Peoria, Ill. (WMBD) – Mild weather has returned to Central Illinois but it will be short lived as a strong storm system will bring wetter and colder weather to Central Illinois for the upcoming weekend.
- Rain develops Friday
- Widespread moderate rain Friday night
- Gradual transition to snow on Saturday
It is still too early to talk about snowfall amounts but it appears the chances of an impactful snowfall are rather low across Central Illinois. A mix of rain and snow is possible along and north of a Galesburg to Peru line Friday evening before changing over to rain as the area of low pressure moves into Illinois. Periods of moderate to heavy rain will develop Friday night before becoming lighter Saturday morning.
Rain showers will be ongoing Saturday morning but should gradually transition to a wintry mix and eventually snow as temperatures fall from the upper 30s into the lower 30s throughout the day. Where snow falls it’s not yet clear how much will accumulate, but the chances of accumulating snow are highest north of a Macomb to Kankakee line…even if it remains light. Those who are planning to travel, especially into Iowa and Wisconsin this weekend, will want to stay up to date on the forecast over the next few days. Regardless of any snow accumulation locally, rainfall amounts region wide are expected to range from 1.0 to 1.5″ through Saturday evening.
Why don’t we know how much snow will fall yet?
In order to make an accurate snowfall forecast meteorologist need know what the Snow Ratio is and how it will change throughout an event. The snow ratio tells us the percentage of water within and sample of snow, in other words, how many inches of snow will fall for every inch of rain. An old rule of thumb states that for every 10 inches of snow there would be 1 inch of water. However snow ratios vary around the country and from storm to storm. Here are some of the variables that affect the snow ratio…
- Depth of warm air between the ground and the snow producing cloud
- Too much “warm” air and the snow will melt or melt and refreeze as sleet on it’s way to the surface. An hour of sleet during a good winter storm could significantly impact snowfall amounts as small ice pellets don’t accumulate like snow.
- Amount of ice in the snow producing cloud
- This is determined by the amount of moisture in the air and the temperature profile within the cloud. When the cloud temperatures are between -10° and -20° Celsius (14° and -4° Fahrenheit) conditions in the cloud are conducive for producing and growing ice crystals. This layer is known as the Dendretic Growth Zone (DGZ). The deeper the DGZ the more ice crystals and snow will develop.
- Wind speeds
- High winds can cause snowflakes to fracture and break apart. The loss of a snowflake’s lacy structure means that each individual snowflake will occupy less space, lowering accumulations.
On top of those variables listed above small scale features that enhance snowfall rates are often not predictable outside of 24 hours. Other factors such as ground temperatures and surface types can impact the amount of snow that accumulates. Given all of these variables the earliest our snowfall forecast will be made is about 3 days out from the start of the event.
Here are some things to keep in mind when you see a snowfall “forecast” on social media
- Peoria averages 24.6 inches of snow a season
- Heavy snow events are not common in Central Illinois
- Snowfall events of at least 6 inches occur only once a year
- Snowfall events of at least 10 inches happen once every 8 or 9 years
- Model snow accumulation algorithms often use simple snowfall ratios to calculate accumulations, doesn’t factor in the constantly changing variables in a storm system. Take output with a grain of salt.