Peoria, Ill. (WMBD) – The National Weather Service has issued Winter Weather Advisories for much of Central Illinois with widespread accumulating snow expected Saturday afternoon and evening.
Timing and Impacts
- Snow expected between noon and midnight
- Heaviest between 3 pm and 8 pm
- Widespread 1-3 inches expected with locally higher amounts possible
- Snow will be fluffy, some blowing snow possible
- Roads will be snow covered shortly after it begins to fall
The weather pattern has become favorable for several rounds of accumulating snow over the next week and the next storm in line will swing through the region on Saturday. Snow will move in from the west after noon and leave the region in the east by midnight. Widespread accumulations of 1-3 inches are expected with the lightest amounts falling east of he Illinois River. Locally higher amounts are possible north of a Macomb to La Salle/Peru line where a narrow band of heavier snow may produce accumulations up to 4 inches.
Uncertainty and the Snow Ratio
Unlike our past events this storm will be strictly snow, so trying to figure out the location of the rain/snow line will not be an issue. However models disagree on the strength of the storm and with dry arctic air in place the snow ratios will be rather high. This means that the slightest difference in rain/melted snow amounts could impact snowfall accumulations by a couple of inches.
What is the snow ratio?
You’ve probably heard of the old rule of thumb that 10 inches of snow equals 1 inch of water (10:1). In reality the snow ratio varies from location to location and from one event to another. In fact it often changes during snow storms, like we saw on Thursday, when the snow started off wet and became light and fluffy at the end.
Given the dry arctic air that’s in place, our snow ratios on Saturday are expected to be somewhere in the 15:1 to 20:1 range. This means that for every inch of water there will be 15 to 20 inches of snow. Thankfully we are not going to see an inch water and therefore won’t see 20 inches of snow from this system, but snow ratios this high means that the smallest error in rain/melted snowfall amounts means a huge difference in snow accumulations.
Here’s an example…
Our current forecast is calling for 0.15 inches of rain/melted snow from Saturday’s storm. This means that with the expected ratio between 15:1 and 20:1, snowfall amounts will range from 2.3 inches to 3.0 inches. If our projected rain/melted snow amounts are off by just a tenth of an inch, the snowfall forecast is then off by one or two inches. You can see the math below…
15:1 Ratio with 0.15″ of water
15″ x 0.15″ = 2.25″ (2.3″ Rounded up) of snow
20:1 Ratio with 0.15″ of water
20″ x 0.15″ = 3.0″ of snow
With the expected snow ratios and 0.15″ of water, snowfall amounts of 2-3 inches are expected.
15:1 Ratio with 0.25″ of water
15″ x 0.25″ = 3.75″ (3.8″ Rounded up) of snow
20:1 Ratio with 0.25″ of water
20″ x 0.25″ = 5.0″ of snow
With just an additional tenth of an inch of moisture, snowfall amounts of 3-5 inches will be more likely.
No matter how much snow falls, snow covered roads are likely by late afternoon and throughout Saturday evening. Wind speeds are expected to blow from the northwest at 10-15 mph which will create some blowing and drifting snow, especially in rural areas.
Once this system moves out the coldest air of the season will be moving in. Temperatures will drop below zero Saturday night and Sunday morning with wind chill values between -15° and -25°. We’ll see some moderation in temperatures through early next week but another blast of arctic air could move in late next week and through the weekend.