RCNYILWX

Good call on the slightly farther southeast track. Interestingly, the models that showed mid 970s mb pressures did well. It was a fascinating system overall - like a tropical system with snow, and an incredible amount of TSSN. Without a strong high pressure system to the north or northwest, proximity to the center of the low was the biggest driver in getting into that core of damaging northeast winds. If the only detail you gave for a forecast for the Chicago area was that a 976 mb low would track north of the Ohio River, you'd say most of the metro would have been a lock for heavy snow. The fact that it was a nearly vertically stacked low meant that the mid level lows were more tucked in than a classic winter storm, keeping the deformation area/fronto banding also more tucked in. The radar for a time was very reminiscent of 2/24/16 and the intensity of the banding resulted in strong low level subsidence and drying that overcame good mid-upper level lift and steep lapse rates in place across the central and northern metro. Just not our winter. Sent from my SM-G998U using Tapatalk

Other unfortunate aspect is that all the model run total maps shown by the TV mets are algorithm driven too, and we all own that forecast. My office didn't put out an explicit snow map on more public facing WxStory graphics (it was available on the probabilistic page) on our home page and social media until Thursday PM, but yet "we" forecasted 10" in the metro. QPF and snow depth change maps, plus forecast soundings with omega are the way to go, plus its important to consider whether the DGZ is supersaturated. Looking at the Cobb output page helps too, although even that scientifically more rigorous methodology can have overdone ratios too. Better to play it conservative (ie. pos depth change including var density HRRR product OHweather mentioned) than to get burned with 10:1 and Kuchera ratios. Sent from my SM-G998U using Tapatalk

The CMC was not good, right for the wrong reasons in the Chicago metro. It was too weak and too far south with the surface low, and didn't capture the dynamic cooling that occurred to support the accums of 2-5" of paste in the southeast 1/3 of the CWA. It only gets credit for having the farther south precip swath. GFS operational did fairly well and then once the ECMWF adjusted, it did pretty well, not a great performance overall though because it was too far north until pretty late in the game. We only have the UKMET via Pivotal Wx, but it did perform pretty well at a longer lead time. NAMs performed the worst of the models we commonly utilize. HRRR and other CAMs did well with capturing the high winds in Central IL and IN. This event was also a good case for the algorithm snow maps to be banned. Forecast positive snow depth change was the best option for snow amounts.

Temps and dew points are solid above the colder guidance. Looking problematic for any accums in our IL counties and even NW IN will probably underperform unless it absolutely rips this afternoon following changeover. Strengthening east northeast flow may help bring in slightly lower dew points and help there, but especially IWX CWA. Up here in the metro, there's forecast to be strong lift in the DGZ, though strong subsidence and some drying in the lower levels northwest of the f-gen banding, per forecast soundings. Sent from my SM-G998U using Tapatalk

Cook County does have 3 zones, as does Will County. The evening shift opted to leave southern Cook out of the WSW, but did put in eastern Will. Sent from my SM-G998U using Tapatalk

They sent the watch at 957 PM EST, and then the warning upgrade at 1021 PM. Interesting. Sent from my SM-G998U using Tapatalk

Surprised IWX didn't join in the upgrade of the watch to present a unified message with us to the west and GRR to their north and DTX to their northeast. I see they did put some counties into a WWA. Sent from my SM-G998U using Tapatalk

18z EPS bumped north, more than a noise level change on the 10:1 snow swath. The "fun" continues lol. 18z vs 12z comparison attached.

Just sent an AFD update. Basically, it's hard to say we'll be 80% confident in warning snow amounts/impacts anywhere in the CWA. Assuming the NAMs are too far NW, general idea is southeast of I-55, *if* the banding is intense enough for dynamic cooling to overcome the marginal BL. Throw convection into the mix and the fact that we're basically trying to precisely place a mesoscale feature (response to f-gen circulation) and say we're confident the dynamic cooling will be enough, it's a tough call. Even with the GFS, we're talking pretty high end impacts for the populous south suburbs, Kankakee county, and NW Indiana corridor. But the Canadians (not that they're great models) have me concerned and getting vibes of the Christmas Eve 2014 bust. If the GFS is right, that's a similar look to what happened on 2/24/16. And again, there's error bars on the exact placement of banding northwest of the compact vertically stacked low. Sent from my SM-G998U using Tapatalk

The 00z Euro bumped back northwest some from the 18z run. It remains a very tough forecast. Impacts will be higher end in the heart of the band within the CWA, barring a UKMET or GEM like outcome. Lapse rates in the DGZ are progged to exceed 7C/km and there's quite a bit of -epv above strong fgen. Sent from my SM-G998U using Tapatalk

Last 3 runs of the GEFS 24-hour 10:1 snowfall mean. Edit: Added 00z GEFS members. Pretty large spread this close to the event.

We're actually technically not supposed to put the totals in the watch text products, according to NWS directives. However, the simplified formatter we went to 4-5 years ago spits out the deterministic ranges and most offices leave them. I think we (at LOT) usually change it to snow totals of 6 inches or more are possible. The unusual watch and warning issuance on the same calendar day is looking likely for this event. Sent from my SM-G998U using Tapatalk

100% agree, we're not talking huge differences in features anymore, but the difference in sensible weather impacts is dramatic. Gonna be a fun day at the office tomorrow lol. Sent from my SM-G998U using Tapatalk

I deleted my post since you beat me to it haha. I think this sort of variability closer in underscores the complex nature of this forecast. Sent from my SM-G998U using Tapatalk

I try to avoid looking at the HRRR at this lead time because it's a hot start model and frequently all over the map for winter systems. I really don't think it was meant to be used for winter forecasting outside 18-24 hours. It's more useful inside 12-18 hours. While it doesn't mean a particular solution in its extended runs can't have the right idea, it's much more often wrong than right this far out and therefore hard to put any stock in it. It was very bad for the ice storm (too warm) and also the event before that (too cold over northern IL). It may have performed decently for the pre Christmas storm at this lead time, but that's the only one that sticks out to me.

Very good agreement between the 18z EPS and GEFS actually with that shift south of the EPS. But that also means there's still a decent # of members north of the operational, similar to the 18z GFS/GEFS. When the 18z Euro did shift south, it was a good reminder to me to not necessarily put too much stock even into a model I tend to trust more, particularly in this type of forecast that is sensitive to relatively small changes in the setup and the mass fields. @OHweather, great post btw. Would you put more trust in the ensemble means at this point? Wondering if that may be the way to go since they've been generally more stable than the operational runs. Sent from my SM-G998U using Tapatalk

I can confidently say that part of the problem with the ice storm forecast out there was the new ptype methodology we were forced into this winter. We had imo a much better system in place for several years up until last year. Sent from my SM-G998U using Tapatalk

It's less than 2 days out though. The Euro was too amped 4-5 days out from the pre Christmas storm but at this range it was fine. A solid majority of the EPS members support the op. Also, different setup, but it absolutely knocked the ice storm forecast out of the park. Sent from my SM-G998U using Tapatalk

Honestly I had a feeling that was the case, but the reason I replied was for the benefit of anyone on here trying to learn from red taggers. All good. As for my take, I prefer the ECMWF/EPS/GEFS, and the NAM also happens to be in that camp. The Euro is usually pretty good this close in and it's a red flag to me that the GEFS mean is again north of the operational and even slightly farther north than 12z. I'm speculating that the GFS may be struggling with convective parameterization. Definitely can't rule out the more southern camp though. Sent from my SM-G998U using Tapatalk

With all due respect, describing it that way makes it seem like there's a predetermined outcome. If your take is that the snow swath will end up south of what the 12z Euro op is showing, that's fine. There's no such thing as a model moving toward a consensus. Plus, the GEFS mean is solidly north of the GFS. And the amount of spread in the solutions means there isn't a consensus anyway.

Not available yet on WxBell. Sent from my SM-G998U using Tapatalk

Additional 18z GEFS content via WxBell and Pivotal Wx. I included the 50th percentile 10:1 snowfall from WxBell. If there's any interest in additional percentile data, let me know. Sent from my SM-G998U using Tapatalk

The Kuchera algorithm will probably be better for this event, but another way to produce an event "floor", particularly outside of the most intense banding, is positive snow depth change. It's basically always lower than 10:1 or Kuchera, so a good way to keep expectations in check, especially in heavy, wet snow settings. Can use soundings and perhaps Kuchera ratios to figure roughly how much you'd want to bump up amounts from positive snow depth change in the prolonged heaviest banding areas. One note of caution with the positive snow depth change is that if the model has an issue with snow depth parameterization, like the GFS did for for the ice storm, the positive snow depth change product will be affected. It's available for most guidance on Pivotal Wx, WxBell, and I assume Wx Models as well.

One of the more recent possible analogs to this storm that I can think of is the late November 2018 winter storm (blizzard warnings that didn't verify due to the wet snow were in effect but it was a high impact event). Sent from my SM-G998U using Tapatalk

No big changes from thoughts posted yesterday about the 3 plausible outcomes. While I wouldn't be surprised if we end up more rain and some backside snow in Chicago and burbs off to the west and southwest, we're still in the game. 12z Euro op fits the idea that this type of system could end up farther NW, even with downstream blocking. That said, the Euro op was too amped at this range for the pre-Christmas storm, also an anomalous setup. Still giving the Euro suite more weight, but too early for a firm call given the large spread in the guidance. Should see the goalposts start to narrow tonight and tomorrow. Sent from my SM-G998U using Tapatalk