Even though the official first day of Spring was yesterday, Mother Nature has other plans for Northern Illinois. Longer range models have been hinting at another possible chance for winter weather across Northern Illinois this coming weekend for about a week or so now, though the location of the heaviest of the snow was a little variable as one would expect being so far out.
The past couple days though the GFS model has been consistent with this system tracking in from the west across North Central Missouri and then to the southeast across Southern Illinois. The track of the center of the low being important, because depending on where that tracks tends to tell you what type of precipitation you might be seeing; whether it is rain, mixed precip, or snow. The freezing line in the upper levels looks to stay to our south for this system meaning we should stay all snow in the upper levels. At the surface though, temps look to be in the 32-35F range from air over the lake advecting across the region keeping us a little warmer. That is something that will need to be watched, as it could play a role in what actually happens at the surface as far as precipitation type and also if it sticks/collects or not. It does appear the areas to our west-southwest that do stay at or below 32F correlates well with the higher model project snow totals in those areas. The temperature also will play a role in the kind of snow that falls as far as wet or fluffy. The warmer the temps, or closer to 32F, the wetter the snow will be. It can snow with temps over 32F if you have a sufficient layer of below freezing temps aloft, but as I had hinted above, if it collects or not is something that will need to looked at.
Though as some may know, forecasting the weather is never as easy as looking at one model and that being the final solution, and that is where the NAM comes in. As far as timing goes, the 0Z GFS and the 0Z NAM runs from today are in pretty good agreement for a start time, which is good. That means you have some consistency between models. There general overall track of this system is also pretty consistent. Where they tend to disagree though is on the strength of the system, precipitation amounts, and the temperature at this time. The GFS has this system a little stronger than the NAM model. The NAM model though doesn’t have the warmer air off the lake advecting as far to the west as the GFS model, keeping the Rockford area at or just below 32F. Which means we should see all snow and not a chance of a mixed precipitation at the surface. Also means that there is a greater likely hood that it could collect, and hence why the NAM model is suggesting a little higher snow totals across the area. The NAM model also does not go out far enough yet to get a full look at this system, but it does show some similar trends to the GFS and is starting to give us a better picture of some scenarios that need to be looked at.
With that being said, I will not mention any projected snowfall totals yet, as things will likely change from now until then. Ground and road temperatures will also need to be factored in and will also play a role in snowfall amounts and how or if it collects. I can say this though, if it does snow it will probably be a heavier, wet snow. Keep checking back for the latest!