A winter storm (also known as snow storm) is an event in which wind coincides with varieties of precipitation that only occur at freezing temperatures, such as snow, mixed snow and rain, or freezing rain. In temperate continental and subarctic climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. A snowstorm with strong winds and low visibility is called a blizzard.

Winter storms are formed when moist air rises up into the atmosphere, creating low pressure near the ground and clouds up in the air. The air can also be pushed upwards by hills or large mountains. The upward motion is called lift. The moisture is collected by the wind from large bodies of water, such as a big lake or the ocean. If temperature is below freezing, 0 °C (32 °F), near the ground and up in the clouds, precipitation will fall as snow, ice, rain and snow mixed (sleet), ice pellets or even graupel (soft hail). Since cold air can not hold as much moisture as warm air, the total precipitation will be less than at higher temperature.

Winter storm warnings will be issued if:

Snowstorms with wind speed of more than 35 mph (16 m/s) and reduced visibility of less than 0.25 miles (400 m) for 3 hours or longer are called blizzards.

Severe winter weather conditions called "winter storms", can be local weather fulfilling the criteria for 24 hours, or large storm systems covering part of a continent for several days. With large, massive winter storms, weather in any part of the area covered by the extreme weather is usually called "storm"; even if meteorological criteria for winter storms are not met everywhere. An example of this is the February 13–17, 2021 North American winter storm with snowfall and below freezing temperatures as far south as Texas and the Gulf of Mexico.

Snowstorms are storms where large quantities of snow fall. 2 in (5.1 cm) of snow is enough to create serious disruptions to traffic and school transport (because of the difficulty to drive and manoeuvre the school buses on slick roads). This is particularly true in places where snowfall is not typical but heavy accumulating snowfalls can occur. In places where snowfall is typical, such small snowfalls are rarely disruptive, because of effective snow and ice removal by municipalities, increased use of four-wheel drive and snow tires, and drivers being more used to winter conditions. Snowfalls in excess of 6 in (15 cm) are usually universally disruptive.

A large number of severe snowstorms, some of which were blizzards, occurred in the United States during 1888 and 1947 as well as the early and mid-1990s. The snowfall of 1947 exceeded 2 ft (61 cm) with drifts and snow piles from plowing that reached 12 ft (3.7 m) and for months as temperatures did not rise high enough to melt the snow. The 1993 "Superstorm" manifested as a blizzard in most of the affected areas.

Severe snowstorms could be quite dangerous: a 6 in (15 cm) snow depth will make some unplowed roads impassable, and it is possible for cars to get stuck in the snow. Snow depth exceeding 12 in (30 cm) especially in southern or generally warm climates will cave the roofs of some homes and cause loss of electricity. Standing dead trees can also be brought down by the weight of the snow, especially if it is wet. Even a few inches of dry snow can form drifts many feet high under windy conditions.