What Is a Blizzard? How the NWS Defines Blizzard Conditions

By
Harris
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Key Points

  • The National Weather Service (NWS) defines a blizzard by wind and visibility, not snowfall: winds or frequent gusts of 35 mph or more with snow cutting visibility to under 1/4 mile for at least 3 hours.
  • A blizzard can happen with little or no new snow. When strong winds lift snow that is already on the ground, the NWS calls it a ground blizzard.
  • A Blizzard Warning means whiteout conditions are expected or occurring, and the NWS advice is simple: do not travel.
  • NOAA says most winter storm deaths come indirectly, from crashes on icy roads, heart attacks while shoveling and hypothermia, and about 70% of ice and snow injuries happen in vehicles.
  • If you are stranded, the NWS says to stay with your vehicle, run the engine about 10 minutes each hour and keep the exhaust pipe clear of snow.

What is a blizzard? It is not simply a big snowstorm. In the United States, a blizzard is a winter storm with strong, persistent wind and blowing snow that reduces visibility to almost nothing for hours. The National Weather Service sets the bar at winds or frequent gusts of 35 mph or more, visibility frequently below 1/4 mile and conditions lasting at least three hours.

That definition says nothing about how much snow falls, so a storm can bring heavy snow and never be a blizzard. Here is how meteorologists define blizzards, how they form, how they differ from other winter hazards and what to do when a warning is issued, based on the NWS and NOAA’s National Severe Storms Laboratory (NSSL).

What is a blizzard? The official definition

The NWS glossary says a blizzard means the following conditions are expected to prevail for a period of 3 hours or longer:

  • Sustained wind or frequent gusts to 35 miles an hour or greater, and
  • Considerable falling and/or blowing snow, reducing visibility frequently to less than 1/4 mile.

All three parts matter. Only when strong wind, blowing or falling snow and very poor visibility come together and hold for three hours or more does a storm meet the blizzard standard.

NSSL puts it in plain terms: blizzards are dangerous winter storms that combine blowing snow and wind, resulting in very low visibilities. Heavy snowfall and severe cold often accompany blizzards, NSSL notes, but they are not required.

The danger comes from what the wind does to the snow. The NWS winter safety page says blizzards occur when strong wind causes blowing snow and whiteout conditions, making roads impassable.

Ground blizzards: when no new snow falls

Blowing snow, in NWS terms, is wind-driven snow that reduces visibility. It can be falling snow or snow already on the ground that strong winds pick up, and it usually comes with drifting.

When blizzard conditions are caused solely by blowing and drifting snow, with no new snow needed, the NWS calls it a ground blizzard and can issue a Ground Blizzard Warning. In other words, dangerous whiteouts can return after the snow has stopped falling if the wind picks up enough to move it.

How blizzards form

According to NSSL, every winter storm needs three basic ingredients:

  • Cold air: below-freezing temperatures in the clouds and near the ground to make snow or ice.
  • Lift: something to raise moist air, such as warm air forced up over cold air along a front, or air flowing up a mountainside.
  • Moisture: to build clouds and precipitation, often from air crossing a large lake or the ocean.

A blizzard also needs wind strong enough to meet the 35 mph threshold. Some of the most intense winter storms strengthen so fast that meteorologists call it bombogenesis; our explainer on how a bomb cyclone forms covers that process in detail. Nor’easters are another example: NSSL says these East Coast storms are notorious for heavy snow, with wind gusts that can exceed hurricane force.

Blizzard vs other winter weather

Blizzards share the season with several other hazards. This comparison uses NWS and NSSL definitions.

Hazard What defines it Typical duration Main danger
Blizzard Winds or frequent gusts of 35 mph or more, falling or blowing snow, visibility often under 1/4 mile 3 hours or longer Whiteouts, drifting, stranded travelers
Ground blizzard Blizzard conditions from blowing and drifting snow only 3 hours or longer Whiteouts after the snow has stopped
Heavy snowstorm Significant snow accumulation; amounts that trigger warnings vary by region Hours to days Deep snow, roof loads, blocked roads
Snow squall Intense burst of moderate to heavy snow with strong, gusty wind Usually 30 to 60 minutes Sudden whiteout and flash freeze on highways
Lake effect snow Narrow snow bands as cold, dry air picks up moisture over the Great Lakes Can persist for many hours Very heavy snow in a limited area
Ice storm At least 0.25 inch of ice on exposed surfaces Varies Downed trees and power lines, glazed roads

Ice storms are driven by freezing rain rather than snow; see our guide to the difference between sleet and freezing rain for how those form.

Blizzard vs snow squall

Both can cause a sudden whiteout. The NWS says the difference is duration: snow squalls usually last 30 to 60 minutes and often arrive with a strong cold front. Accumulations are typically an inch or less, but falling temperatures and quick drops in visibility can ice roads in minutes, and squalls have a long history of deadly crashes. A blizzard must last at least three hours.

Blizzard Warning, Winter Storm Warning and other alerts

Your local NWS office issues winter warnings, watches and advisories. Many of them use local criteria. The NWS gives this example: the amount of snow that triggers a Winter Storm Warning in the Northern Plains is typically much higher than in the Southeast. The blizzard definition, by contrast, is built on wind, visibility and time rather than a snowfall total.

  • Blizzard Warning: issued for frequent gusts of 35 mph or more with falling and/or blowing snow frequently reducing visibility to less than 1/4 mile for three hours or more. Whiteout conditions are likely and travel will be extremely difficult. The NWS message: do not travel.
  • Winter Storm Warning: a significant event with snow, ice, sleet or blowing snow. Ready.gov says these are usually issued 12 to 24 hours ahead.
  • Winter Storm Watch: conditions are favorable for a significant winter storm, which can include heavy snow and blowing snow. Ready.gov says a watch alerts the public to the possibility of a blizzard and is usually issued 12 to 48 hours ahead.
  • Winter Weather Advisory: snow, blowing snow, ice or sleet is expected but should not reach warning criteria. Expect winter driving conditions.
  • Snow Squall Warning: short-fused and focused on a small area, much like a tornado or severe thunderstorm warning.

The full list, with NWS guidance for each alert, is on the agency’s winter weather warnings, watches and advisories page.

Why blizzards are so dangerous

NOAA’s research lab says most deaths from winter storms are not directly caused by the storm itself. People die in traffic crashes on icy roads, of heart attacks while shoveling snow and of hypothermia from prolonged exposure to cold.

NSSL breaks down the injury data this way:

  • Of injuries related to ice and snow, about 70% occur in automobiles and about 25% involve people caught out in the storm.
  • Of injuries related to exposure to cold, 50% involve people over 60, more than 75% are males and about 20% occur in the home.

The NWS adds that, on average, more than 5,000 people are killed and more than 418,000 are injured each year in weather-related vehicle crashes of all kinds. In a blizzard, the wind adds another threat. The NWS explains that as wind speed increases in cold conditions, the body loses heat more quickly, which raises the risk of frostbite and hypothermia. Our guide to how wind chill is measured explains the NWS chart and frostbite times.

How to prepare before a blizzard

The NWS and Ready.gov recommend getting ready before the season starts, not when the warning is issued:

  • At home: flashlights and extra batteries, a battery-powered NOAA Weather Radio, food that needs no cooking, water, extra prescription medicine, first-aid supplies, baby items and pet food.
  • Heat and safety: refuel before you run out, test smoke alarms and carbon monoxide detectors, and never run a generator in an enclosed space.
  • In the car: a survival kit with a phone charger, blankets, a flashlight, first-aid kit, high-calorie snacks, extra clothes, a shovel, a scraper, sand or cat litter for traction and jumper cables. Keep the gas tank near full.
  • Know your alerts: sign up for your community’s warning system. NOAA Weather Radio and the Emergency Alert System also carry warnings.

What to do during a blizzard

If you are on the road

The best choice is not to be there. If you are already driving and visibility collapses, the NWS advises pulling off the road and stopping until it improves. Turn off your lights and set the parking brake so another driver does not follow your taillights and hit you.

If your car becomes stuck, stay in the vehicle. The NWS warns that people who leave quickly become disoriented in wind-driven snow and cold. Its other steps:

  • Run the engine about 10 minutes each hour for heat, with a window opened slightly for fresh air.
  • Clear snow from the exhaust pipe to avoid carbon monoxide poisoning.
  • Turn on the dome light at night while the engine runs, and tie a brightly colored cloth to the antenna or door.
  • After the snow stops falling, raise the hood to signal that you need help.

If you are at home

If the heat goes out, the NWS suggests closing off unneeded rooms, stuffing towels in the cracks under doors, closing curtains, eating and drinking regularly and wearing loose layers. Ready.gov warns never to heat a home with a gas stove or oven and to keep generators and grills outdoors and away from windows.

The bottom line

A blizzard is defined by wind and visibility, not by how many inches fall. When winds of 35 mph or more drive snow hard enough to cut visibility below a quarter mile for three hours or longer, conditions become deadly for anyone caught outdoors or on the road. The safest plan is to prepare early, take Blizzard Warnings literally and stay put until the wind drops.

For real-time alerts, radar and forecasts when the next winter storm approaches, follow the LiveNewsWorld U.S. Weather Live center. NSSL’s guide to winter weather types is a good reference for the rest of the season’s hazards.

Sources