As the cloud cover continues Sunday evening there have been a few reports of patchy freezing drizzle and light snow along the I-39 corridor, up into southeast Wisconsin and northeast Illinois. Locally, a couple reports of freezing drizzle were received between Franklin Grove and Rochelle, as well as around Polo.
It’s likely that the patchy freezing drizzle and light snow flurries are/were a result of an upper level disturbance (trough) passing through northern Illinois and southern Wisconsin. This disturbance provided just enough lift in the atmosphere to squeeze out the little bit of moisture we have in the cloud layer as it passed through. The precipitation won’t last all night and should be ending around 10pm as the trough passes east. The freezing drizzle may be just enough to provide a very thin layer of ice on elevated surfaces, such as bridges and overpasses. With temperatures remaining below freezing overnight those slick spots could continue into Monday morning.
Freezing drizzle forms differently than freezing rain. Precipitation starts as snow way up at the top of the cloud layer, way up in the atmosphere. When that snowflake falls it can fall into an environment with different temperatures. For freezing rain to develop, the snowflake will fall into a layer of the atmosphere that is above freezing. This allows the snowflake to completely melt and become a liquid drop. It remains a liquid drop as it falls to the ground – which is at or below freezing – and freezes on contact. Freezing drizzle, however, can be a lot harder to detect and realize and is often times harder to see on radar. It’s important to remember that just because the temperature at the cloud layer is below freezing, that doesn’t mean that all liquid water droplets are going to freeze into ice. In fact, liquid water droplets can be present in the atmosphere down to a temperature of -40°C (-40°F)! These droplets are known as ‘supercooled’ droplets. They remain in their liquid form because nothing has disturbed them, so to speak, to cause them to freeze. Freezing drizzle forms in the lower part of the cloud, like the ones we have currently Sunday evening, where the lowest portion of the cloud is in the atmosphere at temperatures between roughly 0°C and -10°C, not cold enough to produce ice. These supercooled water droplets (drizzle) then fall to the ground and freeze on contact with surfaces that are below freezing, forming a thin layer of ice as a result.
The impact Sunday evening may be minor, but there could be a few slick spots through early Monday morning.