RCNYILWX

The 18z EPS, while another shift northwest, still nicely shows the range of plausible outcomes for the heavy snow swath. There's roughly 3 camps and the swaths have a similar west southwest to east northeast orientation due to the NE/ENE surface low track. Can use Chicago proper as the dividing line for the 3 camps: 1) NW of Chicago 2) Bisecting Chicago 3) SE of Chicago I think 1 and 2 are about equally likely at this point and 3 is a bit less likely, given recent trends. If you want to give how this season has played out some extra weight, then scenario 1 currently has a bit higher probs than 2. None of the 3 scenarios can be ruled out at this range. There's likely a relative northward bound to how far north it can go, but on the other hand, an earlier negative tilt can result in a farther north approach before track slides east-northeast. Plus, a weaker block can contribute to the above, favoring scenario 1. For the middle ground scenario 2, we basically need to balance out the competing factors in this setup to allow for a sweet spot track for Chicago and closer surrounding suburbs. Scenario 3 would rely on later negative tilt or farther south approach and/or a stronger downstream block to provide stronger confluence. Since the system will be rapidly intensifying early on, a faster occlusion and opening up at 500 mb could potentially contribute to a farther south snow swath. Regarding icing potential, seems like the forecast surface high pressure position would favor extreme northern Indiana and southern lower MI more than out here, due to proximity to the supply of lower dew points with the surface high and strong easterly flow from the surface up to 850 mb providing replenishment. There's also likely to be a sleet zone north of the icing. Farther west, with the fairly marginal antecedent air mass, might be a narrow swath of sleet between the snow and rain areas, vs. pronounced icing. Note on this analysis: It's meant to be a bit more simplified to lay out 3 realistic outcomes. There's obviously more going on meteorologically. Another factor that's always a wildcard is convection in the true warm sector, which looks to extensive and intense in this setup (wouldn't be surprised at moderate to high risks both Thursday and Friday).

Glad to be a part of the discussion Here's some items I've noted: - Anomalously deep surface low for a relatively far south latitude is pretty much a lock with the plotted EPS members all stronger than the 982 mb mean on the IL/IN border at 18z Friday - Bowling ball nature of the 500 mb low supports the idea that the track can be unusually far south for a system of this nature, in the presence of a strong downstream -NAO block. Another late season example of this was the late March 2013 snowstorm that brought record snow to SPI. - The downstream -NAO block is the key to the setup of here and points south still being in the game. - The general idea or trend of 12z suite today of deeper and farther north track also makes sense given the very strong and dynamic 500 mb low/PV anomaly, which can be expected to help pump downstream heights in the northeast a bit. If you look at 500 mb height anomalies, there's positive east of here and negative over Canadian Maritimes trapped under the block, and that'll be the impetus for our primary bowling ball wave to track east northeast and weaken instead of a sharper northeast or north-northeastward track. The presence of the NAO block merely gives us a chance that things could work out. It's a delicate balance with respect to the strength of the synoptic system and "feeling" the downstream block. The warmer and farther north solutions on the EPS and the GFS/GEFS are on the table, though I do think there's only so far north it can go. I'm concerned about a miss just north for us with the heavy snow swath because there will be a tremendous amount of latent heat release to help height rises east of here, and that may overcome downstream blocking to an extent, especially if the block trends a bit weaker. Edit: With 12z EPS 500 mb heights and anomalies valid 12z Friday.

Ask and you shall receive.

Bump north on 18z EPS. Still lots of spread on the individual members, but added several farther north/west and stronger solutions. Edit: Attached are the snow mean and member low locations. Edit 2: Now with individual member snowfall. As an aside, WxBell added percentile precipitation and snowfall data, which is pretty cool. The 50th percentile is lower than the mean, which shows there's several higher members in the top 50% of snowfall totals.

Plus a -NAO, gotta love it when it happens in March. [emoji58] Sent from my SM-G998U using Tapatalk

This event had worse impacts than February 2019. The winds and wet bulbing effects clearly won out over the higher rain rates, which was the challenge to discern. That's my biggest takeaway, to expect warning impacts when the winds are going to be stronger, because of the role in evaporative cooling and accretion efficiency, plus bringing limbs down. Another key factor was that dew points were 2-4 degrees lower than 2m temps despite the high rain rates, leaving room for wet bulbing, and amplified by the winds. Based off the reports, radial ice accums likely ended up in the 0.2 to 0.3 inch range closer to the I-88 corridor and then 0.3 to 0.4 and perhaps locally up to 0.5" in the northern tier (and far northern portion of counties just south). We won't get a final ice accum from UGN because the sensor iced over, so RFD ice accum will have to suffice as highest flat ice report. In hindsight, wish I put warnings out for the northern tier on Tuesday, and maybe then consideration on the midnight shift would have been to expand them south. Regarding the winds on Thursday, the state line counties are unfortunately a wild card as to whether temps rise enough and enough ice melts before the onset of the strong winds. It would help if dense fog develops overnight. Hope power gets restored sooner than later for board members who lost power.

I definitely struggled decision wise, considering DVN had hoisted the ISW including Stephenson County. Competing factors lowered confidence in how much ice would actually accumulate and cause impacts. With how mild it's been, if the more robust warm nose verifies farther north, if temps are marginal the whole time, etc. I'm concerned given the colder and icier trend on the guidance today, but also didn't have 80% confidence in occurrence of widespread 0.25" accums. Official forecast has a couple spots from northwestern Lake across Northern half of Winnebago with ~0.3". Ice Storm Warnings are rare around here, so I didn't want to jump into a warning with lower confidence due to a still plausible scenario where impacts are minimal. Feel confident that the midnight shift will upgrade if warranted. Sent from my SM-G998U using Tapatalk

Inexplicably bad model parameterization. Hoping our Science and Operations Officer (SOO) emails the developers about this issue since its now been multiple runs in a row of generating phantom snow depth in a non snow supportive air mass.

The Euro was showing lingering snow cover in the IL/WI state line area but 00z run backed off and is more realistic finally. So perhaps a reason to put some stock in 2m temp depiction, unlike the GFS phantom snow cover nonsense. Interestingly, the lighter precip rates north plus gusty winds Wednesday evening, if temps are cold enough, would support an uptick in accretion rates. The position of the heavy rain swath remains uncertain, as well as top end magnitude. 6-hour flash flood guidance is around 2", so may need close to or above 2" amounts to have more than minor flooding/ponding.

Year without a winter there - unreal. My parents and brother live in NYC still and it's been extended November. Certainly been mild here but we've had some winter, just less of it than usual. Sent from my SM-G998U using Tapatalk

Put down roots in Naperville - here for the long haul. Sent from my SM-G998U using Tapatalk

The 00z GFS and runs before it are creating phantom snow cover in the model snow depth. In fact, the 00z run didn't initialize terribly with snow depth, but it manufactures phantom snow depth on Wednesday over the southern 2/3 of IA, northern IL, northern IN, and southern lower MI even though the snow accumulation algorithms are correctly not generating any snow accumulations for those areas. That will absolutely affect the 2m temp depiction. Meanwhile, the 18z Euro didn't have the same issue as the GFS, but initialized poorly with snow depth over the area and holds onto that phantom snow cover into Wednesday. Something to keep in mind when looking at these outputs, again even with the better FRAM output. If the temps are wrong, the output will be too.

I grew up most passionate about snowstorms since I came from NYC where the big snowstorms are some of the most interesting weather they get. But I've grown to not get as bothered by not getting as much snow and to appreciate these mild days in the winter. If you love snow, I get it, this has been a bad winter for that across most of northern IL and even worse south of here. On the other hand, relentlessly cold and snowy winters, while memorable, are just a lot to deal with. For it to be uncomfortable being outside for seemingly months on end during the darkest time of year can get depressing. Plus it's not much fun driving on crap road conditions. I still love big snowstorms, enjoy when we do have snow otg and make sure to take my kids out sledding, but I think days like today are a good part of our winter variability and a reason to be okay with these milder winters. Def wouldn't be happy living in a beavis climate. Having experienced some periods of bitterly cold to extreme cold weather while living out here, unless it's historic, pass. Give me a few good snowstorms, some periods of sustained snow cover, but less deep cold and a decent helping of mild days, I'm all set. Sent from my SM-G998U using Tapatalk

One thing to watch for with 2m temps is if models have phantom snow cover and hanging onto it, or if initialization of snow cover improves. Look at the model snow depth to check that. This issue could call into question noteworthy ice accums underneath the more pronounced 850 mb warmth, even with FRAM, and less so with northward extent under the less warm portion of the warm nose. Sent from my SM-G998U using Tapatalk

If you don't want ice in the LOT CWA, a southward trend won't help us, as it would just shift the sleet and icing zones farther south across more of the metro and surrounding areas. Need a substantial jump south to get significant snow into the CWA, which seems rather unlikely at this point. Still certainly enough time for adjustments north and south in the "wintry mess" zone though. The 12z EPS for instance did nudge south on Wednesday-Wednesday evening with the 850 mb warmth, plus the surface low/trough and front position, while the GEFS nudged slightly north. These 12z changes notwithstanding, I'd hedge south with the frontal position on Wednesday, as long as the lead surface wave remains flatter. The warm sector will be potent for this time of year, but plenty of precip in the vicinity of the front, plus the cooler dense air north of the front often wins out even in spring setups, while in this case the air mass north will be supplied by 1035+ mb high pressure to the north.

I'd recommend grabbing the winter sounding from COD (just have to select winter and generate new sounding) or go to Pivotal Weather which automatically will give a winter sounding. On the bottom right of the winter soundings, you'll see layer energy information, which is the Bourgoin method to determine precipitation types. It's analogous to CAPE and DCAPE, positive energy aloft and negative energy low level. The precip. type tab in BUFKIT uses the Bourgoin method. Any positive energy of 25+ J/kg represents full melting, while sleet probabilities increase at -75 J/kg and below. Even with high positive energies (near or above 100 J/kg), negative energies of -100 J/kg or lower tend to allow for sleet to at least mix in and the higher magnitude negative energies can overcome high positive energies to result in primarily sleet. The preferred profile to use is the wet bulb vs. solely temperature based profile. Two examples of this from the 12z ECMWF below, first from interior far northeast IL and the other from near the state line on WI side of the border. Sent from my SM-G998U using Tapatalk

Yep was going to make that point regarding Alek's post, would have to be a much colder air mass to overcome a milder boundary layer from flow off the lake and get any noteworthy ice in Chicago. Still think there's a scenario in which sleet is higher end in this setup farther south into the metro. Probably tougher for that as currently modeled with the magnitude of the warm nose though, would need stronger low level CAD to refreeze into sleet, while southern WI looks more favored at the moment.

The FRAM (freezing rain accumulation model) estimate output. Ice Accumulation Reference - Forecasting https://training.weather.gov/wdtd/courses/woc/winter/fcst-hzds/ice-storm-accum/presentation_content/external_files/Ice%20Reference%20v1.pdf

December 28, 2015 if the low manages to take a more southerly path toward the area. Sent from my SM-G998U using Tapatalk

A zone of significant icing and sleet to its north certainly plausible in this setup, and severe weather in the warm sector. The sharp baroclinic zone and juiced air mass would still be in play with a weaker system, so given the 1035-1040 mb high pressure sandwiched to the north and northeast, that would bring the freezing rain and sleet zones farther south. As it stands, the 12z ECMWF verbatim showing ice storm criteria ice amounts (including with FRAM estimate) and 1-2" of sleet north of I-88 and into southern Wisconsin even with a fairly deep surface low is a red flag for what could happen with a slightly weaker and farther south low pressure track.

Snowing at ORD and MDW now due to low level lake effect, despite being under the dry slot at the moment. Can best see the echoes streaming in on the terminal dopplers. Edit: Joe/Chicago Storm beat me to it

The HRRR stays deeply saturated enough (-10C or colder) for longer, through about 18 or 19z that would hold onto poorer quality snow. But mid-late afternoon, the cloud temps warm up and then have to wait until toward 00z and beyond to flip back to low quality snow. It's a very tough forecast. Sent from my SM-G998U using Tapatalk

The most recent guidance brings a pronounced dry slot centered around 500 mb pretty far north. If you're in the Chicago area and were excited by the 18z NAM kuchera algorithm output, it's definitely overdone based off what's shown on the forecast soundings. There's still uncertainty with how much of the warm conveyor belt (warm advection) precip gets up into northern IL tomorrow morning, with the NCEP guidance more bullish on that note. After that, the models are in good agreement in loss of cloud ice due to the dry slot and this possibly even causing ptype issues up near the WI border (I'm leaning toward there being just enough saturation to stay as a poor quality snow up there). For much of the Chicago metro and points south, some of the guidance brings the warm nose in, and some is colder, but it's kind of immaterial with the cloud temps around -6C. I'd think we'd still have sleet with the low level wedging and steep lapse rates possibly yielding a convective effect driven flip to sleet at times. However, if the WCB precip misses us, concerned that most of the precip tomorrow is a liquid and some sleet mix, with rain/drizzle vs freezing rain/FZDZ dependent upon surface temps. Eventually, the cloud temps cool off to the -10 to -13C range late afternoon and onward, which would shift ptype to mainly low quality snow through Thursday evening. Lake enhancement prospects also seem likely to be affected by the dry air issues aloft. The snow amounts, even in the WSW within LOT CWA, might be overdone due to the mid-level dry air issues. Roads are probably still gonna be shitty for the Thursday PM commute though.

I'm here this evening - not sure if we'll be able to make any changes this shift, but wouldn't be surprised if there's an expansion by the midnight shift. Sent from my SM-G998U using Tapatalk

They didn't have to issue a warning this morning. Could have waited to make the call on the day shift today. My observation is that they as an office feel like they have to make decisions earlier than they need to and sometimes unnecessarily lock themselves into these situations. Hopefully it works out. Sent from my SM-G998U using Tapatalk