RCNYILWX

Yep, this idea was already firmly in the realm of possibility per the ensemble members. It's a pretty fine line - was hoping to convey that to AFD readers today, in simplest terms, the surface low track with respect to the Ohio River will loom large in determining the haves and have nots. Unsurprisingly, the GEFS ticked slightly south vs. 18z, but honestly not bad at this range, with still a solid # of members with a good hit up into the metro. I don't feel any worse locally because of this run, other than that we remain in a precarious spot, and north metro even more so vs. my location on DuPage/Will border. The Canucks need to get their act together with the GEM running late, so us junkies have another data point for the 00z cycle lol.

00z GFS - not what you want to see in northern IL and points west. Short wave gets buried too far south in the desert southwest. It's also a hair less robust so it stays positively tilted for longer and results in a farther south low path that isn't able to track more sharply northeast. Still ends up good for farther south and east in the subforum, because other favorable aspects remain in play. The GFS isn't exactly a paragon of consistency, but it can be used to demonstrate what we don't want to see happen to get a good event up to Chicagoland. Sent from my SM-G998U using Tapatalk

Not to suggest that this event is going to be anything more than a DAB+ sort of deal, but think that the relative "dryness" of model runs and tick back up from 18z Euro is mostly related to guidance struggling with the rather nebulous forcing in this setup. Good proxy is 850 mb and 700 mb RH, which haven't changed much on the Euro and the global models in general. If you have saturation down to 850 mb and 700 mb is saturated with enough lift, it's usually gonna snow, putting amounts aside. I think the wonkiness of the NAM is two-fold: struggling like the other guidance with the forcing mechanisms and also having an issue related to its convective scheme parameterization that I think causes it to hold onto insignificant dry layers for too long. This will be the case more often in modest vs. strong forcing. And since the 3km NAMnest is run from NAM initialization, the dry air issue will carry over. This is probably the case too for the other CAMs running off the WRF core. Also, the RAP and CAMs are inherently less reliable farther out in their ranges. While it's quite possible/probable some areas only get a light dusting, I'd be surprised if there's areas that literally don't snow as implied by runs with 0 QPF response. Otherwise, for eastern IA and Northern IL, thinking 1-2" amounts still attainable, probably not localized up to 3" anymore though.

The ensembles have ticked northwest on the means if you go back several runs. Re. the seasonal trend to be weaker and farther southeast with systems, I think it is a climatological issue. The reason much of this subforum can give exact dates of memorable snowstorms is that we're in a less favored area for everything to come together just right air mass, and storm track and strength wise. It's why in a typical season, a majority of our events feature light to moderate accumulations. The recently concluded storm that nailed the upper MS Valley and northern Lakes didn't follow the seasonal trend after all. Big snowstorms are much more common with north and northwest extent in the Midwest, and of course are essentially a dime a dozen in the Northeast (though they're obviously having a terrible winter out there too). Thinking back to last season, GHD III (if you want to call it that) left potential on the table and had an even sharper NW cutoff because phasing that some earlier guidance (especially GFS) showed did not happen. In a fast flow La Niña pattern absent downstream blocking, that gives reason to be skeptical of the stronger and farther northwest solutions. Even without phasing issues, that could shunt the main shortwave (and surface low) far enough east to keep areas farther northwest out of the game. That being said, there are plausible reasons why this one can work out farther north and west related to the main wave likely being juiced, which can help with downstream height rises from latent heat release. It looks like upper jet dynamics should play a role as well. A lower likelihood of needing to rely on phasing means we're at the mercy of how much the main wave can do on its own. As things stand now, the chance of a mostly non event for the entire subforum has decreased (though is certainly not zero), but for those in the LOT CWA, we're unfortunately firmly in the cone of higher uncertainty, with more wiggle room with southeast extent and less with north and northwest extent.

That's mainly phantom dry air issues from a quick glance at stuff. If the Euro mostly holds, think 1-2" locally maybe 3" in highest swath. I had a point max of 2.2" in the CWA today near I-88 corridor. Sent from my SM-G998U using Tapatalk

If the pieces come together right, the air mass gives vibes of Feb 24, 2016 - think lower ratio and needing to be in the heaviest banding for good totals. Obviously don't want to repeat that nightmarish gradient in the Chicago metro, though for the areas affected, that was probably a top 10 event since 2010. Sent from my SM-G998U using Tapatalk

My personal favorites are Jan 4-5, 2014 and GHD II. I had an account during GHD I but definitely wasn't active in the storm threads for that, if I posted at all. Have read through the GHD I storm threads a few times though. Great stuff.

Best more recent examples of that probably Jan 30-31, 2021 with good near term trends for much of the CWA except far west and southwest, and Feb 14-16, 2021 for Chicago. Sent from my SM-G998U using Tapatalk

You would be incorrect for the EPS mean. I was actually surprised at how excited the Euro members are. A lot of spread, but less than the GEFS and GEPS. Sent from my SM-G998U using Tapatalk

Yep, that was the death knell. If we got a more classic synoptic evolution where the 850 mb low was associated with the eastern surface low, parts of northern IL would've been in the game. The fact that even with the unfavorable 850 mb low placement, we're getting some mixed ptypes this evening shows it wasn't too far off from a wet snow event in parts of the CWA.

The weekend deal is a step back from some of the GFS runs that had earlier phasing. That looks unlikely now. The southern wave gets embedded in the northern stream trough but not quite a full phase until later Sunday-Sunday night off to the east. Overall enough large scale ascent at the mid and upper levels plus saturation in the DGZ for light to moderate snow rates and accumulations. Current global and majority of ensemble guidance evolution would suggest that suppression by the northern stream is less likely but the ceiling for snow amounts is probably up to 3-4" if enhanced banding materializes and a general 1-3" otherwise. Suppression remains on the table if there's destructive interference between the northern stream and southern stream. Also some of the op models and ensemble members, most notably the GFS, are gapping some areas due to subsidence outside of banding.

I'm definitely not a fan of getting rid of advisories - supposedly surveys showed a majority were confused by them because to your point, what is a winter weather statement? What do the TV Mets message on their broadcasts - a winter weather statement is in effect? That is an interesting point though that getting rid of advisories may pave the way to issuing more purely impact driven warnings. Upcoming pattern continues to look solid for fairly regular chances - as long as the mean troughing stays biased west, but with TPV near Hudson Bay, it keeps everyone in the game. No guarantees, and this has certainly been a bad winter for snow enthusasiasts outside of MSP and the LES belts, but would be surprised if there's no widespread decent events in snow starved areas from this weekend through mid Feb. Sent from my SM-G998U using Tapatalk

We should start a movement on the sub forums to discourage posting of the Tropical Tidbits 10:1 snow/sleet maps. While there are flaws and some minor differences site to site with all the algorithm driven snow maps, at least the other sites are relatively similar in binning in QPF as snow. I prefer Pivotal Weather's maps overall, but COD maps are decent too. A few tips: - Algorithm snow maps are not the model producing that much snow - it's the specific algorithm ratio that converts snow QPF into snow accumulation. One of the few explicit model snow outputs is snow depth and then positive snow depth change utilizes the snow depth to give an idea on explicit accumulations. There is a total snowfall output available on Pivotal Weather (for RAP and HRRR) that uses a model parameterized snow ratio and is an explicit model snowfall output. The NCEP HRRR site has explicit model output snowfall as well. - Kuchera ratio is best used in below climo (wet/dense low ratio snow) to climo normal snow ratio scenarios, but is deeply flawed in very cold air masses. The ratio doesn't have an upper limit in extreme cold. - 10:1 ratio is not bad as a first guess in climo normal snow ratio scenarios and even colder air masses because outside of banding or absent strong omega perfectly aligned with a deep DGZ, the Kuchera ratios will often be overdone. 10:1 can give a rough idea and then adjust up where you think mesoscale banding will set up. - Positive snow depth change is a useful metric in low ratio to climo average ratios but will probably be a bit underdone in general. - Tropical Tidbits is much more liberal in QPF to snow so the footprint of snow accumulations in marginal to mixed p-type scenarios will almost always be overdone. It's probably okay when p-type isn't an issue. Sent from my SM-G998U using Tapatalk

The pre-Christmas storm is definitely an example of impact based vs. criteria based WSW. We had that leeway in the past depending on the event, but it's gone more down that road. I'd say more often than not we still try to hold to issuing warnings for heavy snow with the 6"/8 hour and 8"/12 hour criteria in mind. Pre-Christmas was a special case because the combined impacts of blowing snow, high winds, and cold were fairly high end for the little snow we had. Even though advisories are supposed to go away in a few years, we've definitely trended even more impact based for advisories. The thinking is that is there much difference between 2" of snow causing snow covered roads vs. 4" causing snow covered roads. The traditional 3-5" criteria probably was a holdover from the snow advisory when WWAs grouped in the various hazards that used to have their own advisories. WSSI is an advancement in terms of impacts that would help reduce the subjectivity. We noted that it ran a bit hot the first few years, especially higher on the impact scale. An example is the Feb 1-3, 2022 event having top end "Extreme" impacts forecast south of I-80. It was definitely a solid winter storm, but not extreme if you scale it to truly historic events like GHD I and upper echelon storms like GHD II. Some recalibration was done this winter to help with that. The hope is that it can be of more assistance in issuing Winter Storm Watches and Warnings in the future. There's also probabilistic WSSI in development as part of this process.

The element that needs to change is the 850 mb low from the initial Plains sfc low tracking so far north. Even though a stronger surface low eventually evolves farther east, the current forecast 850 mb low path will make it tough to get good snow farther south. Plus the air mass is marginal to begin with, especially with southward extent. In addition, early maturing of the Plains low with a messy transfer and early dry slot intrusion (before considering how warm sector convection might interfere) look to cut into accums even where it does snow. Sent from my SM-G998U using Tapatalk

That can happen, but the exact track of the consoliated surface low might not matter as much if the convection forces much of the warm sector precip to shoot eastward. That plus a more limited duration of sufficient moisture in the DGZ would really truncate the time window of accumulating snow. A problem I'm seeing with the double barrelled low is that it holding on longer ties some of the deformation type precip to that low, with a massive dry slot surging northward. Loop 700 mb RH to see the effects of the dry slot. By the time better moisture wraps back in, the forcing weakens as the consolidated surface low to the east exits off to the northeast. Also that western low may have a pretty wound up 850 mb low resulting in warmer thermal profiles (ie. 12z GEM and 06z Euro). Sent from my SM-G998U using Tapatalk

One of the factors in this setup that's a wildcard is what's likely to be a tremendous amount of convection in the warm sector (as an aside, looks nasty and probably headed for MDT for the Gulf States). An initial double barrelled low pressure setup adds to the complexity and uncertainty of the synoptic evolution in terms of the eventual track of the consolidated surface low. Warm sector convection can cause constructive or destructive interference with the mass fields and there's nothing right now that can provide confidence in either direction. There's also likely to be a very pronounced dry slot, so the farther north the system, the earlier the precip. transitions to drizzle closer to the path of the sfc low. We might see near separation of the warm conveyor belt and cold conveyor belt (deformation area) precip given the already pronounced dry slot being modeled plus the potential for convection to further mess with things.

This one isn't over for the Chicago area, though the Euro/EPS do highlight the concern in this setup. Mild/marginal antecedent air mass and lack of blocking suggest the last couple cycles of the Euro are certainly plausible scenarios. Still far enough out, plus other guidance supporting a colder scenario is a good reason to take a wait and see approach for now. That said, would prefer to see a tick back colder from the 12z Euro/EPS.

I take each setup on a case by case basis pretty much. I don't think we can necessarily hinge on a general trend to play out each time. The above said, part of the perception is related to the difficulty in getting systems to phase as far west as the guidance sometimes shows. Or in the case of pre- Xmas, to get the PV (potential vorticity) anomaly to fully close off and rapidly deepen as far west as some of the runs 4-5 days out. I think for the specific scenario of phasing of short waves, it is worth noting in AFDs that the guidance has a tendency to overdo phasing (either too early to phase or phasing occurring at all). The pre-Christmas storm was a unique evolution. In hindsight, the very sharp angle of approach from where the wave started digging was sensitive to downstream changes, and being skeptical of the aggressively far west bombing out and closing off of the cyclone would have been warranted. That's one of my takeaways from that event. Next week's setup doesn't involve phasing, with it mainly related to the track and strength of the main southern stream wave, and the wave has been forecast to develop a closed 500 mb low fairly early. Magnitude of confluence downstream will be a factor that modulates the track and strength of the wave, which makes it a little less complex and conditional (vs. needing a well timed phase). Finally, our PoPs (at Central Region WFOs) are initialized out beyond 36 hours using the NBM. We already had likely PoPs in the forecast, so there wasn't really any discussion of lowering PoPs. In a vacuum, I probably would have capped them at 50% and trended up or down as needed from there. Anything higher than likely (55-74%) this far out would be too high. Sent from my SM-G998U using Tapatalk

I think a better way think of that comparison is the relative run to run consistency, since the setups are quite different. The GFS has exhibited much larger variability over a period of a few model cycles vs. the ECMWF, similar to the pre Christmas storm. It certainly doesn't mean the operational ECMWF will be right (ie. the ECMWF runs about 4-5 days out from pre-Xmas were much too early to rapidly deepen the synoptic system). However,at this range I'm leaning toward the more consistent guidance from a probabilistic perspective in terms of whether we see meaningful precip over a good chunk of the CWA. There are EPS members with a similar evolution to the last 2 runs of the GFS, which is to be expected at day 5-6 on a 51-member ensemble. A majority of the 12z EPS members and the mean support the general idea of the operational run though. Given the tendency of the GEFS members to more often hug closer to the operational, I consider the GEFS members less useful relative to the EPS members, which do a better job of capturing the full spectrum of plausible outcomes. For today's afternoon forecast package, with the above thinking in mind, felt comfortable with likely PoPs for much of the LOT CWA Wednesday night into Thursday, but that still gives a ~30-40% chance of a miss south, which is reasonable this far out.

Have definitely been surprised at the persistence of the strong El Niño like pattern this month. Looked at other La Niñas in the 2000s that had colder than normal Decembers followed by above/well above normal January, and most prominent examples were 2006 and 2017. The h5 pattern is pretty similar thus far this month to those months. Despite fairly low amplitude in warm phases end of December and through early January, it does appear that the MJO did play a noteworthy role in getting to the current pattern, with a similar progression in the 2006 and 2017 winters, though a much stronger wave in those cases. It's good to see the consistency of the improved pattern in the ensemble guidance, so overall agree with your thoughts, and it appears at this point we probably won't be repeating how the rest of the winters played out in 2006 and 2017, at least to start February. The MJO emerging with more amplitude into colder phases looks like it should get things established for a time and then we'll see if a strong wave maintains back through the warmer phases. For our interests out here in northern Illinois, the EPS has trended to a farther west EPO ridge axis vs a few days ago (and a -PNA toward the end of the latest run), so there's not much concern of a shift to a CAD type pattern, while GEFS had already been pointing in that direction. I think bigger concern would be storm track/p-type related - think something like February 2019 which had a deeply -EPO, but southeast ridge from -PNA flexed just enough to keep the mean storm track too close to us and the extreme snow across IA, MN, WI. That said, would always gamble with a -PNA -EPO coupling and a -NAO can help (if it develops) - late January-February 2021 a good recent example.

While it's a tenuous setup and the antecedent air mass isn't good, it has my attention since there's nothing else to track until possibly reshuffling the pattern later in the month. Aside from the late season like air mass, the synoptic pieces are good if things work out well: A juiced southern stream wave associated with the latest potent California AR event, deepening surface low tracking near the Ohio Valley into the eastern Lakes, and a 1030-1035 mb high pressure spreading in from the northern Plains. Not particularly optimistic yet for the Chicago metro since the improved GFS and ECMWF runs (vs 00z last night) still would put us too far northwest. But at least don't have to squint to see what we'd need to get a swath of heavy wet snow farther northwest: early enough negative tilt of the main wave and/or phasing or constructive interaction with an incoming northern stream wave. Unfortunately it remains a thread the needle type setup due to the air mass and needing things to break right as described above. It does have a more classic synoptic setup than the closed low depiction of GFS runs Saturday and prior that would be heavily reliant on dynamic cooling (while the ECMWF runs had no cold air to speak of), so *maybe* a bit better chance of success.

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.