Illinois State Climatologist Jim Angel rounded up some explanations of last month's record heat. Here's the NWS Chicago office:
The 2.7°C difference between the average temperature for March 2012 at Chicago-O'Hare and the previous record for warmest March is by far the largest difference between 1st and 2nd place between record warm or cold months in Chicago. The second largest difference between 1st and 2nd place is the 1.7°C separating the average temperature of January 1880 (4.3°C) and January 1933 (2.6°C) for the warmest Januarys on record in Chicago.
The 8.67°C departure from normal for March 2012 is the 3rd largest departure from the 1981-2010 normal for any month in Chicago. The only months that had higher departures from normal were January 1877, which with an average temperature of 4.3°C, was 8.9°C above the 1981-2010 average January temperature, and December 1877, which with an average temperature of 6.3°C, was 8.72°C above the 1981-2010 average December temperature.
For a more technical discussion, check out NOAA's analysis, which concludes:
A black swan most probably was observed in March 2012 (lest we forget 1910). Gifted thereby to a wonderful late winter of unprecedented balmy weather, we also now know that all swans are not white. The event reminds us that there is no reason to believe that the hottest, "meteorological maddest" March observed in a mere century of observations is the hottest possible. But this isn't to push all the blame upon randomness. Our current estimate of the impact of GHG forcing is that it likely contributed on the order of 5% to 10% of the magnitude of the heat wave during 12-23 March. And the probability of heatwaves is growing as GHG-induced warming continues to progress. But there is always the randomness.
April, so far, has been bog-standard normal.