Weather

Here’s Dave Epstein’s nor’easter forecast

We should be prepared for the unexpected with a storm this powerful.

Jonathan Wiggs / The Boston Globe, File

By Dave Epstein, The Boston Globe

updated on March 14, 2023

March nor'easter:

Strong low pressure continues to develop Tuesday morning and will increase in intensity throughout the day as it moves toward the coastline, and then eventually pinwheels back out to sea Wednesday.

As of 7:30 a.m., nearly a foot of snow had fallen in higher elevations of Worcester County, and an inch-and-a-half of rain had fallen in other areas. Travel is difficult with the heavy rain and nearly impossible in areas with heavy snow.

Low pressure will continue to impact New England into early Wednesday as it rotates out to sea later in the day. WEATHERBELL

The easiest part of the forecast is actually west of Interstate 495, where the snow will continue and pile up reaching 1 to 2 feet. I won’t be surprised if a couple of those higher elevation towns (over 1,000 feet) close in on 30 inches of snow.

Snowfall totals will reach extreme levels in parts of northern Massachussetts from this nor’easter. DAVE EPSTEIN

Closer to the coastline, the rain will eventually change to snow. The mix and change will happen from roughly 1 p.m. to 5 p.m., depending on where you are.

There are still a couple of elements of which I’m not completely confident. Ultimately, the amount of snow we see in Greater Boston is going to hinge on the exact placement of the low pressure area this afternoon and overnight, and how cold the lowest layers of the atmosphere can get this evening.

We should be prepared for the unexpected with a storm this powerful.

As temperatures fall this afternoon rain will mix with and change to snow even in Boston. WEATHERBELL

The reason temperatures are going to fall is that even though it’s raining this morning it’s snowing above us — and as those snowflakes continue to melt, they’re going to cool the atmosphere.

Think of it like putting a bunch of ice cubes in a warm glass of water, eventually you’ll cool the water down enough so the ice will stop melting. As that happens, you’ll start seeing the rain mix with and eventually flip over to snow.

Temperatures will still be above freezing until around 5 p.m., meaning that even though the snow may start accumulating on the grassy surfaces it will have a tough time doing so on the roads.

This evening, as the low pressure area pinwheels to our east, it’s likely to throw more precipitation toward the coastline. As this happens — as long as it’s cold enough — the snow will accumulate, and this is when most of eastern Massachusetts is likely to see measurable snow.

However, there’s also the chance that the low wobbles a little too close to the coast and actually moves milder marine air back in from the ocean. If this is the case, then it will be difficult for the snow to accumulate, and it may even change back to rain over Cape Ann.

The loop shows rain (warmer colors) flip to snow (blue) this afternoon. Notice how the intensity of the precipitation decreases overnight to scattered light rain and snow showers early Wednesday. NOAA

Notice on the graphic above (from our high resolution model) how the rain changes to snow, and then back to rain from the east this evening. I think this scenario is worth at least mentioning. The track of the storm is highly unusual, ocean temperatures are above average, and it is mid-March. Ultimately, the snowfall you do or do not measure Wednesday morning at sunrise is dependent on these factors.

Power outages will be most prevalent in those areas where snowfall is well over 4 inches. There are also likely to be power outages along the coastline where winds are the strongest. Coastal flooding will be minor at the time of high tide for the next couple of cycles. Because tides are astronomically low, I don’t expect major flooding.