Winter Storm Warning

Late yesterday the track of the storm did turn south, and last night we were upgraded to a Winter Storm Warning. There are a lot of caveats here, but the potential for sticking snow has increased dramatically. The Warning is in effect from 9pm tonight (Friday) through 3pm Saturday for the entire area north of I-80. (Thankfully the evening commute should have no effects.)

For starters, do not fall victim to the "OMG we're going to get a foot of snow" hysteria some people will undoubtedly start spewing. The 7-11" totals referenced in some forecasts, including from NWS, are both somewhat unlikely (in my opinion) and not applicable to the vast majority of people in the forecast area. 

What is clear is that there will be a sharp gradient from the NW to SE along which snow totals will decrease rapidly. Areas near DeKalb or even as close as Kane and McHenry counties have a decent shot at 6+ inches of snow. For the vast majority of the area's population it gets trickier and snow totals will almost certainly be well below that. 

Clouds will thicken this afternoon and precip will start as rain before potentially switching to snow tonight as we approach midnight. The best bet for now:

• 6+ inches of snow for far western and northern suburbs (Kane, McHenry, Lake)
• 3-6" for Dupage and far NW Cook, decreasing rapidly to the SE
• Trace to 2" for areas in Cook and Lake that are within 10 miles of Lake Michigan, including the entire city of Chicago. (I think for the core of the city-- within 2-3 miles of the lake -- there is a 30-40% chance that the snow never really piles up. It is hard to see much more than a couple of inches even if it does accumulate. )

For context, the difficulties with this type of storm are worth noting.

• An earlier or later shift from rain to snow -- a transition that is very hard to pinpoint -- can dramatically alter snowfall totals.
• The ground is still warm after a very mild fall. Even getting the atmospherics right might not be enough to get the snowfall totals right if the snow melts before accumulating. 
• The warm ground effect is amplified in the city. 
• The surface water of Lake Michigan is still near or above 50 degrees (10-12 degrees warmer than at this point last year), and winds initially blowing off the lake could well keep the precip in liquid form over much of the city. 

Miscellaneous:

• Should a 6" snow be recorded officially at ORD, it would be very rare -- it's only happened twice in Chicago (and four times in Rockford) since the late 19th century. 
• It has been 238 days since our last measurable snowfall in Chicago, which is slightly above average. 
• Temps will rebound into the 40s and 50s next week, so any snow won't last long. 
• No major storms are on the horizon here for holiday travel next week.