If you’re in New England and you’ve stepped outside at any point this summer, there was a good chance it was raining. Cities across the region had some of their wettest Julys on record. Flooding devastated Vermont and New Hampshire issued a record number of flash flood warnings.

A lot of that rain has come as downpours. Strong storms are typical in the summer, said Donny Dumont, warning coordination meteorologist with the National Weather Service in Gray, Maine. What’s not normal, he said, is “how frequently it’s occurred.”

While research shows that climate change is driving extreme precipitation in the Northeast, there are also particular weather patterns that settled over New England this summer that contributed to the intensity of the rain.

For most of the summer, low pressure systems — which are associated with inclement weather — have come through New England. Dumont said that’s because of a weather phenomenon known as the North Atlantic Oscillation, which is characterized by fluctuating pressure systems in the atmosphere over the North Atlantic Ocean.

The North Atlantic Oscillation has two phases, a positive one and a negative one.

For New England, positive phases are typically characterized by clear skies and higher temperatures. Negative phases typically bring increased precipitation and lower temperatures.

Typically, New England experiences a persistent negative phase during the winter and fall months, which is what gives us snowfall and cold winds. But New England has experienced this pattern since mid-June, which is when we began to experience more rain.

A negative phase means more low pressure systems over New England. Low pressure systems “mean more instability, more cloud cover, which means more thunderstorm capability, which is what we've experienced,” said Dumont.

Diagrams of the globe show differences between the NAO negative and positive modes. In the negative mode, the U.S. East Coast experiences cold and snowy weather while further north areas warm with less sea ice. It shows a "blocking" effect across the Atlantic Ocean on the jet stream headed toward Europe. The positive mode shows an inverse effect, with the warm and cold areas in the U.S. and Europe swapped and a jet stream that heads more directly across the ocean.
On average, the surface pressure near Iceland is relatively low (L), while the pressure near the Azores Island is relatively high (H). During a negative phase (left), this pressure difference weakens. During a positive phase (right), the difference becomes even stronger than usual. The variation in pressure patterns influences the strength and location of the jet stream and the path of storms across the North Atlantic.
Schematic Adapted From AIRMAP By Ned Gardiner And David Herring National Oceanic And Atmospheric Administration

The low pressure systems also haven’t been able to move much around New England because of other weather systems nearby.

Dumont said high pressure systems in the western U.S. and southern Canada create a “dome” in the atmosphere over these areas. And while these regions may really need the rain that our low pressure systems would bring, they can’t penetrate this dome of high pressure.

“We're kind of stuck for the moment,” said Alexandra Branton, a weather observer at the Mount Washington Observatory, which experienced a new record for summer rainfall this year.

Trying to develop more accurate forecasts as weather phenomena shift

The observatory provides its measurements and data to the National Weather Service to help improve their forecasting accuracy, because on the mountain’s peak, they can actually observe the conditions in the atmosphere before the weather reaches us down below.

Part of Branton’s job is to gauge future weather trends, but she said it’s been harder to draw conclusions recently.

“We're starting to do more research on weather extremes so we can try and figure them out. But when these unpredictable extremes happen, it's a lot more difficult to come to a definitive answer,” she said.

Recently, the National Oceanic and Atmospheric Administration, which operates the National Weather Service agency, announced it will invest $7 million to improve their weather forecasting.

Dumont said weather has already cost the economy billions of dollars.

“So $7 million is really a small drop in the bucket to increase our predictability, to make sure people are prepared and ready for these natural disasters and weather extremes, which are becoming more frequent,” he said.

As for what’s left of this summer, meteorologists are anticipating the rainy weather will continue.

And how does that make Branton feel?

“I would say this summer’s been a bummer. That's pretty much it,” she said.