(8:00PM - 1/18/19) Friday evening update on the winter storm that will be affecting the state tonight into Sunday... Model guidance from this afternoon and evening has finally come into better agreement regarding this winter storm. Observations support the guidance for the overnight into tomorrow morning period, though a little question still remains for the rest of the day on Saturday.
For tonight into tomorrow morning... A large area of snow increasing in coverage and intensity out across Iowa and Missouri is now beginning to push into portions of Western and Northwest Illinois. This activity will quickly spread across all of Northern and portions of Central Illinois through early tonight, continuing overnight, before ending tomorrow morning from west to east. Periods of moderate to heavy snow and snowfall rates of 1-2" per hour will be likely at times...especially around the Quad Cities and for areas along and north of I-88 in Northern Illinois. At this time the heaviest axis of snowfall accumulations of 6-9" will exists from around the Quad Cities and for areas near and north of I-88, with isolated higher amounts possible.
For Saturday morning into Saturday evening...Model guidance has come into agreement showing the storm system intensifying and becoming 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 moderate accumulations, with the possibility of needing to lower or raise amounts in later updates, along with altering the position of the greater snowfall axis.
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, periods of pure lake effect snow will be likely from Saturday morning into Sunday. This will bring the potential for a couple to several more inches of snowfall accumulation in Northeast Illinois, for areas near Lake Michigan. Periods of moderate to heavy snow and snowfall rates of 1-2" per hour will be likely at times. For areas near Lake Michigan, snowfall totals may reach upwards of around to in excess of 12", due to a combination of synoptic snow and lake effect snow.
Below is our current TAM snowfall forecast. Additional updates will be provided tonight and through the weekend; refining the forecast, expected snowfall amounts and giving short term updates.