We are inundated by statements about 100 year floods, blizzards, etc. Examples of 100 year events:
March 5, 2014 Christchurch, NZ flood
June 7, 2012 Colorado Springs, Co flood/storm
Dec 16, 2013 Jerusalem blizzard
July 9, 2011 Phoenix, Az dust storm
Where I live, in 2010 and 2011 water ran over a dam’s spillway about 40 miles from where I live. In 2010, the news called it a “once-in-a-lifetime” event. The following year, when it happened again, people were saying “What? I thought it was a once in a lifetime thing.” Those of us who had lived here for many years also realized that in 1983 or 1984, the water went over the spillway then, though not as spectacularly as in 2010 and 2011. So what is the deal with “once in a lifetime” and “100 year storms”?
Calling weather events once in a 100 year events is quite misleading. The proper statement is the probability of a storm (flood, drought, etc) of such magnitude has a 1% chance of occurring. This probability is per event—like rolling a die. Each number has a one in six chance of showing on a roll and the outcome of one roll does not effect the other rolls.
The terminology “100 year storm” gives a false impression of what is going on. For example, floods in the Midwest USA were 500 year floods and the ones in 1993 were 100 year floods in the same area. The floods of 2007 and 2008 each had a probability of .2%. It’s quite common to have these events two years in a row and then none for another decade or more. There are categories for most rain and snow in 8 hours, 12 hours, 24 and 48 hours.
What does it mean when there are several of these unusual floods or rains in set period? It’s time to recalculate the probability of the events occurring—rework the frequency analysis. The numbers are not rules and changes in frequency should not be ascribed any special significance.
There are several factors that influence the severity of floods, droughts and wildfires that are not part of the weather patterns. Where people live, how much irrigation is used for lawns and farms, how much fuel there is for fires all contribute to the severity of a weather event.