Snow and Weather

Forecasting snow

Snow forecasts are better than they used to be and they continue to improve, but snow forecasting remains a difficult challenge for meteorologists. One reason is that during intense snows, the heaviest snowfall can occur in surprisingly narrow bands, and on a smaller scale than observing networks and forecast zones can see. Also, the extremely small temperature differences that define the boundary line between rain and snow make large differences in snow forecasts. This is part of the fun and frustration that makes snow forecasting so interesting.

Large snow drifts on Washington State Route 27

These drifts along Washington State Route 27 were so high that they nearly obscured traffic signs after a January 2008 snowfall.
—Credit: Charles Haupt, Washington State Department of Transportation

Blizzard in Fargo, North Dakota, December 2008

Blizzard conditions during a December 2008 storm in Fargo, North Dakota, made travel dangerous. Authorities closed Interstate 29, a major route across the state.
—Credit: Becky, flickr

Because conditions in the atmosphere and on the ground can vary, each storm might produce a different type of snowfall. In addition, snow does not fall evenly everywhere. Even during the same storm, one neighborhood may receive deep snow, while an adjacent neighborhood may only receive a light dusting. At the local scale, variations in snow depth are caused primarily by wind during and after the storm, and by melting after the storm. At the larger scale, say across an entire state, it also depends on the storm track. Places in the middle of the storm track may receive significant snowfall, while locations along the edges of the storm may receive much less. Elevation changes also affect snowfall. As a snowstorm travels upslope toward a mountain summit, the storm typically drops more moisture. On the other side of the summit, snowfall is generally lighter.

Weather forecasters use a variety of terms to describe the intensity of snowfall:

  • A snow flurry refers to light showers of snow that do not cover large areas and do not fall steadily for long periods of time.
  • Freezing rain is precipitation that cools below 0 degrees Celsius (32 degrees Fahrenheit), but does not turn to ice in the air. The water is supercooled. When the drops hit anything they instantly turn into ice.
  • An ice storm is a storm with large amounts of freezing rain that coats trees, power lines and roadways with ice.
  • A blizzard is a severe winter storm that packs a combination of snow and wind, resulting in very low visibility. Officially, the National Weather Service defines a blizzard as large amounts of falling or blowing snow with winds in excess of 56 kilometers (35 miles) per hour and visibilities of less than 0.40 kilometers (0.25 miles) for more than 3 hours. While heavy snowfall and severe cold often accompany blizzards, they are not required. Sometimes strong winds pick up snow that has already fallen, creating a ground blizzard.

Forecasters also rate the expected severity of blizzards and other snowy weather using a scale:

  • A Winter Weather Advisory is issued for accumulations of snow, freezing rain, freezing drizzle, and sleet that may present a hazard but does not merit a warning.
  • A Winter Storm Watch is issued to alert the public to the possibility of a blizzard, heavy snow, heavy freezing rain, or heavy sleet.
  • A Winter Storm Warning is issued when a hazardous winter weather event is imminent or occurring, and is considered a threat to life and property.

Blizzards

Blizzards can create a variety of dangerous conditions. Traveling by automobile can become difficult or even impossible due to whiteout conditions and drifting snow. The strong winds and low temperatures accompanying blizzards can combine to create other dangers. For instance, the wind chill factor is the amount of cooling one feels due to the combination of wind and temperature. A strong wind combined with a temperature of just below freezing can have the same effect as a still air temperature about 35 degrees colder. A wind chill chart may be used to estimate the wind chill factor. Exposure to low wind chill values can result in frostbite or hypothermia. Frostbite is a severe reaction to cold exposure that can permanently damage its victim. Hypothermia is a condition brought on when the body temperature drops dangerously low, and can be fatal if not caught in time.

Blizzards also often cause related problems. Strong winds and heavy snow can damage cause tree limbs to fall onto structures or even utility lines, resulting in power outages. Drifts can block roads and sidewalks and make traveling difficult well after the storm is over.

Bomb cyclones

Although they have been discussed more often in recent years, bomb cyclones are not a new phenomenon; meteorologists have used the term for decades. Like all Northern Hemisphere cyclones, these storms move counter-clockwise around their cores. Bringing snow and/or rain, bomb cyclones are characterized by low pressure, large scale, and rapid development. Under the right conditions, bomb cyclones may turn into blizzards.

NWS parking lot in a bomb cyclone
From March 12 to 14, 2019, a bomb cyclone set a new low-pressure record across eastern Colorado. Conditions rapidly intensified into a blizzard, and parts of Colorado, Wyoming, and Nebraska experienced heavy snowfall, stranding National Weather Service employees at the office for 36 hours.
—Credit: Jeff Garmon, Meteorologist-in-Charge, Cheyenne NWS

Thunder snow

Although snowstorms typically occur in the winter, and thunderstorms typically occur during the summer, there are rare instances when meteorological conditions produce a phenomenon called thunder snow. In these instances, temperatures are low enough to generate snow instead of rain, and turbulence in the atmosphere causes the lighting and thunder.

Effects of the polar vortex

Both the Northern and Southern Hemispheres have atmospheric polar vortices—regions of cold air that rotate from west to east at latitudes farthest from the Equator. Because Earth's heavily populated landmasses lie mostly in the Northern Hemisphere, that hemisphere's polar vortex has a larger weather effect on human activities.

The axis of rotation for the Northern Hemisphere polar vortex is roughly situated in the Arctic, and the Northern Hemisphere polar vortex contains both colder air and lower atmospheric pressure than regions to the south. Dividing the colder air of the polar vortex from the warmer air to the south is the polar jet stream, a fairly narrow band of winds that blow from west to east.

Although the Northern Hemisphere polar vortex can exhibit a fairly even circular shape, it is more often irregularly shaped, with troughs of cold air stretching southward, and ridges of warm air extending northward. The locations of these troughs and ridges change over time, as does the position of the jet stream. Storms generally form along the jet stream as cold troughs approach and depart a particular region. The approach of a cold polar trough is sometimes referred to as an Arctic outbreak.

To learn more about the polar vortex, see What is the polar vortex? in NSIDC's Icelights.

For more information see Snow Resources.

Last updated: 10 January 2020