The short-term forecast interest in New England is the threat for showers and the associated storm center moving near Cape Cod Tuesday afternoon and night. Though the public perception question revolves around precipitation chances and amounts on Cape, there is much more to this system. Upon close inspection, it appears as though the storm developing south of Cape Cod Tuesday will be a "warm core" (tropical) system - and as with anything tropical in nature, storm track is critical.
The temperature profile through a deep layer of the atmosphere unquestionably shows a warm core at the center of the storm, even tens of thousands of feet up, which is one big component to taking on tropical characteristics. The second tropical characteristic is symmetry, which is forecast by some guidance to be present with this system, but in reality is likely to be lacking given the presence of some cooler, drier air aloft northwest of the storm track, over New England. (Technical: this is not a barotropic setup in the offshore waters of New England, and even weak but present baroclinicity should mean an asymmetric, warm core system).
The warm core nature of the storm means: 1) the storm may strengthen quickly, 2) wind likely stays over water, on the east side of the cyclone, 3) any steady rain shield will also be a heavy rain shield, wherever it ends up. As of this writing Monday evening, it appears to me a mean steering flow from 200 degrees (mid level winds blowing from the south-southwest) should carry the storm southeast of Nantucket, and the northwest side of this ocean storm probably will not able to develop quickly enough for a big show on Cape Cod, rather, showers will fall for most with generally less than .30" of rain. Nantucket has the best chance of breaking into some heavier downpours, but even that chance seems limited to no greater than 50%.
Offshore, the storm will organize and mariners should be aware of building swell Tuesday evening with storm force gusts from the southeast, then the southwest, near and southeast of "benchmark" 40N/70W Tuesday evening. It's worth noting that as of this post, the official offshore marine forecast via the Ocean Prediction Center for this same region late Tuesday is a 10-15 knot wind! Obviously that's not my forecast, but it does help to illustrate the admitted uncertainty with the evolution of the system. It's early to say just how big breakers will be for southeast facing New England beaches, but 5-7' swell is possible by Wednesday - that will become more clear soon.
In the image below - a forecast from one computer guidance product (NAM), note the 6000 foot altitude wind forecast valid overnight Tuesday night that consists of a 50-60 knot wind core SE of Nantucket, but not much wind on the northwest side of the cyclone, on Cape. That is consistent with an asymmetric, warm core system, and adds validity to the forecast.