RCNYILWX

How things might go tonight through tomorrow night when the winds max out. Remember this event well, January 26, 2014. Shows that not much snow is needed for truly gnarly conditions. We don't often get northern Plains weather (with notable exception of 2013-14 and 14-15 winters), but when it happens, the wide open areas outside of the heart of the Chicago metro are prime for blizzard conditions and significant drifting. The morning AFD update from my office notes that consideration is still being given to upgrading a larger portion of the area to a Blizzard Warning. Think that there's likely to be large sections of north south roads, including some of the interstates, shut down due to blowing and drifting snow tonight through tomorrow night. Hope most people don't chance driving in those conditions. Sent from my SM-G998U using Tapatalk

I fixed that jankiness near the lake in Chicago. Didn't have time to fix it until after all the products were out. Sent from my SM-G998U using Tapatalk

As another note, re. questions about the earlier than usual watch issuance today, a bit of a domino effect took place. That is a challenge we sometimes face in collaborating headlines. In the case of the Monday day shift, most offices we border (and the offices some of our neighboring offices border) preferred to hold off on a watch because we were still well outside the typical 48 hours out before the event issuance timeframe. However, a few offices preferred to issue the watch Monday PM. So with that, and challenges associated with only including portions of CWAs (ie. only western half of LOT, MKX, ILX), ultimately it was decided by most (except GRR and IWX) to issue the early watch. Plus we had to consider what the perception of public safety and media partners and the public would be if we didn't issue and neighboring offices did, also contributing to my office joining in. Ideally we would have waited, because we were gonna hit the messaging hard anyway, and beyond 48 hours, there can be pretty big changes still.

Some perspective is in order I think, and this doesn't apply to all areas and how much snow might fall. The ECMWF did shift in the direction of the other models related to changes over Canada at h5. However, it adjusted less and still shows a 980 mb low over south central lower MI at 12z Friday. That's still a powerhouse low given the very strong high pressure over the Plains. The changes that have been described here may or will hurt totals depending where you're at. It does not mean storm cancel for everyone or that there can't be adjustments back somewhat as the mid and upper pattern evolves over Canada, and some of the features still over the Gulf of Alaska this evening get sampled by the RAOB network. 24 hours ago there was perhaps some concern based off some of the models and mid and upper evolution they were showing that the system would be more amped and track far enough NW to bring rain even into Chicago. Given that this setup is fairly complex but essentially involves the PV lobe piece and the developing main wave over northwest Canada and not some exotic phase (think the failed Feb 2020 storm). If the models are a little bit wrong in the interaction between these and changes in 500 mb height fields, the strength of the main wave, the strength of the intensifying upper jet rounding our main wave, etc, there can be adjustments that oppose what we've seen tonight, just as last night it seemed like everything (except the Euro) was going much more amped. There are valid meteorological reasons why the PV lobe tracking to James Bay might destructively interfere with this setup, but imo it's still too early to feel confident that the changes seen tonight are directional changes that will continue unabated in subsequent model cycles. Even though it is drawing closer and the NWP should have more skill in this part of the forecast, there's still time for meaningful changes. I am not saying that the changes seen tonight won't continue unabated, but that this range before winter storms can and certainly has had shifts back in a positive direction for snow enthusiasts. Describing what the models are showing isn't the same thing as what will actually happen. I don't think if I was analyzing the data tonight at the office (I'm working the day shift today/Tuesday) that I'd have enough info to be completely confident yet. We can see some of the key features on water vapor imagery, but there's not really much observationally right now that can tell you affirmatively the 00z models are on the right track. This still looks like a high to very high impact event for the Chicago area and points south and east at this time due to the wind and snow combined with extremely cold temps. Really, anywhere that gets at least 3-4" of snow will have blizzard conditions in open areas. Finally, as a snow enthusiast myself, hopefully the shifts on Tuesday are back in a positive direction.

And still snowing at the end of the run. The precip type product on Pivotal Weather is a bit stingy on the western edge of the precip shield, with forecast soundings suggesting it's more likely to be a bit west of I-39 on that run. Meanwhile over Chicago at 12z Friday, a deeply saturated profile with a temperature of -3F and ~950 mb winds of 50 kts.

It's honestly hard to get huge QPF as snow in this area. To get 1" liquid equivalent in a snow system is pretty high end, if you go by the top 5 Chicago snowstorms posted above and at least GHD I and II had lake enhancement in those totals. So while the top end QPF amounts have come down since yesterday and there's a meteorological reason for it, it also represents coming down closer to reality. The widespread 2"+ precip. totals from the GFS last night are basically unheard of in this part of the Midwest in a snowstorm, with the exception of the January 1967 blizzard.

As much as the high end Kuchera ratios are overdone (24:1+ in western IL/eastern IA on last night's insane 00z GFS), banking on 10:1 ratios is also underdone. With the exception of 1967, Chicago's #2 through #5 snowstorms all relied on above to well above climo (~12:1) ratios. There are red flags in this setup per today's guidance related to the timing of the low rapidly deepening and wrapping up. That said, by far the most reliable NWP we have, the ECMWF, has 1-1.25"+ QPF for the southeast 2/3 of the Chicago metro (and the ECMWF is not known to have a high bias with QPF). If that verifies, then the ratios will easily verify above 10:1. An average of 15:1 would be attainable, and perhaps maxing out at 20:1 in mesoscale banding. The high winds only do so much to lower ratios. Well aligned strong omega with the DGZ, a deep DGZ, and steep lapse rates from the current ECMWF idea would compensate to result in near to above 15:1 ratios. Unless the system ends up well weaker and east of the current ECMWF/EPS consensus (which is possible but a lower probability outcome at time), this will be a very high impact storm.

Once the GFS jumped strongly to the general idea of the Euro today, it consistently depicted 2-2.5"+ QPF totals in the max swath. Absent lake enhancement, QPF totals like that are very rare in a winter system in the Midwest. As alluded to in my previous post, we saw the GFS do this with both February systems last winter. The 12z ECMWF certainly had a broader swath of high QPF totals than the 00z run tonight, but still those high totals on this side of the lake were in the 1-1.75, locally 2" range. When you have the GFS spitting out obscene QPF and add in the way overdone Kuchera ratios, it creates unrealistic expectations. The 12z ECMWF was probably a realistic best case scenario.

Because the Euro hasn't changed much from the overall evolution, definitely deserves the most weight in forecast thinking. That's not to say that meaningful changes can't happen, but that the GFS is more grain of salt worthy tonight. Now that the models have honed in on the big picture, not all that surprising the GFS is now the most amped and aggressive with QPF and snow, considering how it handled the February 2022 systems. Sent from my SM-G998U using Tapatalk

I buy more pronounced wraparound deformation banding than usual given the extreme negative tilt of the system. Wraparound snow is often overdone, but there are rare cases it produces. Sent from my SM-G998U using Tapatalk

Definitely a plausible outcome for this storm. Sent from my SM-G998U using Tapatalk

I think Thursday morning is a possibility, depending on how much preemptive cancelations are done so that planes don't get stuck at ORD. Wednesday would be safer. I know for a fact that preemptive cancelations are done for big snowstorms, but exactly when that starts probably varies. I wish I knew for sure, but wondering if flights with no connecting flights after arrival at ORD would be more likely to be canceled early than flights that connect to another airport not long after. Sent from my SM-G998U using Tapatalk

They (NCEP) may ask for extra RAOB soundings at 06z and 18z. It's pretty common for that to be done before east coast blizzards in addition to always being done before potential landfalling hurricanes. Don't remember if it was done for GHD I and II. Sent from my SM-G998U using Tapatalk

As currently modeled by the ECMWF and GFS, this storm would be worse than GHD I. The extreme cold in addition to the snow and similar magnitude winds is a game changer. Sent from my SM-G998U using Tapatalk

If consistency continues on this scenario, we in the NWS are really gonna have to hit the message hard about this being a life threatening blizzard, with incredibly cold temps for a system of this nature. Barring a big change in the evolution, the probability is increasingly high for the main swath to see extreme impacts. That's despite it being mainly a Thursday PM-Friday event. Have to give the overall run to run consistency of the Euro some credence at this point. Sent from my SM-G998U using Tapatalk

I'd imagine somewhere between 10-15:1 because the DGZ will be deep, offset by the high winds. The Kuchera ratios are definitely way too high because there's no limiting factor with such a cold air mass, and no accounting for the fracturing dendrites by the wind. Sent from my SM-G998U using Tapatalk

Yep that wasn't any sort of trend toward the other guidance. Small change in the scheme of things and good run to run consistency given the complexity of the setup. Expected more of a shift given how the other guidance really emphasized on the Hudson Bay polar vortex lobe suppressing heights enough to force the storm farther east. The Euro is still letting the main PV (potential vorticity) anomaly/500 mb low do its thing and in this case it was ever so slightly less amped, leading to slightly farther east surface low position over Michigan. As is, high end blizzard for western Lower Michigan and far northwest and north central IN. Out this way, that would be a very high impact storm with warning criteria snow amounts, temps in single digits and winds gusting to 50 mph.

There's also plenty of amped EPS members along the lines of the 12z UKMET. The overall trend vs. the 12z was again a lean to the west with the lower pressures. This is a high risk, high reward setup. It's interesting that 3 consecutive full Euro runs have nailed this area. As a snow enthusiast, it's fun to see obviously, but less meaningful than it seems given the still large spread among the EPS members. On this side of the lake, probabilities of >=3 and 6 inches favor Wisconsin, northeast IA, and southeast MN more than northern Illinois on the EPS, though have ticked up over the past few runs. Long way to go with this one. If we're allowed to post EPS stuff from WeatherBell or Pivotal, can post some later on.

Notable changes from the 12z EPS too vs the 00z and 06z runs, whether the changes are improvements depends on where you live in the sub. Def a better look for northern IL. Deeper h5 low and more of a westward lean to the surface low positions. Sent from my SM-G998U using Tapatalk

. Sent from my SM-G998U using Tapatalk

Edit: The 12z 12/16 and 00z 12/17 runs of the ECMWF are the wildest back to back model runs for this area since I've been out here. Sent from my SM-G998U using Tapatalk

That's inaccurate. Just to give two examples, the ECMWF/EPS far outperformed the GFS/GEFS for the February 2022 winter storms. Sent from my SM-G998U using Tapatalk

The other problem is the GEFS is still too non dispersive to glean much from it when the operational is quite different from the other globals. It's common for ensemble runs to shift in the general direction of the op, but the GEFS hugs closer to the op than the EPS tends to, which does a better job capturing the full spectrum of potential outcomes. Part of that is having only 30 members vs the EPS having 51, though non dispersiveness has been a persistent issue over the years with the GEFS. You can see the issue just comparing the 18z and 00z GEFS. There was a wholesale shift toward the op. Sent from my SM-G998U using Tapatalk

The 12z EPS is rolling out and the operational ECMWF is the most extreme solution SLP wise that far south vs any of the individual EPS members. EPS did trend more toward the general idea of the operational run vs previous couple EPS cycles though in handling some of the key mid-level features. That 12z Euro run is without exaggeration the wildest non fantasy range model solution for a winter storm in this area that I can recall in my time here (since July 2010). But the first paragraph highlights why these operational runs still need to be taken with a large grain of salt at this lead time. The potential in the pattern next week is certainly high as we've been saying, but the complex evolution will take several more days to resolve. Edit: From the New England sub (where the mood is as you'd expect after the non GFS runs today), look at the change in the ECMWF at the same valid time of 06z Friday 12/13 between the 12z runs yesterday and today.

While we can expect to see rather large run to run swings for a few more days, the modeling of the PV anomaly swinging down from western Canada is pretty consistent on the medium range guidance. The 00z Euro is a (fun) best case scenario, but the anomalously deep upper low plus ridge spike along the west coast suggests digging of that nature being a plausible outcome. Meanwhile a 00z operational GFS like outcome would be a letdown vs the potential in the synoptic pattern, but it still would bring a white Christmas to a large portion of the sub that hasn't had a white Christmas since 2017.