purduewx80

09Z jumped up just over 2” at ORD but has a spread from 0-7”, with a bit of a cluster from 3-4”.

This system is a far cry from what was modeled a few days ago and still may try to bomb out Thursday night. Gone are the 40-50-degree snow-eating dew points. The complication with this one is due to the number of shortwaves that make up the trough, and of course all of the blocking. Models are now cutting off the southern wave shown on WV below and are showing increasing separation between it and the trough that affects us. That energy off the coast of Alaska is what ends up triggering cyclogenesis that passes through the area Thursday. The 18Z Euro and EPS made baby steps towards a more southerly solution. With screaming southerly flow ahead of the long wave trough, warm air will at least sneak in for a bit, perhaps after most of the WAA precip has shut off for many areas. Note that guidance is also trending wetter with the WAA and possible comma head precip associated with the new low. Snow can definitely occur closer to the low track than usual due to the amount of lift and dynamic cooling that occur. LOT's concern for a flash freeze Thursday night is legit should all of this pan out.

See the GEFS 500mb height trends for the changes mentioned above. Notice the Rex block off the West Coast (cut off upper low under the ridge), block north of Alaska that is delivering the frigid air, heights trending higher in eastern Canada and lower in the Southeast US.

It's a pretty interesting scenario on the newer guidance for this storm, but as others have mentioned its still on the lower confidence side given so many huge changes that are still ongoing. It looks like the north Atlantic block is slowing down the remnants of our storm off the East Coast, which doesn't allow the ridge to build as much. The trough out west is now splitting on most guidance, with a cut off developing west of Baja and the northern stream trying to dig in. All of this essentially could allow for a more southern track to the low. There are subtle timing differences with all the waves that are important to exactly how this evolves. From a pattern recognition perspective, having a strong 500mb jet exit region (left front quad) going by to the south, with a passing surface low and steep mid-level lapse rates is all a good recipe for thunderstorms in the cold season. 12Z GFS jet below: 12Z NAM Chicago area average sounding valid midday Thursday (7.2 mid-level lapse rates and some modest CAPE, with an isothermal sounding just below freezing in the lowest 3km would = giant wet aggregate snowflakes):

Estimating near a foot from the storm here on the west side. This is my first winter in Chicago w/ a car and I have quickly become familiarized with dibs culture. Not a fan.

It was overdone QPF-wise. Many runs were showing 1.5”+ of liquid equivalent. Models were nearly unanimous in showing the lake enhancement with this one. Lake effect parameters aren’t great, but when you have dendrites falling into the layer it’s a much more efficient process. Most guidance shows 700 mb lift increasing over NE IL later this morning into the afternoon, right when the fetch is most favorable. Likely we’ll see mod to at times heavy snow as a result in the city. ORD also struggling with blowing/drifting snow this morning.

It was always going to warm up above freezing today. The wet bulb temperature is the one to watch. Once you get evaporational cooling from heavy precip falling into a dry air mass, the air temp will drop to the wet bulb temp. It's found on the SPC meso-analysis page under the Winter Weather tab.

Our storm produced hail on the beaches of SoCal earlier today - nice indication of cold temps aloft that will contribute to the steep lapse rates.

On Tuesday, ORD reported blowing and drifting snow when the lake enhancement picked up. Temps were 30-31 and winds were gusting near 30MPH at the time. Looks like ratios w/ that averaged ~11:1 so I would think there may be some of that again this weekend. That evening there was a blast of wind at my place for 10-15 min that caused blowing and drifting as well.

Temperatures are going to be dropping into the upper 20s in the band of heavy snow, at least in northern IL and WI, so I don't see this being as wet as some think. The reason it could be dense and lower ratio has more to due with cloud microphysics and how the smaller or rimed snowflakes are able to compact together.

The coupled jet signature tomorrow evening is just classic for mesoscale shenanigans. Particularly when they're so curved like this. There is also tremendous diffluence (notice how the winds spread away from each other immediately NE-E of the upper low).

Lapse rates above about 600mb are even more impressive with this system than the last. Seeing 7-10 C/km. NAM has consistently been showing potential for upright convection in the area Sat night.

Some of the aircraft soundings showed a decent amount of shear with N winds at 850 and ENE winds heading towards the surface. That's really all we could think of today. Strangely, I drove through a moderate snow shower (possibly graupel) that lasted a minute or two on the way home this afternoon, not too far SSW of ORD.

Props especially to its ensemble mean from way out. There's a reason we don't trust one operational over another from day 3+.

Getting some aircraft effect snow in the western burbs now....they're fast-moving given the 25-30KT northerly steering flow. It'll be interesting to see if the Gary industrial complex plays a part in locally higher snowfall south of there overnight since the two usually occur under similar conditions.

Watch the 3-6PM period in Chicago. RAP and HRRR convergence really blow up and you can see it at MKE already. Should be a more organized/persistent band that transitions southwards and eventually parks IN tonight. Some drying is occurring aloft but I imagine it will persist there in some form. Still unsure about tomorrow but at least coastal Cook should be affected, especially S.

Definite lake enhancement now. Can see it on the terminal Dopplers. ORD has had some 1/4SM vis briefly in that over the last hour.

Just about everything you see on radar there is from the upper low. The lake bands will become more evident once that moves out this afternoon. Note areas well west of any lake influence are also seeing low vis in light to moderate snow. You can see some of the minor showers now over the southern end of the lake, especially on TMDW and TORD.

Haha yep! some of the less rimed ones were pretty but my iPhone can't really handle how small they are.

almost entirely graupel here still

Going to have to watch this dry slot at 500mb closely tomorrow evening. A lot of today's guidance is showing a drying out of the dendritic growth zone and is no longer co-locating the best lift within that layer. Both of those things are negatives when it comes to snow-liquid ratios. The DGZ is just below 500mb but these RH plots are a good proxy for where there is better saturation, based on all the forecast soundings I've seen. GFS is on its own for the Chicago area. If the other guidance is right, precip will be showery by the time it reaches Chicago and could produce grainier snow w/ ratios as low as 5-6:1.

Entirely reasonable to see it wax and wane between Tuesday eve and Thursday AM. We're also bleeding in colder air from the north such that Wed night could be the better performer for lake effect (longest fetch with a connection to Superior, least shear, most favorable temps and some modest upper level support from the next wave). Edited to note there are timing differences on the passage of that upper trough, which will affect when the lake shuts off on the west side.

Agreed. Not sure it's as big an issue these days as it would've been decades ago, but the soundings still provide some value. It has been noted that there are far fewer aircraft soundings since COVID-19 started, which may account for some degradation in model performance over the past year. This image below shows the other observations that are ingested into the ECMWF, for instance.

I will say anecdotally this new HRRR has been bullish on lake effect this season and it has mostly not worked out. That said, the temp profile is becoming much more favorable along with other guidance by early Tuesday.