RCNYILWX

This could be a pretty unique event to get solid advisory snow amounts in a good chunk of the CWA and then advisory+ icing in the same areas. I definitely can't think of any events like that since I got here in 2010. Sent from my SM-G965U using Tapatalk

Donut hole closing fast on KLOT. Most places locally should be snowing by 4pm. That'll help with snow totals. I think 3-5" is a good forecast for much of the metro I-80 and north, with localized amounts up to 6" a decent possibility. Ice accumulations are a bit uncertain but a fairly broad area of 1/10"+ is increasingly likely. Whether we get the swath of 1/4"+ advertised by some of the models will depend on if there's a transition to a few hours of sleet instead of a quicker flip to ZR and then how long temps take to get above freezing. The 18z NAM runs sided with a 2-3 hour period of sleet followed by ZR, which mitigates ice amounts. I don't buy the really big amounts from the RAP/HRRR but still think we could get a fairly narrow swath of around 1/4" or a bit more.

I think the southeast surface winds plus snow cover will provide resistance from temps rising much until the surface low finally gets closer and winds shift to south then southwest. Sent from my SM-G965U using Tapatalk

Donut hole is closing nicely on KLOT. Already good returns aloft not far away on 1.5° scan suggests it won't be long til we see snow reports in the CWA. Latest saturation trends seem to indicate it's a bit ahead of schedule. Sent from my SM-G965U using Tapatalk

Agree there. Maybe a slight bump in snow amounts but then it looks likely the entire CWA will changeover to sleet then freezing rain. The ice accums shown on COD and pivotal wx are probably still overdone but we could get some decent amounts approaching if not a bit over 1/4" in spots. Sent from my SM-G965U using Tapatalk

Have to say upstream trends (radar and reports) over Iowa look good for here. Also, top down saturation is steadily progressing near DVN radar site. As far as how things go into this evening, if the good rates on the leading edge hold, that bodes well for the snow amounts. As saturation occurs, we'll see temps aloft drop from evaporative cooling then rates would temporarily hold back the WAA. We'll have to closely watch the mixing line progression on correlation coefficient this evening.

Euro is basically a compromise between the colder models and the NAM. The pivotal wx soundings suggest that it has the elevated warm layer like on the NAM but not quite as aggressive. Lots of QPF as snow I-88 and north before ptype change. I have no idea how WxBell calculates ice accums but they're impressive. My guess is with northward extent there should be more sleet. The other concerning trend today is slower to warm temps south to north tomorrow night, prolonging frozen p-type. At 06z tomorrow night, the surface wet bulb zero line is still completely south of the LOT CWA on all the operational models. Precip rates where ZR occurs longest in our south could be rather high for efficient accretion but I think realistic shot at ice storm criteria (0.25"+) somewhere I-80 and south.

Been taking a closer look at 12z guidance and the sneaky warm layer on the NAMs comes in at 800 to 775 mb tomorrow evening from southwest to northeast, with 850 mb warmth lagging a bit. The other 12z models with exception of WRF-NMM are not picking up on the 800-775 mb warm layer, but it does seem to have some support from 00z Euro via pivotal wx soundings. We'll see what the 12z Euro shows shortly. I'm not doing the grids today for LOT but am discussing the forecast with winter wx desk and most likely route is we blend the NAM and GFS positive and negative wet bulb energies, which will bring sleet and freezing rain farther north. Could certainly make a case for fully jumping in with NAM thermal profiles but guessing neighboring offices would prefer a blended approach for now. Another interesting thing I'm noticing is the convective look to the precip on simulated reflectivity products later tomorrow evening and overnight. Lapse rates in the 500-400 mb layer increase to over 7C/km and sure enough, 3km NAM is showing small amounts of CAPE, highest far south. Convection could cause ptypes to oscillate where warm layer isn't too pronounced, which would be favored north of 80. Expect winter wx headlines for the entire CWA this afternoon.

And there's the NAM run that does what we feared with a faster changeover to sleet then freezing rain. Sent from my SM-G965U using Tapatalk

How about 2/17/14? Relatively similar setup with air mass not that cold aloft (weather.us reanalysis showed +2C at 850 mb into southern CWA before precip started), broad low pressure west, departing surface high to the east, and southeast surface winds.

We definitely have to hope for earlier saturation to get some bonus snow, should the feared overperforming WAA occur. It's really a razor thin margin. Regardless of every other NAM run struggling with lower level dry air, I'm buying that we'll get into good precip rates, likely snow for at least a couple hours near/just north of 80 and then longer 88 and north. Sent from my SM-G965U using Tapatalk

Have noticed it's very common with the NAM. It has some threshold for dry air in the sounding that it seemingly automatically bins out those areas from having QPF despite most of the sounding being saturated enough. The fact it has a swath of ~1-2" of snow by 00z from SW WI to northwest IN, a band of 0 snow south of that and then another band of 1-2" south of that illustrates the issue. Given the orientation of the precip shield, that doesn't make much sense. Sent from my SM-G965U using Tapatalk

The 00z NAM would indicate a wall of WAA precip during the late afternoon and evening and then dry slot with lighter rates 06z and beyond. Because of the support for heavy rates due to very strong isentropic ascent, dynamic cooling could in that case keep a bulk of the QPF as snow for much of the Chicago metro. The transition over to sleet and freezing rain would happen when precip rates lighten as the mid-level dry slot moves overhead. Sent from my SM-G965U using Tapatalk

The GFS and Euro both want to keep the central and northern Chicago metro mostly all snow until flipping to rain late Friday evening or overnight, which explains the jump in amounts the past few runs north of I-80. With the strength of the southerly flow aloft, that would mostly be purely dynamic cooling beating back the WAA after initial evaporative cooling from top down saturation. Still having a tough time believing the WAA won't win out earlier. Interestingly enough, the trend today has been to keep 925 mb temps colder for longer, especially with northward extent. If we assume that the 850 mb temps will verify warmer than modeled, and the 925 mb temps verify closer to correct, then a decent chunk of the metro would more likely flip to sleet before any freezing rain and then rain. Of course, the 925 mb temps might also verify warmer than modeled, which would bring the freezing rain risk farther north quicker. FWIW, the NAM tends to do better with capturing the WAA, but it too keeps 925 mb quite cold deep into the night. I do believe the NAM is overdone with dry air issues given the consensus of the other models.

The energy method works on that basis because it is so intuitive and mirrors severe weather forecasting. An integrated value tells you a lot more than a point value, like CAPE and CIN and iterations of both, versus LI, K Index and totals totals. Not that there's no use for more simplistic measurements, but they oversimplify the atmosphere. -4 to -5 C is definitely a decent rule of thumb or proxy and one I've used before. You'd figure to have that magnitude of cold underlying a warm layer, it would have a decent amount of negative energy. Sent from my SM-G965U using Tapatalk

Great, informative post. Have learned a lot about long range forecasting from reading your posts over the years. Sent from my SM-G965U using Tapatalk

18z Euro and Euro ensemble came back up with snow accums I-80 and north in Illinois. Sent from my SM-G965U using Tapatalk

The reason for that is they still use the first generation top down methodology for p-type derivation, max wet bulb temp aloft. It's fine as a proxy but the modified Bourgoin wet bulb energy method provides better results with less potent warm noses but enough positive energy for full melting and not enough low level negative energy for refreezing. In max Tw aloft, probabilities are set to certain temp thresholds, so anything below 2C aloft has increasingly high snow probs and less sleet, 2-3C has max sleet probs and above 3C has highest ZR probs. The work done by my co-workers, who are getting a paper published on modified Bourgoin, has real sounding cases that show the max Tw aloft method causes too much sleet and snow and not enough ZR in a lot of cases. On the other hand, when a fairly potent warm nose is undercut by a pronounced low level cold wedge, that method won't produce enough sleet. The energy method is the way the agency is going and it's what's being used for experimental probability of weather type grids in the NBM, but they still haven't brought all offices on the same page. So for offices west of GRB, MKX, LOT, ILX, they're still using max wet bulb temp aloft, creating an unfortunate discrepancy. Once confidence increases in the scenario closer in, I'm sure they'll do more to have better interoffice consistency with the weather grids.

Check Don S's stats on strong MJO waves in the warm phases. Almost every historical case (7 of 8) since 1950 I believe of p4 wave at or above this year's amplitude progressed into phase 7 and 8. Most cases featured the wave being at least at an amplitude of 1 on the RMM plots for at least 14 days, which implies that the ECMWF ensemble should correct toward bringing the wave into phases 7 and 8 and potentially 1 and 2. CFS has gone to this idea. Also, phase 7 composites for January are colder for the east than the trimonthly composites on the CPC website. This doesn't mean anything for snow chances down the road, just that pattern should be going to more persistently colder. Sent from my SM-G965U using Tapatalk

GFS definitely trended back south with surface low vs 00z and 06z runs. If the GEFS and rest of the guidance doesn't jump NW, would think that trend will continue. Sent from my SM-G965U using Tapatalk

The recent trend that's not good for us is slowing down precip onset into later Friday PM. However, with the magnitude of isentropic ascent forecast, we have to keep in mind the possibility the models are too slow to saturate the modified Arctic air mass. It's not always the case, but sometimes saturation happens quicker than forecast. Using the 12z NAM as an example again, by about 21z Friday I-88 and south, the dry layer is centered below 850 mb and saturation starts around 850 mb. A cloud base of around 850 mb/4-5kft is a decent rule of thumb for being low enough to allow snowflakes to reach the surface. Sent from my SM-G965U using Tapatalk

Take h84 of the NAM fwiw, but it's still cold aloft valid 00z Saturday with the mixing line south of LOT CWA, colder than I would've expected. Warm layer in sleet zone across central IL as depicted in ptype map is between 850 mb and 700 mb. Sent from my SM-G965U using Tapatalk

Several of the NWS offices, including mine, have done good post mortems on this event along with what went wrong in the forecast. I'll try to address the various parts that the forecast didn't work out (or worked out okay but differently). I think convection played a noteworthy role in wreaking havoc throughout the forecast. It's likely too simplistic to focus on this factor alone, but I think helps hit the main ideas of what went wrong. This event was essentially impossible to forecast well from a professional meteorologist perspective because the moving parts were constantly changing even in the shorter ranges when RAOBS typically help build the consensus. We rightly guessed that convection may ultimately modulate what happened. There were a few "hunch" posts made about how the cold side of the system would be lackluster, and how Friday night would be primarily rain. Those were good calls clearly. Much of my forecast analysis generally didn't work out too well with the exception of holding onto idea of surface low tracking farther southeast for various reasons, including convection modifying mass fields. But hopefully was still informative for the meteorology beyond this event. Friday Night Precip: We had an unusually convectively active front for January along the inverted trough axis, due to the extreme warmth and moisture under the amplified southeast ridge. In more spring-like setups of this nature, we often look to areas within relatively close proximity to the boundary for the highest rainfall totals due to higher instability and getting the convective rates. It's common for precip to be less than forecast well up on the cold side of the boundary and the precip amount gradient usually ends up sharper than forecast. This is largely what happened, the heavy amounts of 2-3" verified but confined closer to the front in the southeast 1/3 of the LOT CWA. The sharper cutoff northwest due to moisture transport robbing led to an earlier end to round 1 of precip and less snow and ice than earlier forecasts. That said, in the NWS, we never forecast the extreme ice amounts some of the models like the NAM kept showing with good reasoning to support why we didn't. That part of the forecast worked out, but then the earlier end and narrower zone of freezing temps still resulted in forecast ice accums in northwest CWA being overdone. Saturday daytime and Saturday evening: As of the Wednesday night/early Thursday model runs, the models had trended hard toward the 500 mb wave becoming negatively tilted and supporting a deepening 990s mb surface low lifting north-northeast. This is when I discussed potential similarities to the December 28, 2015 event and possibility of significant sleet amounts. I think a good case can be made that Friday night's convection helped assure a completely separate round 1 precip by: development of multiple competing sfc low centers and delayed deepening of the system as main wave stayed positive tilt for longer. As a result most of the daylight hours Saturday were devoid of steady precip (except for the odd LES under the warm layer into NE IL), precluding any sig ice, sleet and snow accums as cold press from north deepened. On Saturday PM, there was finally a respectable area of backside/deformation snows with the finally deepening mid-level trough streaming toward the area. Models had hinted at this and it seemed we'd finally get a solid period of snow. However, at the same time, an extensive intense squall line developed over the lower OH valley down to near the Gulf Coast. Despite the support for heavy precip rates that were expected to accompany the Saturday evening snow, it seems likely that the convection had to play at least some role in the deformation precip getting shredded like it did. What part of the forecast did work out: The lakeshore flooding forecast: based off reports, photos, and videos of impacts, likely the most significant lakeshore flood episode since the Feb 87 event, surpassing Halloween 2014 due to near record high lake levels. Calumet Harbor lake level gauge came within 1" of the all time record. If anything, the lakeshore flooding was more significant, with inundation noted a few blocks inland in the South Shore neighborhood. We now have a good baseline for future higher end events in this near record high lake level regime. Final Thoughts: Based off the available ingredients for the system, the more significant potential outcomes were reasonable forecasts given the preponderance of the data at hand. One could probably have found ensemble members that mirrored the actual outcome, but those were thought to be outliers at the time. As NWS MKX showed in their what went wrong graphic, the snowfall ended up close to the 10th percentile of the ensemble distribution. Perhaps we could have done more to collaboratively (with WPC and surrounding WFOs) tighten up the rain forecasts based on conceptual model of a convectively active front, and forecast a smaller footprint off of the charts extreme amounts for January. That would've helped produce better river forecasts. As for the winter side, once to headline phase, it's tougher to couch things with as much uncertainty, despite the inherent challenges with this forecast. I'm not sure we could've totally avoided a bust in this regard, aside from playing things a bit more conservatively because of the possibility that downstream convection would destructively interfere with the setup. Have to say this too, that when the typically most reliable model and ensemble, the ECMWF struggles this mightily for an event, it does affect the quality of the forecasts. No model did extremely well, some less bad than others, namely the UKMET and GFS. Despite the failures and frustrations, it's an interesting case to study and learn from for the future to do better next time. Sorry for this being so long, but thank you if you did take the time to read through it.

I think that, however much one wants to ascribe CC to things, the reality of the winter climate on the west side of the lake at this latitude (Chicago area and west) is one of volatility. These forums became really popular back in the late 2000s and early 2010s. 07-08 through 10-11 happened to be the best 4 year stretch for winter enthusiasts in this area since the late 1970s. Since that stretch, it's been much more mixed. The highs of the highs (13-14 and GHD II) and the lows of the lows (11-12 and 16-17) and a lot in between. Last year was extremely cold and active just west and north of here through Feb after a break in December-early January and we had a good stretch of winter from mid January through beginning of Feb. 17-18 had fairly lengthy doldrums but also had a memorable stretch of 9 straight days of measurable snow and the bitter cold of late December-early January. This really isn't unusual for our winter climo. Think back prior to the 4 year run, the 00s were not great until then. 00-01 had one great month, 01-02 was one of the worst winters until March 02 (which still sucks when you think about it), 02-03 was only good east and southeast of here and 03-04 was also not good here. 04-05 had one good month in January and 05-06 was torchy with only 26" at ORD. The 90s also were very mixed, with the best event of the decade being of course the 99 blizzard. The 80s were mixed for snow as well, 81-82 the big snow season of a decade otherwise known more for its extreme cold shots. So I understand why there's a lot of complaints about this winter, it hasn't been good for winter interests after the early start. If it doesn't turn around in the snow department, then it would rightfully deserve a low grade, but it wouldn't be unusual in the context of winter climatology. Stringing together good stretches and getting things to break our way with storm tracks or not is what separates the good from the mediocre winters and often one or two events separate good/mediocre from bad. It's the winters like 13-14 and the late 70s are the exception to the rule and are once a decade or every couple decades type phenomena. Having 4 consecutive good winters like 07-08 through 10-11 is also rare on the climo spectrum, but a lot of us are constantly hoping for a stretch like that.

Kuchera totals Sent from my SM-G965U using Tapatalk