A small area south of Boston took a big hit with epic rain and flash flooding. Here's why it was so rare.
The flooding south of Boston Thursday will be a day to remember in Massachusetts. It was a very rare and somewhat historic weather event.
Most of the rain fell in just a few hours, right during the morning commute. Three-to-six inches fell in a 2-to-3 hour deluge over a relatively small area. It was a highly localized event. The highest impact was only felt over a swath of land about 5-to-10 miles wide.
The first downpour was right around 5 a.m. Then, in the next few hours, torrential downpours continued to "train" over the same areas, dropping 2-to-3 inches per hour at times.
Finally, by about 9 a.m., the rain began to decrease in intensity, but the damage was already done. Interstate 93 was completely shut down in Quincy and traffic was at a complete standstill.
How rare is rain like this?
If you look at the highest rainfall totals, in parts of Weymouth and Hingham where more than six inches fell, in a timespan of less than six hours, that equates to about a 1 in 200-year event.
To put it another way, a flash flood of this magnitude has about a 0.5% chance of occurring in a given year.
Why so much recent flooding?
Flooding has been a headline in the news, locally and nationally, as of late.
This begs the question - is there something more to all of this? Is this related to climate change?
While it is difficult to solely blame any one event on these global issues, we have seen plenty of evidence that events like these may be happening more and more frequently in the years to come.
Our friends at have been researching the frequency of these "higher end" rainfall and flooding occurrences and the data shows some startling results.
Over the last 50 or so years, the average rainfall intensity has increased by nearly 10% in Boston. It's much higher in other parts of the country.
If you look at just the high-end events, the heaviest 1% precipitation days, the Northeast has seen the biggest increase, more than 60% in fact.
Perhaps the main reason for this is there's more water vapor in our atmosphere now compared to the recent past.
As global temperatures increase, over land and ocean, that allows for more water to be stored and eventually wrung out of our atmosphere.
For every 1 degree increase in temperature the air can hold 4% more water vapor.
So, simply put, when it rains, it pours!
