Safety Awareness Week Continues... 

The National Weather Service in Burlington, VT has declared March
14 through March 20 as Flood Safety Awareness Week. Each day during
the awareness week will feature information about a different flood
related topic. Todays topic is Flood Hazards in the North

Northern New York and Vermont are at risk from flooding all year
long. During the winter, the flood threat is from ice jams and a
sudden snowmelt. The risk for snowmelt flooding grows into the
spring as temperatures warm and the snowpack melts. Summertime
thunderstorms can produce torrential downpours and cause flash
flooding. In late summer into the fall, tropical storms can spread
prolonged heavy rain across the region.

When melting snow is a major source of the water involved in a
flood, it is considered a snowmelt flood. Unlike rainfall, which
either runs off or percolates into the soil immediately, snowfall
accumulates all winter long. when snowmelt finally occurs, it can be
the equivalent of an entire season`s precipitation running off all
at once. Most often, spring melt of the snow pack is a relatively
slow phenomenon. Snowmelt rates are usually comparable to light to
moderate rainfall. However, if the snow melts quickly enough and is
accompanied by moderate to heavy rainfall, flooding can result. Such
was the case on April 15, 2019 when thunderstorms and melting snow
led to flash flooding across portions of southern Vermont and the
Otter Creek basin.

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. The NWS in
Burlington issues a biweekly Winter/Spring Flood Potential Outlook
which summarizes the risk of spring flooding based on the depth and
water content of the snowpack, river flow and river ice conditions,
and weather forecasts.

Ice jams occur when the solid cover of river ice is broken up by
higher river flows, rather than slowly melting away. The large
blocks of ice are carried downstream until they are stopped by an
obstruction in the river channel such as a sand bar or island,
bridge abutment, of thicker ice cover on a deep pool or lake. The
ice piles up to form an ice dam, forcing water levels to quickly

Ice jams are unpredictable, and the depth and speed of rising water
impounded by an ice jam can vary greatly. In March 1992 an ice jam
developed in Montpelier, VT at 7:00 am, and by 8:00 am the downtown
area was flooded. During the next 11 hours, the business district
was covered with 4 to 5 feet of water.

Summertime thunderstorms can produce torrential rainfall in a short
amount of time, in addition to hazards including strong winds, hail
and deadly lightning. Runoff from the heavy rain can quickly
overwhelm drainage areas, and cause small creeks and streams to
become raging rivers. Runoff can damage roads, especially rural dirt
roads, by washing away the road surface and carving out deep
gullies. In order to produce the heavy rainfall, a thunderstorm may
remain stationary over one location for an hour or more. Other
times, a succession of several thunderstorms may track over the same
location, a phenomenon known as training.

When it comes to tropical cyclones, a generic term for a hurricane,
typhoon, or tropical storm, wind speeds and 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
occur when tropical cyclones have a slow forward speed or antecedent
soil moisture conditions are high. In Vermont, the two most
significant floods in the state`s history were a result of storm
with tropical origins, including the devastating impacts from
Tropical Storm Irene in 2011.



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