Slow-moving storm pulling away today; more rain with Laurentians snow on Tuesday; first widespread lower elevation snowstorm possible late week; extended cold and mostly dry spell likely after

Plain-language summary:

 

After producing substantial rain and mountain snow in southern areas but little precipitation in northern areas, the slow-moving storm over the Northeast U.S. is finally moving east out of the region today, with lingering rain and snow showers over western Maine and eastern Quebec. After a dry Monday, another storm will bring widespread rain on Tuesday, except wet snow and isolated freezing rain over the Laurentians and a bit of snow over higher elevations of the Eastern Townships of Quebec and western Maine. It turns modestly colder afterward before possibly the first widespread snowstorm of the season occurs Thursday into Friday. It then turns colder and likely drier for an extended period of time with a slower-moving weather pattern, though the coldest air will likely stay to the west of our region as it often does early in the winter.

 

Meteorological discussion: 

 

A slow-moving storm that has been bringing rain and higher elevation snow to the Northeast U.S. and parts of southeastern Canada to a lesser extent is finally pulling off to the east. It has been a weird, upside down storm with heavy precipitation, even heavy snow to the southwest and much lighter rain to the north and northeast, all possible due to a strongly blocked pattern (-NAO) across at least half of the Northern Hemisphere. Though the cold air associated with the storm cut off and weakened, a reinforcing shot of slightly colder air is causing rain showers to change to snow showers over eastern Quebec and western Maine. However, accumulations will be light.

 

Source: TropicalTidbits

 

Source: Climate Prediction Center

As the storm moves off to the east, it will dry out for Monday, but another storm will already be approaching from the Great Lakes and strengthening. The low-pressure system will track into the St. Lawrence Valley and just off to the north on Tuesday, bringing southerly winds, slightly warmer air, and rain to the St. Lawrence Valley on south, with wet snow and isolated freezing rain in the Laurentians near and north of the low-pressure track and with upslope cooling and precipitation enhancement. Higher elevations of the Eastern Townships of Quebec and western Maine could see wet snow as well. With the blocking weakening (-NAO becoming less negative), this storm will move quicker than the storm over the past few days, and it will dry out by Wednesday with another surge of somewhat colder air with blustery westerly winds, though it will not be true arctic air, and daytime temperatures will still be above freezing in lower elevations.

Source: TropicalTidbits

 

The modest cold air mass on Wednesday and Thursday, as well as a disturbance moving eastward along the southern branch of the jet stream across the southern and central U.S., will set the stage for what could be the first widespread lower elevation snowstorm by late Thursday into Friday. With the lack of a true cold air mass in the area, temperatures will likely be near or just above freezing at the low-levels for most of the storm, so snow accumulations could be somewhat terrain-dependent despite the widespread synoptic-scale area of precipitation, with rain possibly mixing in during lighter precipitation periods at lower elevations. There are sort of 3 pieces of energy interacting in this setup, though the intensity and how far north the storm gets seems to be more determined by location and intensity of pieces 1 and 2 while in the western U.S. The more aggressive ECMWF 12z yesterday had piece 2 lagging behind and close enough to piece 1 to inject into and strengthen piece 1 (the main storm with all the moisture), leading to a stronger storm cutting into southern Quebec, with the heaviest snow over the Laurentians and snow changing to a wintry mix and then rain from Vermont on east. On the other extreme, this morning's GFS 06z had piece 2 much faster and piece 1 much slower, with everything getting stretched out and there hardly even being a storm. I expect the models to gradually get into better agreement on an in-between scenario, and at this point, it appears that higher elevations of northern New England and eastern Quebec are the most likely locations to see a significant snowstorm.

 

Source: PivotalWeather and TropicalTidbits

Source: TropicalTidbits

 

 

Source: TropicalTidbits

 

 

With the still -NAO and blocking ridge in Nunavut joining with western Atlantic ridge built downstream of the snowstorm, storm won't be escaping east out to sea easily. Instead, it will turn north, occlude, and meander in northeast Canada for several days with snow up there, leading to a persistent northwesterly flow of cold, dry air to the southwest, including our region, likely blocking any other storms from reaching our region for a while. The blocking pattern could also prevent the coldest air over central Canada from reaching our region, with subtle moderation from the influx of Atlantic air into the tightly wound, occluded storm over northeastern Canada. Still, with a +PNA, -NAO, and -EPO, we have an extended period of upper-level ridging over western Canada and Alaska as well as Greenland, with an upper-level trough over central and eastern U.S., a classic configuration for northwesterly flow of cold arctic air into the eastern U.S. Depending on how long this lasts for, this could be the coldest December since 2017 for our region. There are some signs that the cold pattern could break down somewhat after the first week of December, but that is too far out to have much confidence in.

Source: TropicalTidbits