The highest temperature in 2013 so far was a 43 degree high on both January 10 and April 3. You would need to go back to 1979 or 1881 to find a year with a maximum temperature lower than that so late into the year.
In 1881 it did not reach the mid 40s until the April 15, although, just a week later the first 70 degree day arrived right near the current average. In 1979, that first 70 degree day did not arrive until May 16 and April recorded mainly highs in the 40s. Persistent high temperatures in the 40s even lingered in May that year with nearly one inch of snow falling on May 5. Plus, although, no significant individual rain events were recorded, from the middle of April to the middle of May a total of 15 days recorded some rainfall (or snow).
So not only was it a cold spring in 1979, it was also, cloudy and wet much of the time. The odds favor our pattern changing before the middle of May this year, but if it happened once, it could obviously happen again.