WINTER COMES TO TOWN

If you hear knocking at the door Saturday night, ignore it, it's just winter wanting in. Actually, it's more of a scout checking the lay of the land and sending a message, "hey, my boy winter is alive and well and there's more where this came from". So while it will get windy and cold with snow showers, this is just a wake-up call and by next Tuesday temperatures will be on their way back up. Sorry for the interruption.

So, after much talk and speculation, our first system with wintry implications is rolling in from the west. A clipper is streaking east along a cold front that brought blustery conditions and a few showers to the north Friday evening. With the front stalled over Missouri, it will act as a guide to steer the surface low towards central Illinois Saturday night. Here's the expected track. Since it's a fast mover, precipitation should only last about 8 hours once it begins.

Here's a depiction of how the day unfolds.

12 NOON Saturday:

Saturday starts out with clouds on the increase but dry conditions. Temperatures will modestly warm during the morning reaching peak levels around noon of 41 north to 48 in the far south. Just before noon, you can see a swath of rain approaching my western counties, but the majority of my area remains dry.

3:00 PM SATURDAY:

Saturday afternoon rain spreads from west to east across the region. Just to the NW of my area, the rain is shown mixing with or changing to snow. Temperatures with the onslaught of precipitation will slowly decline, with readings around 37 north to 44 south.

6:00 PM SATURDAY:

By early evening, the clipper's surface low has crossed into WC Illinois. Rain is still shown falling from the Quad Cities southeast, while rain is mixing with snow and perhaps changing to snow in the northwest. Temperatures are in the range of 34 north to 40 far south.

12 MIDNIGHT SUNDAY:

At midnight, the system is over EC Illinois and only light precipitation remains. A rain snow mix is indicated in many areas except the far south. In parts of the north, a brief, but complete transition to all snow is possible before precipitation ends. Temperatures at midnight should range from 32 north to 36 south.

As was the case 24 hours ago, there is significant variance among models regarding snowfall. A couple of weeks later, a similar track would have produced a solid 2-5 inch snow. But in this case, despite 850 temperatures in most areas below freezing Saturday afternoon, some in the north -2 to -4 C all day (usually a sure bet for snow), surface temperatures remain above freezing much of the day melting the flakes just before they hit us in the boundary layer. That's not to say some slushy flakes won't eventually mix in, that's eventually likely to happen in the north. However, it remains questionable that a full transition to snow takes place towards sunset and beyond, and if it happens how soon? The sooner, the better the chances of meaningful accumulations. There is a scenario where evaporative cooling allows the transition to happen faster, and 2–3 inches could occur on grassy and elevated surfaces. Currently, that is not the favored solution, but I can't count it out. However it plays out, at midnight only light rain, snow, or a mix remains.

Much like yesterday, models show variations in how much snow might fall. The EURO remains essentially accumulation free, but has the potential for a mix in the north that does not stick.

The GFS does have snow west and east of my area, where it thinks temperatures will be slightly colder. That leaves most of the region north of I-80 with accumulations of less than an inch.

The 3K NAM is still the most aggressive and really came in hot north of I-80, with accumulations (dare I say it) of 3-6". Even the south sees measurable snow. Again, I was surprised at its stronger intensity and higher amounts. I'm not saying with certainty it's wrong, but odds are high it's too high with its depiction. I would not count on it.

The HRRR just came in, and it's now looking more like a laid back version of the 3K NAM. It and the 3K are both hi-res models that are supposed to be the cats meow this close to the event. We'll see, but it really bothers me the EURO is a nothingburger and I hate to bet against it.

This is the NBM, a national blend of various models. It could have some merit as its solution is kind of a low to mid-range compromise.

Needless to say, there remains high uncertainty on how this plays out, it may take much of Saturday morning before we see how the mesoscale thermal parameters are progressing before a solid assessment can be made. I still think at the very least most spots should at least see a transition from rain to a rain snow mix. And, it's looking more likely, (especially north of HWY 30), there could be a full on change to snow for a period.

If this system fails to deliver snow, on the back side of the clipper, a fast moving polar vortex drops nearly straight south from Canada to central Illinois Sunday. This is the core of some of the coldest air in North America relative to average. The depth of the cold air aloft and some vorticity wrapping into the circulation will kick up snow showers, especially Sunday afternoon and evening. These fast moving streamers could briefly reduce visibility and produce a nice dusting in spots. The northeast half of my area is most favored to see the snow showers. Here's what the EURO indicates for light accumulations from that event.

Temperatures will take a pretty good hit and by Sunday morning should be back in the low to mid 20s with wind chills of 10–15 degrees. Afternoon high will struggle to get out of the upper 20s to low 30s, with wind chills no better than the mid to upper teens. Monday morning, some spots in the north could hit the upper teens, with wind chills in the single digits, easily the coldest weather of the season so far.

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ENDING ON A WARM NOTE

As I alluded to earlier, the core of the cold kicks northeast rapidly and by Tuesday, return flow is underway. Temperatures will rapidly moderate and by next weekend we could be back in the 60s enjoying another round of Indian Summer. Nothing like the Midwest for extremes.

Well, that's the best I can do under some unique and rather challenging conditions. I'm anxious to see how it all plays out. Have a great weekend despite old man winter knocking on the door. Roll weather...TS