RCNYILWX

Euro ticked back up for the main swath in northern IL as well vs. 18z run. Sent from my SM-G965U using Tapatalk

GEFS still nice for northern IL, so we have that going for us. I think the best way to look at this event now is one of managing expectations. A few days ago, it looked like it could be something special. Now it trended away from that but it still should be the single largest snow event for many of us in the metro in a while. Also the winds look like they will perform. The lake assist should be nice for parts of Lake and Cook County and LES should continue beyond the synoptic snow Tuesday night into Wednesday. So taking it for what it is, it won't be a bad event. Sent from my SM-G965U using Tapatalk

That dry air at 500 mb can clearly be seen moving into southwest TX on mid-level W/V imagery. Looking at the various soundings, it certainly can be a negating factor to ratios tomorrow evening as you point out. If you look at some of the 00z soundings from the southwest like El Paso, Midland, Del Rio, and Albuquerque, also being advected northeastward with the dry layer at 500 mb will be very steep mid and upper level lapse rates, which can be seen on local forecast soundings tomorrow evening (lapse rates of 7.5 to 9C/km, depending on which model you look at. The possibility of upright or slantwise convection as the snow arrives tomorrow evening is a wild card in terms of the rates we'll get. If convection occurs, it's difficult to predict how the profile will be modified. If the dry air wins out, that will certainly be very detrimental to snow ratios and amounts tomorrow evening. On the other hand, I'm not sure we can safely assume that in the presence of such steep lapse rates that convection won't enable re-saturation up there and help produce heavier rates. Something to watch.

Agree without looking too deeply into it yet (which I'll be doing tonight) wherever the LES has longest residence time could even see up to 2-4", possibly 3-5" on the high end. Sent from my SM-G965U using Tapatalk

The LES signal on Wednesday is pretty solid. Wherever the banding sets up could need an advisory. It checks the boxes: Good convergence Decent lake induced thermos Synoptic ascent with 500 mb wave overhead

Excellent point. As an agency we're also moving away from always trying to strictly adhere to criteria for headlines and leaning toward impact based headlines. The people don't know why we have certain criteria. If we get close to forecast snowfall in this event, plus the wind, that will feel like a warning event. We'll see what they do but my gut is they go WSW. Sent from my SM-G965U using Tapatalk

Here's the Pivotal Wx Kuchera output for the 12z Euro Sent from my SM-G965U using Tapatalk

Our criteria for WSW verification is 6"/12 hours and 8"/24 hours. That might be tough to accomplish verification wise, or maybe the pendulum does swing back guidance wise. Either way, I think there's enough evidence to largely stay the course with amounts. Our primary QPF input is WPC and they apply time lagging to their QPF. I also think they mostly stay the course in light of the GEFS and prior runs of the EPS justifying that. Hopefully the new EPS doesn't cut back much. Regarding what headline we issue today, we've noticed that issuing an advisory after having issued a watch can be found confusing to the public. I'm not part of the decision making process on the headline, but my speculation is that our intent is to issue a warning even if we don't technically meet WSW criteria. I'm very confident DVN will issue a WSW. When you throw in the wind impacts on top of having the largest snowfall for most of the metro since 2018 or 2019, that should support going with a WSW.

I'm sure it would be for the Monday-Tuesday period for the event. Any additional snow could be counted as a 4-day total but I think the 2-day total would be what we use for PNS and LSRs. Sent from my SM-G965U using Tapatalk

Astounding when you consider that it wasn't as bad as it could have been out this way. Had it maintained the massive descending RIJ driven blowdown structure across northern Illinois instead of transitioning to prolific QLCS mesovortex tornado producer, there's no telling how much higher the damage number would've gotten. Sent from my SM-G965U using Tapatalk

This quote from TSSN12's sig via baro from GHD I. Such perfection, it's amazing how much more rare it is to get everything to come together like that at our latitude and longitude. It's just about time for my annual tradition of reading Gino Izzi's AFDs going into that event. Sent from my SM-G965U using Tapatalk

I think maybe some equipment issues? Such bad timing for these RAOB outages. Hopefully not a problem for the all important 12z runs tomorrow. Sent from my SM-G965U using Tapatalk

It's funny that the Euro, much more consistent than the other operational guidance so far, would have its first more noticeable shift closer in to the event when we're probably headed for watch issuances in the morning. Just to spice things up a bit lol. But in all seriousness, to your point, if you think of the operational models themselves as part of a super ensemble and also just one member among their own ensembles, the runs tonight are all well within the ensemble envelope. We have a pretty good general idea of the areas most likely to get hit hardest with some error bar padding either side. Even with the prior operational runs focusing more north of your area, you were still in the threat zone for the heavier snow, just like the northern tier is still in it despite what the 00z Euro just showed. Another thing to consider with tonight's runs is that unfortunately there were several missing RAOBs out in the southwest this evening, so maybe that's hindering getting a better consensus. Sent from my SM-G965U using Tapatalk

Yeah agree, that's why thinking closer to climo would work for the various negating factors including those you mentioned. I'm briefing the mid shift that I think our ratios are a bit low for Monday PM for the all snow areas. Sent from my SM-G965U using Tapatalk

So have been taking a closer look at BUFKIT soundings and Cobb output from PSU and starting to think the initial burst late afternoon into evening will have higher ratios than you'd think given the surface temps. Attached below is the 18z NAM BUFKIT sounding for ORD valid 03z Tuesday. GFS is similar but faster, implying heavy snow as early as 22z Monday. The white line to the left of the T/Td trace is omega and yellow line on the temp trace is the DGZ. We look for good alignment of strongest lift with the DGZ to feel confident about higher snowfall rates. In the case of this sounding, the omega is strong at greater than -20 ubar/sec and very well aligned with the DGZ. Another factor is the very steep mid upper lapse rates above 550 mb at near or over 8C/km, which is another favorable element for heavy convective snow rates. As mentioned above, the GFS is faster with this signature seen on NAM sounding and even a hair stronger omega between -25 and -30 ubar/sec. Given all these favorable factors, I think the Cobb ratio should be given consideration, because the Cobb takes into account favorable lift through DGZ. Output for 18z NAM and GFS is 13-15:1 for several hours after onset of initial burst of snow. I'm not sure I'd yet go 15:1 ratios, but I think these soundings and Cobb output make a good case to go with around climo (~12:1) and certainly no lower than 10:1 SLR late Monday afternoon through Monday evening. Edit: And forgot to add, the DGZ itself is pretty deep at about 3kft, which certainly doesn't hurt.

One aspect I'll be taking a closer look at tonight is the low and mid level frontogenesis progged. The models do a decent job of forecasting the existence of fgen, but much less good in placement and also maintenance, especially this far out. Even with a shearing h5 vort, you wouldn't expect the mesoscale banding to disintegrate because that's more driven by low-mid level thermal gradient, which will remain tight. That's a way that the big dog type totals could get farther east into the LOT CWA, with longer than forecast maintenance of intense fgen banding. It seems as if some of the guidance is starting to hint at that possibility. Sent from my SM-G965U using Tapatalk

I was briefed in on DVN's thought process today and I don't mind sharing some details: First off, they had to go into unplanned service backup, so ARX actually had to spin up and do all grids and products, except probably graphics. I think the word is that they felt they had time for the counties east of those they put in the watch to give next shift one more look. DVN has been aggressive in the past with watch issuance (they were very bullish with early watch for failed event last Feb), so I was personally surprised they didn't issue up to the LOT CWA. Our lead forecaster today would've been fine going with a watch into our CWA. Sent from my SM-G965U using Tapatalk

For those interested in the lake effect/lake enhancement, 18z GFS keeps it going on IL side into Wednesday lol. Sent from my SM-G965U using Tapatalk

Some Chicagoland/LOT northern IL specific comments re. 12z EPS: Looking at all 51 members, it's pretty clear a few outlier duds are skewing the mean a bit. A majority still bring 6+ at 10:1 across the whole metro. If we saw a lot more duds would be more concerning. The op seems to be leading the way toward prolonging snow on Tuesday with the 700 mb forcing hanging back. My guess is the duds don't have that and merely dampen eastward from shearing of 500 mb vort. If 18z op holds serve from 12z, we should see more EPS members hinting at prolonged snow on Tuesday for the 18z run. Sent from my SM-G965U using Tapatalk

Here you go Sent from my SM-G965U using Tapatalk

New EPS is more concerning for east of Chicago. A majority of members still keep the precip swath intact into Chicago proper, but also majority dampening quicker eastward. We'll have to watch that trend related to shearing of the h5 wave. Feel like at this longitude we'll be okay still, but a bit worried for posters to our east. Sent from my SM-G965U using Tapatalk

My polite suggestion moving forward is to try to not wear your emotions on your sleeves as much with every post. As red taggers in here, we can add to our credibility by posting in measured tones. Of course everyone in here loves snow, we wouldn't be posting in this thread otherwise, so nothing wrong with alluding to the desire of it to work out for ones area. Just to try to be more objective when discussing the meteorology, which also is to the benefit of everyone trying to learn. Sent from my SM-G965U using Tapatalk

I think we're still a bit far out to be concerned about the finer scale details of a particular operational run. There's a pretty good consensus developing on the big picture, which is I-80 and north favored and northern tier looking much better than it had been the previous few days. New ECMWF in a vacuum is less favorable with southward extent, though I'm sure when the ensembles roll we'll see wiggle room within the general consensus of the EPS, that's been rock solid consistent. Takeaway from the prolonged mid-level forcing on Tuesday is it would be a nice way to tack on higher ratio fluff with 850s down around -10C.

Secondary low forms with the 700 mb circulation and really keeps precip going for a while. Sent from my SM-G965U using Tapatalk

I think ratios will be too low overall for during the heaviest rates for true blizzard conditions out this way. We didn't verify in the November 2018 blizzard warning because the visibilities never tanked too low even though the winds were more than supportive. The magnitude of wind speeds and gusts progged should still increase the severity of the impacts. After starting out with 10:1 or less ratios, the snow later on should be more 12:1 and that would be on top of a more dense layer beneath so blowing and drifting probably becomes a bigger issue at that point.