Winter Storm Warning

As expected we have been upgraded to a Winter Storm Warning that takes effect at 6pm tonight and runs through 9pm on Friday night. The conviction on this snow has been high and growing for several days, and the 27+ hour duration of the Winter Storm Warning itself is another sign of the potential impacts.

 

Some details and considerations:

 

• The entire area north of I-80 – including the entire city of Chicago and Cook, DuPage, Kane, Lake, and McHenry counties – is in the bull’s-eye for snow. This is still a tricky forecast (see below) but there is good reason to put the over-under on snow totals in that area in the 8-9” range. A few spots – especially far southern suburbs – could see totals closer to 2-6”, but there are far more people who are going to see totals in the 10-12” range. Everyone in the warned areas has at least a 15-20% chance of seeing more than a foot of snow. As I noted yesterday – and as the models have shown in the past few hours – this set up tends to surprise on the high side.*
• The cold air at the surface this morning is going to increase snow totals and hurt the snow removal efforts on the roadways. Temps are going to climb into the teens with a few flurries today before the main snow approaches toward sunset. The snowfall this evening will be light for most locations. Snow during the overnight hours will be moderate but could approach 1” per hour in some spots in the early hours of Friday.
• The Friday morning commute is going to be a problem. With several inches down and snow still accumulating the roads are going to be tough. School cancellations are possible. The airports are already cancelling hundreds of flights; neither airport is especially likely to close entirely (winds will be on the lighter side) but the impacts to flight schedules will still be severe.
• This is a “forced boundary” snow – a warm front is draped from west to east across the area, and the collision along the front of warmer/moist air with cold air is creating the snowfall. Just to the south of the front there will be almost no snow, and just north of the main snow band there will be almost no snow. The area of heavy snow will be at least 200 miles wide from west to east but only ~30-50 miles from south to north. If the track were to wobble by just a few dozen miles it would mean the difference of 2 inches versus 12 inches. The “bust potential” is low – heavy snow is very likely in the area – but there is bust potential for specific areas (especially south) since the details across a 10-20 mile corridor are difficult to pin down. If the storm does miss it looks more likely to be on the northern side (i.e., the northern half of the city and suburbs toward the Wisconsin line have the highest odds of heavy snow).

 

I will send an update this afternoon if anything changes, but at this point I wouldn’t count on it.

 

* As an aside, I thought I took a screenshot of yesterday’s NWS probabilistic snowfall projections but somehow I lost it. In any case, last night the “chance of snowfall >12” for Chicago Downtown (or anywhere else in the region) was literally show at zero. This morning it’s at 25%. That is a painful failure.

 

 

 

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