Chicago's normal high temperature for April 17th is 16°C, which by strange coincidence is the new record high for January 29th:
The warm front associated with the strong low pressure system passed through the Chicago area between 2 and 3AM on it’s way north and at 6AM is oriented east-west along the Illinois-Wisconsin state line. South of the front south to southwest winds 24 to 45 km/h and temperatures in the upper 10s°C prevail – Wheeling actually reported 15.6°C at 6AM. North of the front through southern Wisconsin and farther north, winds were east to southeast and temperatures near freezing. Milwaukee at 6AM was 3°C.
Moreover:
The 18°C high projected for Chicago Tuesday easily replaces the day's previous 99-year record high of 15°C set in 1914 and is a reading just 1.1°C shy of the city's all-time January record high temp of 19°C set back on Jan 25, 1950. Only 5 of the 34 January 60s [Fahrenheit] on the books have made it to 18°C.
Temps in the 60s [Fahrenheit] in January are incredibly rare—a fact which can't be overstated! In fact, just 21 of 143 Januarys since records here began in 1871 have produced 60s.
The city's last 16°C January temperature took place 5 years ago when the mercury hit 18°C on Jan 7, 2008.
Ordinarily in the middle of winter in Chicago it would be customary at this point to say "It was last this warm in..." and throw out a date from last summer. But no, this is the new world of climate change, so I can say: "It was last this warm December 3rd."
Of course, it can't last. Here's the temperature forecast starting at noon today (click for full size):
January to April to January in three easy steps...