Peoria, Ill. (WMBD) – Over the past week we have talked about the weather pattern trending more active and wetter for the start of February. This pattern centers around one particularly strong storm system that will develop over Texas and move through Midwest Tuesday night through Thursday.

It all starts with a Canadian cold front that will move through Central Illinois on Tuesday bringing a few rain showers to Central Illinois. The main system developing in Texas will then follow this front northeast into the Midwest bringing a prolonged period of snow, sleet and freezing rain to Central Illinois. The position of the Canadian cold front, which is uncertain, will play a key role in determining who experiences freezing rain, sleet or snow when the main system arrives.

As the storm intensifies, cold temperatures will plunge southward from Canada while warm and moist air is drawn northward from the Gulf of Mexico. The end result will be a prolonged period of moderate to heavy precipitation across the Midwest and Illinois, much of which would fall as freezing rain, sleet and/or snow.

Here’s what we know…

  • There will be a storm, likely significant, that will impact Illinois Tuesday night through Thursday of next week.

  • Where heavy snow and icing occur, significant impacts to travel and power outages will likely follow

  • Models are trending colder and therefore favor more wintry precipitation. You can see this trend in EPS (European Model Ensembles) which more and more members of the ensemble are producing heavier snow totals. The exact amounts at this point are not important but the trend is, notice a lot more pink and purple in the last frame of the gif loop below
EPS Ensemble snowfall 51 member snowfall output for Peoria

Here’s what we don’t know…

  • The end position of the Canadian cold front. Right now the position of the front varies from Central Illinois to southern Illinois, this will have a huge impact on what kind of wintry precipitation falls and where.

  • Ice and snowfall amounts. Since we don’t know the end position of the Canadian cold front, it’s unclear how much precipitation will fall as freezing rain vs sleet or snow. While models are ramping up snow accumulation across Central Illinois, these models typically underestimate how much of that snow will fall as sleet. If an area were to see a prolonged period of sleet, snow accumulations would be significantly less. A difference of 1° can be the difference between freezing rain, sleet and snow. Knowing the exact temperature profile of the atmosphere this far out is impossible.

  • Locations of mesoscale features, such as frontogenesis bands, that result in higher rates of snow. Parts of Central Illinois experienced the impacts of challenging mesoscale features Friday morning when 1-2 inches of dry fluffy snow fell over parts of Tazewell, Woodford, and McLean Counties. While we often know days in advance that these features will be present, accurately forecasting the exact location of them typically can’t be done until you’re within 12 hours of the event.

  • Impacts from wind and how much blowing and drifting snow there will be

  • What the snow ratios will be throughout the storm. More details on this below

What is a snow ratio?

There’s an old rule of thumb that states 10 inches of snow equals 1 inch of rain (10:1). This “rule” is highly inaccurate as snow ratios vary from place to place and from storm to storm. In fact, recent studies have suggested that an average snow ratio of 12:1 may be more representative of snow events across the Midwest.

That said, snow ratios tend to vary significantly throughout the duration of a storm which makes projecting snowfall totals a challenge. Here are some of the variables that impact snow ratios…

  • The amount of ice in the snow producing cloud. The more ice in the cloud the higher the snow ratio
  • The depth of the warm layer from the surface to the snow producing cloud. The warmer it is the lower the snow ratio will be
  • A deep cold temperature profile throughout the atmosphere favors higher snow ratios, especially if the layer is between 14° and -4°. This is a temperature layer called the Dendritic Growth Zone (DGZ)
  • Higher wind speeds could also lower snow ratios by tearing apart snowflakes as they fall to the ground. A smaller snowflake takes up less space resulting in lower accumulation

In the end there’s too many moving parts to narrow down specifics at this point in the process. We know models are trending towards a more impactful event, but that could also change in the next few days. For now it is important to monitor forecast from trusted sources and be wary of extravagant snowfall totals on weather apps and social media.