Gale warning for Buzzards Bay and the sounds runs 8 p.m. Monday to 8 p.m. Wednesday

NEW BEDFORD The National Weather Service upgraded its gale watch for Buzzards Bay and the nearby sounds to a gale warning early Monday. The warning runs from 8 p.m. Monday, Oct. 19, to 8 p.m. Wednesday, Oct. 21. It calls for northeast winds of 20 to 30 knots, gusts up to 40 knots and seas of 4 to 7 feet, according to the weather service's Boston/Norton office, which issued the warning at 2 a.m. and raised the gusts from 35 knots in its 10:03 a.m. update.

The warning covers Buzzards Bay, Rhode Island Sound, Block Island Sound, Vineyard Sound, Nantucket Sound and Cape Cod Bay. "Strong winds will cause hazardous seas which could capsize or damage vessels and reduce visibility," it says. Mariners are told to "remain in port, seek safe harbor, alter course, and/or secure the vessel for hazardous conditions."

Thirty knots is about 35 mph, and 40 knots is about 46 mph.

What changed since Sunday

On Sunday afternoon the weather service posted a gale watch for these waters, meaning gales were possible. Its forecasters wrote Monday morning that they upgraded it "given increased confidence in gale-force winds impacting those areas." The watch had allowed for winds up to 35 knots and gusts to 40; the warning's sustained winds are a step lower for the bays and sounds, its gusts match the watch's 40 knots, and seas are unchanged at 4 to 7 feet.

Farther out, the gale warning for the coastal and ocean waters off Cape Cod, Nantucket, Martha's Vineyard and Block Island, out to 60 nautical miles, stays in effect for the same hours, with northeast winds of 25 to 35 knots, gusts to 45 knots and seas of 9 to 14 feet, the weather service says. Narragansett Bay, which takes in Mount Hope Bay off Fall River, Somerset and Swansea, is under a small craft advisory from 8 a.m. Tuesday to 4 p.m. Wednesday, for northeast winds of 15 to 25 knots, gusts up to 30 knots and waves of 2 to 4 feet. "Conditions will be hazardous to small craft," the advisory says.

How the wind builds on Buzzards Bay

The Buzzards Bay forecast issued at 10:03 a.m. builds the wind in steps. Monday night brings northeast winds of 10 to 15 knots, turning north at 15 to 20 knots with gusts up to 30 knots after midnight, and seas of 2 to 4 feet. Tuesday, winds turn northeast at 20 to 25 knots in the afternoon, with gusts up to 35 knots and seas of 4 to 6 feet, and stay gusty Tuesday night. Wednesday is the peak, with northeast winds of 25 to 30 knots, gusts up to 35 knots and seas of 4 to 6 feet. Winds ease a little Wednesday night, and north winds of 20 to 25 knots with gusts to 30 knots are forecast Thursday through Friday night. The forecast's gusts on Buzzards Bay top out at 35 knots, below the 40 knots the warning allows for across the bays and sounds.

The storm and the shore

The storm is a nor'easter deepening well to the south, off the Mid-Atlantic and Carolina coast, and it appears likely to sit over the same waters for most of the week, the forecasters wrote. They said its main effects here would be marine: "rough seas, beach erosion, and strong winds." Beach erosion on east- and northeast-facing beaches may become an issue, they wrote.

For New Bedford, the forecast updated at 9:16 a.m. calls for north winds of 9 to 16 mph Tuesday with gusts as high as 33 mph, and northeast winds of 16 to 20 mph Wednesday with gusts as high as 31 mph. Highs are in the upper 50s to low 60s through Wednesday. The only rain in the city forecast is a slight chance of showers, about 20%, from Wednesday night into early Thursday morning. The forecasters said the best chance of rain is on Cape Cod and the Islands for the second half of the week, and that the storm's track is still less certain than usual.

Late-week outlook

The forecasters expect a trough from Canada to steer the storm out to sea toward the end of the week, letting seas go down and the winds ease. The weather service can change or cancel a gale warning at any forecast update. The current marine alerts and forecasts are posted at weather.gov/box. South Coast 360 will update this story if the warning changes.

Label: Reported by us, from the town's posted records.