Though temperatures will still climb above normal Tuesday, cooler air has undoubtedly spilled into New England, and will only become more established in the days to follow. What Tuesday's air lacks in cold temperatures, it makes up for in low dewpoints - that is, the air in place across New England still may afford an opportunity to average near 40° for afternoon highs, but dry air will cool quickly when night falls or moisture is added. Overnight Tuesday night, we'll do both: cool the air overnight and bring moisture in from the south. With dew point temperatures in the lower 20s Tuesday night, air temperatures above freezing early on will plummet below freezing in most locales, meaning as precipitation begins late Tuesday night and continues through Wednesday morning, it will predominantly fall as snow.
There will be exceptions to a snowy scenario - the immediate South Coasts of Connecticut and Rhode Island will likely be just a bit too warm for more than a burst of a dusting to an inch of snow before mixing with sleet and then rain. Communities southeast of the Cape Cod Canal likely will be too warm for anything but rain, save for a few flakes mixed in, owing to a significant easterly component off the ocean. A similar fate may be in the cards for immediate South Shore of Massachusetts communities, given a projected wind direction out of the east, but the difference here is that the wind doesn't pick up in velocity until later in the game - starting nearly calm when precipitation begins, meaning two or three inches of snow is possible before warmer, maritime air starts to drift in. Though available moisture will decrease farther north, colder air will be better in place, affording a large area of a couple of inches.
One final, important note: Though total moisture expected to fall with this storm normally would warrant on the order of five to six inches in the highest accumulation zone, the temperature profile aloft - about 12,000 feet in elevation - favors a poorly accumulating snow type, and that's why I'm not going higher. If the forecast thermal profile changes by a few degrees - warmer or colder - I can make an argument these numbers need to go up. As of right now, however, that part of the atmosphere is projected to produce about the poorest accumulating snowflakes one can generate. Another factor not to be ignored: soil surface temperatures have warmed over the last several days, though I don't expect this to be a major factor.
Southern New England Early Estimate Map (Tuesday night through Wednesday):
Northern New England Early Estimate Map (Wednesday):