Well as we stated early last week, the weather pattern we are heading into had the potential to lead to several systems producing wintry weather across the Midwest, including here in Illinois. The first one brought light snow accumulations today to the northern half of the state and now all attention is on a potentially more significant system arrive later tomorrow. (with another possible Thursday/Friday of this coming week...stay tuned on that one)
This system will most likely produce several different precipitation types across the northern half of the state and the heart of this winter storm will be affecting northern Illinois, favoring along and north of I-80 as things look now. The one point to get across at this stage of the game as that this is a very tough and tricky forecast from just south of I-80 to the Illinois/Wisconsin border and 25-50 miles can mean the difference between rain, freezing rain, and accumulating snow.
As you can see below by just our first thought on initial snow accumulations from this system, there is going to be a sharp gradient from potential several inches of snow to little/no snow.
Several area National Weather Service offices have already issued Winter Storm Watches and Winter Weather Advisories, which you can see in the image below. The watches are in effect for northwest Illinois in darker blue with advisories in effect across north central and northeast Illinois, including the Chicago area and further downstate in purple.
I dig into the details below on our system below....
As we can see by taking a look at a United States water vapor image, our already strong system, or upper level low is currently across northern California late this afternoon and will begin to move more east over the next 24 hours, eventually taking aim on us here in the Midwest.
The loop below shows the track our system will take as it will cross the Four Corners region mid-late morning tomorrow before it crosses the Rockies and emerges into the central/southern plains late tomorrow afternoon and evening.
As our strong upper low crosses the Rockies later in the day tomorrow, cyclogenesis (the birth of a cyclone) will get going with our area of low pressure developing across the plains. As pressures fall responding to the developing low, low level winds will strengthen across the southern plains. This wind response will lead to strong isentropic lift and warm air advection east of the developing cyclone (shown in red below).
As low pressure continues developing across the plains during the day tomorrow, an uptick in precipitation developing/blossoming to the north in the cold air will gradually occur thanks to that continued warm air advection and insentropic lift along with large scale forcing from the upper low itself. This will initially occur to our southwest and west tomorrow morning and afternoon. The loop below runs from 6am to 3pm tomorrow, showing the potpourri of precipitation across the plains and slowly lifting northeast into Iowa, Missouri, and eventually Illinois. There could be a brief period of freezing rain with light accumulations downstate as you can see in the loop, before precipitation changes over to all rain.
As we go into the late afternoon and evening hours tomorrow, precipitation will continue lifting northeast into most of Illinois with the brunt of the wintry precipitation expected across northern Illinois while the rest of the state remains all rain due to warmer temperatures both aloft and at the surface. As you can see below (valid at 9pm tomorrow night), there are several precipitation types laid out across northern Illinois, mainly north of I-80 with predominately snow across far northern Illinois, freezing rain/possibly sleet across the I-88 corridor and rain south of that. That is why this remains a tricky forecast and trying to nail down the locations of each along with expected ice and snow accumulations across northern Illinois. Not only is there the threat for several inches of snow across portions of northern Illinois but there could also be an axis of formidable freezing rain/ice accumulations.
As the surface low moves northeast through portions of northern/central Illinois and into northern Indiana late tomorrow night/overnight, we could see a lull in precipitation early Tuesday morning until the area of low pressure rapidly strengthens to our east across Michigan. This would aid in pulling in colder air into the northern half of Illinois and changing precipitation over to all snow and what could be a period of accumulating snow Tuesday afternoon and evening. If this were to verify as shown below, not only would snow return to portions of the area but winds would really pick up in strength due to the strengthening low pressure system tightening the pressure gradient across our area.
Remember, all these details remain up in the air and we should be able to nail down the finer details and timing as we go through the day tomorrow.
Below is our first thinking for freezing rain/ice accumulations across the state. As you can see, this will mainly be affecting the northern portions of Illinois, along and north of I-80 but cannot rule out some ice south of I-80 as well. .
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