Peoria, Ill. (WMBD) — It’s an active week of weather across Central Illinois with two storm systems impacting the region. While the first storm is strictly a rain producer, the second storm is expected to track much further south and could have enough cold air to produce a period of freezing rain and snow over Central Illinois.

Key Takeaways

  • Models trending colder Wednesday night and Thursday
  • Period of light freezing rain possible Wednesday night and Thursday morning
  • Accumulating snow possible over much of the area

The track of the system remains uncertain as some models show a path from St. Louis to Champaign while others bring the system much further south, passing the far southern tip of Illinois. The storm track will have significant implications on temperatures, precipitation type and the placement of the heaviest snow.

Northern Track
A northern track will keep surface temperatures above freezing resulting in mainly light rain Wednesday night and Thursday morning with a gradual transition to snow Thursday morning and early afternoon. With this scenario the heaviest snow will remain well north of the area with only some minor snow accumulations locally.

Southern Track
A southern track will allow colder temperatures to settle further south resulting in surface temperatures being near to below freezing. Light rain is still likely to develop Wednesday night, but with surface air temperatures below freezing, light freezing rain would be possible. This track would also bring a quicker transition to snow and higher snow accumulations to areas west of I-55.

Current Forecast

The current forecast is for the area of low pressure to track along or just south of I-70. This would bring some light rain and freezing rain to areas along and west of the Illinois River late Wednesday night and Thursday morning. That freezing rain/rain mixture is then expected to transition to snow by mid morning Thursday. Snow is then expected to continue into Thursday evening before ending no later than midnight.

Snowfall & Freezing Rain Forecast
We currently expect snow accumulations to range from a Trace in the east to as much as 3 inches in parts of Knox, Henry, Bureau, and La Salle Counties. Ice accumulations are generally expected to remain confined to areas west of I-55 and below a tenth of an inch. However, both snow and ice accumulations could increase if the southern storm track verifies. This is just a first attempt snowfall forecast so expect additional adjustments on Wednesday.

What Determines Precipitation Type?

Whether precipitation falls as rain, freezing rain or snow depends on the temperature profile of the atmosphere. Often, the difference between freezing rain and accumulating snow comes down to a degree or two difference in the air temperatures aloft.

Snow falls when the entire temperature profile of the atmosphere is below freezing.

Sleet, also known as ice pellets, form when temperatures aloft are warm enough to melt snow as it falls towards the ground. However, the rain falls through a deep layer of cold air before reaching the ground causing it to refreeze prior to contact with the ground.

Freezing Rain
Similar to sleet, freezing rain develops when temperatures aloft are warm enough to melt snow as it falls towards the ground. However, unlike sleet, the layer of subfreezing air is much shallower and rain does not have a chance to refreeze before reaching the ground. instead, rain freezes on contact resulting in surfaces becoming coated in ice.

Rain falls when a substantial part of the atmosphere is above freezing.