RCNYILWX

Nothing says jaded snow enthusiasts like no posts on this thread for an hour when the NAM has run and looks like the 06z GFS at the end haha. FWIW, the 06z ECMWF operational, while not quite headed toward a 06z GFS type solution if extrapolated out, was an improvement from the 00z run with respect to the Hudson Bay PV lobe being farther north and less suppressive. Also upper jet configuration in that run suggests coupled jet streak would be probable and increase chances of a deeper secondary surface low. On the 06z EPS, noted a similar signal that the members with the 6"+ snow swaths were the deeper solutions. It makes sense, but it's also appearing that unlike GHD III, this event would be less likely to produce big amounts farther south if a farther southeast and weaker secondary low pans out.

As noted earlier, very large spread still on the GEFS, with many members northwest of the operational and the mean. Compared to the 18z GEFS, solid cluster of member lows west/northwest of the mean Wednesday night into Thursday. The 00z ECMWF (EPS) ensemble also definitely left the door open for the main wave and associated secondary low pressure to track farther northwest. Comparing to the 18z EPS, similarly to the 00z GEFS, the 00Z EPS has a solid clustering of member lows northwest of the ensemble mean Wednesday night into Thursday. Along the lines of my post yesterday, a distinct majority of the GEFS and EPS members with 6"+ snow swaths are tied to the deeper solutions. And sure enough, the 06z GFS came in way deeper and farther northwest with the secondary low. Given the trend on the 00z suite and continued on the 06z GFS/GEFS, I wouldn't at all hang it up yet in eastern Iowa, NW IL, northern Chicago burbs and southern WI. Plenty of time to go on this one with multiple moving parts.

*Longer post with some explanation of ensemble analysis for this event* Things to consider regarding sensitivity of the setup to key features: - Obvious one being strength of southern stream/main short-wave, also entailing whether wave is negative, neutral, or positively tilted. Most recent ECMWF and GFS solutions favor neutral to negative tilt in southwest trending to neutral to pos tilt in Midwest. - If there is phasing with the northern stream or at least constructive interference/interaction between northern stream and main wave. - Strength of downstream positive height anomalies. One of the things you've probably seen referenced in LOT AFDs over the past year is ensemble cluster analysis. WPC has an experimental page for this: https://origin.wpc.ncep.noaa.gov/hmt/wk2/day_3_7/view.php?type=day_5_&field=cluster_qpf&reg=_central&regstr=CENTRAL (linked to day 5/24h period ending valid 00z Friday) Abbreviations to note on the attached images: C: Canadian ensemble members E: ECMWF/EPS members G: GEFS members T: I believe UKMET members but not 100% sure I'll focus first on clusters 2 and 3, which are wetter farther northwest Wednesday night and Thursday, our time frame of interest. Cluster 2 has the highest % of GEFS members at 37%. Cluster 3 has the highest % of EPS members at 36%. The drier clusters farther NW, 1 and 4, have the highest % of Canadian members, but note that cluster 1 has a noteworthy minority of GEFS, EPS, and T (UKMET?) members. You can then see the ensemble cluster mean 500 mb height anomalies vs the multi-model ensemble (MME) and the 500 mb height differences between the clusters and the MME. My interpretation of h5 w.r.t the cluster precip difference vs the MME is that the favorable clusters either have stronger downstream positive height anomalies (cluster 3) or better northern stream interaction (cluster 2) with the main wave via phasing or constructive interference. This is kind of an obvious point, but in light of the 06z GEFS and EPS, what I'm seeing is that: Of the members with heavy (6"+) snow swaths, a distinct majority of these are farther northwest with the swath. There are only a few that are solidly farther southeast/ie. congrats IND. My take from the above is that the more likely way to get a heavy snow swath out of this is represented by ensemble cluster 2 or 3, or a combination of those. I'm not sold that there will be a warning worthy snow swath in a flatter/more progressive/farther southeast scenario, which is essentially what most recent GEM runs have shown. A flatter solution will have much narrower deformation area cold sector precip and will contend with being undercut by encroaching dry Arctic air mass from the NW. As far as which scenario I'm currently leaning toward at this point, I'd say *slightly* toward a decent (warning worthy) event within the LOT CWA and parts of surrounding CWAs, given the EPS and GEFS lean toward this. I'm certainly worried about the GEM like solution however. We'll see here shortly if the various clusters diverge much with the 12z cycle today.

Unfortunately the southern stream wave is in strong agreement to get squashed far enough south to preclude much in way of additional snow sans lake effect (see my post above), but with decent synoptic lift maybe low quality seeder feeder in your area. Guidance continues to focus the convergence axis onto the IL side after brief period this evening over Lake County IN.

Yeah, feel pretty confident the HRRR is overdoing things again.

Doesn't look nearly as favorable as last week's event due to much lower inversion heights and less instability, but the convergence is quite good and persistent and good agreement that it'll be nearly stationary for a while tomorrow into tomorrow evening. Part of the event looks to be seeder feeder due to synoptic lift (upper jet forced) upstairs with DGZ saturated, and then low level lake induced lift, with DGZ partially saturated (up to about -15C) down there. Tomorrow evening we'll lose seeder feeder with a transition to low level saturation only and still up to about -15C. Thinking is this all shouldn't result in an intense single band for most of the time but a focused area of snow showers with embedded heavier snow showers. Potential exists for a band to get going tomorrow evening. Due to lake induced portion of the sounding not being fully saturated up to -18C (top of DGZ), expectation is smaller, lower quality snowflakes that should keep rates in check, but good convergence could result in heavier snow showers producing solid rates and sub 1SM visibility (small flakes tend to reduce visibility as or more effectively than quality dendrites). We'll probably have Lake County in an advisory with the evening update. In the update to convert central and south Cook over to an advisory, mentioned accumulation of 1-4".

If anyone from LOT CWA has reports from their area that you'd like to go out as LSRs, reply to my post here. Include estimated time you measured. Thanks. Edit: I had a storm total in Naper Carriage Hill of 7.8" on DuPage side (I back up to 87th St), but then a neighborhood friend just south of the county line in Will reported 8.5", so seems like anywhere from 7.5" to 8.5" in my general area. Good storm.

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