Forecast Office Toronto - Long range outlook #2

First, let’s quickly look at the current setup. We are expecting a powerful system around the 29th, which was first forecasted about 10 days prior at FOT. This system is also associated with a weakening of the mean zonal winds in the lower stratosphere, allowing some cold air to spill south and briefly support a -AO, -NAO pattern. That cold air is then driven to the surface behind the system’s cold front. If you look at the mean zonal winds graph, you can already see weakening during this timeframe, however, a much stronger signal appears around the 1st, which is exactly what we will discuss next.

Looking ahead to the 3rd-4th timeframe, ensembles are consistently forecasting a surface low along with prominent blocking over Quebec and the Canadian Prairies, ensuring that sufficient cold air will be in place. The jet stream during this period also supports a favorable track for a significant snowstorm. This setup aligns very well with the stratospheric signal, as strong storms are often followed by strong cold fronts which is being reinforced by the stratospheric pattern. However there is an even stronger storm signal around the 8th which we will discuss next.

Looking ahead to the 8th timeframe, we see the PNA rising rapidly while the AO also turns negative. A positive PNA supports colder conditions in the East, and a sudden drop in the AO can indicate stratospheric influence, which appears to be the case here. When we see rapid cooling across a large area, it is often a sign of a strong storm accompanied by a powerful cold front

On the ensembles, you can clearly see a weakening signal in the mean zonal winds at 50 hPa. While this signal is not as robust as what we see around the 29th or the 3rd, that is expected given the longer lead time, as ensemble spread increases farther out.

This storm is already showing up on the AIFS ENS, even at this extended range, and the overall setup is in place with a surface low developing somewhere over Colorado or Texas, and prominent blocking over Quebec. While the jet stream may not look particularly impressive at first glance, the 500 mb pattern is clearly present. The placement of the strongest winds suggests a more SER-like pattern rather than a purely zonal flow. As of now, even deterministic models are beginning to pick up on this system, though they continue to push it out to sea, likely due to the prevailing zonal flow.

Looking even farther out, especially into mid-January, I have a strong feeling we will transition into a very active and well-established -PNA SER pattern. During this timeframe, the polar vortex is expected to be quite strong, making sustained -NAO / -AO conditions difficult to achieve. It’s important to remember that this represents the overall pattern and that at times, it can certainly break down, as stronger storms can bring colder air behind them into the East.

At face value, the SER may appear extremely strong, and it likely will be, but the storm track will be the deciding factor. If the jet stream shifts slightly south, and if the SER weakens enough to allow for well positioned snowstorms for Toronto. Nonetheless, this pattern carries significant potential, but also a high margin for error.

After the polar vortex strengthens to this degree, it is reasonable to expect it to eventually weaken or collapse, likely in late January or February. When this occurs, depending on details that we cannot accurately forecast yet, it could trigger a major pattern shift toward a +PNA / -NAO / -AO type setup. This signal is already evident on the EPS 500 mb pattern and is also hinted at in the teleconnection graphs, which gradually trend in a more favorable direction.

Overall, the upcoming period from late December through mid-January is shaping up to be active, with multiple strong storm signals supported by both stratospheric and tropospheric patterns. The system around the 29th sets the stage by bringing cold air south behind a strong cold front, followed by another likely impactful storm around the 3rd–4th, aided by blocking over Quebec and the Prairies. A potentially even stronger system appears around the 8th, coinciding with a rising PNA and negative AO, indicating further stratospheric influence.

Looking farther out into mid-January, the pattern may evolve into a well-established -PNA SER setup, which carries high potential for significant snowstorms, though the exact storm track will be critical. The strong polar vortex at that time makes prolonged -NAO / -AO conditions difficult, but temporary breakdowns could still allow for cold intrusions. Later in late January or February, the polar vortex may weaken or collapse, potentially triggering a major pattern shift toward a +PNA / -NAO / -AO regime. While the overall pattern shows high potential, there is also a significant margin for error, meaning careful monitoring will be required to determine specific impacts.