RCNYILWX

We're lucky there's so much precip before the warm air really arrives. Hopefully when that happens most of the storm is over. But regardless it's so hilarious to see another of these late big amped/NW pushes, which makes sense since we don't have any blocking to force a good track and the mean trough is so far west. The easterly winds will warm us up for a time east of the city. It can be as cold as it wants now but that wind shift warms us right up. But it will be brief. 

Re. 'we don't have any blocking', with all due respect the NAO is currently at about -1.5 according to CPC monitoring. It would be more accurate to say the blocking is not configured in a favorable way to keep the warm nose from surging in eventually along the coastal plain. But H5 anomalies and the teleconnection indices tell you all you need to know about whether there is any blocking.

The current -EPO/-NAO/-AO/-PNA regime will transition to a temporary +PNA later this week as the NAO block breaks down. Long lived AK ridging (-EPO) will retrograde to Aleutian ridging, with deep troughing along the west coast up to AK, setting up a +EPO and amplifying the ridging downstream.

This entails a mild week next week, though how far AN will depend on getting a system with higher Td into the region and melting the extensive snow and ice cover. It appears possible but too far out to be a lock.

The warm-up does look to be relatively short-lived, however, as the EPS, GEFS, and GEPS are in good agreement in the typical El Niño Aleutian troughing being displaced well to the south/southwest, which will set up another round of -EPO, though 2 of the 3 (EPS and GEPS) ensembles indicate that pairing with a +PNA, which would tend to be a drier, clipperish, LES pattern. The GEFS is far enough west with the AK ridging to have an interesting look for more of the sub-forum. It appears we're looking at a +AO/+NAO for that stretch, unlike now.

We're talking out at the end of these ensembles though, so wouldn't put too much stock in either outcome yet. Main point is that the warm-up next week does not look to be the end of winter temps, and we'll see on the details, barring changes from the general pattern progression.

Where is that exactly? Mind if I share with my office? Could be posted on social media and/or the top news write-up of the cold wave.

Extreme cold can definitely be be beautiful - looks like I caught the sundog in the photo. TWC app says -10/-33 here, guessing on a local wunderground site. Still -11/-37 at KLOT. The cold definitely overperformed.

Looks like it could be a rain/snow mix for us? It's too warm to be all snow.

With a wet bulb zero height of 950 mb plus the exceptionally steep low level lapse rates, even though the sounding shows 43 degrees, that could still produce snow or graupel.

We've had squalls out here in the fall and early spring that temps reach the low 40s in between squalls coming through and quickly back down to the 30s under the squalls.

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Forecast vs. Actual

The extended rain up into the central metro uglied things up, but storm total forecast worked out pretty well due to the early thump plus the overnight fluffier snow. Happy with where we messaged the 12"+ probs.

13.3" in La Salle seems really high, though I'm off this weekend and can't speak to the process behind going with that report.

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Edit: re. [mention=18064]mannynyc[/mention] post of sim ref a few posts above

Looks good for a squall there tomorrow. Too warm in NYC and LI to accumulate but surface wet bulbs and wet bulb zero heights are low enough for it to probably be snow if it stays coherent as it comes through, which both the HRRR and NAMnest are hinting at.

If this happens, could be some brief fun, with heavy snow and 40-50 mph gusts. These soundings are pretty classic, featuring very deep mixing, very steep low to mid level lapse rates, sufficient moisture, and instability. Could get a quick coating over interior sections.

I feel like we all need a Lessons learned debrief after this one.  What went well, what did not....uggghhh this hobby sux

The morning thump was legit and for me, observational trends were still unclear enough to not drastically change the forecast overnight. Was leery of making a big change based on guidance (especially the HRRR/RAP), that's often unreliable until even closer in.

But once observations (such as 3-hour pressure falls) showed that the northwestern-most low would stay dominant much longer and as part of the process the 850 mb and 700 mb low tracks also farther northwest, how things played out makes total sense.

This scenario remained on the table even though a lot of the guidance didn't show it, though a few of the recent ECMWF runs did hint at it. These dynamic systems often surprise for better (the morning thump and legit higher end winter storm conditions) or for worse, lately for worse for us on the whole unfortunately. So it goes.

Another challenge for us from a public notification and impacts perspective is that while the snow enthusiasts will be mad at us if we bust high, a significant under forecast is worse than an over forecast.

Not that I want to cry wolf, but to an extent as a meteorologist with a public safety responsibility, and we're right on the line, it's not the worst to err on the too snowy side. If we fully go with a warmer solution and the colder, higher impact scenario played out this afternoon, and there's major traffic incidents as a result, that ends up being more problematic.

As a good example, on Tuesday, we leaned a bit on the too optimistic side down in the heart of the metro. With the changeover back to accumulating snow/parachutes happening faster in the early afternoon, the roads deteriorated pretty quickly and we had to reissue the WWA for areas that we canceled more quickly than expected.

Ultimately, it's all part of the learning process and the challenge for us as forecasters in weighing all of these competing factors. I get the disappointment, and I'm personally disappointed that it didn't work out better during the day down this way and into the city and south.

Glad the northwest burbs got into the goods though, a part of the forecast that worked out nicely. Trying not to beat myself up too much, because the warning was definitely warranted this morning.

Finally, we've had misses that were much worse than this, without even the morning fun. One of the tops CIPS analogs was 12/19-20/2012, the MSN blizzard, and that one had all the rain and drizzle and no front end snow for us, and the deformation area fell apart as it tried to push across the metro.

Miami rule porks us, but Chicago doesn't have an issue with a Brownsville rule. 

We actually missed the goods west and northwest lol. Some 2-3" LES like rates early this morning though and a gravity wave/wake low feature produced a 48 kt gust at ORD. So short lived fun and then rain while western IL cleans up, with Moline, IL up near 15".

Totals for the heart of the metro today were only in the 4-7" range (around 5" for the Chicago sites), while the far northwest burbs got up around 10". We should get some light accums tonight but definitely a disappointing near miss. There's 0 snow left OTG in downtown Chicago, as they warmed up to the upper 30s and had close to half an inch of rain.

The lake had nothing to do with the track of either storm system. The lake is also not the main reason for the warm intrusion with each storm system either, that's more-so a storm track issue and lack of antecedent quality cold air issue. Yes, the lake is making things worse in the metro/NE Illinois, but for both storm systems temperatures have even gotten into the mid-30's across much of N Illinois/S Wisconsin...and E Iowa with the first one earlier in the week.

Agree it's not the lake for this one, the lake would've been primarily an issue for the Chicago shore accumulations if the track was more favorable. Riding the line with a marginal air mass and the near term synoptic track being farther north is what did it.

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Just woke up and it's unfortunate HRRR did a decent job - obviously something that can happen but knowing when is challenging.

Regardless, the northern low being dominant for longer is what messed up things for us here after the morning thump. With a farther north track you have to adjust everything a bit north and west.

I thought things would be more of a mix between the globals and some of the mesos because I still bought into a favorable enough track of the system including 850 mb low, which resulted in change to rain/mix relatively far north but not far enough.

Certainly can never discount the strength of WAA, but I believe that a with a more favorable SLP evolution, things would've worked out a little better locally by now with phase 2 of the event. Small details make a big difference when the antecedent air mass isn't that cold, which we knew.

Hoping the change to snow happens soon enough to get some new accumulation on top of what will become an impenetrable glacier lol.

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Looks like the 9:31 AM updates mention 3-5 additional inches in the North parts of IL rest of the storm? That band over the Mississippi River looks nice if it can not miss north!

Your office running on coffee and Redbull?

Based on the hellacious thump we had up there, 3-5" additional would line up pretty well with the forecast grids I did overnight (Gino did the AFD to help free me up to help out with our morning briefing graphics). It's been a week lol. Lots of extra hours for everyone. Not gonna lie, nice to be done for the week.

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For the Chicago metro it all comes to what falls from the sky this afternoon, where that rain/snow line sets up, and when it crashes southeast. This period has been the concern with the storm for awhile.

I wouldn't be surprised if something somewhat similar to Tuesday happens. It was raining pretty hard down to the I-80 corridor, and we had parachutes not long after. Not sure if it'll light up to that extent, but I wouldn't be surprised if the HRRR is overdoing things as usual.

LOT threw in the towel. Morning burst was nice, but in the end looks  like event will come up well short of expectations.

It became clear that the AM round would feature the heaviest rates for the metro as a whole and a majority of the snowfall, especially with southward extent. That was not well out of expectations from yesterday and working again overnight.

Maybe we could've conveyed that even better than we had, but it looked like north would be better for the afternoon part, though I do think we're going to get more snow this afternoon through tonight and end up pretty close to the storm total range.

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Looks like you can see it from ILX Radar.
 

From my coworker, one of our lead forecasters (Carlaw):

"I think this is one of those subsidence induced high wind events. Acts kinda like a wake low but without the wake low haha. Think my FWD SOO had a nice presentation on it. Would make sense since I don't think this is a dryslot coming in, but just a brief subsided regime."

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Well the last few hours were probably the best winter storm conditions locally since 2021 if not before that. Drive home from LOT took about 50 minutes, normally ~20-25.

Heaviest snow here I'd say since Feb 9, 2018. A bunch of branches came down when the wake low like feature hit, had to be 40-50 mph gusts. Measured 4" a little after 7am and it was still snowing heavily at the time but since slowed down on the back side of the heavy band.

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Not a Met but feels like warning criteria down in IKK. Don’t think wife will be able to get out of the driveway. Snow piling up over the wheel wells 

We were on the fence there because of concern that transition would be relatively quick followed by a long lull with rain before changing back over. It was a tough call. Put it in the watch and hindsight could've put Kankakee in the warning but wasn't confident enough.

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Today was a whole other kind of crazy and will be going back in at 11pm. Probably won't have enough time to chime in much, but will try to respond on any forecast stuff of note this evening.

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Ride the GFS/Euro blend. GEM has been consistently a bit southeast of that since the GFS locked in similar to the Euro/EPS. UKMET has also been consistent with the SLP track for this the last several cycles.

Feeling pretty decent away from the lake in the metro as things stand now, and hopefully things work out cold enough to help near the lake. Will be doing the forecast for the storm again tomorrow, and possibly tomorrow night too, as there's a chance I double back and work a midnight shift so we have 3 people there.

Rapid cooling of Lake Michigan


.

If we can get another one after the Arctic blast, then the lakefront will be in business.

I spent no more than a few minutes looking at the NAM today. It's just not worth trying to figure out when it's right when it's usually wrong until much closer in. Wish it had been discontinued earlier. NCEP stopped updating the code for it at least 2 years ago.

Crazy how the lake I assume can reduce amounts and then you go just west a little and you are at 10 inches. That sucks for us near the shore or in downtown. Is there anyway that the shore or downtown can get in that 10 inch range ? 

I'd feel better about it if the lake were colder and the antecedent air mass was 2-4 degrees colder and also colder at 850 mb.

Unfortunately paying for the warm December and dealing with issues more common to early or late season storms.

I'd say if it really rips, the extent of marine influence could be more muted, but if you're along the shore and probably a mile or two inland if not a bit more than that, experience (yesterday, November 25-26 2018, November 2015) tells us that it's gonna be tough to max out accums there.

And as mentioned earlier, including 18z Euro solution, if the 850 mb warm nose temporarily pushes too far north, that'll also make it even tougher to get the higher end accums.

So north of I80 and near the shore like downtown you are predicting around 5inches which could be on the high side? 

Here's the raw storm total forecast output, available at: https://www.weather.gov/lot/winter . The dark blue is 4-6", the light yellow is 6-8", orange is 8-12" and the darker orange is 12-18", in this case between 12-13".

For watch purposes today, was okay with this, but certainly might be too broad brushed and how far inland the marine influence extends is a challenging aspect of the forecast.

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Thoughts on "less slop, more snow"? Hard to really tell, but looks like precip/dynamics may be shutting down by the time the arctic air gets pulled into the system and profiles cool. Could this be another repeat of Mon-Tue when even Rockford was plagued by marginal temps, inefficient accumulation, mixed precip and lousy snowfall for 3/4 of the storm?

I think even more than the temps, the Rockford area was plagued by subsidence holes yesterday between the prominent banding associated with the 700 mb deformation (eastern IA to far NW IL and SW WI) and the band that set up just north of the 850 mb low track and dry slot and hit parts of the interior metro hard. When you have marginal temps and subsidence, it's a recipe for an underperformer.

As things stand now, I think the RFD area is in a better spot to avoid that but too early to call, since mesoscale banding is one of the tougher things to pin down, especially at this range of the forecast when the system track is still in flux a bit. Then Friday evening, should still be enough forcing on the back side to wring out as much as a 0.2" QPF while ratios greatly improve.

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The AFD update was great. Nice putting the "Forecast Uncertainty" in there too...
As to the timing... You guys were spot on with the N IL impacts of the last storm between 12-4 PM. My daughter's school canceled classes and at 11 am it looked like things were not that bad on the roads. But from 11-3 it just RIPPED here (Northern Kane County in Elgin - 3 miles south of I-90) and the roads were awful. Buses (especially in my rural area) would have had a hell of a time. 
Looks like the hourly forecast for the next storm has the snow really starting around me by 8 AM Friday and going through the whole day. In your 100% "not representative of the US Government" opinion ;-) it is looking like the second snow day this week for my daughter?
I've said it before, your posting here is so awesome. Thanks for taking the time. 

Thanks for the good feedback on the AFD! Tried something different there with the more limited amount of time I had. Glad to share insight on here over the years too. As far as school Friday, always tough to say how they'll handle the Friday morning part with the more marginal temps.

With it being the last day of the school week I guess I wouldn't be surprised at a snow day, or early release even, because things should get more hairy in the afternoon like they did yesterday. Then they have to account for everyone getting home safely, including teachers who might live farther out. There does seem to be less of a break than yesterday did before the ramp up 11am-4pm, and with a warning pretty likely to be in effect.

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