As the sun gradually lowers in the sky in the Northern Hemisphere during the fall, cold arctic and polar air masses intrude farther and farther south into the United States. Disturbances forming along the boundary between the cold polar air and the relatively warm, tropical air sometimes turn into winter storms. These are usually large, intense low-pressure systems that may cover tens of thousands of miles. Illinois' location in the Midwest and its great north-to-south extent place it in the path of many of these storms. When conditions are right, these storms can strike Illinois hard, leaving snow and ice over all or parts of the state.
Severe winter storms in Illinois produce more total damage than any other form of short-term severe weather, including tornadoes, lightning, and hail. Central Illinois has the distinction of being in the nation's primary area for severe freezing rain (ice) storms. However, any part of the state is apt to have a severe snow storm or ice storm.
What is a severe storm? How does it impact our daily lives? How can we prepare for it? What should you do and not do when a winter storm strikes?
Illinois, on the average, experiences five severe winter storms during the November-April period. These storms may be those with only heavy snow, or with snow and ice mixed, or with ice (glaze) only. Although the average is five per winter, as many as 18 storms have occurred in one winter (1977-1978) and as few as two (1921-1922). The number of times severe winter storms have occurred in the various sections of Illinois is shown in Figure 1.
January is the most favored month for severe winter storms, although December, February, and March are close behind in numbers of storms. The earliest a severe winter storm ever hit Illinois was on October 28-30, 1925, and the latest one ever to hit was on May 1-2, 1929. The maps in Figure 2 depict the snowfall patterns resulting from these storms.
A study of the number of times that severe winter storms have occurred on each date reveals that December 24, 25, and 26, and March 2 and 3 are high incidence periods. There are 2 chances in 10 that a severe winter storm will occur somewhere in the state on these dates. In contrast, low storm incidence periods are December 3-4, December 15-16, January 3-5, January 23-28 (normally the January thaw period), February 20-24, March 15-17, and March 21-24. Although historically these are low incidence periods, this does not mean that storms will never occur on these dates. In fact, in 1977 two severe winter storms hit Illinois during the December 2-6 period.
Scientists at the Illinois State Water Survey studied 304 severe winter storms that occurred in the years 1900 to 1960. Some features of these storms were averaged together to form a model of the typical severe winter storm in Illinois. The model (Figure 3) depicts the pattern of freezing rain, sleet, and snow that typically exists with severe winter storms. The snow area of 4 inches or more is about 215 miles long and 70 miles wide, lying with a southwest-to-northeast axis somewhere in Illinois.
Many severe winter storms strike Illinois will probably not have the exact rain, sleet, and snow patterns depicted by this model. However, the model is useful in that it shows us several relationships. First, the very heavy snowfall (12 inches), if it occurs, usually is in a relatively small area that is less than 3 percent of the area affected by the storm.
The model also shows us that different types of precipitation can fall in the same area. Note how the sleet area overlies the southern half of the heavy snow area. The fall of snow is preceded sometimes by the sleet or occurs at the same time as the sleet in the initial stages. Farther to the south is the freezing rain zone, and the model shows it is not uncommon to have freezing rain and sleet together. The center of the freezing rain zone is usually 50 miles or so south of the 6-inch snow area.
The storms that affect Illinois during the winter most often do not develop in or near the state. Rather, these storms start to take shape hundreds or even thousands of miles away. Low pressure systems traveling east from as far away as Asia may move into the United States from off the Pacific Ocean. Many of these disturbances end their travels in the mountainous Southwest, dying out as they cross the rough terrain. Some, however, do make it over the mountains and redevelop just east of the Rockies.
One of the most favored areas for this re-development is in Colorado and the surrounding region. Water Survey climatologists discovered that this area was where most of our severe winter storms in Illinois come from. There are also two other areas that favor development of severe winter storms, although to a lesser extent. These areas are the Province of Alberta (just east of the Canadian Rockies) and the Texas Gulf Coast. As shown on the map in Figure 4, there are five storm types for Illinois based on:
Not all low pressure systems that develop in the favorable areas during the winter become severe storms. Thankfully, only a relatively small percentage of all lows that form, regenerate, or intensify in these three source areas actually lead to severe winter storms in Illinois.
Storms tend to form where there are great contrasts in temperature. During the winter months the lee side of the Rockies and the western Gulf Coast provide these battlegrounds for the air masses. Once storms develop, the winds in the upper atmosphere determine where and how fast they move, and ultimately if Illinois will be their target.
The severe winter storms that affect Illinois tend to follow certain paths. The most damaging storms, on the average, are those that originate in Colorado and end up moving just south of Illinois (Types 2 and 4 on the map in Figure 4). Cold air north of the storm center in the Great Lakes or upper Midwest can then feed into the storm - the necessary ingredient for snow or ice.
Precipitation with most major low pressure systems tends to fall east and north of the storm center (see Figure 5). If the air is cold enough in the winter, the precipitation will be snow, sleet, or freezing rain. The reason the Type 2 and 4 storms usually result in the most damages is that the paths they follow tend to keep Illinois in the area of maximum precipitation for the greatest amount of time. In the record-breaking winter of 1978-1979, nine of the 18 severe winter storms were Types 2 and 4, and five of the most damaging storms during the winter were Types 2 and 4.
When the word "snow" appears in a weather forecast, it arouses a great deal of concern for most people, no matter what amount is expected. One inch of snow may result in minor travel delays, while 10 inches can close down a city.
One type of severe storm in Illinois produces a snowfall of 6 inches or more in 48 hours or less somewhere in the state. However, the severity of a particular snow storm is not measured solely by the amount of snow that falls, but also by:
The amount of water in a snowflake is determined largely by the temperature of the air it forms in. If snow is falling with the air temperature at 32 °F, 1 inch of that snow, if melted down, would yield on the average 0.10 inches of water. However, if the snow was falling at, say, 10 °F, that 1 inch of snow would yield an average of only 0.05 inches of water if melted. In other words, at lower temperatures a given amount of snow contains less water, making it lighter and fluffier, than the same amount of snow at higher temperatures.
Many of us know what it is like to shovel heavy, wet snow from the sidewalk or driveway, and how comparatively easy it is to clear the same amount of snow when it is much drier and lighter. This same relationship also makes it easier for the wind to pick up snow from the ground. Snow rarely drifts when the temperature is near the freezing point as the snow is falling. As the temperature falls farther below freezing, however, the snow is prone to being blown into drifts.
Our typical severe storm: 6 inches or more of snowfall that occurs when the temperature is near freezing will result in considerable inconvenience and disruption of daily activities, but the snow can normally be cleared from roads and walks without too many problems. But a 6-inch snowfall at a temperature of 10 °F with strong winds is going to result in considerable blowing and drifting of snow, which will choke highways, strand travelers, and isolate towns. Even after the snow stops falling, the snow on the ground may continue to blow and drift for hours and perhaps days, depending on how long the winds remain high. Conditions may be just as bad as when the snow was falling, preventing snow removal from streets and highways.
Freezing rain occurs when rain developing in a relatively warm (above freezing) layer of air falls through a layer of air that is below freezing (25-32 °F). The rain is "supercooled" (still liquid) as it falls through the cold layer near the surface of the earth. When the supercooled but still liquid raindrops strike the ground or an object already below freezing, they freeze on contact. The resulting coating of ice is commonly known as glaze.
A heavy accumulation of ice can topple power and telephone lines, television towers, and trees. Highways become impossible to travel on, and even stepping outdoors on foot can be an extremely risky undertaking. If you have lived through an Illinois winter, chances are very good that you have already had some encounter with freezing rain.
The severity of an ice storm (that is, the amount of damage) depends on:
Urban areas tend to suffer more damage than rural areas because of the concentration of utilities and transportation systems (aircraft, trains, buses, trucks, and cars), all of which may be affected to a great degree by the ice storm.
The area most likely to experience freezing rain in Illinois is a west-southwest to east-northeast band extending from Pike County in western Illinois to Iroquois and Vermillion Counties in eastern Illinois (see Figure 6). This area corresponds well with the average motion of ice storms. The worst icing situations tend to result from storm Types 2 and 5 (Figure 4), moving from the west-southwest or southwest. In a study of 86 ice storms, the maximum number of storm motions were from the west-southwest. Ice-only storms (no snow) tended to move from the southwest. On the average here in Illinois, we will experience 15 severe ice storms in a 10-year period.
Since 1976, residents of Illinois have lived through three of the most severe winters of the century. The winter of 1977-1978 holds the distinction of being the worst, followed by the winter of 1981-1982 and then the winter of 1978-1979. These winters produced a total of 53 severe winter storms.
One of the most severe ice storms to hit central Illinois since 1967 began the morning of March 24, 1978. Freezing rain continued until the morning of March 25, and by the time the rain ended, 1/2 to 2 inches of ice coated a 90-mile-wide belt west to east across central Illinois. One million people were without power at least 24 hours, and some outages took up to two weeks to repair. Over 1000 auto accidents occurred, and there was an estimated $20 million in tree losses. Twenty-four counties in Illinois were declared disaster areas. Figure 7 shows the snow and ice pattern for this storm.
The winter of 1981-1982, the second most severe winter in this century, featured 18 severe winter storms, 8 in January alone. The most damaging storm of that winter struck on January 29-31, 1982. Ten to twenty inches of snow fell on an area extending from the southwestern counties of the state to east-central Illinois (Figure 7). The storm resulted in 10 deaths, and the National Guard was called out to help with snow removal in southern Illinois. In the ten days following this storm, these same areas received another 5 to 15 inches of snow, leading the Governor of Illinois to declare 15 southern Illinois counties disaster areas.
The disabling effects of large amounts of snow or ice on daily activities are quite extensive. Metropolitan transportation systems are usually affected first and hardest. Transportation is the backbone of our economy, and the storm effects lead to impacts on domestic and commercial activities.
Losses due to a major winter storm may result from lost revenue to the state from people not working and businesses closed (lost taxes) and from increased expenses for snow removal and salting operations. The delivery of goods and services may be sharply curtailed. The effects of a severe winter storm may impact many facets of our lives.
However, not all the impacts are detrimental. A bad winter storm keeps people indoors. Crime rates tend to drop (although police are kept busy with accident calls), and the incidence of colds and flu may sometimes be reduced because people have to stay indoors and public meetings or other gatherings are called off. Stores may profit from sales of sleds, toboggans, snowmobiles, and skis. Sales of snow shovels, snow blowers, and snow tires typically increase, especially if the storm occurs early in the winter. However, the undesirable impacts of these storms far outweigh the desirable ones.
Much of the material in this report was based on research at the Illinois State Water Survey and is described in more detail in the following Water Survey publications: