Has it ever rained as hard, as often, as it has this year?
I keep saying, “I have never seen it rain this hard, ever.” It isn’t just raining cats and dogs, it’s been raining bobcats and wolfhounds.
Well, it’s not just my imagination. There are facts, and some pretty reliable predictions, telling us that heavy storms are heavier, and happen more often, than they did 20-30 years ago.
In our region, the Nashua River Basin, autumn days with more than 4 inches of rain have increased from “never” in the 1970s to once in 10 years in 2019, predicted to increase to once in 3 years in 2030. Bigger picture: extreme rainfalls in the Northeast happened 40% more often between 2001-2012 than during the 1950s. (An extreme rainfall refers to a 2-day rain total that happens about once every 5 years).
And big rainstorms are now more dumping more rain on us.
In 1990, 5 inches of rain was a 100-year storm. By 2010, a 100-year storm was 6 or more inches of rain.
Before I started this project, I thought a “hundred year storm” happened every hundred years. I wondered, “how do they know that?” It turns out, that’s not what a hundred year storm is. It is a storm that is predicted to have a 1% chance of happening this year.
In 1990 a storm as severe as Hurricane Irene, which dumped more than 5 inches of rain on us and put us in the dark for weeks in 2008, was predicted to be a 450-year storm. By 2010, a storm that big was predicted to be a 100-year storm – more than 4 times as likely.
To make those odds a little more personal, imagine a raffle with 450 tickets. You’ve got one ticket. Do you feel optimistic? OK, now imagine there are only 100 tickets, which means you have 1 in 100 chances, or 1%, of winning. Feeling more optimistic, right? Now imagine there are only 10 tickets. Winning feels really likely, doesn’t it?
Now let’s talk about what you win: a big storm. Current predictions are as follows:
1/500 chance of a storm this year that drops 10” rain in 24 hours
1/100 chance of a storm this year that drops 7.6” rain in 24 hours
1/10 chance of a storm this year that drops 5” rain in 24 hours — and 1” in 15 minutes.
1/1 – certain to be normal – 2.7” in 24 hours this year.
That’s a lot of water. Where does it go? The ground can’t drink it that fast, so it roars on by making gullies and ponds on its way to the river. Those gullies can make a mess of roads (I’m talking about you, Kendall Hill). Floods on roads and in cellars. Chemicals, sewerage, and other undesirables floating by. Ground so saturated that roots are loosened, trees come down. The harder the rain, and the longer the rain, the more impact.
Fortunately, because we have these predictions from NOAA, we can prepare by taking control of runoff via ditches and culverts. We do pretty well now with a normal 2.7” rainfall. Are we prepared for 5” – the rainfall that is a 10% chance this year? Should we be planning to handle the 1/100, 1% chance rainfall of 7.5”? Hurricane Irene was a 1/100 storm. Once the storm hits, we might all wish we were better prepared. This isn’t an easy decision. Preparation costs the town money now: doing nothing costs the town and perhaps your own money when the storm hits.
One thing we can do, for free (nearly), is figure out which ditches, culverts, and floods have been problems. Then we can consider what it costs to fix them or what it might cost to ignore them until a storm hits.
Data for this article came from NOAA. Who is NOAA? The National Oceanic and Atmospheric Administration is the source of the weather data and modeling that are used to predict weather, and that virtually all TV meteorologists rely on. It issues flood, hurricane, tornado and blizzard alerts, and small craft advisories, which towns like Sterling use to alert residents. Their predictions are quite accurate for the big picture, like how much rain we’ll get this year, and where a hurricane is likely to reach land. They are also pretty good for 24-48 hour weather prediction.
How you react to a prediction and its consequences is personal. Take an umbrella, or put up with being wet? How our town reacts to predictions is a matter of public interest.
On Target 