RCNYILWX

6.1" here as of 930 am. The rates when I woke up at a bit after 8am were really impressive, some of the heaviest since I've been living in the Naper Carriage Hill subdivision (since May 2015). We've since eased back to -SN/borderline SN with a slight recent uptick. Had told neighborhood friends expecting 6-8" locally, so should fall right in that range. My top end/90th percentile range was 10-12" which would've required that band to be a lot more slow moving. Nice event.

Looks like the banding push extending back to far southeast IA is and should fill in places currently just northwest of the heavier band over the metro. Tilt 2 on KLOT almost completely filled in. Sent from my SM-G998U using Tapatalk

Pouring snow here now, heaviest rates imby since Jan 30, 2021. Sent from my SM-G998U using Tapatalk

Broke the seal here in southeast Naperville a few blocks north of WillCo border. Sent from my SM-G998U using Tapatalk

My general perception is that more often than not dry air effects are overdone rather than underdone. Tonight-tomorrow will have the additional effect of f-gen sharpening up precip gradient, so I buy the sharp cutoff somewhere over northern IL (hopefully north of most forecasts for places up north that legit need the precip anyway). Later tomorrow night and Thursday won't be a heavily banded setup up farther north, so I would tend to think the dry air effect would be overdone on a day like that, unless everything is squashed south, which is possible. Last February 14-16 was a great example of favorable jet positioning aiding in a precip shield much farther north and west than earlier forecasts. Link to SPC mesoanalysis archive: https://www.spc.noaa.gov/exper/ma_archive/action5.php?BASICPARAM=300mb.gif&STARTYEAR=2021&STARTMONTH=02&STARTDAY=15&STARTTIME=22&INC=-6 Textbook right entrance region of an anti-cyclonically curved upper jet streak. In addition to the strength of the mid-level wave, this magnitude of large scale ascent played a big role in both the expanded precip shield and the intensity of the lake effect into NE IL and far NW IN.

No prob! Glad to help with analysis on here. I'm back 3pm-11pm tomorrow so for evening update AFD and any headline changes (if needed) would be me or MTF. Sent from my SM-G998U using Tapatalk

18z GFS 250 mb winds, isotachs, and heights valid 18z Thursday This is a pretty classic favorable jet right entrance region for northern Illinois, northern Indiana, and SW MI. Jet level forcing can do a lot of the work to cause a more expansive precip shield (and in this case would be doing a lionshare of the forcing). If the jet configuration verifies similar to this, don't be surprised if there's snow farther north, kind of like what the RGEM has been showing. My friend who's a lead at OKX (WFO NYC) said they deal with this all the time with east coast storms. He said anecdotally the GFS is always too far south and east with precip tied to jets. As it stood on the 18z run, there was a slight tick north vs 12z. Something to watch. Getting synoptic snow father north would also help the lake enhancement part, plus this good synoptic lift is a favorable factor for lake effect alone.

Some thoughts on locations closer to northwest edge of precip shield: In synoptic precip events, being in the favorable right entrance region of the upper jet is a factor that tends to result in a more expensive precip shield to the north and west. Unfortunately, we have the very dry incoming Arctic air mass to counteract that, so this is of a question of whether synoptic lift can overcome the dryness 850 mb and below. Most model guidance has the 700 mb front (and maximum h7 f-gen) separated well to the north from the 850 mb front (and maximum h8 f-gen). Because of this, I suspect that there will be multiple embedded mesoscale bands tonight into Wednesday, with a decent shot at two dual dominant bands. You look to the warm side of the maximized f-gen to where you would get the most intense mesoscale bands. This is one of the more challenging aspects of winter forecasting. But with consistent northern position of H7 front and f-gen and large separation from H8, I'm leaning toward there being a strong f-gen band positioned not far distance wise from the northwest edge of the precip field. The presence of strong fgen also tends to sharpen this gradient, so it might not end up too far between warning level snow amounts to much lower amounts to little if any snow. The hope for areas consistently right on the northwest fringe is that the dry air influence is being overdone a bit, allowing large scale (jet entrance region) and mesoscale (f-gen and low stability to unstable conditions in the DGZ) to overcome the dryness. For the above reasons, I was definitely in agreement with the midnight shift issuing the warnings for areas left in watch and including DuPage and Kendall in the warning, with a tier north/west of that in advisory. If the models are much overdone with the dry air influence, will have to watch for banding to set up decently farther north than expected. The opposite can be true as well regarding the dry air. Will be interesting to track obs and radar this evening to see how this all unfolds. Sent from my SM-G998U using Tapatalk

I'd call initial inversion heights sub marginal. Since you will certainly be saturated and have good synoptic forcing with right jet entrance region, the enhancement will probably be of speed convergence variety for the most part. As delta T and instability improves and inversion heights come up to or above 850 mb, then it's more marginal to conditional with continued speed convergence onto land. Because there's synoptic forcing in addition to boost in low level lift from the lake, I'd expect a modest boost in totals due to lake enhancement, maybe 1-3, 2-4" type stuff? Last Feb actually didn't have great inversion heights, but synoptic lift and lake induced instability and convergence resulted in lift being centered in deep DGZ. This doesn't have nearly as favorable a setup but it's not nothing for a time. Jan 31, 2021 actually had pretty good lake enhancement despite marginal inversion heights, for example. Sent from my SM-G998U using Tapatalk

It's basically nowcast time, in terms of position of key features, with the short range/high resolution guidance used as a tool. You can check obs and radar vs the hourly RAP/HRRR runs. Can also check the short range forecasts of the globals vs obs and radar to have an idea which one has a more correct trend for tonight. SPC mesoanalysis will be a good place to check the position of maximum low and mid level f-gen. For the gradient area in Northern Illinois, frontal timing and position, how quickly the dry HP is building, and positioning of the surface frontal trough post fro-pa are all very important to whether we over or underperform. This is in addition to radar trends for mesoscale banding position and orientation.

To be fair, comparing the models through 00z Thursday, the RAP isn't wildly different than the other non NAM models. 18z NAMs by far have the sharpest cutoff over northern Illinois. The incoming dry Arctic air mass will clearly play a role in causing the cutoff that all the models are depicting, but I think the NAM is likely being too aggressive with the razor sharp northwest edge. I've seen multiple times where NAM soundings that are supportive of snow do not show any QPF/snow at that location. This on/off precip binning likely has something to do with the NAM's BMJ convective scheme according to one of our leads. (O/T but this issue doesn't only arise with snow either. Those on here who chased April 14, 2012 will recall that the NAM did not convect on the KS dryline, which led a decent amount of chasers to go with the Nebraska warm front target.) Sent from my SM-G998U using Tapatalk

Have noted a bit of a northward shift of the general forecast placement of axis of maximum 700 mb f-gen. So immediately on the warm side of the f-gen you might get a solid northern band as long as the dry air issues aren't too much to overcome. The location of mesoscale banding is one of the most challenging of many challenging elements of winter weather forecasts. Sent from my SM-G998U using Tapatalk

Hate to see it Sent from my SM-G998U using Tapatalk

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