Wet snowstorm starting for many, with rain/snow mix in southern areas; snow showers linger until early Tuesday morning, then dry and chilly through Thursday before turning milder at times but perhaps rather dry until possible major storm early the following week

Plain-language summary:

 

Rain will change to a wet snow for almost everyone tonight, with a dense, sloppy accumulation at lower, southern elevations and a significant wet snowstorm elsewhere. Snow showers will linger along and northwest of the Appalachians through early Tuesday morning, and then it dries out and stays chilly through Thursday. It turns milder with possible light precipitation from later Friday through the weekend. A cooldown could follow for early the following week, but an uncertain major storm could provide heavy rain and a brief thaw as well, with the weather pattern looking overall mild and faster-moving afterward but also very uncertain.

 

Meteorological discussion:

 

A surface low pressure over Lake Erie is slowly strengthening and is producing an expanding area of precipitation extending from Wisconsin to Maine. A modestly cold surface high pressure to the north has provided just enough cold air for snow in Maine, higher terrain, and north of the U.S./Canada border, but elsewhere, low-level temperatures are above freezing, and precipitation is starting as rain. Areas of eastern Ontario are experiencing freezing rain and ice pellets due to low-level cold air channeled through the St. Lawrence and Ottawa valleys while temperatures aloft are still warm. As the storm slowly strengthens and moves northeastward, southeasterly winds off the Atlantic Ocean is causing warmth and moisture advection into the region, causing the widespread precipitation. Often, in this warm advection setup, any snow would change to rain. However, the warm advection isn’t that strong, and temperatures aloft are not that warm. Instead, mid and upper-level height falls will cool down temperatures aloft, and heavier precipitation will lead to dynamic cooling from the top down, overcoming any warm advection. In the low-levels, cold air from the northeast in Maine will seep into central New England and spill into the Champlain Valley to some extent as a secondary low pressure develops south of New England, cutting off any true warm air. By early in the night, rain in the valleys will change to a wet snow, except in the southern St. Lawrence valley of northwestern New York and coastal areas. Later in the night, the primary low pressure will move to Lake Ontario and northwestern New York and eventually the Champlain Valley, causing a brief changeover back to rain from the central Champlain Valley on south at lower elevations. In addition to higher elevations just being colder, the modest easterly flow will lead to some upslope enhancement along eastern slopes and downsloping, leading to slightly warmer temperatures with slightly less precipitation, in the Champlain and southern St. Lawrence valleys. Colder air will filter in Monday afternoon as the storm pulls offshore, with winds turning to the northwest but being light and not pulling in dry air yet, so there will be lingering snow showers along the northwestern slopes of the Appalachians and backing into parts of the St. Lawrence and Champlain Valleys due to the blocked flow. Most higher elevations will start as snow and stay all snow.

 

Source: TropicalTidbits

 

Snowfall accumulations are still rather uncertain as except in northern Maine and at higher elevations, surface temperatures will be near or just above freezing, meaning that accumulations are very sensitive to slight deviations in temperature or precipitation intensity. A best guess would be a wet 6-10” (15-25 cm) from Ottawa to Montreal with 8-12” (20-30 cm) in the immediate Laurentian foothills just to the north, and only a coating to 2” or 5 cm of slushy, sloppy snow in the southern St. Lawrence valley of northwestern New York. Central Maine and higher terrain of the Adirondacks, Green Mountains, and White Mountains will receive 6-12” (15-30 cm), with 1-5” (2-13 cm) in lower elevations of northern New England away from the immediate coast. Note that the 10:1 ratio map shown below will be an overestimate due to the wet, warm nature of the snow.

 

Source: PivotalWeather

 

It will gradually dry out later Tuesday and through Thursday as a slow-moving, modestly cold surface high pressure parks to the north, providing chilly temperatures, especially along and northwest of the Appalachians, where high temperatures will stay below freezing. The slower movement is due to a blocking -NAO pattern. By Friday, an Alberta clipper will dive southeastward, and southerly flow ahead of it will cause a warmup to begin. Another Alberta clipper could track a bit farther north Saturday and Sunday, but it is unclear if either clipper will bring much if any precipitation given the dry air masses from the west associated with the clippers. The clippers will also bring mild air from the west, but the mild air will be tempered a bit by the west-northwest flow aloft. 

 

Source: TropicalTidbits

 

However, a major storm ejecting from the Rockies next weekend associated with a trough in the western U.S. could pump a ridge in the eastern U.S. and track to our west, bringing a lot of warm air and rain to our region early the following week, melting the snow that will fall tonight. This is still speculative though, and there is still a chance that the storm is weaker and farther south, bringing more wintry precipitation instead. Models are in even more disagreement afterward, but the erosion of the -NAO blocking pattern means that systems will be moving faster again, with alternating warm and cool spells, though most indications are that it will be overall mild as we head into mid-December, with subtle troughing in Alaska and the western U.S. associated with a slightly negative PNA promoting an eastern U.S. ridge and mild weather in our region.

 

Source: TropicalTidbits

 

 

Source: Climate Prediction Center