When severe weather events like flooding or hurricanes occur, you may hear terms like “100-year storm” on the news. The “100-year storm” label makes it sound like that amount of rain only falls once every 100 years, but 100-year storms can actually happen two years in a row or even multiple times in the same year. 

A “100-year storm” refers to the amount of rainfall that has a 1 in 100 (or 1%) chance of occurring at that location in any given year. As of now in Pittsburgh, 2.56 inches of rainfall within 1 hour or 4.92 inches of rainfall within 24 hours would both be considered 100-year storms.

Storm sizes are estimated for each region based on a statistical analysis of local rainfall data. The Pittsburgh region only has 60 years of historical rainfall data, which makes the estimation of a 100-year storm less accurate. As we gather more years of rainfall data and as climate change continues to shift our weather patterns, the amount of rain associated with a 100-year storm in Pittsburgh will likely change.