Forecasting models reveal more details on December 24 disruptive storm

Friday, December 19, 2014, 2:10 PM - On Wednesday, the forecast centre at The Weather Network started paying attention to the possibility of a storm system impacting eastern Canada around December 24-26.

So, where are the more recent weather models?

The ECMWF forecast model still develops a strong low pressure system for December 24-25 (Christmas Eve into Christmas Day) with the main cores of the lows tracking up the Appalachians impacting the Maritimes, Quebec and eastern Ontario with residual effects across the rest of Ontario and merging into one system. The ECMWF solution has the storm system with a deep low pressure core leading to strong winds across the affected regions.

The Global model brings two low pressure centres into Canada with one into Ontario and the other up the east coast into the Maritimes and Quebec. This shows a less intense low pressure system with regards to how deep the pressure is and, therefore, how strong the winds are.

What map below depicts: Global forecast model showing two centres of low pressure with the energy being split between the centres and a less windier solution for Ontario than Wednesday's solution (December 25).

The GFS forecast model wants to bring two cores of energy into Ontario and Quebec which would still give us the threat of windy conditions across Ontario, Quebec and Atlantic Canada.

What map below depicts: GFS forecast model showing two centres of low pressure with windy conditions throughout northern and southwestern Ontario as well as the Maritimes and Quebec (morning of December 25).

When forecasters see two separate centres of low pressure (as the Global and GFS are showing) you can think of it as they’re sharing the energy within the whole system. When this occurs, the tendency is to see a less focused area of intensity so winds will be weaker, but still have an impact. When it shows a concentrated area of low pressure, like the ECMWF solution, a more focused energy leads to stronger winds.

What do forecasters make of these three different solutions? Therein lies the conundrum. You can visibly see in the two images above the two similar, but yet very different solutions to the looming storm system. You may be saying, "Well, this certainly doesn’t look as bad as it did Wednesday!" And you wouldn’t be wrong. In the previous article I discussed the different solutions the models were giving and each new model run on each new day leading to Christmas will continue to give us new and more accurate solutions. The Thursday solutions indicate a stormy period mid-next week for eastern Canada but the intensity has backed off a little bit. We have also noticed the Thursday model runs are demonstrating less of an impact for southern Ontario and more of an impact for the Maritimes.

There are still a lot of unanswered questions which will become clearer over the next several days. Once we have a better handle on the track and intensity, then we can start talking what we may be looking at in the way of precipitation. In this situation, it is highly likely like some impacted may see heavy precipitation in the form of rain or snow where others will likely see little to no precipitation. At this point, it is too early to forecast precipitation types and amounts.

The forecasters knew the models on Wednesday were indicating a strong storm system for next week. The main weather models we take a look at for long range forecasting update two to-four times daily. The two main updates (morning and night) are the ones forecasters really pay attention to when looking longer into the future (a week or more out). On Wednesday, each of these forecast models (GFS, ECMWF and Global) showed a very strong low pressure system but they all differed in positioning of the storm. Some had more of an Ontario/Quebec impact where others painted a stormy picture for Atlantic Canada with moderate to strong wind for Ontario and Quebec.

The term weather bomb, or bombogenesis, was used to explain what the models were indicating for the aforementioned days. Weather bomb is an actual meteorological term with a specific criteria. If you would like to learn more about the phenomenon, visit the following link for a complete definition.