As the center of a strong low pressure system tracked east out of Iowa through central Illinois into Indiana Thursday, a distinct line of demarcation between rain and snow set up in the vicinity of and along Interstate-80. Up to a half-inch of rain fell to the south, while nearly an inch of rain/snow/sleet mix fell to the north. As the low pressure drew away from the Chicago area, northerly winds pulled colder, snow-laden air out of Wisconsin into northeast Illinois – with the changeover from rain to snow occurring in the afternoon during the Thursday evening commute. Rush-hour traffic to the north and west out of the city was slowed considerably. Heaviest snowfall through mid evening was 9 inches just north of the Illinois-Wisconsin border at Kenosha – with 8.5 inches at Beach Park and 7 inches at Gurnee with snow still falling. Closer to Chicago snow amounts decreased sharply – tapering to just a couple inches in northern sections of the city to barely a trace of snow in southern Cook County.
At the same time the low pressure system tracks east into Pennsylvania Friday, a strong low pressure system will moves north up the east coast . When the cold air associated with the Pennsylvania low pressure merges with the warm moisture-laden low pressure along the east coast, hurricane-force winds will create blizzard conditions as snowfall that could top 2 feet brings travel to a standstill in the northeastern U.S. late Friday into Saturday.