During the entire month of May, NWS Chicago only issued two severe thunderstorm warnings and no tornado warnings, making it one of the quietest severe weather May's for warnings dating back to 1989.
May 2015 was abnormally void of severe weather across north central and northeast Illinois as well as northwest Indiana. In fact, dating back to 1989 there have only been 3 years (0 warnings in 1992 and 2002 and only 1 warning in 1990) with fewer severe thunderstorm and tornado warnings issued during what is typically one of our region`s busiest months for severe weather.
During May 2015 only 2 severe thunderstorm warnings were issued (one on May 5th, one on May 8th) and no tornado warnings were issued.
Dating back to 1989, the average number of convective warnings (tornado and severe thunderstorm) issued by our office in May is around 28, so 2015 was certainly well below average. The busiest May in terms of severe weather was back in May 2004 when 131 convective warnings were issued, which includes 26 tornado warnings!
Through the end of May, it has also been an unusually quiet severe weather season with a total of only 24 convective warnings issued by our office, which is tied for the 7th quietest start to severe weather season. Of those 24 warnings, nearly all of them (22) were issued during the April 8-9th severe weather event.
Interestingly, dating back to 1989, the top 10 seasons that had the fewest warnings issued up through May 31st also tended to see the trend for below average severe weather continue for the remainder of the year. In fact, of the 10 years with the slowest starts, 6 of them saw a below average number of warnings issued after June 1st...3 were near normal...and only one year (2010) saw an above average number of warnings after June 1st.
As was evidenced by the April 9th tornado outbreak, it is not necessary for a severe weather season to have below or even near average number of warnings in order for there to be destructive life altering severe weather events. In fact, prior to the April 9th tornado, the previous violent tornado in our area was back in August of 1990, another year which featured a below average amount of severe weather through may as well as for the entire year.
While May was unusually quiet locally for severe weather, the same cannot be said for the southern plains where many offices in Texas and Oklahoma have been hammered time and time again by severe weather during May.