Quiet and cold weather along with sunny skies has been the norm so far this weekend but attention still looks to be on the system that is going to bring accumulating snow across a large portion of Illinois beginning tomorrow night and into Monday morning. This will not be a major snowfall but it could drop several inches. I dig into the forecast details and discussion below.
Our system or wave of interest is still well northwest of the Midwest late this afternoon up across far northwest Canada across The Yukon. Over the next 12-24 hours, this wave will quickly dive southeast across a good part of the Canadian provinces and eventually into the northern U.S. by tomorrow afternoon. Across the local area, the sun will again be out tomorrow morning and into the afternoon hours with highs reaching the upper teens to mid 20’s north to south across most of the area but clouds will be on the increase by late afternoon.
This wave will continue southeast into North Dakota and Minnesota by tomorrow afternoon (see below) along with it amplifying and strengthening some as it digs southeast into the Midwest. This wave will provide the large scale lift needed for snowfall production.
Also, a smaller shortwave within the flow out ahead of the stronger main wave will help to saturate the atmosphere earlier on. It will do this by delivering a period of warm air advection and isentropic lift across the northern half of Illinois. Conditions at first will be too dry for snow but with this period of warm air advection expected, the column should saturate at a steady pace during the afternoon and into the early evening hours tomorrow. You can see this period of advection in red below ahead of the southeast bound area of low pressure.
Another good visualization and learning tool to see the atmosphere saturate is by using and looking at forecast soundings. Below are four forecast soundings at different times from the early afternoon into the late evening hours over northern Illinois. At first, you can see the atmosphere is quite dry. You can see this by how far apart the red (temperature) and green (moisture) lines are from each other. The further apart they are, the less saturated the atmosphere is. Over time tomorrow, you can see those lines getting closer and closer. This is showing you the atmosphere becoming saturated and by the last sounding image of the loop, the are close together and nearly saturated and this is when snow begins to reach the surface.
Snow will begin across the region tomorrow evening once saturation occurs and right now that is looking like between 7-10pm depending on your given location with the further west you are meaning you’ll see snow first and further east it will arrive during the mid-late evening hours. Once it starts, it will snow continuously through the overnight hours and through the morning as well. The question then becomes when does it stop. Some new model guidance has trended a little further south with the location and track of the surface low. This then leads to snow continuing into the afternoon hours the further north you are, more so across northern Illinois. If this trend continues with new model guidance tonight and again tomorrow morning, snow could hold on as long as Monday evening, especially across northern and northeast Illinois so we will keep you up to date on snow stopping times as well.
Another factor to the system could be the blowing and drifting snow from the gusty winds, more so by Monday afternoon and evening across central and northern Illinois. This will be a fairly dry snow given the cold temperatures at the surface and also higher up in the atmosphere so it will be easy to blow around. Winds really look to strengthen out of the west by Monday afternoon after the front moves east through the region. This could lead to quite a bit of blowing and drifting snow, especially across rural areas. Temperatures will also drop as well Monday evening with a fresh snow pack and cold air advection with single digits expected.
Below is our latest thoughts on snowfall amounts. These might need to be upped slightly tomorrow if we see more of a consensus among model guidance for slightly higher amounts but for right now this is our thinking.