The radar Wednesday evening is beginning to get a little more active as precipitation quickly lifts from south to north across northern Illinois. While surface temperatures remain above freezing, we are beginning to see a little more blue show up on radar. Surface reports from south of Rockford are indicating that rain, sleet, and some snow, has started to fall.
The mixed precipitation developing is due to a process known as ‘evaporative cooling’. This is where the atmosphere cools due to evaporation of a liquid (in this case, the rain/snow). As a result, it causes the freezing level in the atmosphere to lower (the 32-degree line), transitioning the rain showers over to more sleet and snow. This process may last for a couple hours before we begin to see rain spread back north through the evening. The heavier nature of the precipitation is also helping with the transition over to the wintry precipitation.
Snow accumulations will remain light area wide, although a slushy inch or two is possible over south-central Wisconsin and northwest Illinois where Winter Weather Advisories have been issued. Wind speeds are also expected to increase, gusting 30-35 mph later this evening.
The steadier precipitation will come to an end later tonight as drier air wraps in around the low. This will lead to more drizzle and fog for Thursday morning. Thursday afternoon a mixture of rain and snow are likely once again for the afternoon. Be careful if out driving this evening, as well as Thursday morning.