NOAA issues Winter Weather Awareness Week tips – 4

Flooding and Ice Jams

One of our deadliest winter weather hazards is flooding.

In the state of New York one usually associates snow, ice, and bitter cold with winter. But sometimes nature throws a curve at us with unseasonably warm temperatures, and heavy rain

A number of different factors work together to produce floods in winter.

When unseasonable warmth comes to the region it will often melt much, if not all of the snow on the ground especially in the lower elevations. The melting snow can saturate the ground and also swell the rivers. If the combination of unseasonably warm temperatures, heavy rain, and snow melt occurs, rivers may rise above their banks producing floods.

Some of our worst winter floods are created by an intense low pressure system that tracks from the Ohio valley northeast, down the Saint Lawrence Valley in Canada. These storms bring a lot of warm and moist air into the region from the Gulf of Mexico and the Atlantic.

Flooding during the winter months can also be caused by ice jams. When river flows increase, water levels rise. Ice is less dense than water, thus ice floats. When enough pressure is applied by the river below, the ice will break. Ice typically breaks into slabs and floats downstream. When the ice interacts with an obstruction such as a bridge, bend in the river, island, or a wide shallow area, the ice will often pile up into an ice jam. The ice jam then prevents water from freely flowing, and forces the water to rise, creating a flood. Ice jams can form any time during the winter season.

Flooding on roads and poor drainage areas can also occur when mounds of plowed snow and ice block grates and storm drains. Standing water can cause dangerous black ice if it freezes.

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