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Yesterday’s storm, which brought lots of snow, rain, and wind to the area has now moved up into the Gulf of Maine. This storm will continue to rotate for another day, bringing clouds and still the chance of some rain or snow showers.

You might have noticed that snowfall accumulation had an extensive range with this storm and while we’ve seen this before, you might be wondering how the city of Boston can receive an inch of snow while Wilmington gets 20 inches. The answer actually lies in a uniquely coastal phenomenon particularly prevalent here in the Northeast.

A strong storm continued to rotate in the Gulf of Maine Tuesday.
A strong storm continued to rotate in the Gulf of Maine Tuesday.COD Weather

When storms move up along the coastline, their fuel is cold dry air to the north mixing with warm and humid air to the south. These low-pressure areas rotate in a counterclockwise manner, having a southerly flow of air on the right side transitioning to a northerly flow of air on the left. Consider that there needs to be an inflection point where the winds shift from southerly to more easterly, and then eventually northeasterly and finally northerly as these storms rotate.

It is within this transition that a boundary sets up between the easterly fetch of air and the more northerly. This boundary is called the coastal front and is very common during nor’easters.

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Observed snowfall totals.
Observed snowfall totals.NWS Boston

The Monday storm featured a coastal front that snaked its way from Cape Cod to Boston and then onshore over Cape Ann.

On the right side of the front, marine air brought temperatures into the 30s, while to the left readings were in the 20s. The contrast between the cool and the warm air brings an additional lift to the atmosphere enhancing snowfall on the colder side. When the coastal front stays stuck in one place, even dividing a town, you can have large discrepancies of snow in a couple of miles.

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Late afternoon Monday saw a coastal front set up through eastern New England.
Late afternoon Monday saw a coastal front set up through eastern New England.Dave Epstein

In addition to where the snow falls on either side of the coastal front, the texture of the snow on either side is very different. The snow that fell in the milder air was heavier and wetter and the ratio between the moisture and the snow small. For example, in Mansfield, there was a 3:1 ratio (snow to liquid equivalent), meaning 3 inches of melted snow was nearly an inch of liquid. The snow was heavy and wet and hard to move.

Contrast that with interior towns in the cold air, where there were a 15:1 ratio and an inch of water equivalent, thereby producing 15 inches of snow. I have seen ratios as high as 20:1, and some of the storms back in February 2015 had so much snow because of the low snow to liquid equivalents. Sometimes this is referred to as the fluff factor, but people use this term because it’s a simple way to convey snow to liquid ratios.

In conclusion, the reason why the accumulation map looks like it does is because of the coastal front and the big contrast in temperature that occurs on either side of it. Next time there’s a coastal storm you can see if you can find the coastal front. It’s a fun way to forecast along with other meteorologists.