Unusually strong fall storm to bring a wind-driven drenching to southern New England

A wind tunnel on Franklin Street in Boston on Oct. 10.
–Lane Turner / The Boston Globe, File

The first major fall storm of the season is set to lash the Northeast, bringing strong gusts and heavy rainfall as it rapidly intensifies south and east of New England on Wednesday afternoon and night.

The system should barrel through New England during the overnight hours Wednesday into Thursday morning and could cause pockets of wind damage, power outages and even some isolated flooding.

The incipient storm was riding along the jet stream, the river of swiftly moving air that separates cold air to the north from milder air to the south, in the Upper Midwest on Tuesday. As the jet stream dives south, it will begin to transfer its energy to a new zone of low pressure forming near the Mid-Atlantic coast.

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The Mid-Atlantic low will intensify rapidly as it charges northeast, meeting the criteria of a “bomb cyclone.” That means its central air pressure will plummet at least 24 millibars in 24 hours. The lower the pressure, the stronger the storm. Winds around the storm – driven by changes in pressure – will increase in response, reaching storm force (at least 58 mph) in some areas near or just offshore the New England coast.

Most models project this storm will have around the lowest pressure on record in southern New England for the month of October.

The storm looks to start cranking off the Delmarva coast after lunchtime Wednesday, barreling northeastward and arriving south of New England by nightfall. While cities along the Interstate 95 stretch from Washington through Philadelphia are expected to see a period of rain and gusty winds during the day Wednesday, the worst of the weather is expected farther to the north, from New York City to Portland at night, where the storm will have grown stronger.

While the rain should end in New York City by the Thursday morning commute, showery weather may linger Thursday from Boston to Maine, with the storm departing Friday for the Canadian Maritimes.

In the wake of the storm Thursday into Friday, strong gusts are likely, reaching 30 mph or so in the Mid-Atlantic and 40 mph or so in New England.

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The combination of heavy rain and high winds with fully leafed trees may cause scattered bouts of wind damage and power outages – perhaps even rivaling that of a more traditional winter storm. In wintertime, all the leaves have fallen from the trees, meaning the trees don’t catch the wind like sails.

Because of uncertainty in the storm’s exact track, wind speed projections are still coming into focus, but what follows is a general guide, which may need to be fine-tuned in future forecasts.

Sustained 25 to 30 mph southeasterly winds will build around dinnertime in western Connecticut and Massachusetts, quickly spreading east and increasing with gusts to 45 mph by about 9 or 10 p.m. Gusts may hit 50 mph in the higher elevations of the Berkshires and the Worcester Hills. Winds will then abruptly switch around from the northwest west of the Hartford to Springfield corridor around midnight, with cooler air wrapping in on the departing side of the storm.

Farther east, winds will continue to wax closer to the coast, especially as the a zone of particularly strong winds, known as a low-level jet, roars in from the south. Those from Rhode Island or eastern/southeastern Massachusetts are probably used to those rainy systems where you can hear the low-level jet stream roaring just a thousand feet above you, but relatively little of that momentum makes it down to the surface as strong winds. This is unlikely to be one of those systems.

Instead, gusts of 45 to 50 mph are possible in eastern Massachusetts and Rhode Island, including Boston and Providence, with 55 mph along the immediate shoreline from Cape Ann southward through Cape Cod. Isolated gusts to 60 mph can’t be ruled out over the Cape and Islands, with even a low risk of one or two 65 mph gusts.

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The worst of the wind is likely during a six-hour period between about 9 p.m. Wednesday and 3 a.m. Thursday in eastern Massachusetts. The low-level jet will then shift northeast, buffeting the midcoast and Downeast Maine before sunrise. Gusty northwesterly winds over 30 mph may linger for the morning commute, but the bulk of the precipitation will have ended.

It’s not out of the question that higher-elevation inland areas see some gusts approaching 50 mph; the Blue Hills Observatory in Milton could gust above 55 mph. A very similar storm Oct. 28-29, 2017, brought a 73 mph gust to the observatory, along with 3.12 inches of rain.

The storm will be moving quickly enough that widespread flooding shouldn’t be an issue. However, heavy downpours will still drop some hefty rainfall amounts and may produce isolated pockets of flooding in poor drainage areas.

Like wind speeds, exact rain amounts will depend on the exact storm track, but at least an inch of rain is likely for most locations in the Northeast.

An inch to two inches is possible in eastern New York, with a localized zone of enhancement on the east slopes of the Berkshires. Up to 2.5 inches may fall within the Worcester Hills, with lesser amounts of one to two and a half inches to the east.

Toward Boston and Providence, around two inches seems most likely but up to three inches can’t be ruled out (and is the National Weather Service forecast).

Areas to the east and southeast may see about an inch and a half, though these amounts are all subject to the final track of the storm.

Model differences in the exact storm track introduce some uncertainty as to where the worst of the impacts will occur. The European model has been consistent in its suggestion that the storm’s center will track inland somewhere between Interstate 84 and Interstate 95 – which would bring strong winds farther west. The American GFS model, meanwhile, depicts more of an offshore track, with the highest winds in coastal locations.

If you split the difference between the two models, the storm would track near the Cape Cod Canal. This scenario is reflected in the National Weather Service’s forecast, which calls for the low to cross eastern Rhode Island or south coastal Massachusetts just after midnight Thursday before passing near or just south of Boston toward daybreak Thursday.

The strongest winds will occur near and to the east of the center (i.e. where the low tracks), with the swath of heaviest rainfall likely to the west of the storm’s track.

Most storms that swing through New England this time of year are dubbed “nor’easters,” because their strong winds come from the northeast as a result of their offshore track (winds spin counterclockwise around storm low-pressure centers). This time, because a pass over land has become more likely, the southeasterly winds expected to prevail qualify it as a “sou’easter.”