FIRST ALERT: Monday brings heavy rain and severe flood threat to New England

We’re entering the critical hours as a potent weather system moves through New England. It will carry the potential for life-threatening flash flooding throughout Monday.

Sunday brought widespread rain and flooding from Northern New England to New Jersey. Between four and eight inches of rain fell through Eastern Pennsylvania and New Jersey. Due to the overall forward motion, and orientation of the storm complex, southern New England was majorly spared on Sunday. The same can’t be said for Monday though.

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As the front approaches, we’ll already have a fairly vast wingspan of showers and inundating rain from Southern Vermont into Central Massachusetts. We’ll likely have early morning flash flooding from Windham County, Vt., and just across the state line into Cheshire County, New Hampshire, as rain begins to fall near 7 a.m.

Sunday’s thunderstorms dumped heavy rainfall through the Monadnock Region, like 2.15” of rain in Keene. This has laid the foundation for a saturated to super-saturated ground, and the ground will have difficulty absorbing any additional moisture.

It’s at that point that the upper level low is moving through, which will likely induce more widespread heavy rain, leading to a high risk of flooding throughout Northern Vermont and the Champlain Valley. While the threat will modify, and potentially lessen throughout Monday night, it isn’t completely diminished until just before sunrise on Tuesday morning.

When does the rain start?

Across Southern New England, and the Boston metro, widespread rain is set to start just before 8 a.m. Monday. We’re not up against the severe flooding for a host of reasons. First and foremost, the significant rain from Sunday was displaced to our north. So the ground hasn’t been overly stressed with rain as of late. The strongest showers are also displaced well north and west.

What will be of concern for areas between Worcester and Interstate 495 is the heavy rain that arrives Monday afternoon. Stronger thunderstorms will spark between Franklin, Norwood, Framingham and Northbridge around 2 p.m. Boston should see moderate downpours between 1p.m. and 6 p.m., with most activity diminishing after 9 p.m. Coastal Communities across the Cape and Islands aren’t removed from rain, but should be spared of the worst of the storm threat.

Because of the nature of saturation of the rain, downed trees and powerlines become a concern; specifically where strong winds correspond to downpours. Though we’re not expecting widespread severe winds, rain will loosen soils, weakening roots of older and rotting trees, thus adding to the threat of trees coming down. So watch for downed powerlines. If you come across a downed wire, treat it as live…it could still carry a current and cause electrocution.

Localized flooding around Boston today, to life-threatening flash flooding in VT with a rare @NWSWPC high risk for flooding. I'll have more as we swap coverage from @NBC10Boston to @NECN at 7am! pic.twitter.com/6wxvofEzH8

— Pamela Gardner NBC10 Boston (@Pamelanbcboston) July 10, 2023

Airport delays likely

Airport delays and cancellations will likely mount Monday, too. Nearly 2,000 flights were cancelled within, into, or out of the United States according to FlightAware.com. Specifically from major Northeast hubs like Newark, JFK and LaGuardia. Airlines are playing catch up from Sunday, and the same storm system will be in our hair today, making it all the more difficult for flights to depart and land on time.

What will influence and limit Monday’s storm threat are cooler temperatures. With little sun and ample cloud coverage, the environment will start off much cooler than yesterday. The rain from Sunday has also helped that cooling, too. That said, areas that do see exposed sun, especially as the front nears, should be on guard for a slightly stronger storm threat in the mid-afternoon to evening.

How much rain?

Rain totals are expected to near an inch for MetroWest and tail off toward the east. West of that, with terrain, we expect totals to peak near 2” for Central Massachusetts, and 3” for the Berkshires. Isolated spots of 3-4” of rain will occur through the Green Mountains and Northeast Kingdom of Vermont.

Construction zones where the natural flow and drain of water has been tampered with, will see water began to pool. Areas of topography will also induce heavier rain and downpours. Areas in hilly terrain where low water crossings exist, will be dangerous to cross and potentially deadly in heavy rain situations.

Roadways and intersections with standing or flowing water should be avoided at all costs—especially today, with heavy tropical downpours overhead.