It's a nice, clear evening out in the Stateline, but sadly those clear skies are only going to allow those temperatures to drop even more as we head through the night. Highs were only 2° this afternoon in Rockford, and those are the kind of highs that we can expect for our area for the next several days.
We're already in the negatives, these temperatures being reported at 6pm, and they're only going to get colder as we head into the night. We dip to around -8° for our lows tonight, with wind chills around -20° during the overnight. The only relieving factor is the cloud cover that'll move in, which will keep those temperatures from cooling below -10°.
As we head through the evening we'll start to see some clouds move in after 8pm, which will continue building as we head into the late-night hours.
A weak, mid-level low will move into our area after midnight, bringing us chances for flurries during the overnight, but any light snow that forms will end up staying west of the Stateline area as our atmosphere is fairly dry. Chance for flurries will stay with us through Sunday morning, but then we'll see the cloud cover decrease for the afternoon, allowing for some sun.
The reason we've been seeing such cold temperatures is because of the Polar Vortex. Usually, it's fairly strong and keeps itself together up in the Arctic, which in turn keeps any cold spells short-lived since it's harder for us to get access to the arctic air in that scenario.
However, this winter, the Polar Vortex is weaker. This means it cannot keep itself together, and ends up spreading away from the Arctic down near the mid latitudes. This gives us easier access to the polar air, and allows cold spells to last longer. This is the pattern we've been experiencing the last several days, and is why this cold spell has lasted so long.
Also, when you factor in the placement of this high pressure system, that only allows more arctic air to push into our area.
This will be occurring for the next several days, but luckily the atmosphere always finds a way to make up for what it's lost. In this case, it's warmth.
As we head towards the end of the week, our jet stream will become oriented in such a way that allows for warm air to start moving back into our area. At this point, this looks to occur on Saturday, and if the jet stream can hold its placement around that time, we could be looking at highs near 20° by the start of next weekend.
As we head through the evening we'll start to see some clouds move in after 8pm, which will continue building as we head into the late-night hours. 
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