The forecast of 50s for Saturday quickly evaporated on Wednesday as model guidance abruptly changed gears to a weaker weekend storm. Weaker means less ‘oomph’ to surge mild air northward, and so all the sudden we’re now looking at widespread 30s and just a collection of 50s along the South Coast. Frustrating for meteorologists and holiday travelers alike, I’m sure.
So what are we now looking at? Unfortunately, a lot of wintry hijinks. It starts on Friday with snowfall, but switches over to a much more challenging situation Friday night and Saturday – ice. Let’s take it day by day and break down what’s to be expected during this busy time.
Up first, the cold settles in. A shallow layer of arctic air slides into place on Thursday with highs stuck in the upper 20s and lower 30s. And the area of high pressure delivering it will be the key player for our weekend. It will help to keep the cold locked in across much of New England. We call this kind of setup ‘cold air damming’ as the dense arctic air settles in for the long haul and blocks milder air from surging north. Instead, that warm air takes the easier route over the top of the cold. The result is an initially snowy and then icy situation.
High pressure centered over northern Maine and southeastern Canada is a hallmark of New England ice events. Add in a weak approaching wave of low pressure and it looks like things could get ugly this weekend.
We can see this with model ‘soundings.’ These show a picture of the atmosphere from the surface all the way up to the tropopause tens of thousands of feet above us. What we see for Saturday is a large layer of above freezing air, nearly 10,000 feet deep, hanging out above our heads. No snowflakes can grow in there; it’s all raindrops. But right at ground level, the cold air is hanging tough and draining out of Maine and New Hampshire. So we end up with a tiny sliver of temps 28-32F on the ground. And that allows the rain to freeze on contact.
This is a snapshot from the NAM model in Worcester Saturday morning. I’ve annotated it so you can see the deep layer of air above freezing which cuts off the snowfall, but the freezing temps right at ground level which could result in freezing rain. How cold the surface stays is the key.
Now the *exact* surface temperatures are going to be the biggest part of this story. Hanging around 32F or 33F isn’t going to make for a big ice storm, because the mild layer above it is extremely mild and deep. To end up with a significant and potentially damaging ice storm, it will need to be more in the upper 20s category. I wouldn’t be surprised to see these ground-level temperatures reach that point, especially toward northern Massachusetts up through New Hampshire and Maine.
The national Weather Prediction Center (WPC) currently thinking the highest ice risk is across New Hampshire and western Maine with this storm. http://www.wpc.ncep.noaa.gov
Let’s focus on each day and what I’m currently expecting in terms of accumulation and impact.
Friday will be our snow day, but a manageable amount is expected. Areas of snow should begin breaking out Friday morning from west to east across the region and continue on/off until the evening. The steadiest and heaviest snowfall will move across Vermont, New Hampshire, and Maine with a widespread 3-6″ of snow piling up (maybe even some higher amounts across the highest elevations). For northern Massachusetts, we should see 1-3″ of snow. No biggie by our standards but it should slow down travel, especially for those heading northbound. South of the Pike we may see some novelty flakes in the air with some coatings but that’s about it. All told, Friday isn’t a high-impact day locally. And the farther south you go, the easier traveling will be.
The warm air aloft will be pushing north on a low-level jet Friday evening, and will overtake the area from south to north. By 9pm this warm nose of air should be up to the Pike, and then to the Massachusetts border by about midnight. At the same time, the steadier snow will have tapered off. So what falls Friday evening until Saturday morning should generally be a light freezing drizzle away from the coast. It won’t come down at a fast clip, but it should get quite icy outside and caution will be needed for any overnight travels.
This shows the air above freezing aloft nosing north Friday night (snapshot at 1am here). This cuts off the snow and changes everything to ice across the interior…and this change should move all the way up across a good portion of Vermont, New Hampshire, and Maine.
This is the highest impact day but also the most uncertain. A warm front jams up across the coast and moves up to about the Providence area, but likely won’t get any farther. The immediate shoreline and southeast Massachusetts will be talking about run of the mill rain. But interior areas may hover right near the freezing mark, if not the upper 20s, for a good part of the day.
The difference-makers between this just being an icy day for travels across the interior or a significant ice storm with damage and power outages will be how much precipitation actually falls and whether or not we can get a few degrees below freezing at the surface. The highest risk zone of significant ice is the Merrimack Valley region up through New Hampshire. If you live in any of those spots, I’d keep a very close eye on the forecast and make sure your generator is ready to go just in case. The brighter pink on this map shows zones where I think there is a *risk* we could see ice accumulations over a quarter of an inch. In reality, it likely wouldn’t be this entire zone. But I made it give a general idea of the threat area.
Farther south into Connecticut it should also be very icy, particularly northern Connecticut. There’s a decent chance it will change over to regular rain for a time there. And we may see regular rainfall all the way up through the 128 corridor. It’ll be a fine line so stay informed on this one. We’ll also need to watch for the colder air sinking south again on Saturday night as the wave of low pressure passes, bringing the subfreezing line down through Boston and towns south of the city. Even if you get plain rain in these spots as expected, it could get real icy Saturday evening as temperatures fall.
If historical analogs are our guide, CIPS Analogs is a good tool to use. It shows the highest risk of 6+ hours of freezing rain, when looking at previous similar setups, is across NE Mass, New Hampshire, Vermont, Maine, and eastern New York. And that we should also be wary of freezing rain getting all the way down to the Connecticut coast (or very close to it) Friday night.
It looks like we’ll get a break with dry weather on Sunday. A chilly, partly sunny day in the 30s for most of the region.
Here we go again! Another wave of low pressure will be swinging up the coast. At least with this one, it looks more like a rain/snow story than a snow/ice story. A good portion of guidance points toward this storm tracking near the coastline, which would produce some significant (6″+) snows inland and a rain/snow mix in eastern New England. It would arrive late Christmas Eve and last into at least the start of Christmas Day.
That being said, there are still some signs that point to it possibly missing out to sea. Considering we are still 5 days out, there is a lot of uncertainty. So we can’t lock it in just yet but I’d favor a solution that brings this mix of snow and rain into New England for now. Another challenging travel day across the region.
European model ensembles clustering near Nantucket on Christmas Day, which is tight enough to the coast to bring mainly rain in the Boston area and snow farther inland (especially NW of 495). A little wiggle west and it’s all rain locally. A little wiggle east and there’s a lot more snow in play. The UKMet model agree with this idea, though the GFS and its ensembles are pointing out to sea. Definitely one to watch.
Getting new skates or ice fishing gear for Christmas? Good! A very cold stretch is shaping up for Christmas week and it should last into the start of January. This will be a long stretch of highs that often will stay in the 20s with very cold nights to go along with it. There are also indications it could wind up being active in terms of storms and snow chances. Front loaded winter? You betcha.
A very broad area of cold temperatures should encompass Canada and the northern tier of the U.S. to wrap up 2017 and start the new year.