Great Flood of 2014: How historic was it?

DETROIT – Following a catastrophic weather event like last week's floods, it's only natural to ask, "How bad was it?" 

There are many ways to answer that question. 

For example, last Monday's rain shattered Detroit's all-time Aug. 11 rainfall record (was 2.06"...we more than doubled that). Another way to answer the question is by saying that our 4.57" rainfall (officially at Metro Airport) is the second highest all-time daily rainfall record in Detroit history. 

But how often should we expect something like this to happen? 

I computed those numbers for southeast Michigan, and the answer is astounding. 

After checking all of the rainfall reports from last Monday, I found that four different weather reporting stations measured six inches or more rain, and the National Weather Service reports that as happening in a 12-hour period. 

So, I ran the numbers for a six-inch rainfall in 12 hours on a NOAA website, and this type of rain event is STATISTICALLY expected to occur only once every 500 years.  That's right -- you just lived through a once-every-500-year rain event. 

Of course, that statistic is based upon a climatology of past rain events and, as we know, our climate is changing.  So, could this type of event happen again in less than 500 years? Possibly. 

Let's just hope it doesn't happen again in our lifetime.