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I for sure became more optimistic yesterday on round 2 also being big up here, but round 1 has always been the more likely way to get warning level snows in the metro. There's not a lot observational wise that can tell us which outcome is more correct, so we have to go by the overall guidance consensus, which is an upper level pattern evolution less favorable to get round 2 farther north. But on the other hand, aside from the GFS, there has been a general positive trend today for round 1 in the metro. Given some past fairly similar events trending more positive in the near term for what would be round 2, giving some more time on that, but certainly did not trend favorably today. Here's the IL/IN zoom 10:1 ratio snowfall totals from the Euro runs going back to the 00z 1/30, showing that near and southeast of I-55 continues to be most favorable and a better look for northeast Illinois with the unfortunate exception of the far NW burbs. Let me know if you'd like a Kuchera ratio gif or a a gif with a Midwest zoom. Sent from my SM-G998U using Tapatalk

A meteorologist or weather enthusiast's understanding of the issue vs. most of the general public's understanding of it are two different things. The amount of snow left by a storm for most people's intents and purposes is gonna be snow depth where they live. For those of us very invested in exactly how much snow falls in our backyards, it's a different consideration. For this particular event, it appears likely there's going to be a meaningful gap in the heaviest snow rates, assuming part 2 makes it far enough north. That gap is important to the overall impacts, such as for snow removal, and will result in settled snow being less than what's measured via 6 hour board clearing. I understand the point my coworker was trying to make but I wouldn't have made it because the varying level of weather aptitude among followers. Sent from my SM-G998U using Tapatalk

Two points I'd make: 1) I wouldn't have framed the tweet like that. I would've urged caution at the maps because a. they are based on an algorithm that doesn't account for important factors that determine SLR and how much snow falls, and also don't adequately account for sleet b. they include snow from a second part of the event that's still uncertain at this time 2) On the issue of snow amount vs snow depth, we're inconsistent on it. The official sites use 6-hour board clearing methods to lessen the effect of compaction, but also arguably inflate totals above what most in the general public would witness, which is snow depth. COOP and CoCoRaHS observers aren't required to measure more than once in 24 hours. Some observers, especially CoCoRaHS, will measure and report more frequently during a storm. Most COOP observers emphatically don't measure frequently, since many of them are now at water plants and other public works buildings. This can create inconsistency between amounts being reported, and the 24 hour measurement will essentially be reporting snow depth. Because of all this, I wouldn't have waded into the waters of snow amount vs snow depth and instead focused on point 1 as the reason to not get too caught up in individual model run snow totals.

I think GFS did pretty well for GHD II, did it waver less farther out than ECMWF? I do recall the ECMWF started honing in on the more extreme QPF/snow output. The stronger surface low was a pretty late detail in the forecast that led to us issuing the blizzard warning. Sent from my SM-G998U using Tapatalk

Too early to say that, it's not uncommon at all to have pretty large swings at shorter lead times. See the east coast bomb 2 days out for one of many examples. Sent from my SM-G998U using Tapatalk

A few additional points: - Don't want to sleep on round one, the f-gen enhanced overrunning. We have plenty of experience with strong fgen bands producing low to mid end warning amounts on their own (for recent LOT CWA history, think 11/22/15, 2/8-2/9/2018, initial late Nov 2018 snows just to name a few). This current setup has a lot going for it, with high PWATs, sharp low and mid level baroclinic zone, and steep mid-upper lapse rates including in the DGZ. Barring major changes, expecting 1-2"/hour type stuff wherever the heaviest banding sets up. - For these long duration extreme events, the ingredients in play for past events, overrunning followed by strong southern wave ejection, is certainly the case here. That's why we've seen GHD I & II on the analog lists. - I don't know if there is anything to this, but for major Midwest events, there was often a big east coast system within a week in advance of the Midwest system. Prior to GHD I and II, there were major noreasters/blizzards less than a week out in the last week of January. Sent from my SM-G998U using Tapatalk

I'm off the next 2 days, so I won't be on day shift AFDs and headline issuance, but I'll have more time to contribute to the analysis on here. Thanks to those who posted some nice words on my discusssion/dissertation today. Glad to be able to help everyone learn. Anyway, have to echo [mention=525]OHweather[/mention], and I already mention this in the AFD: if the southern stream wave comes in as strong as now modeled by the GFS, ECMWF, and a majority of their ensemble members, I fully expect a northward adjustment to the second round, assuming properly timed ejection. This is due to antecedent pronounced eastern ridging, tendency for strong PV anomalies (potential vorticity anomaly is another name for the short waves we track) to track north and west of earlier forecasts via further pumping downstream ridging, and addition for this setup of +NAO. We've referenced past great events that tracked north of earlier guidance, such as GHD I & II, January 4-5, 2014. Only have to go back to last Feb 14-16 for another example of strong southern wave coming much farther north than had been modeled just a few days prior. That was despite the NAO still being negative. The current temporary block east of Greenland caused by the departing east coast bomb is forecast to dissipate, so the Hudson Bay PV lobe is now forecast to escape northeast a bit quicker. At this time based off the GEFS/EPS mean as shown by op runs, northern stream trough left behind is less likely to outpace the southern wave, with some interaction between the two. This is as opposed to the main wave getting buried, with only sheared out positive tilt southern stream energy shunting everything south. While it is great to see the much stronger EPS support (should see another bump on 18z EPS after further improved op run), setup for the 2nd part is still conditional to the strength of the southern stream wave and proper trough ejection and favorable timing of ejection. Feeling increasingly confident in a major event including a good chunk of the LOT CWA but we still have a ways to go and lots of fun/stressful model watching.

Possibly relevant: GOES-West data is all jacked up at the moment. That would be important for satellite sampling of what would be the main wave for part 2 of the event. Sent from my SM-G998U using Tapatalk

Will be doing the AFD again for the event on the day shift today at LOT. Sent from my SM-G998U using Tapatalk

While not a top CIPS analog, this setup reminds me of Jan 4-5, 2014. We weren't even in the game really in the Chicago metro just a couple days out, and it ended up being the largest single event of the season at ORD (the botched measurement on 1/1/14 notwithstanding). The fronto snows ended up being well north of most short range guidance and the main wave also north. Here's a direct link to the ECMWF ERA-5 reanalysis dataset for 3pm 1/4: 500 mb: https://weather.us/reanalysis/ecmwf-era5/usa/geopotential-height-500hpa/20140104-2100z.html MSLP: https://weather.us/reanalysis/ecmwf-era5/usa/sea-level-pressure/20140104-2100z.html From there you can check out those and other parameters at different times that day. My gut feeling is similar to yours, that the PV won't have too much influence to at least allow the overrunning part to be closer to GFS depiction. I feel like the PV influence is often an element that's overdone by the guidance. The PV in the 2014 event of course went on to produce the brutal shot of cold air into Jan 6-7. Sent from my SM-G998U using Tapatalk

Yep, key improvements aloft that get the good overrunning back north, with farther north Hudson Bay PV and better upstream ridging. The GFS run was close to a best case scenario, but we'd still take a good overrunning event Tuesday night into Wednesday night. Sent from my SM-G998U using Tapatalk

FWIW, the 12z Canadian Ensemble (GEPS) is well north of the operational model with multiple big hits across Chicago metro. Sent from my SM-G998U using Tapatalk

If you're learning about this stuff, an easy way to see the important changes at 500 mb is looking at run to run trends of the h5 anomalies. Pivotal weather has a nice model trend loop tool that goes back 3 days worth of runs. Very easy to see the changes just from 06z to 12z run. The PV lobe over Hudson Bay being farther north allows the eastern positive height anomalies (ridging) downstream of trough to our southwest. These big picture changes are in addition to a much more favorable short-wave orientation and ejection described in some of the recent posts. And to top it all off as part of all the favorable changes for a stronger system, textbook right entrance region of an anti-cyclonically curved upper level jet streak that peaks at over 200 knots.

Noteworthy shift south. From a LOT CWA and my backyard (as a snow enthusiast like most on here lol) perspective though, we're in the threat zone. It's hard to tell if this particular run was under dispersive, though it kind of seems that way. Nonetheless, still several significant hits up into the metro and I think key takeaway is confidence continues to grow in a swath of impressive precip/snow, wherever it ultimately sets up. I wouldn't argue with feeling a bit worse about chances at this juncture in the far north CWA and northwest CWA and points west, though also still plenty of time. It was roughly around this lead time I believe from GHD II, that several guidance members shifted main axis south. This is not to imply that things will go a certain way because of how it went on some past occurrences, but that we're still pretty far out.

Excellent point. To see QPF output of that magnitude, have to assume a tremendous amount of latent heat release. With a juiced baroclinic zone to work with due to finally open Gulf trajectories advecting northward to it, you can get sufficient downstream height rises to counteract suppression from the TPV lobe over Hudson Bay. You don't want that feature to be overly suppressive, so the risk is dry Arctic air mass really sharpens cutoff, but without it, the background pattern could support a much farther north baroclinic zone. Sent from my SM-G998U using Tapatalk

The full operational ECMWF runs have actually had quite a bit of variability the past few days. Just last night had a similar situation. Changes aloft were overall fairly subtle from the 12z run that resulted in farther south orientation of the heaviest banding. We're still well far out enough that various aspects of this setup of far from being nailed down. You can point to a particular feature on the operational runs to be rightfully concerned about if that's how it plays out. We still have plenty of other data to say that this run and the other operational runs fall well within envelope of solutions that we've seen of the ensemble members. Sent from my SM-G998U using Tapatalk

I know the Canadian ensembles (GEPS) aren't nearly as commonly cited as the EPS and GEFS, but they too were more juiced with the 00z run than the 12z run. The operational run is not something that can be fully discounted, but viewed in the context of the ensemble shows that there's a majority of members with an evolution that results in main snow swath farther north. This is one of the reasons I don't really get too caught up in the UKMET. Without access to the ensemble, there's no way to know how it fits within the spread of the ensemble output.

00z GEFS members, mean (with sample points), 6"+ probs, and 12"+ probs. Hopefully 00z ECMWF and EPS hold serve if not improve. Sent from my SM-G998U using Tapatalk

Lurker on this thread (originally from College Point Queens) taking a short break on a busy day out here at NWS Chicago. Have been thinking that the snowfall distribution may end up somewhat similar to February 2013 and late January 2015 (hopefully a hair west of that one). The models have been consistent in depicting the 700 mb fronto bisecting Long Island. You'd expect the heavy snow band to set up just west of the modeled f-gen band on the cold side of the f-gen maxima. In events like this with a lot of wind, one of the impediments to getting widespread Kuchera ratios (among multiple flaws with the methodology) is fracturing of the dendrites. Also you'll have some subsidence either side of the heaviest band. But then within that band, the Kuchera ratios should be attainable despite the wind. Assuming the mid-level fronto does intersect Long Island, would expect beefy totals under the band with 18"+ certainly possible, especially if residence time of the most intense banding can be maximized. Elsewhere you should have ratios at or a bit above climo (so figure 11-13:1) or so.

And eastern region offices still do them. I think what we settled on (using winter storm watch and warning for higher end LES) is probably more confusing to some users. My preference would've been either to stick with status quo like eastern region went back to, or to simplify the name to a Winter Weather Watch, Warning, and Advisory, and specify the phenomena in the What bullet in the text. Storm implies a larger scale system, vs a mesoscale convective phenomena like LES. Sometimes it can be both, such as last Feb for Chicago and NW IN, so LES warning or advisory wouldn't have been ideal either. Sent from my SM-G998U using Tapatalk

It's not super practical from a going into an event time-frame to do that sort of consulting with other offices. More common would be to have webinar presentations where tips and suggestions for LES could be shared by forecasters in some of the more LES prone offices. Some of our forecasters also spent part of their careers at MQT, GRR, IWX, to name a few. So they bring that experience with them. Additionally, while big LES events are less common overall in our area, we do tend to get at least a few lower end to moderate events, so our forecasters with several to many years of experience have that in their mental database. We also have had some good west and south side of the lake events within the past 10 years to build from, of course including last February. Gino just shared a lake effect pointers presentation with the staff as a refresher for our experienced forecasters and to assist our newer forecasters. Just the near term changes with tonight's event show the challenges we face in forecasting and trying to message these events. Sent from my SM-G998U using Tapatalk

Fun/busy shift today. Had we not inherited a watch for Lake County Indiana, might have done things differently. Fresh in my mind, had the Sunday morning event that I worked during, that had warning type impacts into Lake County and probably an hour or two away from that south of downtown Chicago. Scenario of a more progressive band and/or earlier mesolow causing heaviest snow to slip back out over lake or band going nearly stationary for a few hours were all plausible scenarios. So with the watch already for Lake County, opted to expand into Central and South Cook zones and treat it more like a warm season convective watch. Lake effect is convection, so always higher uncertainty than synoptic at shorter lead times due to mesoscale uncertainties that can't be ironed out until some of the observational trends show the cards of how it may play out. Not too concerned about the HRRR/RAP trends yet until seeing the rest of the 00z CAMs. We did hint at higher impacts getting into northern Cook zone (north shore burbs) though decided to just keep watch for the zones we went with at noon and let the evening shift resolve the headlines. We have two of our better LES forecasters there (Gino, and Kluber, who made warning upgrade for Lake IN on Sunday), so we're in good hands this evening. Edit: In case anyone comes across this later, didn't want to mix messages with the updates done by the evening shift tonight. Great example of the nature of the beast that is LES forecasting. We trended toward the more western convergence placement on the day shift, but that still wasn't enough as it appears from latest guidance and upstream obs. We know these sorts of shifts are within the realm of possibility, but also still can only go by the guidance we have at hand to put out the forecast at the time.

With a super busy shift at LOT today dealing with LES threat, tried to do most I could to add some general details in extended. Asked neighboring offices if they'd consider tossing in some sleet and freezing rain chances and they opted against it, so I blanketed CWA with sleet/freezing rain chance late Tuesday night through Wednesday night. In reality obviously not the whole area would be in threat zone for icing on top of sig snow threat, but the surface and lower level pattern strongly supportive of a zone of sleet and freezing rain with cold drain from 1045 mb high to north. Hopefully most of the CWA ends up mostly snow and we get a big event (or an appetizer round) followed by the main course. Sent from my SM-G998U using Tapatalk

The EPS has been more unstable lately than I'm used to seeing it. GEFS has been more stable lately, but that doesn't mean it'll be right either. [mention=14]Hoosier[/mention] are we allowed to post the WxBell teleconnection charts on here? Large divergence of the EPS and GEFS becomes apparent by about 1/30-1/31. Prior to that they both show EPO popping back to positive around 1/27. This is only temporary on the GEFS as it goes back negative on the 31st and stays that way through the end of the run, while the EPS never gets back to a -EPO (though it's not strongly positive). PNA handling also quite different, with GEFS going -PNA by the 30th and then staying deeply negative, while EPS has only a temporary -PNA until 2/4 and then is neutral to slightly positive thereafter. The active period next week with deep southwest trough and downstream ridge amplification is pretty high confidence, though the details will be low confidence for a few more days. Beyond that, it'll probably be at least a few days until we see a decided jump toward one of the main ensemble suites or a relative compromise. For our purposes, would prefer the recent GEFS depictions later in the run to the EPS. 12z EPS runs have for whatever reason had a worse look than the 00z runs, though GEFS depiction still more favorable regardless with -EPO/-PNA/-WPO.

After my post late last week, there's certainly been more volatility out later in the ensemble runs, particularly on the EPS. Perhaps this is partially related to the MJO being stuck in the COD, reducing medium range predictability That would be due to several of the MJO phases (especially when MJO wave has higher amplitude) have well correlated mid level pattern configurations associated with them in DJF and JFM. Just the past two days, the EPS has alternated from a not great look late in the run, to serviceable/not bad last night, to objectively bad today. It's a note of caution about the lower predictability anyway that far out. Meanwhile the GEFS has seemingly been more stable later in the run and hasn't backed off -EPO/-WPO/-PNA. I know the EPS generally verifies better than the GEFS but I wonder if more stable/persistence is the way to go for a bit. The window of favorability is still there in the first week of February, but it's far from a sure thing to benefit any given part of the subforum. The nature of the pattern with a -PNA trough digging into the southwest and downstream ridge amplification could be overamped, but the hope is the heights could be suppressed enough by the TPV near Hudson Bay to avoid a warm cutter. There's been a signal on some of the operational runs out toward and beyond 10 days of larger storms that would fit the pattern thoughts, so I'd say give a few more days to monitor the signal on the ensembles before getting too concerned about what the op runs are showing. After the first week of February, it's really low confidence in the way things are headed since the ensemble means are quite divergent at the end of their ranges and the teleconnection forecasts are also quite divergent on the EPS and GEFS. Until our potential favorable window, looks like we'll be in a mostly quiet period away from the LES belts and watching that east coast beast late this week. I remain cautiously optimistic in getting a good storm during early Feb. Interested in seeing thoughts from other long range gurus/enthusiasts on here.