The Northeast had its first major wintry wallop of the season Sunday night, but it’s not clear yet. After more than a foot of snow fell in some locations, more is on the way late Monday afternoon into Tuesday morning, as the storm’s brutal backside is set to lash many in New England and the tri-state area (of Pennsylvania, New York and New Jersey) with slow-going commutes.

Bands of moderate to heavy snow are forecast to redevelop Monday afternoon and evening, pivoting west and potentially stalling over New York City, Providence and Boston. It’s a high-stakes forecast that will have major implications for anyone traveling Monday evening through Tuesday morning.

On Monday morning, it was a breezy 42 degrees in Boston with a bit of light rain falling. It didn’t feel like a snowstorm.

But for many in the Northeast, the deceptively quiet weather won’t stay that way for long. As the same slow-moving storm that brought the first round of wintry precipitation continues to wobble offshore, cold air drawn in behind it will cause temperatures to plummet Monday evening in New York City and Monday night in Boston. Meanwhile a band of heavy precipitation will develop on the storm’s backside that will quickly turn to all snow as it spreads over the region.

The heavy precipitation will be embedded in a feature known as a “comma head,” the wraparound apex common in intense coastal storms. These features are challenging to forecast, but they can produce disruptive, heavy snowfall.

Forecast for New York, New Jersey and Pennsylvania

In New York City, light intermittent showers were ongoing Monday morning, with sleet pellets and snow just west of town. By midafternoon, however, precipitation will become more intense, consolidating in a southwest- to northeast-oriented band that will probably end up as heavy snow just west of New York City and in northern New Jersey and northeast Pennsylvania.

There’s a shot that the band, if it develops, could produce snowfall rates closing in on 1 to 2 inches per hour. That could snarl the evening commute, seriously hamper operations at JFK, LaGuardia and Newark airports, and pose big problems on the roadways.

The greatest impacts will be just west of New York City, but a difference of only 5 or 10 miles could place the heaviest snowfall over the heart of downtown. There is an increasing risk of greater impact to New York City proper, with temperatures already a bit lower than anticipated — favoring primarily snow.

At the moment, forecast data show a wide range of possible accumulations, from 1 to 6 inches over New York City, with 2 to 7 inches for Newark and northern New Jersey. It’s an enormous range, but there is high potential here for both boom or bust scenarios, meaning snowfall amounts ending up near (or even above or below) either the high or low ends of forecast ranges depending where very localized bands of heavy snow set up.

The latest high-resolution computer models favor some of the higher-end totals in New York City. Three or 4 inches is probably a best guess for now in the Big Apple.

Heavy snow may briefly reach down to southern New Jersey tonight, but amounts will taper off as one heads farther south.

While the Poconos are likely to experience heavy snowfall Monday, only lighter snow probably makes it as far south as Philadelphia, where an inch or so might fall, mainly on grassy areas.

Forecast for Massachusetts, Rhode Island and Connecticut

Light snow should break out across much of southern New England, particularly between Interstate 84 and Interstate 90, by midafternoon. It could become briefly moderate for a time though Monday evening. In Boston and Providence, the precipitation may be mixed with sleet and/or a cold rain, but just a few miles west, all snow is forecast. Hartford is expected to remain all snow.

Connecticut may miss out on the worst of the comma head when it really ramps in the wee hours of Tuesday morning in extreme eastern New England. The key time frame for Boston and Providence looks to be from about 3 a.m. to 9 a.m. Tuesday — and about one or two hours later for Southeastern Massachusetts, the Cape and the Islands.

During this time frame, snowfall rates may briefly peak at 1 or 2 inches per hour, with gusty winds, very low visibility and the potential for isolated thunder and lightning. However, there is a chance the heaviest of the snowfall hovers slightly east of Boston, sparing the city the worst.

For the Cape and Islands, the heavy thump of precipitation will start as rain but transition to heavy, wet snow before ending.

Snow should taper down during the midmorning hours, but flurries are possible through late afternoon in extreme eastern areas.

Additional snowfall totals of an inch or two are possible from the Connecticut River east to the Blackstone Valley, with 1 to 3 inches from Providence north along Route 495 in Massachusetts.

Closer to the coast, 3 to 6 inches are possible (assuming the heavy snow band extends far enough inland), including in Boston, Plymouth, Brockton and Taunton. One to 4 inches is possible on Cape Cod — with the greater totals near the Upper Cape — and a slushy inch or so on the Islands.

The Tuesday morning commute is likely to be significantly disrupted, with travel difficult to impossible at times.

Maine and New Hampshire

For Maine and New Hampshire, the substantial snow mostly focuses on Tuesday. Winds won’t be terribly strong and the precipitation won’t be as heavy, with a steadier, more moderate snow likely near the coast and lighter rates inland. Additional totals near the coast of 3 to 7 inches are possible, with lesser amounts of 1 to 5 inches inland.

What has already happened

From the shores of Lake Erie to the coast of the Maine, a widespread 6 to 10 inches fell in the first phase of the storm. Seven inches accumulated in Syracuse, N.Y., while 14.5 inches was measured at Albany International Airport.

A few places saw even more like Readsboro, Vt., which picked up 18 inches, while a few 20-inch totals were observed near Saratoga Springs, N.Y.

The Berkshires in Massachusetts also received substantial amounts, winding up with more than a foot. Boston officially only got 2.5 inches, but inland suburbs 15 miles or so away saw closer to 8 inches. Similar totals were posted in Worcester County after a heavy snow band swept through the region Sunday evening.

Much of the snow fell in only a short window after sunset Sunday, piling up rapidly amid hefty snowfall rates before many places changed over to sleet or rain. A band of thundersnow preceded the transition, the booms heard throughout much of northern Connecticut and central/western Massachusetts.

That potent snow squall with thunder was thanks largely to what’s known as frontogenesis. It sounds fancy, but it basically means “building a front.”

As a dry slot of warmer air began to encroach at the mid-levels of the atmosphere over Southern New England on Sunday night, the temperature gradient — how quickly temperature changes over a distance — got tighter. That essentially created a front, which helped to enhance lift and snowfall rates. In addition to producing a band that dumped more than an inch per hour, all the upward lift built a strong enough electric field to touch off a few lightning strikes.

After that, many spots south of the Massachusetts Turnpike and east of Interstate 95 flipped to sleet and eventually purely rain. That melted some of the initial snow that fell, but more is on the way. The lull coastal New England was experiencing Monday morning was merely an intermission.