Plain-language summary:
A powerful storm will bring wide-ranging impacts to the region Thursday night into Friday night. Due to the storm tracking to our west, there will be a brief thaw with heavy rain in almost the entire area on Friday. There will also be a rapid freeze-up at the end of the storm as well as strong winds. However, higher-elevation areas and areas from the Champlain Valley on northwestward will receive front and back end snow, the most falling in eastern Ontario and the Laurentians. On Saturday, snow will only gradually decrease in coverage and intensity with a lot
of wraparound snow from the storm itself and enhanced by lake-effect
snow in the Laurentians and areas just to the northeast of the lakes. It turns much colder behind the storm but not nearly as cold as areas farther west. An Alberta clipper could bring light to moderate snow in the middle of next week before it turns quite mild through the beginning of January.
Meteorological discussion:
The seeds for the well-advertised storm late this week are currently in the Pacific Northwest where there has already been heavy snow even in lower-elevation cities such as Seattle, Vancouver, and Victoria. Extremely cold arctic air currently over British Columbia, the Canadian Prairies, and the northern U.S. Plains will push southward and will amplify the weather pattern in a dramatic way. This arctic air pushing southward to the Gulf of Mexico, accompanied by an upper-level disturbance, will lead to a quickly intensifying storm moving into the eastern Great Lakes. The upper-level trough will go negatively tilted and cut off.
 |
| Source: Aviation Weather Center |
 |
| Source: PivotalWeather |
Since this is occurring so far west, the center of the storm will likely pass over southern Ontario and then into western Quebec as it matures and occludes, too far west for the heaviest snow in our area. There will be a brief thaw with heavy rain in almost the entire area on Friday. The warmth will be especially pronounced from Vermont to southern Maine, where temperatures could briefly reach 55F (13C). With strong low-level southeasterly winds, there will be a pronounced terrain effect on precipitation amounts, with very heavy rain along and just east of the Laurentians and spine of the Green Mountains and White Mountains due to upslope flow, and much less precipitation with downslope flow in the Champlain Valley and eastern St. Lawrence Valley, where it might be completely dry for several hours. However, everyone will see at least a brief period of heavy precipitation along the arctic cold front due to the strong low-level convergence along the front. The 90 kt low-level jet at 850 mb also indicates that there will be widespread strong winds with gusts up to 50 kt being common, and possibly even higher than that along the favored western slopes. One limiting factor is the warm air aloft advecting over the snowpack, which will stabilize the lower atmosphere and prevent the strongest winds aloft from mixing down to the surface. How much wind reaches the surface is highly dependent on mesoscale features, terrain, and lower atmospheric stability, and will be somewhat uncertain until the storm moves in. The southeasterly wind will shift to a southwesterly wind behind the cold front, but will still be rather strong with gusts up to 40-45 kt. The area of strongest wind will shift to the St. Lawrence Valley as the wind will be almost parallel to the valley axis.
 |
| Source: TropicalTidbits |
 |
| Source: PivotalWeather |
Despite the storm track being mostly unfavorable for snow in our area, there will be modest cold air in front of the storm and very cold air behind the storm. This means that in southern and eastern Ontario, as well as the Laurentians, closer to the storm's track, there will be a period of potentially heavy front end snow enhanced by upslope Thursday night, as well as a brief period of heavy snow on the back side Friday evening followed by an extended period of intermittent snow showers, enhanced by lake-effect snow off Lake Huron and Lake Ontario. This area could see 6-10" (15-25 cm) of wind-blown snow. The St. Lawrence Valley, Adirondacks, Green Mountains, and White Mountains will see a brief period of snow before changing to rain, and the St. Lawrence Valley and Adirondacks will see a brief period of snow at the end of the storm, with total accumulations probably in the 2-6" (5-15 cm) range. In the Champlain Valley and southern Vermont through southern Maine, snow will be minimal due to the strong southeasterly flow advecting in warm air from the Atlantic before the storm starts, as well as downsloping effects throughout the storm in the Champlain Valley. Everywhere will experience a rapid freeze-up at the end of the storm as arctic air rushes in immediately following the heavy rain along the cold front.
The details are still subject to change, but there is high confidence in the overall picture. There has been a trend for the storm to strengthen later and therefore be farther southeast over the last few model runs, which would push everything to the southeast and increase front and back end snow potential.
 |
| Source: TropicalTidbits |
As the storm occludes, matures, and slowly moves northward over western Quebec, snow will only gradually decrease in coverage and intensity with a lot of wraparound snow from the storm itself and enhanced by lake-effect snow, with areas just to the northeast of the lakes being favored due to southwesterly flow. Most of the snow will be confined to the Laurentians, Ontario, and northern New York, closer to the storm and Great Lakes. Some snow will also make it into the St. Lawrence Valley, with little or no snow from the Champlain Valley eastward due to downsloping and being farther away from the storm and Great Lakes. Since the arctic air would have traveled an indirect route from the Rockies to the Ohio Valley and over the Great Lakes before reaching our region, it will have greatly modified, with few if any lower-elevation areas dipping below 5F (-15C), especially since it will be too windy for good radiational cooling and it will be mostly cloudy northwest of the Champlain Valley. Due to the intensity of the storm, the temperature structure will become convoluted, with much warmer air to the north of much colder air and the cold air coming from the southwest!
 |
| Source: TropicalTidbits |
After the storm leaves, an Alberta clipper will dive southeastward, rotating around the strong vertically-stacked storm that by this time will be located over eastern Hudson Bay or northwestern Quebec. If this system becomes more amplified with a stronger ridge over the western Atlantic, and/or a piece of the polar vortex digs south over the Canadian Prairies at the same time, it would lead to another strengthening storm, this time just off the coast with the snow being closer to the coast. However, the
upper-level pattern for now does not seem quite right with the Hudson Bay storm being too far east and probably just overwhelming everything, and there is not much ensemble support for a strengthening coastal storm. But the chance might not be zero since it is still 8 days away. Even if a bigger storm does not occur, the clipper could bring light to moderate snow by itself.
 |
| Source: PivotalWeather |
After next week, there is good model agreement that the Pacific jet will become quite strong through the beginning of January and flood the contiguous U.S. and southern Canada with mild Pacific air. A thaw or two with rain is almost inevitable. However, in January, it does not need to be colder than average to get snow. This mild pattern also tends to lead to above-average precipitation, since warm air can hold more moisture than cold air. While there is little suggestion of it in the modeling now, snowstorms can actually result from such mild patterns if we get a well-timed and well-placed storm and there is not too much warm air. Also, the positive PNA and negative NAO and AO indices suggest that the warmth might not be too intense or long-lasting.
 |
| Source: TropicalTidbits |
 |
| Source: Climate Prediction Center |