As with any weather event, there are numerous factors that need to be considered when forecasting a nor'easter. These include the storm's strength, how rapidly the storm strengthens as it approaches New England, how much cold (or mild) air is in place over New England as the storm approaches and the overall size of the storm. There is one factor, however, that is the most important: the track.
The track of a nor'easter working up the coast is critical in determining who gets snow, who gets rain, who gets a mix and how much of each precipitation type will fall. There is one point on the map that helps forecasters determine what will happen. This point is referred to as "the 40/70 benchmark," "the nor'easter benchmark" or, as New England Storm Center often uses, just "the benchmark."
The benchmark is a singular point in the Atlantic Ocean, located at the coordinates of 40° north and 70° west. This coordinate is located about 80 miles south of Nantucket Island. Where the center of a nor'easter passes in relation to this point plays the biggest role in who gets what and how much.
Nor'Easter tracks west of the benchmark
If a storm tracks west (or inside) of the benchmark, it draws warmer air over the ocean into eastern New England. This typically results in a more mild storm, with rain over eastern and coastal New England, snow further inland and in the mountains with a mix somewhere in between.
How far west of the benchmark the storm tracks will play a massive role in determining how far inland and north the mild air is pulled. If the storm tracks between the benchmark and Cape Cod, this can result in rain across the larger Metro areas of southern New England as well as coastal areas of northern New England while dumping heftier snow totals further inland.
If the storm tracks over Cape Cod, it can mean a big snowstorm for New England's ski country while leaving the rest of the region more wet than white. If the storm tracks well inland, most of New England could see rain, or at least mixed precipitation.
The March 2023 nor'easter is an example of this kind of track. That storm tracked between Cape Cod and the benchmark, leaving northern and western New England with 20-40+ inches of snow (Readsboro, Vermont saw 42") while areas closer to the coast saw less than 10 inches, with coastal southern New England not seeing more than a couple inches at most (less than an inch fell at Logan Airport in Boston).
This storm was strong enough to create its own cold air, which did bump up snow totals in the higher elevations of southern New England as initial rain turned to snow. This is one of the many other factors that determine snow totals.
March 2023 Nor'Easter snow totals:
Nor'Easter tracks east of the benchmark
If a storm tracks to the east (or outside) the benchmark, it can be seen as a close call for New England. This track usually places the storm too far away to have a significant impact on the region.
Depending on just how close the storm comes to the benchmark, it could give Cape Cod and the south shore a larger snowfall while everyone else just sees light snow showers or just overcast skies. This is the one storm track that will give Cape Cod (and other coastal areas) the most snow while all other tracks typically give them the least.
Nor'Easter tracks over the benchmark
If a storm tracks right over the benchmark (or close to it) it can result in a major snowstorm for much of New England. In cases where there is enough cold air in place ahead of the storm, this track can result in blockbuster blizzards for the larger Metro areas of southern and central New England while leaving ski country comparatively left out. This track typically allows for Cape Cod to switch to rain, leaving them with less snow as well.
Some of New England's (and the east coast in general) most memorable blizzards took this track including the Blizzards of 1978, 1996, 2003 (Boston's biggest snowstorm), 2013 and 2018. All these storms deposited their most snow across Connecticut, Rhode Island, eastern Massachusetts and southern New Hampshire.
The Blizzard of 2013 saw 20-40 inches of snow fall across Connecticut, Rhode Island, eastern Massachusetts, southern New Hampshire and southern Maine (Hamden, Connecticut saw 40 inches). Areas north and west generally saw 12 inches or less (Burlington, Vermont saw about 8 inches).
Blizzard of 2013 snow totals:
Non-nor'easter storms
The benchmark can also be used to help determine potential impacts from non-nor'easters as well. If a storm moving west to east through the Mid-Atlantic tracks north of the benchmark, it can draw more mild air into New England. If the storm tracks to the south, it can keep the precipitation away from New England. This north-south track is similar to a nor'easter tracking to the west or east of the point.
Conclusion
This is one of the reasons it can be difficult to determine snowfall amounts from a large storm in New England. A difference of just 50 miles in a storm's track can have massive implications in the outcome of a storm.
It's important to remember that there are many factors that go into not only where it snows, but how much snow falls. The track of the storm in relation to the benchmark is very important, but other factors will always come into play as well.