It's only late November but Mother Nature never likes to care what time of year it is and she's getting ready to throw a major winter storm together and impact all of northern Illinois during the day tomorrow and into the evening as well. It is very rare to get significant winter storms across this area during this time of year but tomorrow will be the exception with possible blizzard conditions along with 6-9" of snow, locally close to a foot in places across northern Illinois by late tomorrow night. I dig into the details below....
As usual, I like to get a first look at a CONUS water vapor loop and we can see that our system of interest, which looks fairly impressive on satellite, is slowly spinning its way across Wyoming with associated jet streak across portions of Utah and Colorado late this afternoon. This upper level low is forecast to continue southeast, crossing the Rockies tonight and emerging into the plains early tomorrow morning, before turning more easterly and it moves through the central plains.
As we go through the overnight and early morning hours tomorrow, low pressure will develop on the lee side of the Rockies across eastern/southeast Colorado before turning and moving east along the KS/OK border. As this is occurring, low level winds will be responding to the pressure falls occurring across the plains to the east of the developing cyclone along with strengthening as well. The strengthening will increase warm air advection and moisture transport northward up into the heart of the central plains (Nebraska/northern Kansas) and this in conjunction with large scale lift from the wave/upper level low itself, a large precipitation shield (including snow) will develop overnight tonight and into tomorrow morning. Also as you'll see in the loop below, cold air advection will also get going on the north side across the northern plains as colder air advects southward.
By late tomorrow morning and afternoon, the strong surface low and associated precipitation shield on the northwest side will be turning more to the ENE as the system moves through the state of Missouri. Right now, there is fairly good agreement on the surface low tracking from extreme southeast Kansas to just north of the St. Louis area from mid-morning to late afternoon tomorrow. With a storm moving ENE along this path, it is usually a favorable track for heavy snow across northern Illinois.
As the upper low and cyclone strengthen into tomorrow afternoon, low wind winds will really increase across most of Illinois, especially across the southern half of the state in the warm sector. This will lead to very favorable and continued moisture advection/transport up and over into the cold conveyor belt across portions of Iowa, Missouri and into northern Illinois.
Another favorable aspect of this event is the very impressive mid-level frontogenesis signal that will develop over portions of northern Illinois and this will act to lead to and enhance snowfall with a SW-NE or WSW-ENE heavy band of snow to setup somewhere across the area. Models are good in a lot of ways but they are not that good in being able to pin down the exact location, at least yet of where this potentially narrow band of extremely heavy snow will be. Even so, the signal is there for it to develop. This frontogenesis in tandem with very steep mid-level lapse rates and very strong large scale forcing could lead to some intense snowfall rates greater than 2"/hr and also the possibility of some thundersnow as well.
All these above ingredients mentioned will lead to period of potentially very heavy snow across northern Illinois tomorrow favoring the late afternoon into the late evening hours as rain changes over to heavy snow from west to east, as the surface low moves through central Illinois and into Indiana. As mentioned, there could be a several hour to 6 hour period over the Chicago metro area, at least way from the immediate downtown Chicago proper, where snow is falling at rates of 1"/hr or more and this could lead to rapid accumulations. There is also going to be chances for thundersnow as well in the deformation band tomorrow evening, also leading to impressive snowfall rates.
Surface temperatures will be marginal at first for accumulations but as the surface low passes to our south and southeast, cold air advection will continue to advect colder temperatures at the surface more southeast into more of northern Illinois. This will lead to snow ratios slowly becoming more favorable by evening along with snow being able to stick and accumulate easier.
Another big factor in all of this will be the strengthening northeast winds as the surface low approaches northern Illinois from the south and southwest. Numerical model guidance is forecasting winds just off the surface tomorrow night to be as high as 50kts, which is reminiscent of some of the more recent blizzards we've seen across this area, most notably the GHD I blizzard of 2011 and the GHD II blizzard of 2015 where both those events had winds as strong and even a little stronger than what is forecast for tomorrow night. As the pressure gradient tightens, winds at the surface will strengthen to 30-40mph by late afternoon and more so into the evening hours with gusts potentially as high as 45mph.
Snow will end from west to east early Monday morning, especially across northeast Illinois with cold air advection continuing on Monday, and with a fresh snowpack down, expect well below normal temperatures and frankly downright cold air the start the work week.
All in all this is going to be a major winter storm for northern Illinois and the Chicago metro area with a large swath of expected snow totals in the 6-9" range and a narrower swath of 9-12" expected, given all the favorable ingredients mentioned leading to the potential for some seriously impressive snowfall rates in both the frontogenesis band tomorrow afternoon and the deformation band tomorrow night. Below is our latest thinking from ISC on expected snowfall totals. We will have a full update after model guidance comes in this evening.