Heavy rain causes flash floods in Greater Boston, inundating roads

Heavy rain inundated roadways and snarled traffic on Thursday morning in Boston and communities to its south.

While the heaviest rainfall from the storm had passed by 10 a.m., the National Weather Service said flood waters will take time to recede. Sections of I-93 were shut down on Thursday morning as highways became impassable.

NWS said the rain had the potential to overwhelm small creeks and streams, streets, highways and underpasses, as well as other poor drainage and low-lying areas.

Communities under a flash flood warning included Boston, Quincy, Weymouth, Braintree, Randolph, Milton, Stoughton, Dedham, Hingham, Canton, Scituate, Winthrop, Holbrook, Hull, Cohasset and Avon.

NWS reported instances of flash flooding in communities south of Boston on Thursday morning, including "significant" flooding on the Southeastern Expressway in Quincy, and flooded roadways in Sharon, Walpole and Hull.

The Massachusetts Department of Transportation closed the north and southbound sides of I-93 at exit 3 in Milton due to flooding just before 9 a.m., according to a post on social media. The route was reopened at about 9:45.

The agency also said it closed exit 8 northbound and exit 10 southbound on I-93 in Quincy and the I-93 northbound HOV lane in Quincy. The southbound lanes reopened at around 11 a.m.

Flooding on the Red Line between Quincy Adams and Braintree caused the MBTA to call in shuttle buses service to replace the subway just before 9 a.m. The shuttle buses were then canceled, overwhelmed by street flooding and heavy traffic. The MBTA announced it had restored Red Line service at 11 a.m.

In their flash flood alert, NWS instructed residents not to cross flooded roadways or drive on flooded streets, writing "most flood deaths occur in vehicles."

Brockton, Milford, Foxborough, Sharon, Walpole and the Attleboro area were included in a previous warning, which was set to expire at 9 a.m. The weather service had issued a flash flood watch Wednesday evening for the eastern half of the state, along with the entirety of Rhode Island and part of northeastern Connecticut.

While heavy rainstorms that cause flooding are not a new phenomenon in New England, climate change is making them more extreme. As the atmosphere warms, the air is able to hold more water — increasing the amount of rain and the frequency of severe downpours.

According to the most recent National Climate Assessment, the Northeast has seen a 60% increase in the number of extreme precipitation events since the 1950s.

Beyond localized road flooding, heavy rains can trigger so-called "combined sewer overflow" events in the 18 Massachusetts communities where stormwater and sewage from homes and businesses travel through the same pipes.

When the volume of water exceeds the capacity of the pipes, the system is designed to overflow into a nearby river or stream. In many cases, the sewage-laden water is treated with chemicals before being released, but in some communities, a heavy rainstorm means some amount of raw sewage will end up in the water.

Not all CSO outfalls are monitored in real time, so data can lag. But so far, CSOs have been reported in the last 24 hours in Boston, Somerville, Fall River, and New Bedford.

State officials advise staying away from contaminated bodies of water for at least 48 hours after a CSO event.

This is a breaking news story. This post has been updated to reflect new information from the National Weather Service.