From the U. of Chicago:

January 2017

University of Chicago Crime Lab

… Between 2015 and 2016, Chicago experienced 58 percent more homicides and 43 percent more non-fatal shootings. Annual increases of this size are not unprecedented among American cities, particularly in recent years, but are rare for a city of Chicago’s size.

One striking feature of Chicago’s increase in gun violence is how sudden it was: as of December 2015, there was no indication that gun violence was on the verge of rising sharply. But in January 2016, homicides and shootings surged relative to their 2015 levels and remained higher in almost every month that followed, threatening 20 years of progress on violent crime in Chicago. …

What caused Chicago’s sudden surge in gun violence in 2016 remains a puzzle. Weather cannot explain the surge in homicides and shootings, since monthly temperatures in 2016 were close to their historical averages. City spending on social services and public education did not change much in 2016 compared to previous years, and while the state budget impasse disrupted funding for many community organizations, this did not seem to change sharply in December 2015.