The Party’s Over – Here Comes the Snow

We haven’t had a decent-sized snowstorm around here since late November. Well, that is about to end, much to the dismay of most of you. We’ve got several different concerns with this system, so we’ll try and touch on all of them.

Winter Storm Watches and Warnings are in effect for most of the Northeast with a Coastal Flood Watch for parts of the Massachusetts coastline. Image provided by Pivotal Weather.

We’ll start with what’s going on now. High pressure is building into Quebec, and this will bring cold air into the region. Tonight will be chilly, with lows in the teens, maybe even some single numbers. Meanwhile, low pressure is moving out of the Southern Plains. This low will move towards the Appalachians on Saturday, then off the Mid-Atlantic coastline Saturday night. This brings us to forecast problem number 1. Where does the low track from there? This is a critical point, as it will help determine what type of precipitation falls across the area. Some models keep the storm offshore, south of the Cape and Islands, while others bring it right across southeastern New England. We’re thinking it tracks pretty close to the Islands, but stays just to the south. By Sunday, it moves into the Gulf of Maine, and takes all of the precipitation away.

As for the timing of that precipitation, there’s not much disagreement among the models. Snow should move in during the evening hours (7-9pm) from west to east, and end during the afternoon (3-5pm) on Sunday, except across Cape Cod and parts of southeastern Massachusetts, where it may linger into Sunday night. The heaviest precipitation will likely fall between about 11pm Saturday and 8am Sunday, so if you don’t have to be out then, we’d recommend that you stay where you are.

The High-Resolution NAM model shows the progression of the precipitation across the region. Blue is snow, orange is sleet, purple is freezing rain, and green/yellow is rain. Loop provided by

What type of precipitation is going to fall? Well, that is a BIG question, that is still in doubt for a large portion of the region. It should start as snow for everyone. It will likely change to rain across Cape Cod and the South Coast, and parts of southeastern Massachusetts. In between? That’s where things get really complicated. As we usually see during storms, we’re going to have a “coastal front” set up. Basically, the milder air from the ocean will push inland a bit. If you are south and east of this coastal front, temperatures will be near or just above freezing. If you are on the other side of the front, temperatures will be in the teens. Eventually, this front will collapse to the coast on Sunday, bringing the cold air in everywhere (more on that later). But the surface is only part of the equation. Warmer air will also move in aloft. How far inland it moves is something that that models disagree on right now. With warm air aloft and cold air at the surface, the precipitation will change to sleet or freezing rain, depending on how thick the layer of warm air is above the surface. Obviously, this will have a significant impact on snowfall amounts. Everything should go back to all snow Sunday afternoon as the coastal front collapses toward the coastline.

The high-resolution NAM model shows a well-defined coastal front setting up in the Merrimack Valley and the I-495 belt Sunday morning. Image provided by WeatherBell.

We’ve got more concerns than just what falls from the sky though. The full moon is Sunday night, which means that tides will be astronomically high. That is usually enough for some minor coastal flooding in a few spots. However, when you add in east to northeast winds of 15-25 mph and gusts to 40-50 mph (or more), then you get coastal flooding in a much wider area, possibly even some moderate flooding in the more vulnerable locations. This is mainly a concern for the high tide that occurs Sunday morning. The winds will also be a concern inland, as it will create blowing and drifting snow, making driving even more hazardous.

We mentioned earlier that the coastal front would collapse to the coastline during the afternoon, bringing cold air back in everywhere. As that cold air comes rushing in, we expect everything to quickly freeze up Sunday afternoon and evening. This will create black ice on the roads, but more importantly, any snow still on your driveway, cars, roads, etc, will quickly turn into cement. You’ll want to get outside and quickly clear everything off, because the longer you wait, the more difficult it will become. Temperatures will continue to drop Sunday night, and as high pressure builds in, skies will clear out, which may allow for viewing of the total lunar eclipse Sunday night. On Monday, temperatures won’t rise that much, with many places likely staying in the single numbers or lower teens. It will still be breezy, so wind chills may stay below zero all day long. Temperatures will start to moderate on Tuesday, but longer-range indications are that we are in a colder (and stormier) pattern now, so winter has finally arrived.

This is the forecast high temperatures for Monday based on the GFS model. It’s going to be cold. Image provided by

OK, finally, the part you’ve all been wondering – how much are we getting? As we’ve said, the amount of sleet and freezing rain will have a significant impact on snow accumulations, so we’re going to keep our ranges somewhat wide for now, and if need be, we’ll put out an update on Saturday.

Cape Cod: 2-4″
Southeastern Massachusetts/Southern Rhode Island: 3-6″
I-95 corridor (including Boston and Providence): 4-8″
MetroWest/North Shore: 6-12″
Merrimack Valley/NH Seacoast: 8-14″
Southern NH/Southern ME (Nashua/Manchester/Portland): 10-16″
Central NH (Concord): 12-18″

The NAM model is probably closest to our thinking right now, though some of these numbers may be a bit higher than our thoughts. Image provided by the College of DuPage.

If time allows, and/or there is a significant change in the forecast, we’ll update this on Saturday.

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