(12:15PM - 1/18/19) Friday mid-day update on the approaching winter storm for the state this weekend... Overnight and morning model guidance has come in with some big changes, but disagreement and uncertainty on the overall situation for many areas still exists. 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 into this afternoon and evening as the winter storm takes shape to our west and moves in later today. We will have more updates refining the forecast during the afternoon and evening as we see needed, especially given the uncertainty for many areas.
At this time what is known is a winter storm will affect the state this afternoon into Saturday evening, with lake effect snow possible for areas closer to Lake Michigan from Saturday into Sunday morning. 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 this evening into Saturday morning, and then portions of Central and Southern Illinois Saturday morning into Saturday evening.
Morning model guidance and observations have come into agreement on having the frontogenetic driven snow to be focused further north across Southern Wisconsin this afternoon and evening. If you remember back to last nights forecast update, the agreement at that time was to have this over Northern Illinois, but model guidance and observations suggest that threat has shifted north. However, it does still look like this band will quickly brush areas near the Illinois/Wisconsin border this afternoon and evening, before other snows move in later. Later this evening a large area of snow will overspread portions of Western and Northwest Illinois, spreading east with time across all of Northern and portions of Central Illinois, and continuing overnight and into Saturday morning. Periods of moderate to heavy snow will be possible at times, along with snowfall rates of 1" per hour at times. There is still disagreement among model guidance on where exactly there heaviest corridor of snow will this will be with this period of snow, but as of right now that looks to be from the Quad Cities area and then along/north of I-88 in Northern Illinois, with lower amounts south of there and into Central Illinois.
Further south, model guidance has come into agreement showing the storm system intensifying and becomes better organized as it pushes east-northeast across the Ohio Valley. This will lead to an area of snow developing that would traverse portions of Central and Southern Illinois Saturday morning into Saturday evening, leading to moderate accumulations. However, there is still disagreement on location and exact snowfall amounts for this axis of of snowfall. For now we will go with light to moderate accumulations, with the possibility of needing to lower or raise amounts in later updates.
In addition to the main storm system, lake enhancement to the snow is likely across portions of Northeast Illinois near Lake Michigan from this 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 Sunday morning. This will bring the potential for a couple to 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 around to in excess of 12" 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 is still poor at this time. Additional updates will be provided this afternoon and evening, refining the forecast and expected snowfall amounts.