A few days ago I posted an article that we were seeing the early signs of a major pattern change – one that will bring sharply colder conditions – to take place within December’s first 10 days. All indications are that is going to happen, and all signs point to the pattern sticking around for quite awhile. This will not just be a brief shot of cold air, but a prolonged period of mostly below normal temperatures, the kind of pattern where temperatures in the 30s will be a relief. Yes, sadly, its coming.

The main driving force behind this will be classic “Greenland block” pattern. The simple gist of it is a large dome of warm air parks over That region and stays there, not allowing air masses to move. The result is cold air is forced down into the eastern US, and Illinois is in the dead center. A similar ridge will develop over the pacific northwest. Cause the cold air to be lodged over our part of the region for days to come. Its a classic setup for prolonged cold, and it is illustrated very well on some global plots.

With Illinois centered right in the middle of it all (at least by next weekend)

What to expect? Highs struggling to reach 30 north, 40 south, and lows in the teens-20s across much of the state. At least with this initial blast of cold air. Right now some models suggest the dome will flex and re-charge later in the month, bringing our first chance for temps near or below zero for this winter season that technically hasn’t started yet, but should pretty much right on cue.

As far as snow goes, this particular pattern doesn’t favor deep low pressure development that would bring Illinois big snows, and some indications are it could be mostly dry, however, we will be in somewhat of a NW flow regime which opens the door for potential clipper systems to bring us bursts of snow. These systems typically move quickly and only lay down a couple inches, but sometimes they can line up one after another and add up quickly. Currently there is little indication of that happening, but forecast models have a hard sniffing out those systems this far out so we’ll have to wait and see.

It appears with high confidence our string of above normal and mild days are soon over. Get the winter gear ready.

So when can we expect this drastic transition? Guidance is in pretty strong agreement with a doomsday date of December 5th as a strong cold front sweeps across the county. Its pretty easy to spot on forecast surface plots.

After that passes, game over for those hoping winter would somehow skip us. Some rain and maybe even thunderstorms may actually accompany this front before it passes, so storm lovers may get one last round of thunder for the year. Right now there are no indications of anything severe, but we’ll be following the precipitation potential with front more closely in the coming days. Right now the big pattern change is the bigger, more confident story to address.

Expect more updates on just how cold it will get, and any snowfall predictions in the coming days!

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