The GFS American model shows rainfall totals approaching nine inches in some locales. (Weatherbell.com)

A flood watch is up for a narrow zone in southern New England, where a “long duration heavy rainfall event” is set to unfold through Friday.

While rainfall amounts will walk a tight gradient, a bull's eye of three to five inches, with localized eight-inch amounts, is possible in southeastern Massachusetts, Cape Cod, and surrounding islands.

A high wind warning is also up for the Cape and islands, where gusts could approach 55-60 mph and cause some damage.

It’s all part of a hybrid early-season storm system with a hint of tropical nature to it. The instigating area of low pressure has even been monitored by the National Hurricane Center. While it has only a 20 percent chance of development and is “unlikely” to develop any established tropical characteristics, it underscores the connection to the tropics that’s present — mainly in the plume of deep tropical moisture it’s dragging all the way to New England.


An impressive plume of moisture streams north, setting the stage for heavy rain in coastal southern New England. (Weatherbell.com)

In the image above, notice the streamer of yellow wrapping up into a juicy atmospheric bowling ball off the New England coast. That ball represents locations that have close to two inches of water vapor sitting in the air above them. That means any bands of rain that form will have the potential to be exceptionally heavy, tapping into a reservoir of big-time moisture.

Tropical downpours with high rainfall rates can be anticipated where the heaviest bands of rain set up. And with speedy low-level winds scooping more of the air mass north, that moisture bank will constantly be refreshed. The stage could be set for a top-tier rain event in the hardest hit spots.

Rain is already falling from Boston and to the south and west, reaching to near New York City. The bulk of the action will remain focused south of the Massachusetts Turnpike on Wednesday, before spreading a hair farther north and west Thursday.

It’s important to emphasize that this is primarily an eastern and especially a southeastern New England event; you’d be hard pressed to see much, if any, shower activity west of the Hudson River on Thursday morning onward.

Bands of heavy rain are expected to rotate ashore as the system stalls well south and east of Cape Cod on Thursday, with rain moderate to heavy at times. Winds from the east around 25 to 35 mph, gusting to 55 mph are possible along the Outer Cape and Nantucket as well as Martha’s Vineyard. Elsewhere on the Cape, gusts to 50 mph will be the story. From the Canal through Scituate, and along the tip of Cape Anne, gusts may hit 45 mph.

Winds from the due east are atypical for traditional fall nor’easters. That fact, coupled with fully leafed trees, will raise the potential for at least isolated power outages.

The wind could also pile up water along the coastline, with minor to moderate coastal flooding possible in Nantucket Harbor. A few instances of coastal flooding are possible in eastern Massachusetts, as well, as is beach erosion.

Waves just offshore could top 25 feet, combining with 60+ mph winds off the coast, which could pose a significant danger to mariners.

By Friday, precipitation intensity will be waning some as the system starts to meander north, with perhaps a few downpours sneaking ashore from the New Hampshire seacoast up through coastal Maine into the weekend.

While exact rainfall amounts are uncertain but expected to be high, the placement of the jackpot zone carries more confidence.

“There is definitely a big wide range,” said Joe DelliCarpini, science and operations officer at the National Weather Service in Boston. “Our forecast right now has the heaviest rain over Rhode Island and Southeast Massachusetts.”

Some of the higher resolution models suggest that the heaviest rain may fall in a south to north axis somewhere in Plymouth and/or Bristol counties. Depending on where this sets up, flood-prone locales such as Fall River, New Bedford, or Taunton could wind up with some high-end totals. The National Weather Service’s Boston office wrote that “significant urban and poor drainage flooding is likely."

Getting bouts of tropical moisture sneaking into the Northeast in the fall isn’t common, but isn’t unusual, either.

Some of the most impressive rain events in places such as Boston, Hyannis or Worcester can come during this transitional season, when periodic episodes of summerlike, tropical moisture overlap with an increasingly winterlike jet stream. If they come together just right, it’s possible to cook up a hybrid sort of system, with both subtropical and mid-latitude traits.

“With this one, you have the jet dynamics in play, and it’s a large system,” DelliCarpini said. So while its center will remain farther offshore than most big New England snowstorms, for instance, it can “still bring impacts into [the region],” he said.

Similar storms have occurred in recent years. More than five inches fell in Hyannis during an event on Oct. 25-26, 2017. Five inches also fell in Worcester in mid-October, 2005. 4.24 inches came down on Oct. 21-22, 2016.

“I remember a setup in October ’96,” DelliCarpini said. “Hurricane Lili was way offshore, but we got a moisture feed in and got some big rainfall.” Boston measured 7.89 inches over 48 hours associated with that episode.