Winter storm early next week? Not so fast!

Weather Blog

Peoria, Ill. – You may have seen several posts floating around social media talking about a potential winter storm early next week. Weather hobbyists and “crap apps” are already calling for heavy snow and ice, when in reality, there’s far too much uncertainty to know what is going to happen.

Here’s what we know…

  • Rain and snow showers are possible Saturday
  • Another storm looks to form east of the Rockies this weekend and track east Sunday through Tuesday of next week
  • A period of below-average temperatures could follow this potential storm

While models agree on the overall pattern, a large scale trough moving across the Midwest, they disagree on the overall track, timing and strength of the storm system that will come along with it.

With the storm being five days out we are using three separate groups of models to make this forecast. Those models are the American GFS, the European ECMWF, and the Canadian GEM. Each model gives us an idea at how things could play out in the future, and it’s not uncommon for those scenarios to vary significantly.

In this case, the American GFS model is calling for a powerful winter storm to move through central Illinois while the Canadian and European models show a far weaker system that passes 300 miles further south. If the northern track is correct, snow and ice could very well impact Central Illinois while the southern track would bring only light snow showers to the area.

So which model is correct?

Well…neither! No model is ever 100% correct, though some handle various scenarios better than others. In order to increase confidence in one solution or another, we rely on model ensemble forecasts to hone in on a solution.

You can think of a model ensemble as a family with individual members. While the members of these families are similar to one another, there are small differences in their DNA (algorithm) that give us a slightly different look.

Here’s how many members are in each Ensemble…
GFS Ensemble (GEFS) – 21 Members
ECMWF Ensemble (EPS) – 51 Members
GEM Ensembles (GEPS) – 21 Members

While there is value in understanding how each member of each family behaves, we are more interested in where they group together. While not perfect, if more members of a certain ensemble start to coalesce around a certain solution, and that solution is similar to other ensembles, then the confidence in a particular outcome increases.

Let’s look at the GFS Model Ensemble for this upcoming storm system…

The image on the left is the raw model output for snow accumulation (10:1 ratio) from the deterministic GFS model through 6 am Tuesday, December 17th. On the right, we have the projected snow accumulations (10:1 ratio) for each individual member of the ensemble. The blue line represents the deterministic model (model of the left) while the black line is the ensemble mean.

The deterministic models are often the ones shared on social media but they don’t tell the entire story or paint a complete picture of a developing storm. In fact, by looking at the image above, you can see that the heavy snow produced by the deterministic GFS model is an outlier solution that is much higher than many of the other members.

When we compare GFS Ensemble to the European and Canadian models, we try to find where the models agree. By doing this we can find the most likely outcome and keep the forecast consistent while the individual ensemble members jump around.

While the American GFS model is advertising a powerful winter storm, the European model, Canadian model and the ensemble models suggest a southern track and a weaker storm are more likely. The storm has yet to form, and the pieces of this storm are scattered across the Pacific Ocean. Once the system actually forms and starts to creep closer to the west coast, the models are likely to come into much better agreement.

At this point, we will continue to call for scattered snow showers on Monday. While this system could evolve into a winter storm, the model data as a whole doesn’t support one. As we get closer to the weekend the models should start to come into better agreement and the impacts of this storm will be better known.

If you see model snowfall projections on social media, don’t take them too seriously. Monitor forecasts from Your Local Weather Authority as the details of this potential system become clear.

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