To Snow or Not To Snow
That is the Question
Typical La Nina Winter Pattern
While we look ahead to the month of November and the inevitable return of winter cold and snow for the Chicagoland area, it will be difficult to forget this year’s incredibly active hurricane season. In terms of accumulated cyclone energy (ACE), the National Hurricane Center determined that September 2017 was the most active month on record when compared to the 30-year climatology data of 1981 to 2010. As the hurricanes churned away in the Gulf of Mexico and Atlantic, warm, tropical moisture flowed northward into the eastern half of the United States, resulting in pleasant, yet above-normal temperatures.
Typically, remnant moisture from decaying storms can cause heavy rainfall in the Chicago area. However, the storm tracks from Harvey and Irma remained well east of Illinois, resulting in a prolonged period of dryness. In fact, after a rather wet June and early July, our weather turned incredibly dry, with less than one inch of total rainfall from August 3 to October 7, and September was the fourth-driest on record since 1871.
In addition to the dry conditions, temperatures soared to record levels during the second half of September, with seven consecutive days in the 90s, all record highs. The combination of the dry soil and the blocking pattern due to the active hurricane season helped foster a remarkable run of late-season heat. In fact, the month of September recorded more 90s than June, July and August combined. This has happened in only two other seasons.
So we’re all wondering how this sets up the winter season. Winter and summer seasons are never quite the same from beginning to end. With an increase in climate variability, it becomes difficult to characterize how a season will be from a temperature and precipitation standpoint. Last winter, an early season six-to-eight-inch snowfall occurred on December 4, followed by less than an inch of snowfall for all of January and February combined. The heaviest snowfall of the winter occurred in mid-March, when a 10-to-18-inch, lake-enhanced snowfall covered the eastern sections of the Chicagoland area and Northwest Indiana. Temperatures never got below zero after December.
Research shows that in a warmer winter preceded by a mild autumn, the possibility for lake effect snow increases during the first one or two arctic outbreaks. This might be contrary to public perception, because most people associate milder winters with less snow. However, it’s been shown that the longer the Great Lakes remain unfrozen, the more lake-effect snow there is. Although snowfall has generally averaged 35 to 40 inches in the Chicago area since 1871, annual snowfall has averaged 46 inches in the last 10 years. While the trend in the Chicago area is up, snowfall across the southern Midwest has actually decreased in the last 30 years.
The important keys to this winter’s outlook are, as always: 1) Ocean temperatures are an important piece to understanding the overall warmth of the atmosphere. 2) The warm September and October will lead to warmer temperatures in the Great Lakes and the possibility of several early season significant lake-effect snow events in December. 3) A strengthening La Niña can often allow a greater intrusion of Arctic air as the El Niño jet stream is minimized. 4) A subpar typhoon season in the Pacific can often lead to fewer Pacific storms migrating into Canada and the upper Midwest.
With these factors in play, it’s currently looking like the winter season will begin rather late in the Chicago area, with less than four inches of snowfall in December. January appears to be quite active, with several large mid-latitude cyclones moving through the region, capable of producing 12 to 16 inches of snowfall, followed by several rounds of Arctic cold. Colder than normal conditions will continue into February, with eight to 14 inches of snowfall, followed by repeated Arctic cold. A highly variable month of March is likely due to the warmer-than-normal oceans interacting with a very cold pool of air over the Northeast.
In summary, expect 28 to 32 inches of snowfall south of I-80, with 32 to 38 inches for the Chicago area, and upwards to 48 to 52 inches close to the Wisconsin border. A longer season of snow cover is expected, which will result in 10 to 12 days of below-zero weather. For those that live or play in the lake-effect snow belts of Michigan and Indiana, snowfall could range from 55 to 85 inches. Up north in the Lake Superior snowbelt, totals could approach 150 inches—but at that rate, who’s counting?
Rick DiMaio is a professional meteorologist and climate scientist, specializing in aviation meteorology and environmental sustainability. Since 1985, DiMaio has served the Chicago area as a TV and radio broadcast meteorologist, college instructor and flight operations aviation meteorologist. He is currently heard on The Mike Nowak Show.