Hoosier

Yeah, the normal cold spots could get out of control if conditions are prime/nearly prime, as they look to be. Will be fun to watch. I'm even thinking minus mid 20s are possible at ORD. Normally would not entertain anything much under -20 there on a night so dependent on radiational cooling, but I feel like some non traditional thinking is warranted in this case with how low the starting point will be on Wednesday evening. If any "radiational cooling setup" could pull off temps well under -20 at ORD, it's this one.

Starts cold and then big moderation. In fact, could be an epic change. As an example, the 00z GFS has a 75+ degree turnaround in parts of IL from Thursday to Sunday.

Good question. There are diminishing returns to the impact of snow depth as the numbers get bigger. Having 0" or 6", having 2" or 8", having 18" or 24", all are a difference of 6" but that difference matters a lot in the first case, less in the second case and I would say virtually no impact between having 18" or 24".

You want to qualify the -20 as daytime high in case of a midnight high?

LOT lowered ORD to -23. Even that value with no downward adjustments would be the coldest temp since 1/20/1985.

Good idea

So here's my first call for Wednesday's low at ORD. I have spent the last couple days looking at the models and looking back at historic cases, and based on those things, I believe any guidance showing a low warmer than -25 is probably too warm. So then the question is how low? Progged 850 mb temps and the amount of expected snowcover at ORD and upstream generally match or exceed the coldest historic cases. In this case, the coldest temps aloft also come in during the night/early morning, so pretty good timing. It was tough coming up with a number given the variance in the guidance for 2m temps, and that is where the historic cases were helpful. I'm going with a low of -29 to break the all-time record low of -27 from 1/20/1985. I'd be lying if I said I had a lot of confidence in that particular number, and I certainly didn't want to stray too far above an established all-time mark (here's looking at you FV3). I would give about a 60-70% chance of the record being tied/broken, so more likely than not but still a very reasonable chance that it comes up a little short.

Oh yes. It can be aggressive over snowpack.

The FV3 is still like a coldpocalypse. Has about -37 at ORD on Wednesday.

Well, anything is possible lol. It is noteworthy though because the NWS is typically conservative with forecasting records, and we're talking about an all timer.

LOT is explicitly forecasting the all time record low max to be broken in Chicago. With it still being 3 days away, the potential is there for further downward adjustments.

Sorry, don't have data for Gary.

00z Euro has a spot of -38C 850 mb temps on the IL/WI border at 12z Wednesday. Will be curious to see which model's 2m temp progs do best with this.

Besides temps, dews will be something that is rarely seen so far south. Widespread -30 to -40 dews, maybe lower in some spots.

MEX mos continues to trend colder and now has -25 at ORD on Wednesday morning. Also only recovering to -14 in the afternoon.

GFS at 12z Wednesday

Here's actual temps. Keep in mind lows would tend to occur 1-2 hours later.

If there is a midnight high, I just hope it's under -11.

So keeping in mind that this situation is so rare/virtually without precedent in northern IL, I think there's a decent shot at the high not getting warmer than -15 at ORD on Wednesday. -15 would break the all-time record low max by 4 degrees. A couple things that could interfere with that. One, the airmass modifies a bit in the coming days and it just isn't able to stay that cold during the day. The other would be the dreaded midnight high. Right now, it looks like the cold comes just quickly enough to prevent the midnight high scenario but it's something to monitor.

Give it time... MKE might trend toward -27.

Looking back at 1/20/1985 when Chicago broke their all time record, it appears that 850 mb temps were around -32C at the time that the record was broken. There was 7" of snow on the ground at ORD with plenty of snowcover upstream. In this case, 850 mb temps are progged at that level or even colder, and ORD will have deeper snowcover and again a nice snow field upstream. Based on this historical case and the current model progs, there is every reason to think that the -27 record will be approached (by approached, I mean within a couple degrees) with a real chance to tie/break.

.LONG TERM... 158 PM CST Tuesday through Saturday... Record breaking to perhaps historic cold outbreak still on track for mid week. Medium range models remain generally in good agreement in bringing in a period of record breaking cold, though there are some minor differences in the details in the models. For days now, the GFS and GFS-FV3 have remained on the cold end of the spectrum with 2m advertising fairly widespread -30F temps Wed morning across the northern CWA and keeping temps at or below -20 all day Wednesday. These readings don`t just break records, but in some cases shatter the all-time records. The ECMWF has been and the latest continues to be more reasonable with 2m temps but still advertises widespread temps at or below -20F Tues night/Wed AM and again Wed night/Thur AM. The ECMWF, GEM, FV3, and GFS all continue advertise all time record cold 850mb temps and 500mb heights based on the SPC sounding climo tool. These forecast values have shown some slight variability from run to run over the past several days, but the variability has mainly all been within record territory and just a matter of how much lower than records the values progged are. Given the run to run consistency and the consistency among the various medium range models and their respective ensembles, it is growing increasingly difficult to refrain from forecasting near all time record temperatures. Generally speaking, raw 2m temps from the various models outperform any MOS type products during periods of extreme temperatures. Have continued to gradually trend the forecast closer to the model consensus of 2m temps, but still stayed a bit warmer than the mean and much warmer than the coldest guidance (GFS & FV3). While remaining warmer than the mean of 2m temp guidance, we still are now officially forecasting record tying all time cold high temps Wed at both RFD and ORD. Wind chills should easily drop into the -35 to -45 range with some -50 degree wind chills possible, especially Wednesday morning. There is a bit more uncertainty with temps heading into Thursday as the latest GFS moderates temps in response to an approaching clipper. ECMWF and GEM aren`t on board with this idea, so stayed a bit more above the 2m model consensus for Thursday, but still officially below zero at RFD and Chicago. Assuming temps stay below zero Thursday, then Chicago and Rockford both could be looking at 72 continuous hours (or more) of sub-zero temps, though the second half of that cold will come with lighter winds and less extreme wind chills later Wednesday night onward. Medium range models do indicate that this extreme cold outbreak will break quickly late week with temperatures potentially rebounding back up to the freezing mark next weekend. - Izzi

12z Euro has about -33C to -35C overhead at 850 mb from 6 am Wed through Wed afternoon.

Don't expect this to verify of course, but still, things you never thought you'd see on a 4 day model for $1000, Alex