What is the definition of a blizzard and how does a blizzard form?

A blizzard is the combination of cold temperatures, strong winds, and poor visibility due to blowing snow. Specifically, the Sioux Falls Forecast Office issues a blizzard warning when the following conditions are expected to occur for 3 hours or more:

  1. Sustained wind speeds of 35 mph or more, or frequent gusts to 35 mph or greater;
  2. Considerable falling and/or blowing snow that reduces visibility frequently to 1/4 mile or less.

There is no set temperature requirement for issuing a blizzard warning but often very cold arctic outbreaks that produce blizzard conditions are accompanied by temperatures in the single digits or sub zero readings. These temperatures combined with the wind produces life threatening wind chills making blizzards one of the most dangerous weather situations to affect the northern plains.

One misconception is that blizzards are often accompanied with heavy snow. A surprising fact is that often very little snow falls with a blizzard because there simply is not a whole lot of available moisture associated with these arctic outbreaks. There are occasional exceptions but for the most part it appears worse because the driving wind whips up existing snow reducing visibilities to less than 1/4 mile. In fact, we can have a blizzard without any falling snow! These are termed ground blizzards and can occur when strong winds cause blowing and drifting of loose, existing snow cover. In these cases, poor horizontal visibility is confined to a few feet above ground and often you may be able to see the sun under cloudless skies directly above.

As noted above the main driving force for a blizzard is very strong winds associated with an arctic outbreak. Typically, weather systems that cause blizzards tap extremely cold air that has been pooling over interior Canada and plunge straight south delivering the full brunt of the cold to the northern plains. Often, a jet, or strong northwesterly flow, develops near the surface and aloft ushering in this cold air and causing blowing snow. One of the strongest blizzards to affect South Dakota since the infamous January, 1975 storm struck just last year on January 9-10, 1997. Termed the "Blizzard of '97" this event lasted for 48 hours and shut down both I-29 and I-90 for several hours. One woman was stranded for 30 hours in northeast South Dakota. There were 12 to 18 foot drifts and wind chills in the -70 to -80 degree range were common.

Here are a few weather charts that show this blizzard in its full fury. The surface chart at the height of this event showed temperatures in the teens below zero with northwesterly wind gusts of 35 to 40 mph across the region. The upper air chart for conditions around 5,000 feet shows that a bitterly cold arctic airmass had spread over the Dakotas and southward into Kansas with temperatures of -24 to -28C. Also very strong northwest winds between 35 and 50 knots affected a widespread area of the northern plains. At around 20,000 feet the coldest air of the winter was poised over the northern plains with temperatures approaching -40C. However note that at Rapid City the temperature was -29C with warmer air over the west. This spectacular change in temperature, or gradient, resulted in a strong jetstream. This can be seen at the jetstream level where a core of 90 knot winds (red arrow) was diving along the Rockies helping to drive the cold air south.

The upper air balloon launched at Aberdeen, SD for Friday, Jan 10th showed just how deep the cold air was. The blue line shows a trace of temperature in Celsius as the balloon rose. You can see that from near the surface to 5,000 feet the air temperature was between -20 and -30C with slightly "warmer" air above 6,000 feet. On the right side the wind barbs show strong 40 to 45 knot northwest winds up through 5,000 feet.

To get a better representation of winds through the atmosphere we use information from a wind profiler. This is like a radar that is pointing straight up and shows us wind direction and speed versus time. One profiler located at Neligh, Nebraska represents conditions in the east while another located at Merriman, Nebraska shows conditions over the west. In both displays time increases from right to left. Both profilers show strong winds at the lowest levels but the Merriman profiler shows a deeper layer of stronger winds since the jetstream was overhead across the western Dakotas and Nebraska. Note the 120 knot wind around 21,000 feet (yellow arrow) at Merriman.

This event was typical of blizzards that affect South Dakota and will long be remembered for its severity and impact to those who live in the northern plains.

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