Potential for impactful snow in and around Ottawa looms

Saturday, November 19, 2016, 9:11 AM - Heads up, National Capital region. The Weather Network Forecast Centre is tracking two powerful systems set to potentially impact the Ottawa and Montreal areas Sunday and Monday.

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The active weather expected for this weekend and early next week is being driven by a classic fall storm system, a Colorado Low, currently tracking across northern Ontario. This system is bringing a wide swath of impacts, from blizzard conditions in the States to over 30 cm of snow in parts of western Ontario.

As a blast of arctic air pours in behind the system, we expect to see falling temperatures and stiff northwesterly winds develop late this weekend across Ontario and Quebec. This should bring the lake effect snow machine roaring to life in the snowbelts, but, even though they're further from the lakes, Ottawa and the western St. Lawrence Valley aren't out of the woods.

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As a secondary low develops near the Atlantic coast, a zone of enhanced convergence between this new low, and the Colorado low, will set up across eastern Ontario and western Quebec, leading to a persistent, slow moving band of snow.

Though there is still uncertainty about the specifics of exactly how and where this feature will develop, it could potentially bring impactful snow accumulations to Ottawa, Montreal, and the western St. Lawrence Valley late Sunday into Monday.

The ultimate fate of the forecast will hinge on what happens after the Colorado Low tracks into Quebec. It gets a bit technical, but essentially the uncertainty boils down to one question: Does a second area of low pressure develop off the Atlantic coast, as the Colorado Low departs?

If a secondary low does not develop, the original Colorado Low should track far enough north into Quebec that the most significant snowfall will miss our heavily populated areas.

But, if a secondary low does form off Cape Cod late this weekend, the forecast may look very different. This new low, developing in response to the upper level trough tracking out over the Atlantic, would potentially wrap back into upstate New York. This would create a zone of enhanced convergence – what forecasters call a “deformation zone”. With cold air already in place, this would lead to a band of heavy snowfall to the northwest of the new surface low.

These two images illustrate the different scenarios.

The first shows the more limited snowfall expected if the there is no deformation zone setup. The second shows a band of higher accumulations pushing much farther south, thanks to the enhanced convergence from the secondary low. As always with long range forecast guidance, it’s important to focus on the overall pattern, not specific amounts.

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Deformation zones are tricky to forecast, especially this far in advance, but they can lead to surprisingly high snowfall amounts. If a deformation zone does develop over Ottawa later this weekend, an extended period of heavy snowfall would be expected, continuing at least through the Monday morning commute.

Deformation zones are also notorious for very tight snowfall gradients. Amounts can vary greatly over just a few kilometers. We’ll need to wait until we have higher resolution short range guidance to get an idea of just who, if anybody, will bear the brunt of the heaviest snowfall.

The key message is to keep a close eye on your Sunday and Monday forecast, particularly in and around Ottawa and the western St. Lawrence Valley. We’ll continue to refine our snowfall projections based on the latest forecast guidance, and if there are big changes we’ll be sure you to keep you informed.