What has been a very chilly Chicago April will end on a warm note Monday, as brisk southerly winds send the mercury into the 70s for only the second time this month. April will finish as the city’s fourth coldest dating back to 1871. The month’s chill, which produced nine days with observed snowfall, was still noticeable early Sunday, when widespread freezing temperatures numbed the area, including record-tying lows of 27 at Rockford and 31 at Chicago. May will open much warmer, with temperatures reaching the lower 80s Tuesday, the city’s warmest readings since Oct. 9. The warmth will set the stage for Chicago first thunderstorm activity since February. Frequent rounds of showers and thunderstorms later this week could deliver heavy rainfall as the delayed-growing season finally revs up.