Parts of the Northeast have received more than a foot of rain since the start of the month, with more than a week left to go in July. Relentless rounds of rain and thunderstorms have made Boston look more like Seattle, and the waterlogged pattern shows no signs of breaking.
As of July 21, Boston has picked up 9.39 inches of rainfall this month. The city has received four times more rain than typical; the average through the third week of July is only 2.18 inches. On just two days — July 15 and 20 — has there been no rain in Boston.
Boston now only has 0.07 inches to go for first place when it comes to wettest Julys. And it’s not just Beantown that is swimmingly wet.
Worcester, a hilly city 50 miles west of Boston, has seen more than a foot — 12.7 inches to be exact. It is at nearly 500 percent of its average month-to-date. Worcester has experienced its wettest July since 1948, when record-keeping began. Only three days in the month have been rain-free, but five have featured an inch or more. So far, it has beaten the old wettest July, in 2009, by more than two inches.
It hasn’t been much better in Hartford, Conn., which has recorded 9.71 inches of rain. And a couple hours’ drive down Interstate 95, Central Park in the Big Apple is sitting at 9.20 inches.
Central Park is 2.69 inches shy of achieving its wettest July since record-keeping began in 1869. The current month stands at fourth place, but a single thunderstorm could easily be a tipping point in the rankings.
Why so much rain? It’s been a combination of a favorable atmospheric pattern and a few dashes of meteorological randomness.
The month has featured heat dome after heat dome, or the anchoring of warm high pressure, over much of the West and the central Lower 48. That has allowed the jet stream to arc southward over the Northeastern, permitting some cooler, drier air to periodically punch in.
It’s those colder pushes running into humid air masses that have helped produce more showers and thunderstorms than is typical.
The month also began with Tropical Storm Elsa, whose moisture supercharged downpours over part of the Northeast the day before the center passed.
“It’s been a little combination of [shower and thunderstorm activity and frontal systems],” said Rob Megnia, a meteorologist at the National Weather Service serving Boston and surrounding parts of southern New England. “Any time we get the moisture and the forcing available in the afternoon in the summertime, you’re going to get the convection.”
The term “convection” describes the rising of heat through a fluid — in this case the atmosphere — and in meteorology is used as an allusion to downpours and thunderstorms.
In recent weeks, the rain has been heavy but has been distributed over a long enough window so as to not create too many flooding issues. That said, urban and small-stream flooding has occurred, and periodic flash flooding also has occurred.
“Last week in portions of central and eastern Connecticut, they did receive three to five inches of rainfall with one of our convective events,” Megnia said. “One of the streams flooded out and washed out an entire road after reaching major flood stage.”
Downpours early in the month also lashed New York City at rush hour, flooding subways and leaving roadways ripe for white water rafting.
With another wet pattern looking probable by the middle to the end of next week, any additional rain could lead to flooding, given increasingly waterlogged soils.
“We are pretty saturated,” Megnia said. “We’ve only had a couple of dry days this week. As of right now, our flash flood guidance is pretty low.”
In other words, the ground has a low threshold for flooding.
“Right now, for a large portion of western Mass, we’d only need an inch and a half or two inches in one hour for flash flooding to begin,” Megnia said. “With [more thunderstorms,] flash flooding wouldn’t be too difficult to achieve.”