5:30 p.m. radar showing snowfall just west of St. Louis and generally along/north of I-70.
NWS St. Louis Winter Weather Advisories.
HRRR simulated radar for 8 p.m. as light freezing drizzle/snowfall continues to fill in.

Current radar trends suggest the atmospheric column is saturating as 700mb frontogenesis is occurring over central Illinois. Snow should continue to fill in from the northwest and expand as far south as Mt. Vernon along I-64. As the column saturates, there could be a brief period of freezing drizzle resulting in a fresh glaze of ice on any untreated road surfaces. In fact, the National Weather Service in St. Louis has issued Winter Weather Advisories (see counties impacted above) for a glazing of ice and snowfall accumulations up to 1″. This activity will continue into the early overnight hours. Any plans for travel this evening and the early commute tomorrow should proceed with a great deal of caution. The southern extent of impactful wintry precipitation this evening should remain north of Carbondale but I will continue to monitor radar trends throughout the evening in case frozen precipitation develops farther south than I currently expect. 

Two lobes of energy noted on the 500mb vorticity chart that will give us to focal points of precipitation Tuesday night into Wednesday.
GFS Simulated radar showing mixed precipitation potential.
Euro Simulated radar showing mixed precipitation potential.
3km NAM Simulated radar showing mixed precipitation potential.

Attention will then turn towards yet another round of winter weather tomorrow afternoon through Wednesday morning. With the system just now coming onshore from the Pacific there are still some details to be ironed out but right now it looks as though this sytem will have two main areas of energy. The weaker lobe looks to pass north of I-70 providing all snow and the stronger lobe of energy should track through the Ohio valley which is where the greatest concern for icing will exist. There is the potential that St. Louis and points east, north of Mt. Vernon may see very little, if any wintry precipitation but to be conservative, I’ll mention light snow up to 1/2″ at this point in time. Icing potential for the southern lobe of energy will depend on how far south the surface freezing line can migrate during periods of precipitation. The last couple model runs have trended farther south with this adjustment resulting in up to 1/10″ of icing for areas near Mt. Vernon increasing to 0.15″ farther south from Carbondale and Paducah. Additionally, Snowfall accumulations of 1-2″ may also fall from Mt. Vernon and points south with the favored area for snowfall being similar to that of icing potential. 

Conditions will begin to deteriorate rapidly once the onset of precipitation begins as untreated roadways will quickly glaze over with ice. This will last through Wednesday morning where the morning commute may likely be hazardous as well. Slow down and use caution when getting to work in the morning.