(8:00PM - 1/21/19) We are continuing to eye the potential for a period of freezing rain across portions of Illinois tomorrow and into the early evening hours, favoring northern Illinois at this time. These freezing rain setups can sometimes be tricky to get a handle on and forecast given northern Illinois usually isn't very prone to bigger freezing rain events. Even so, tomorrow looks quite iffy in that regard with also the threat of some accumulating snow across portions into northern Illinois as well tomorrow night.
I dig into our latest thinking and some new details below...
As expected, our main weather system, this potent upper level low below has come ashore California and has continued its track eastward and is currently located near the Four Corners region this evening. This strengthening upper low will continue off to the east and northeast through the central plains by tomorrow afternoon and eventually the Midwest tomorrow night.
As this occurs, a strong area of low pressure will develop across the plains and move in the same direction as the upper level low, pushing into the Midwest by tomorrow evening. Strong warm air advection and isentropic lift will lead to a widespread area of precipitation north and northeast of the area of low pressure by tomorrow morning as it lifts northeast.
Across Illinois, temperatures will be quite cold during the morning but the mentioned strong warm air advection will lead to rising temperatures into the afternoon hours. Even so, high resolution model guidance continues to advertise temperatures below freezing. As temperatures remain above freezing, temperatures just off the surface a few thousand feet up will be above freezing and this will lead to increasing chances of freezing rain and possibly some sleet as well, at least for a few hour period during the late morning through mid-afternoon time frame as precipitation moves in from the west and southwest.
For this reason mentioned above, several National Weather Service offices have issued winter weather advisories for a good chunk of Illinois for the threat of freezing rain on the front side of precipitation and also possibly some accumulating snow tomorrow night as temperatures drop completely back below freezing.
Expect precipitation to move into western and northwest Illinois during the late morning hours tomorrow and then continuing east and northeast into the rest of northern Illinois and portions of central Illinois during the early to mid-afternoon hours. Below is one forecast model valid at 3pm tomorrow afternoon, showing fairly widespread wintry precipitation in the form of freezing rain, sleet, and even some snow.
Temperatures will continue to warm however through the afternoon so most places should go over to plain rain for at least a time, especially south of the I-88 corridor and all the way into southern Illinois seeing just plain rain. As the area of low pressure continues northeast tomorrow night moving through the area, colder temperatures will advect back into northern Illinois, potentially changing rain back over to snow with some light accumulations possible, favoring far northern Illinois at this point.