As we continue to analyze and look at the latest model guidance from this afternoon and so far this evening, it continues to look like a fairly sizable, possibly significant freezing rain/ice threat will get going across portions of the state, favoring northern Illinois during the mid-late evening tomorrow night. We are slowly getting more concerned about this event as well and that there could be significant amounts of freezing rain/ice accumulations as you'll see below in our latest TAM/ice accumulation forecast.
I dig a little more into the details below....
This event will be brought on by a low amplitude wave rippling along in the mean mid-level southwesterly flow from the southern plains on up into the Midwest by late tomorrow afternoon and early evening. As it is moving along, it in conjuction with a period a fairly strong warm air advection will aid in developing a widespread area of precipitation initially to our southwest. You can see the strong warm air advection, strong low level flow in the image below (in red).
Precpitation will initially develop to the southwest across portions of Iowa and Missouri tomorrow afternoon before continuing northeast up into portions of the state by the evening hours as it moves in from west to east. As you can see in the loop below, this high resolution model guidance is hinting at some really high reflectivity in the model, indicative of the potential for some convection (thunderstorms) as well. If this were to occur, precipitation rates could become quite heavy across portions of northern Illinois. Also, if we do see convection, this could throw a wrench into favored precipitation type as thunderstorms have strong enough vertical motion and cooling to even get graupel, sleet, or snow to the surface when freezing rain is expected.
The tricky part of this forecast is figuring out the exact precipitation type during the height of this quick hitting event with some guidance hinting at some sleet and some hinting at mostly freezing rain. As we take a look at a forecast sounding below, more so a prediction of the temperature profile of the atmosphere, we can see that a warm layer aloft exists. This means that precipitation falling into this layer would change to rain given temperatures at that level are above freezing. As the rain gets closer to the ground, temperatures of the column return back to below freezing at the surface, leading to freezing rain/rain freezing on contact at ground level. You can also see in the bottom right of this image, based on the forecast thermal profile, the model is predicting "freezing rain".
Lastly, looking at forecast precipitation type, it is pretty much a wintry cocktail across the region but across northern Illinois, model guidance continues to show precipitation being in the form of mainly freezing rain, showing it moving in from west to east during the evening hours. It initially will get into northwest Illinois between 4-6pm, north central Illinois between 6-8pm, and northeast Illinois including the Chicago metro area, around or just after 8pm as things look now. This will most likely lead to very hazardous travel conditions across the area, even if it arrives just after rush hour. We will nail down the timing aspects with continued updates through the day tomorrow.
Our latest thinking on freezing rain/ice accumulations is below. Due to growing concern of significant icing across portions of the state, we have added a swath of up to a 0.50" of ice, which could cause significant to widespread travel problems along with tree and power line damage/outages as well.