(12:15AM - 1/18/19) Thursday night update on the expected winter storm to affect the state this weekend... While this storm is likely and snow is less than 24 hours away for some areas, there is still major disagreement within model guidance regarding snowfall amounts and placement. What this means is that our TAM snowfall forecast at the bottom of this post is based off of current model guidance and observations, but will likely change based on further model guidance and observations, which hopefully will come to more of an agreement by morning. We will have an update during the morning, to hopefully provide a clearer picture regarding the situation.
A this time what is known is a winter storm will affect the state Friday afternoon into Saturday evening, with lake effect snow possible for areas closer to Lake Michigan from Saturday into Sunday. Snow will be likely across most of the state at some point during this period. Snows will likely affect all of Northern and portions of Central Illinois Friday afternoon into Saturday morning, and then portions of Central and Southern Illinois Saturday morning into Saturday evening.
At this time the only thing that model guidance has been consistent in showing and is in some agreement on is a period of frontogenetic driven snow across portions of Northern Illinois from Friday afternoon into Saturday morning. However, there is still less than average agreement on the exact location and amounts within this axis. For now we will go with a blend of guidance for location and amounts, with 6-9" expected along and north of I-80. Snowfall rates of 1-2" per hour will be likely at times during this period as well.
Further south, there are two solutions in play for portions of Northern Illinois south of I-80 on into Central Illinois and Southern Illinois. The first solutions is some synoptic snow to push through, allowing for light to moderate snowfall accumulations, higher north and lower south. The second solution would be if the storm system intensifies and becomes better organized as it pushes east-northeast across the Ohio Valley, which would lead to a larger area of snow to develop that would traverse portions of Central and Southern Illinois, leading to moderate to higher end accumulations. For now we will lean towards the first solution, with more lighter to moderate snowfall accumulations expected, higher north and lower south.
In addition to the main storm system, lake enhancement to the snow is likely across portions of Northeast Illinois near Lake Michigan from Friday evening into Saturday morning. This could lead to some enhanced totals for those areas during that time. After the synoptic snow exits the area Saturday morning, pure lake effect snow will be likely from Saturday morning into the middle of the day on Sunday. This will bring the potential for several more inches of snowfall accumulation in Northeast Illinois, for areas near Lake Michigan. Snowfall rates of 1-2" per hour will be likely at times. For areas near Lake Michigan, snowfall totals may reach upwards of 12-15" due to a combination of synoptic snow and lake effect snow.
Below is our current TAM snowfall forecast. As the disclaimer said at the beginning of the post, this is based on current model guidance and observations, and that model agreement in poor at this time. Another update will be provided in the morning, hopefully one that will portray a clearer picture regarding this storm.