A cold front is currently moving east through the region and will push offshore this evening. Behind it, a sprawling high pressure system will build in straight from Western Canada bringing with it our first real shot of cold air. On Tuesday, warm air from the tropics blasted up the coast by the storm currently causing trouble in Texas will smash into the cold air and a good burst of snow will result. Here’s how I think it will play out and why.
The Cold Air: Tomorrow
Highs will remain below freezing tomorrow for everyone except maybe far southern York County. Highs near the international border will struggle to get out of the single digits. That doesn’t include the NW breeze which will definitely be felt. This is our source of cold air for Tuesday. Prepare for January like temps and wind chills which compared to this torch of a December will feel quite chilly!
The Moisture: Monday Night
At the same time, the storm that is bringing a slew of problems to Texas will be moving North through the Ohio valley. This will set up a feed of warm moist air from the Gulf of Mexico right up the coast to Maine. This will cause some concerns as the moist air also comes with warmth which will cause concerns for mixing towards the end.
The Collision: Tuesday
The cold air comes tomorrow and the moisture arrives on Tuesday. The collision is what will cause the snow. At the moment this looks like a front end thump of snow during the day Tuesday followed by a lighter period of mixed snow/sleet/freezing drizzle/drizzle on Tuesday night. Let’s dig into the snow first.
As warm air plows northward, it will run into the cold air that arrives tonight into tomorrow. This will allow for heavy snow to form on the leading edge of the warm air as it is forced up and over the cold air. This map from the 12Z GFS shows strong forcing (that’s how the moisture in the air is squeezed out) Tuesday morning. Expect slick roads and heavy snow starting Tuesday morning and lasting through Tuesday afternoon/evening. This is when most of the snow will fall. A general 4-8″ is expected with some seeing more and some less. See the snowfall map below for more details.
Southwest winds bringing the warm moist air into the region will eventually bring in warm enough air to start causing mixing problems especially in Southern New England. My current thinking is that Warm air aloft gets as far north as the MA state line during the tail end of the steady precip. Lighter precip will linger through the evening and areas as far north as Portland could see some light sleet/freezing rain overnight Tuesday. If warm air moves in earlier/farther north than forecast which is a possibility, totals could be cut down in SW ME and NH. Northeast of route 26 is likely safe from any impactful sleet.
The other concern for precip type will be along the immediate coast where a coastal front is likely to form. There will likely be a tight temperature gradient along this front and if you end up on the SE side, temps could rise high enough to change the precip to a rain/snow mix. This threat is greatest along the coast south of Portland and along the tips of the peninsulas. Wherever this sets up, look just to the NW for heavier precip associated with the warm ocean air moving up and over the cold locked in at the surface. Warm air rising over cold air is what is causing this whole mess on a large scale, the same thing happens on a small scale.
Residual Light Precip
After the main snow event moves east Tuesday evening, lingering low level moisture will contribute to light mixed precip lingering through the first half of Tuesday night. As warm air completes its invasion aloft Tuesday evening, this could fall any of a number of ways, snow, sleet, freezing rain, or rain. The precip will be light so the impact on snowfall totals will be limited.
I’m thinking that because the moisture is below the warm layers, it stays mostly snow. The warm air moves in at 700mb (10,000 feet or so) but the cold stays tough below that (the blue lines on the map above indicate subfreezing temps at 5,000 feet). The moisture is in the low levels (where the cold air is) and thus even though 700mb is warm, I could definitely see another inch of snow as the snow falls in the low level cold layer. Regardless, this will keep roads slick through Tuesday night despite the heavy precip being gone.
Looking at the final snowfall amounts, I expect a general 4-8″ for Northern New England with less to the south. I think someone sees 10 or 11″ and the best chance looks to be either in the mountains where deep cold stays really locked in contributing to a drier snowfall that accumulates faster or near the coast where the coastal front contributes to more liquid falling from the sky. The coastal ‘jackpot’ area largely depends on the ocean air staying farther SE and the sleet being kept at bay. The northern Jackpot is dependant on enough moisture moving that far north. A coastal low developing Tuesday evening will try to steal some of the moisture from NW areas.
I’ll have another update tomorrow evening on the storm, details on tomorrow’s weather will come tomorrow morning.