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Trend continues to show stormy start to March for the east

Brad Rousseau

Sunday, February 28, 2016, 10:35 AM - It's another messy day across central and eastern Ontario Sunday with the potential for another stormy setup mid-week to kick off March.

Medium and long range models continue to show a stormy pattern for next week as depicted below. The darker blue and purple shaded region over Ontario and nudging into the Maritimes is showing that a broad upper level trough is forecast to setup over the region. It looks to usher in an unsettled pattern and forecast models have been advertising a potent low pressure system associated with this upper pattern for several days.

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Within the upper trough we track an area of spin called vorticity. These areas of vorticity are what provide that strong dynamical energy that fuels these large low pressure systems. Below shows one model's forecast showing how the upper level energy associated with this system will become Wednesday.

There are two main bundles of energy with this system. The stronger bundle as circled in yellow below will trough through the southeast U.S. up into the northeast with a second bundle of energy tracking southward through The Great Lakes as circled in red. How these two bundles of energy interact will ultimately determine how the surface low will evolve and where it will track.

Should the northern bundle of energy drop south too fast, it could kick the southern one out to the east. Should the northern bundle drop south much later than the southern one, it could kick north a little faster which would track the low into Ontario, ushering in a messy system for Ontario, Québec, and the Maritimes.

With this much uncertainty still in place, we continue to track what the ensemble forecasts are showing.

Remember that ensemble forecasts are done by taking a model, adding slight adjustments to the initial conditions and running them out in time. By doing so we get a variety of scenarios and can come up with a mean or average outcome.

With the uncertainty still relatively high with this system, the ensemble forecasts are still quite useful. Below shows the GEFS ensemble run for Feb. 25 showing the surface pressure field for Thursday morning. The light orange area centered over southern Ontario shows where the average low pressure centre is among all the ensemble members and the Ls show where each individual member places the low.

Below that is the latest ensemble run from the Canadian GLB forecast model. We can see that the newer ensemble has now trended the low to the south and east of Ontario. Similarly, the latest GEFS ensemble (not shown) has trended as such. The current track that the ensembles are showing is one that is favourable for heavy snow across southern and eastern Ontario, as well as for southern Québec with more of a mess for the Eastern Townships and the Maritimes.

Does this mean this is how the low will pan out? Possibly not. The ensembles as well as the operational forecast models have been back and forth quite a bit with where the exact low will track and it will continue to be adjusted leading up to the event.

For now, the consistency and the latest advertised track seems reasonable given what the upper air forecast is showing as seen above. However, how this system evolves depends on how the energy aloft interacts. Below shows our preliminary precipitation forecast for Eastern Canada.

The take away for now remains that the upper air pattern for mid-to-late next week looks to support another large winter storm and based on the ensemble data, it could potentially track into Ontario. Preliminary forecast is for a mainly snow event in Ontario and Québec with mixing possible for southwestern Ontario and the Maritimes.

It is important to note that this is a preliminary forecast and once the low pressure system itself develops early next week over the south central U.S., we will be able to get a better indication on its exact track, precipitation types, and accumulations.

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