Metro

Dave Epstein

Sleet or snow: What are we going to get this weekend?

How much snow ends up on the ground Sunday will depend on the day’s temperature fluctuations.
David L. Ryan/Globe Staff
How much snow ends up on the ground Sunday will depend on the day’s temperature fluctuations.

Perhaps it’s the low snowfall totals we’ve accumulated so far this winter that’s making the curiosity around Sunday’s storm so high, but let’s just say right off the bat that there’s nothing really unique about this system.

It’s not a monster storm. It won’t break snowfall, tidal, or wind records. Fifty years ago next month, it snowed for 100 hours in Boston without a break. This storm will likely precipitate for around 24 hours.

The storm has the potential to bring a high volume of mixed snow and sleet to the eastern part of the state, including Boston, but since the storm is still 72 hours away, you can expect some changes in the forecast.

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Travel will be most impacted Saturday night and Sunday with newly formed snow banks dotting the landscape Monday.

A storm will develop late this weekend and bring the first significant snowstorm to all of southern New England this year. (COD Weather)

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A bit of light snow (a coating to 1 inch) falls Friday morning, unrelated to what we are going to see Saturday night into Sunday. The main event gets underway Saturday evening, most likely after 8 p.m.. From that point forward, it will be snowing all the way down to Cape Cod with moderate to heavy accumulation by sunrise Sunday. Most of the snow that will fall will do so by the time you get up Sunday morning.

After that point, things get more complicated.

During the day on Sunday, warm air will be pushing northward at 5,000 to 10,000 feet. At the same time, it will still be cold enough for snow down here on the ground. Those little snowflakes will be melting above us and then refreezing into pellets of ice called sleet. To get an inch of sleet, you need about four times the moisture as you would for snow. There is likely some additional accumulating sleet during the day or even freezing rain.

This is the most difficult part of the forecast.

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Right now with the change to a mixed bag, the heaviest snow will stay over northern and western New England. This is a typical snow event for most of us, not too big, but rather mess. You will want to clear everything up as quickly as possible Sunday before it all freezes.

COD Weather
In this image a profile of the atmosphere on the left is predicted to be just cold enough for snow at higher levels Sunday.

How much warm air floods into those midlayers is critical when it comes to precipitation. If the warmth doesn’t make it to Boston, then snow will fall instead of sleet and we’ll get another 4 to 8 inches on top of what falls during the first part of the storm. On the other hand, more warmth means we get rain on top of snow.

If the precipitation was 100 percent snow, we’d be getting 8 to 16 inches.

Currently, the models are tending warmer, so all that snow is a lower possibility. It would likely be easier to deal with than mixed precipitation. Better clearing your driveway with a shovel instead of an ice pick. Most everyone will have some sleet and ice to deal with.

It will be windy on the coast Sunday and there will be minor to perhaps moderate coastal flooding, typical of a storm taking a track over — or just south of — New England. Tides are astronomically high this weekend as well, making it easier for those shore roads to flood.

NOAA
Tides are high late morning Sunday.
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Monday will be a blustery and cold day so break out your warmest gear to enjoy that fresh snow. The cold isn’t prolonged, just intense. By midweek, readings are breaking freezing again.

A cold MLK day is likely this year.
WeatherBell
A cold Martin Luther King Jr. day is likely this year.

Follow Dave Epstein on Twitter @growingwisdom.