Mark Sirois

The storm system coming from the US Midwest this weekend will once again be a mixed precipitation event as the center of the system will glide along the St Lawrence Seaway as it passes through. Because of this track, much of the area along the seaway will see mixed precipitation with a constant risk for freezing rain as it passes. One area in particular between Ottawa and Montreal will be most at risk for ice accretion (5-10 mm). Snow will remain north where significant accumulation is expected (10-20 cm). For those the the south and west of the St Lawrence, this will mostly be a rain event. Next storm system expected Dec 17th. I’ll have a post on that in a few hours.

A clipper system will drop 2-6 cm of snow over the greater Southern Quebec area starting Wednesday morning (drive time) and wrapping up in the early hours of Thursday. In some of the higher elevations along the US border you could be looking at 5-10 cm. Eyes are on this weekend as a mixed precip storm will bring snow transitioning to freezing rain and rain. The storm should start late Saturday and stick around until late Sunday/early Monday. This storm will approach from the ...US Midwest. At the moment, the models have agreement that the center of the storm should pass through west of Ottawa. Areas to the west and north of the center could see around 10 cm while areas to the south and east will see less snow due to the mix of rain. There should also be a period of freezing rain for some areas along the St Lawrence. I’ll have a map tomorrow showing how it should break down. See more

Storm update..... As of 8:55 pm, we have a dense, wet snow falling over most parts of Southern Quebec with the exception of areas east of Sherbrooke where the snow has not started yet. The Cornwall, ON area is starting to see ice pellets which indicates a progressive transition to rain along the seaway (Cornwall to Trois Rivieres). Between 11:30pm and 1 am we should see the first rain moving in from southwest to northeast. The image below is predictive radar which shows what it should look like by 3 am. Green is rain, blue is snow, and violet is ice pellets.

5:05 pm radar has some strong storm cells containing hail, heavy rain, frequent lightning, and strong winds stretching from Quebec City to Ottawa. These storms are moving towards the southeast and will affect Montreal, Monteregie, and the Townships over the next couple of hours.

The storm this weekend will do everything it was supposed to do, except the storm track shifted east giving the big snow amounts to New Hampshire, Maine, and Bas Saint Laurent. In the end, the GFS model tracked this better than the Euro model and this only points to some great improvements they have made to the GFS over the last 2 years. For a long time the Euro was far more accurate but things have changed. Below you will see two snapshots of both models taken for the same t...ime period (11 pm Saturday). Both are essentially identical in position and geographical coverage. Both show a pretty intense storm which bombed out somewhere between New York and Maine. So the blocking pattern from the NAO does it’s job in developing this nor’ easter but I think if the NAO had been negative sooner, we could have kept it further west. It all comes down to timing sometimes. Bottom line, if you live west of the line between Trois Rivieres, you’ve got nothing to really worry about this weekend. East of that, things get spicy the further east you go. (See snow map). Things get colder after the storm and we should get more into an early December weather pattern in regards to temperatures going forward. We’ll try again with the next storm.

PRELIMINARY SNOW MAP A very interesting day tracking this system as we now get into crunch time. Highest confidence is in the areas highlighted below, but make sure it is understood that this is preliminary as all the ingredients are there for this to be a much bigger storm. The models are having a hard time agreeing on the track and phasing of the system with another system expected over the Great Lakes at the same time. The exact track of the storm will determine whether or... not this storm will become the season's first nor'easter. A nor'easter is any large storm that brings northeasterly winds along the Atlantic coast of North America. There is potential for this storm to go through the process of bombogenesis, or a rapid strengthening that occurs when the central barometric pressure of a storm nose dives within 24 hours. When a storm undergoes this level of intensification, it is referred to as a bomb cyclone. If that type of significant strengthening takes place, the storm could bring high winds and heavy snowfall across the interior of the Northeast. So this, by all means, is not set in stone. Snow totals could shift or increase. But for now, if traveling this weekend, avoid the Eastern Townships starting Saturday afternoon. Next update later this evening.

With variable skies and unseasonably warm temperatures this weekend, it should be great for leaf peeping. Just be mindful of the COVID alert situation for each area.

Update on Hurricane Teddy. Latest model guidance would have the storm eye making landfall between Noon and 6 pm next Tuesday between Dartmouth and Peggy’s Cove. With expected barometric pressure between 961mb and 972mb prior to landfall, that indicates the borderline between Cat 2 and Cat 3. But with expected sustained winds at 115 km/hr, it will likely be a Cat 1 to 2 storm by the time it makes landfall.

No change to the expected snow map since yesterday for the upcoming storm. As of 1:10 pm, radar shows light snow just about to start in the Ottawa area. Montreal should see the first flakes between 4 pm and 6 pm. For Quebec City and Sherbrooke it should be 8 pm tonight. Do not be surprised to see a mix with ice pellets along the St Lawrence Seaway between 10 pm and midnight as the transition to rain happens. In regards to the bigger cities, Ottawa should end up being the winner when it comes to snow. 13-17cm are expected with less of an ice/rain mix.

Update on the Sunday night/Monday storm..... I’ve made a slight change to the snow/rain line as well as to accumulations. When it comes to snow, the winners are all areas well north of the St-Lawrence Seaway with 12-24 cm expected. Up to 30 cm is possible in isolated areas. Along the seaway, (including Montreal, Trois Rivieres, and Quebec City) this will be the transition zone where we will see a mix of snow and rain with some freezing rain during transition (around 1am Monday). 5-10 cm of snow is likely in this narrow zone, but any fluctuation in the storm track could either increase the amount of snow or decrease it. This is an interesting storm to monitor and it will establish the snow base in northern areas which likely will stick around until Spring.

Generalized measurable snow likely next Wednesday..... Some of the models this morning are pointing to the potential of a 5-10 cm snow event that would impact most of Southern Quebec next Wednesday. Take this as a heads up rather than an official forecast as there still is not unanimity between all the models. Stay tuned....