Why the Vermont flooding was so bad: 5 key factors

Publish date: 2024-08-19

The prolific flooding that inundated portions of New York’s Hudson Valley and Vermont on Sunday and Monday after about two months’ worth of rain fell in two days wasn’t triggered by a tropical storm or hurricane, but rather by low pressure and a cold front associated with a more typical summertime weather system that encountered a powder keg of atmospheric conditions.

Rainfall that saturated the ground ahead of the storm, double the normal amount of moisture, an atmospheric traffic jam and Vermont’s flood-prone terrain were among the elements that contributed to the catastrophic flooding. On top of that, scientists suspect that human-caused climate change boosted the intensity of the rainfall.

Here are five key ingredients that came together to create a historic flood event.

Ground was already saturated

Plenty of rain had already fallen across the region during the previous month, especially in the two weeks immediately preceding the flooding. Many locations had received 4 to 8 inches of rain since the last week of June, or about 150 to 300 percent of what they normally get during that time.

With the ground already saturated, there was nowhere for so much rain — more than 8 inches in some places — to go in such a short period. The result was nearly 300 reports of heavy rain and flooding in two days and some of the worst damage since Hurricane Irene in 2011.

Additional flooding was occurring late Monday and into Tuesday in Vermont as the Winooski and Lamoille rivers surged to major flood stages. River volumes across the Northeast had already been increasing because of the above-normal rainfall in late June and early July.

Monster moisture plume

At the middle levels of the atmosphere, the storm’s circulation transported a saturated plume of moisture from the tropical Atlantic Ocean all the way north to New England. The amount of precipitable water, which indicates how much moisture there is in the atmosphere at a given time, reached nearly 2 inches in some locations, which is about an inch above normal for the time of year.

Such plumes of extreme moisture are not unlike the atmospheric rivers that are common on the West Coast and pummeled California with historic amounts of rain and snow earlier this year.

A very warm Atlantic Ocean may have contributed to the intense rainfall as well. The coastal waters from Florida to New England are experiencing a marine heat wave, with water temperatures near Florida simmering in the 90s amid record-breaking ocean heat worldwide. Warmer waters can evaporate more moisture into storms, resulting in heavier rain.

Storm slowed by atmospheric traffic jam

The overall weather system moved slowly because of a blocking pattern in the atmosphere that led to the storm’s low-pressure circulation getting stuck behind a large area of high pressure over Greenland. A similar pattern helped create the conditions that trapped smoke over the Northeast in early June.

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The weather system’s slow movement and wind direction contributed to the massive amounts of rainfall.

Storms formed parallel to a slow-moving cold front, oriented south-north, approaching from the west. At the same time, the winds that steered the storms also were blowing from south to north. That allowed storms repeatedly to develop and track across the same areas, a situation known as “training” because storms come through one after another, like the rail cars of a train.

Vermont’s vulnerable valleys and mountain towns

Vermont is especially vulnerable to flooding because of how its steep terrain funnels water. Water draining into a valley surrounded by mountains piles up faster and higher than water spread over wider, flatter spaces. This not only increases the risk of flooding, but it also heightens the likelihood of landslides.

The terrain-intensified runoff means streams and rivers are “flashier” and faster-rising than they might be in larger watersheds or flatter areas.

The state’s vulnerability to flooding is compounded by the fact that its interior experiences heavy rainfall less frequently than most of the country, meaning rainfall of the magnitude seen on Monday was much less common than it would have been elsewhere. Because the frequency of heavy rain informs flood codes, infrastructure and natural streambeds, a 6-inch rainfall in Vermont had much more potential to be damaging than it would have been in a place where such rain is more common.

The role of climate change

I've plotted the rain totals and average recurrence intervals (ARIs) of the astonishing storm that's continuing to cause severe flood impacts across the Northeast.

In Vermont, this was a widespread 50 to 200+ year rain event. pic.twitter.com/2heqzfXgxb

— Jacob Feuerstein (@Jacob_Feuer) July 11, 2023

New data from the nonprofit First Street Foundation indicates that climate change is making bursts of heavy rain and flooding more likely in parts of the country. Heavy rain events that used to occur approximately once every 100 years are happening more frequently in some locations.

Human-caused climate change — temperature increases linked to human activity — is increasing the amount of water vapor in the atmosphere that is available to storms. The Intergovernmental Panel on Climate Change projects that heavy precipitation will increase by 7 percent for every 1.8 degrees Fahrenheit (1 degree Celsius) of warming.

There is also evidence that atmospheric blocking patterns such as the one that was in place this week are connected to climate change. For example, a new research paper finds that diminished snow cover in North America because of climate change is linked to summertime blocking over Greenland, although the relationship between such blocking and climate change has been a matter of debate.

Jacob Feuerstein and Jason Samenow contributed to this report.