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The Rainfall Every Hundred Years

RainfallEarlier this year, the Fort Calhoun nuclear facility in Nebraska was surrounded by the Missouri River, which jumped its banks due to a strong winter runoff and heavy spring rains. That flood was characterized as a 100-year event. 

Is a 100-year event like the cicadas that swarm the Washington area every 17 years? No, not at all. The cicadas arrive on a predictable schedule. Seventeen years elapse and there they are again.

This is a better description of a 100-year event:

Of course, return period doesn’t mean that we won’t see that kind of rain in those locations for several decades (or centuries). A 1 in 100 year rain means that there is a 1% chance of seeing that amount of rain in any given year. A 0.1% chance is true for a 1 in 1000 year event.

I assume that the 1 in 100 number changes every time something similar takes place – obviously, the odds of it happening in a given year drop if it happens more frequently. In any event, what is being discussed here is the rainfall in the Washington area last week.

On Thursday, September 8, Ft. Belvoir [in Virginia] received an astounding 7.03” of rain in three hours. According to the National Weather Service (NWS), that amount of rain in that amount of time was an “off the charts above a 1000-year rainfall (based on precip frequency from Quantico).”

The rain every thousand years – good title for a novel. There’s more (a bit edited – see the link for more):

  • The Bowie IFLOWS gauge recorded 4.57 inches in 3 hours, which is about a 200-year rainfall.
  • For Upper Marlboro and near Ellicott City, Wednesday’s rains were a roughly one in 50-100 year event.
  • For Westview , Wednesday’s rains were a roughly one in 10-25 year event.
  • The Kingstowne IFLOWS gauge in Fairfax County recorded 5.47 inches in 3 hours, which is approximately a 500-year rainfall for that timeframe.
  • The Reston IFLOWS gauge in Fairfax County recorded 6.57 inches in 6 hours, which is also approximately a 500-year rainfall.

These places are all in Virginia and Maryland.

One might be tempted to ask: Global warming?

Although the ties between climate change and extreme weather events often elicit absolute statements from advocates on various sides of the climate change issue, the reality is that while much can be said, much remains unknown. The challenge for reporters as well as scientists is to accurately convey both the scientific findings and the uncertainties surrounding them.

So perhaps, perhaps not. But:

The physical mechanism behind the link between warming global average land and ocean temperatures and more frequent heavy precipitation events is rather simple to understand - as air warms, it can hold more water vapor, which means that more water can then be squeezed out of the atmosphere as liquid or frozen precipitation.

So, um,maybe more perhaps – in intensity if not frequency – so it may be that a strong rainfall previously thought a 100 year event becomes a 50 year event because the intensity of rainfall generally increases. Or the floods along the Missouri become at least somewhat more frequent. (Tornadoes need not apply – different dynamics.)

A report published by the Bush administration in 2008 (based in part on the work of the U.N. IPCC, but focusing specifically on North America) had already concluded that human activities are associated with changing characteristics of certain kinds of extreme weather events, and the evidence keeps on accumulating.

And keeps on accumulating. The world has not stopped working on this issue, and the United States – industry and government and plain old folks – are still working on mitigating the effects of climate change. While there are countervailing forces, certainly, there is also a strong forward momentum.

Nuclear energy has different parts to play in different issues, but it plays its part in this one just by existing and expanding its share of U.S. electricity generation – if not quite yet, soon. So do solar, wind, hydro and other energy sources.

In the meantime, the river will rise and the rain will fall.


It doesn't affect the article, but it is worth noting what the weather bureau means when they talk about a 1000 year rainfall, or for that matter when they say that there is a 50% chance of rain tomorrow.

Weather forecasts, including those of extremes, cover areas, not a single geographic point. So on average there will be one thousand year rainfall event per year, per every thousand locations covered. Similarly if the weather forecast for your area is a 50% chance of rain tomorrow, the expectation is that half the area buildings will get rained on. (Those people who move around a lot will tend to get rained on part of the time.)

Are there more than one thousand areas in the US that will have significantly different weather over the course of a year? Probably, although a lot of them will be in Alaska. So a couple of extreme (local) weather events per year are not surprising.

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