RCNYILWX

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

00Z GEFS shifted a bit south. For my money at this range, giving the ECMWF/EPS the most weight (overall best NWP and 51 member high res ensemble), followed by the GFS/GEFS (for all its flaws it's not terrible and has done okay this year and the ensemble has improved with 30 members at higher resolution). Then the GEMs, then the UKMET (not having access to its ensemble precludes giving more weight to its forecast), then the NAM.

Resist the urge to look at the NAM for now. The parent 500 mb wave is completely off its domain, so the initialization over the Pacific is from the 18z GFS. Even when the main shortwave is on its domain, the NAM is still prone to wonkiness.

The 18z EPS, while another shift northwest, still nicely shows the range of plausible outcomes for the heavy snow swath. There's roughly 3 camps and the swaths have a similar west southwest to east northeast orientation due to the NE/ENE surface low track. Can use Chicago proper as the dividing line for the 3 camps: 1) NW of Chicago 2) Bisecting Chicago 3) SE of Chicago I think 1 and 2 are about equally likely at this point and 3 is a bit less likely, given recent trends. If you want to give how this season has played out some extra weight, then scenario 1 currently has a bit higher probs than 2. None of the 3 scenarios can be ruled out at this range. There's likely a relative northward bound to how far north it can go, but on the other hand, an earlier negative tilt can result in a farther north approach before track slides east-northeast. Plus, a weaker block can contribute to the above, favoring scenario 1. For the middle ground scenario 2, we basically need to balance out the competing factors in this setup to allow for a sweet spot track for Chicago and closer surrounding suburbs. Scenario 3 would rely on later negative tilt or farther south approach and/or a stronger downstream block to provide stronger confluence. Since the system will be rapidly intensifying early on, a faster occlusion and opening up at 500 mb could potentially contribute to a farther south snow swath. Regarding icing potential, seems like the forecast surface high pressure position would favor extreme northern Indiana and southern lower MI more than out here, due to proximity to the supply of lower dew points with the surface high and strong easterly flow from the surface up to 850 mb providing replenishment. There's also likely to be a sleet zone north of the icing. Farther west, with the fairly marginal antecedent air mass, might be a narrow swath of sleet between the snow and rain areas, vs. pronounced icing. Note on this analysis: It's meant to be a bit more simplified to lay out 3 realistic outcomes. There's obviously more going on meteorologically. Another factor that's always a wildcard is convection in the true warm sector, which looks to extensive and intense in this setup (wouldn't be surprised at moderate to high risks both Thursday and Friday).

Glad to be a part of the discussion Here's some items I've noted: - Anomalously deep surface low for a relatively far south latitude is pretty much a lock with the plotted EPS members all stronger than the 982 mb mean on the IL/IN border at 18z Friday - Bowling ball nature of the 500 mb low supports the idea that the track can be unusually far south for a system of this nature, in the presence of a strong downstream -NAO block. Another late season example of this was the late March 2013 snowstorm that brought record snow to SPI. - The downstream -NAO block is the key to the setup of here and points south still being in the game. - The general idea or trend of 12z suite today of deeper and farther north track also makes sense given the very strong and dynamic 500 mb low/PV anomaly, which can be expected to help pump downstream heights in the northeast a bit. If you look at 500 mb height anomalies, there's positive east of here and negative over Canadian Maritimes trapped under the block, and that'll be the impetus for our primary bowling ball wave to track east northeast and weaken instead of a sharper northeast or north-northeastward track. The presence of the NAO block merely gives us a chance that things could work out. It's a delicate balance with respect to the strength of the synoptic system and "feeling" the downstream block. The warmer and farther north solutions on the EPS and the GFS/GEFS are on the table, though I do think there's only so far north it can go. I'm concerned about a miss just north for us with the heavy snow swath because there will be a tremendous amount of latent heat release to help height rises east of here, and that may overcome downstream blocking to an extent, especially if the block trends a bit weaker. Edit: With 12z EPS 500 mb heights and anomalies valid 12z Friday.

Ask and you shall receive.

Bump north on 18z EPS. Still lots of spread on the individual members, but added several farther north/west and stronger solutions. Edit: Attached are the snow mean and member low locations. Edit 2: Now with individual member snowfall. As an aside, WxBell added percentile precipitation and snowfall data, which is pretty cool. The 50th percentile is lower than the mean, which shows there's several higher members in the top 50% of snowfall totals.

Plus a -NAO, gotta love it when it happens in March. [emoji58] Sent from my SM-G998U using Tapatalk

This event had worse impacts than February 2019. The winds and wet bulbing effects clearly won out over the higher rain rates, which was the challenge to discern. That's my biggest takeaway, to expect warning impacts when the winds are going to be stronger, because of the role in evaporative cooling and accretion efficiency, plus bringing limbs down. Another key factor was that dew points were 2-4 degrees lower than 2m temps despite the high rain rates, leaving room for wet bulbing, and amplified by the winds. Based off the reports, radial ice accums likely ended up in the 0.2 to 0.3 inch range closer to the I-88 corridor and then 0.3 to 0.4 and perhaps locally up to 0.5" in the northern tier (and far northern portion of counties just south). We won't get a final ice accum from UGN because the sensor iced over, so RFD ice accum will have to suffice as highest flat ice report. In hindsight, wish I put warnings out for the northern tier on Tuesday, and maybe then consideration on the midnight shift would have been to expand them south. Regarding the winds on Thursday, the state line counties are unfortunately a wild card as to whether temps rise enough and enough ice melts before the onset of the strong winds. It would help if dense fog develops overnight. Hope power gets restored sooner than later for board members who lost power.