Winter Storms

Winter storms can bring snow, sleet, and freezing rain. Blizzards occur when strong wind causes blowing snow and whiteout conditions, making roads impassable. Thousands of people are injured or killed every year in traffic accidents related to slippery roads from winter storms.

The National Weather Service Winter Safety website is designed to teach you how to stay safe before, during and after a winter storm. You will find information on winter alerts, science and hazards, snow coverage maps, and information describing the different types of winter storms

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Winter Hydrology: Snowfall & Snowmelt


Throughout the winter, much of our attention is directed to weather forecasts to determine if snow is on the horizon, or how much snow fell in the past storm. Just as snowfall is a major source of water during the winter months, the snow on the ground (called the ‘snow cover’ or ‘snowpack’) is a significant storage of water during the winter months. When the snow cover melts much of the water flows over the land to streams, however there is some ground water recharge during the snowmelt as well. Even with the ground frozen, there are spaces where water can fill and flow down to the water table. This ground water recharge is critical to replenishing the aquifers for the upcoming spring and summer seasons.
 
There are a number of different ways to measure snowfall. One simple way is to use a snowboard and a ruler. A snowboard is simply a flat board that is placed on the ground. After the snowfall has accumulated, the depth is measured and recorded. Each time a measurement is taken, the snowboard is moved to the top of the snow. The rate of snowfall can be determined by taking measurements every hour or every six hours for example.
 
Another measurement of snowfall is to collect the snow and ice in a common cylindrical raingage, without the center measuring tube or top funnel. After the snow and ice are collected, the gage is placed in a warm area and melted. Then the melted precipitation is poured into the measuring tube and recorded. The result is the precipitation (as total water) in inches from the snowfall, which is the method that the CCWRA Volunteer Rainfall observers use.
 
The rule-of-thumb is that about 0.1 inches of precipitation (or water) results from every 1 inch of snow. A heavy, wet snow may be closer to 0.2 or 0.3 inches of precipitation to an inch of snow while a light, powdery snow may be less than 0.1 inch.

What Makes a Winter Storm?


Cold air:


Below-freezing temperatures in the clouds and near the ground are necessary to make snow and/or ice.
 

Moisture:


To form clouds and precipitation. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of moisture.
 

Lift:


Something to raise the moist air to form the clouds and cause precipitation. An example of lift is warm air colliding with cold air and being forced to rise over the cold dome. The boundary between the warm and cold air masses is called a front. Another example of lift is air flowing up a mountain side.
 
For more information, check out the National Weather Service Winter Storm Safety web page.
  

How a Typical Nor’easter Forms


The classic winter storm in the Mid-Atlantic region is called a Nor'easter. A low pressure area off the Carolina coast strengthens and moves north. Wind-driven waves batter the coast from Virginia to Maine, causing flooding and severe beach erosion. The storm taps the Atlantic's moisture-supply and dumps heavy snow over a densely populated region. The snow and wind may combine into blizzard conditions and form deep drifts paralyzing the region. Ice storms are also a problem. Mountains, such as the Appalachians, act as a barrier to cold air trapping it in the valleys and adjacent low elevations. Warm air and moisture moves over the cold, trapped air. Rain falls from the warm layer onto a cold surface below becoming ice.