Weather Impact Alert: Mix of rain and snow ending overnight

ST. LOUIS — A Weather Impact Alert has been issued and will remain active until Monday at 4 a.m.

SUNDAY EVENING UPDATE: Cold rain and wet snow are currently entering the region this evening. As precipitation starts, temperatures are falling, with observations of sleet or graupel indicating atmospheric cooling. Expect a wintry mix of cold rain and occasional wet snow, with potential accumulating snow north of Interstate 70 until approximately 3 a.m. Monday. Travel may experience slight disruptions. Areas along and south of I-70 will likely see rain or a mix of rain and snow, with heavier downpours in some localized spots.

For Monday morning’s commute, only light rain or drizzle should remain, leaving roads damp rather than heavily soaked.

This weather system is not anticipated to be a major winter storm.

Tuesday will see temperatures rise into the 60s, with mild conditions persisting throughout the week. The forecast suggests active weather, including several periods of rain, possibly accompanied by thunder towards the week’s end. While severe weather isn’t a strong possibility at this time, we will monitor the progression of the late-week system closely.

Access the complete Weather Impact forecast by clicking here.

5 On Your Side meteorologists deploy Weather Impact Alerts to notify you of significant or hazardous weather conditions as soon as there is a high degree of certainty about disruptive weather in our region.

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Timing: The period from Sunday evening through Monday morning is critical, primarily due to the potential for slick roads and the necessity of monitoring updated weather forecasts. 

Snow Accumulation Potential: The greatest chance for accumulating light snow is expected north of Interstate 70, where colder air is more established. Residents in northern Missouri counties or west-central Illinois should be aware that this corridor faces a higher risk of travel disruptions if the system becomes colder or the snow band shifts southward. While forecasts may change, some northern areas could experience accumulating snowfall.

This is not a straightforward “all snow” event. The atmosphere is structured like a layer cake, with cold air at the surface and a warmer layer attempting to intrude higher up. This atmospheric setup presents three significant forecasting challenges:

1. Precipitation type: A slightly more intense warm layer aloft increases the likelihood of sleet or freezing rain, whereas a more profound cold layer promotes frozen precipitation. Even a minor temperature fluctuation several thousand feet up can drastically alter the outcome, and current forecast models often struggle with such precise details this far in advance.
2. Storm track: Minor shifts north or south in the system’s path can reposition the most intense precipitation band and the rain-snow boundary from one county to another, a frequent occurrence in the days preceding the system.
3. Road impacts: Road temperatures may remain above freezing even when air temperatures drop to near freezing, or they can fall rapidly after sunset. The rate of precipitation is also a factor; heavier precipitation can freeze surfaces more quickly than lighter precipitation, even at borderline temperatures. 

As previously noted, a primary challenge in forecasting this system stems from the significant lack of widespread cold air in the region. Even at its coldest, hundreds of miles away, temperatures are only 5-10 degrees below freezing, which is typically where precipitation type is less ambiguous. Locally, while we anticipate temperatures to approach or dip below freezing for a few hours, the southward reach of this cold air will be crucial in determining the exact weather scenario from Sunday into Monday.

Currently, the forecast through midday Thursday aligns with the details outlined above. There’s an uncertain zone where the precise southern limit of the cold air remains to be determined. Although this highlighted area might experience several rounds and variations of a wintry mix, it’s anticipated that temperatures will struggle to stay at or below freezing for an extended period. Nevertheless, some localized travel issues and impacts are still possible from Sunday night into Monday morning.

For those with Sunday evening engagements or a Monday morning commute, it’s advisable to have alternative plans. The uncertainty surrounding the “rain/snow line” near St. Louis highlights the importance of staying updated: as new information emerges, we will refine predictions for the precipitation type changeover, the southward extent of the snow line, and whether the primary concern for the metropolitan area will be isolated slick patches or more extensive issues. At present, the likelihood of travel disruptions seems mostly restricted to damp roads.

This Weather Impact Alert will be updated as forecasting confidence improves, particularly regarding the exact location of the most significant precipitation band. Continue to monitor forecasts throughout the weekend and ensure your devices are configured to receive alerts should any advisories or warnings be issued.