David Epstein: What to expect from the 4th nor’easter in March

There's going to be a very sharp cutoff between a couple inches of snow and absolutely nothing.

Boston, MA- March 21, 2018: Trees begin to flower at the Public Garden in Boston, MA on March 21, 2018. 
As Massachusetts prepares for the fourth major winter storm in three weeks, forecasters have adjusted snowfall totals and the timeline for Wednesday’s storm already dubbed the “four’easter.”
Snow is expected to start falling around the evening rush hour in the Boston area, and could potentially bring 6 to 8 inches to the region by the time it ends on Thursday, said Eleanor Vallier-Talbot, a meteorologist with the National Weather Service. (CRAIG F. WALKER/GLOBE STAFF) section: metro reporter:
Trees begin to flower at Boston's Public Garden on Wednesday. –Craig F. Walker / The Boston Globe

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Afternoon update:

During the afternoon hours additional information became available, and it now looks like the heaviest of the snow is definitely going to stay south of Boston and, perhaps, make this even less of an event than I had expected this morning. I’ve now push the 2-to-5-inch totals very close to the city of Boston, with the 5-to-9-inch totals south of the city. There’s going to be a very sharp cutoff between a couple inches of snow and absolutely nothing. The line could end up getting pretty close to the I-495 belt and Mass. Pike.

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There’s still a pretty good-sized storm system that will move underneath us, so I don’t want to completely write off the storm. However, if this trend continues, some of you could wake up to not much additional snow on the ground, compared to what there is when you go to bed. I hope no one is disappointed.

Initial:

The midweek nor’easter won’t get underway in earnest until overnight Wednesday, so you should be able to get through the work day and get home without too many road issues, beyond the usual traffic.

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The challenge with this particular forecast is in determining the axis of heaviest snowfall and whether it winds up in the southwest suburbs of Worcester or closer to the southwest suburbs of Boston.

If you’ve been watching the forecast, you know there’s going to be a storm stretching from Philadelphia to New York City on up through the southern coast of Connecticut. What’s less clear is how the northern edge of that heavy axis will affect Massachusetts and Rhode Island.

For now, plan on 5 to 9 inches of snow in most spots, with lesser amounts north and west of Interstate 495 and on the Outer Cape, where it will just be too warm for all the precipitation to fall as snow.

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The exact placement of the axis of heavier snow will determine if Boston ends up on the low or high end of that range. Different models show subtle but important variations that will ultimately determine how much you shovel.

With more data coming in this afternoon, we should know how those variations will play out.

Expect the snow to be heavy and wet, especially along the coastline, where the power outage possibility is highest. Temperatures around sunset will be within a few degrees of above freezing, so some of the snow won’t even stick as it begins, but as the night progresses it will turn colder and therefore snow will pile up.

Temperatures will be marginally cold enough for snow over coastal areas this evening.

Strong winds and coastal flooding

Winds will be strong and gusty over Cape Cod and the Islands, where there is a high wind warning in effect, perhaps as high as 60 mph overnight.

Coastal flooding is likely at high tide, but most of it will be minor. The coastline, of course, has been taking quite a beating over the past month, and there are pockets of compromised beaches that could sustain even more damage. Tides are running astronomically high once again, but not as high as they were at the start of the month.

Cold pattern

The weather shows no signs of a warm up anytime soon. The 8- to 14-day outlook shows a high probability of colder than average conditions in the east. This doesn’t mean no spring like weather, but I think we have to keep our guard up in terms of snow. We likely are not done just yet.

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As for Thursday’s conditions, the storm will wind down during the morning hours. Even if it’s still snowing after the morning commute, don’t expect much additional accumulation on the roads.

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