Extreme Rainfall Events Growing More Common And We're Not Ready

Flash flooding in Richmond, Vermont back in 2013.
Extreme rainfall events are becoming more common
nationwide, and our infrastructure isn't ready for it.

 This week, we watched the skies and radar images in Vermont for signs torrential rains were about to lead to flash flooding.   

We lucked out this time, with no reports of any major problems, though there was some flash flooding in  neighboring New Hampshire. 

This was just one local weather event, or potential one.

We know, however, that extreme rainfall events are increasing in much of the world due to climate change. That's true of most of the United States, and it's true here in Vermont. 

The latest bit of news on this comes from non-profit First Street Foundation. The gist of this latest peer-reviewed study is that emergency and flood planners are not taking into account a "new normal" that is bringing more extreme precipitation events to most of the United States, including here in Vermont.  

As the Washington Post reports: 

"Americans should consider the rising number of catastrophic rain events in recent decades not as outliers, but rather 'as the new normal for many area," Monday's report finds. 'What this means for communities today is that their understanding of risk is often underestimated,' the group wrote, 'and in many locations the infrastructure in place or that is currently being built to protect communities, property and individuals is built to an insufficient standard.'

The study starts with basic physics. As air temperature increases, it can hold more and more water vapor. For every 1 degree C increase, 7 percent more water is carried in the same air volume. 

This can have a dramatic effect on the intensity, duration and frequency of heat rain events.  

First Street looked at 795 accurate rainfall recording stations across the United States to reach their conclusion that 1 in 100 year rainfall events are actually now more frequent than the term implies. 

By the way, for accuracy, a 1 in 100 year rainfall event doesn't mean you'll have to wait 100 years for the next big flood. It just means that each year, there's a 1 percent chance of such an extreme event. 

In any event, what were 1 in 100 year events are now 1 in 50 year events. Or even sometimes more frequent than that. 

According to First Street: 

"The analysis reveals that a significant number of highly populated areas are experiencing higher flood risk than what the local communities consider a 1-in-100 year event. This highlights the urgent need for accurate rainfall data to inform infrastructure design and investment decisions. Furthermore, the analysis shows that cities like Baltimore, Dallas, Washington DC and New York City face substantial increases in risk."

The Washington Post gets into specifics: 

"By First Streets estimates, more than half of Americans now live in an area that is twice as likely to experience such a rain event than is predicted by Atlas 14, produced by the National Oceanic and Atmospheric and considered the gold standard of precipitation frequency estimates across the United States."

Current standards for building flood protections, proper drainage, and bridges and culverts large enough to handle floods cling to the old normal. The "new normal" makes these building standards obsolete, leaving communities more vulnerable to flooding. 

NOAA is working on an "Atlas 15" that would incorporate the changes in rainfall intensity and include estimates on future climate conditions. This takes a lot of research, testing and smart people, so Atlas 15 won't be available until 2027 or so. 

The First Street Study concludes that while we in Vermont are definitely at risk from these heavy rainfall episodes, other parts of the nation are in even worse shape.  Places like the Gulf Coast, much of Appalachia, some of the big cities in the Mid-Atlantic, and sections of the Midwest are most at risk. 

Here in Vermont, precipitation has increased by 21 percent since 1900, according to a 2021 Vermont Climate Assessment from the UVM Fund Institute for Environment.

The Vermont Climate Assessment states:

"Vermont now experiences 2.4 more days of heavy precipitation than in the 1960s, mostly in the summer. With flooding expected to increase, improved stormwater infrastructure and planning is required to reduce damage to homes, roads, bridges and farm fields. Heavier rainstorms will impact farm and forestry operations."

Vermont has been at risk for flash flooding almost all week.  As noted, we managed to avoid trouble this time.   Obviously a very good thing.  But when will our luck run out. 

Given the changing climate, the answer is probably sooner rather than later.