C’mon. You know you’ve done it. We’ve ALL done it. So fess up.

You’ve watched hours and hours of coverage of the Storm of the Century on the Weather Channel. You’ve drooled over the satellite imagery with the perfectly formed hurricane eye. And you’ve definitely rooted for the hurricane to move closer to the coast so that the whole eye shows up on the land-based radar.

Storms of the Century are great to watch … from afar through your TV set or now your computer monitor or your phone or iPad.

You likely have experienced one. And now you’re thinking you are safe. It won’t happen again in your lifetime, much less during the life of your mortgage. Um, no.

Storm of the Century doesn’t mean that the storm will occur once every 100 years. It’s a risk number. It means that there is a 1% chance of a storm of that size and duration (it’s a combination) will occur in any year. (1% = 0.01 –> 1/0.01 = 100-year storm; that’s how the math goes). A 500-year storm is a storm (rain depth and storm duration combination) that has a 0.2% chance of happening in any given year.

But that’s the probability in any one year. What about the chances of having one of these unlikely events during my mortgage life? As the graph below highlights, the likelihood of having a 100-year storm (1% probability of occurrence in 1 year) in the duration of a 15-year mortgage is about 14% and in my 30-year mortgage it is about 26%. For the 10-year storm, which is used as the storm size for sizing stormwater pipes in my borough, the likelihood of that system “failing” by being full and ponding in the streets is 80% during a 15-year mortgage time frame and almost 96% during a 30-year mortgage.

Time to take a breath, though, and remember that the storm sewer system being full does NOT mean that I should look for an ark and trying to gather up the animals two-by-two. A 10-year storm has a 10% chance of happening in any year. It’s a common storm and it is one where having a storm of that size usually results in a little bit of street flooding – maybe the entire road is under less than 1 – 2 inches of water.

It’s the rarer storms – the 100-year and 500-year or 1% and 0.2% storms – that will cause more damage and more likely to have people die due to flooding. In my borough, we’ve had 2 500-year storms in the last 6 years. Yes, that’s right. TWO 500-year storms in 6 years. Yes, the fact that a rare storm occurred one year does not stop it from happening again … and soon. And we can calculate that too! The chances of having 2 100-year storms (# of events = 2; red line on graph) in 6 years is slightly more than 0.1%, in a 15-year mortgage is 1%, and in a 30-year mortgage is about 3.2%. Still rare, but not THAT rare.

Well, we got 2 500-year storms in 6 years (Tropical Storm Lee in 2011 and a microburst storm in 2017). How likely is that? 0.006%. In a 15-year mortgage, the likelihood is 0.04%, while in a 30-year mortgage, the likelihood is 0.18%.

What does this mean to me? Consider flood insurance, even though I don’t live in a FEMA floodplain. (Urban flooding is material for a future post).

And we are just darn unlikely. We had 2 rare events in 6 years and that likelihood was 0.006%. Someone find the broken mirror or black cat or spilled milk. Fix our luck!