eduggs

I'm happy it's the GFS showing the miss and not the CMC, ECMWF, or UK. But big snowstorms are uncommon - everything has to go right and usually doesn't. So it's reasonable to be on guard for what might go wrong. Any signs of trending towards the GFS should be concerning if you want a big snow event.

If we trend towards the GFS-family, this could be a non-event. The 12z ECM-AI was actually a step towards that solution aloft, despite what it printed out in terms of QPF. If we get a 6z ECMWF, 12z UK/ICON/CMC event, then this is definitely a NESIS/KU event with significant snow from Richmond to Boston. I'm far from comfortable characterizing reasonable QPF expectations at this point. First I want to get more confidence that the 12 GFS solution is unlikely.

The models and ensemble members that have a better phase are slower with the overrunning arrival and longer in total duration (CMC, UK, 6z ECM-AI). The worse phasing models (GFS, GFS-AI, 12z ECM-AI) are faster to arrive and shorter duration.

The 12z ECM-AI clearly has more wave interference than 6z. Slower ULL ejection and worse phasing. It's not as favorable a solution as last run, which was very good.

Everything is dependent on the height field orientation and evolution. The cold air is in place to the north. There is ample moisture to the south. If the isoheight lines are oriented mostly west to east (zonal) on Sat & Sun from North Texas to North Carolina (e.g., GFS), then this is likely a southeast and mid-Atl snowstorm. If the isoheight lines evolve with more of a north-facing component (meridional), the moisture will track further north. The more meridional the height field, the more QPF we will likely get up in our region. The placement of surface features (highs, lows, precip. field) are a reflection of the mid-and upper-level height field. To get a favorable height field we need the ULL to eject east and a favorably timed northern stream shortwave to partially or fully phase.

Most of the area is over 0.75" liquid on the CMC. ELI and parts of SWCT are over 1". It's much "wetter" than the GEFS mean.

Get the arc of significant overrunning to OH and PA inside 84 hours on the CMC, AIGFS, and EPS-AI, and I'll be a believer in a big event. Until then I'm on guard for wave interference and a non-ejecting ULL.

Good signs: - Relatively stable solutions run-to-run on the UKMET and CMC. - 12z ICON adjusted towards the ECM/UK/CMC solution - Storm evolution has trended towards a longer duration event in recent cycles - EPS, EPS-ICON, GEPS have trended north with QPF in recent cycles Not great signs: - The AIGFS has moved away from a big storm idea over the past few days - 12z GFS reversed a multi-cycle positive trend with a sudden shift towards wave-interference - EPS, GEPS still relatively dry (though that's typical considering ensemble spread at this lead time)

But worse than and south of 6z. Not terrible but still room for improvement.

The CMC would be the best storm for most in many many years. It has literally all the ingredients that we love: cold temperatures, all-snow, long duration, includes part of weekend, high QPF. Anyone saying it's not a big storm is crazy. It might not drop record snow anywhere/everywhere, but in a few spots that could rival the big ones even as depicted.

I gotta admit, the GFS/AIGFS make me nervous. That ULL in the Southwest has to eject. AND it needs to be well timed with northern stream shortwaves dropping south through MT and the Dakotas. There are always failure modes.

UKMET looks sweet so far at first glance too. Same trends as other models. I didn't look closely yet. Hope I'm not wrong.

Really nice improvement on the GEFS too vs. 18z.

Hours 144 - 168 are probably undermodeled there in terms of QPF on the CMC. But it doesn't really matter since it's so far out.

That's a sweet CMC run. And at 150hr when most of the precipitation appears done, there's more energy back in the midwest suggestion this could be more extended than shown. It looks like the 18z ECM at the end of its run except with the southern stream ULL escaping east first to deliver a major snowstorm.

I agree that even modest QPF can make for a good snowstorm with cold temperatures. But I don't think we can write off a big storm either. It's early. IMO the 18z ECM was the best run in several cycles for the model. Ensembles, GFS/CMC/ECM, and AIs are all solid or workable.

FWIW, the 18z ECMWF and EPS look like they were going to deliver a decent hit after hour 144. The mid-levels support extended precipitation after the end of the run. Probably not huge QPF, but solid especially considering the airmass.

IF the latent heat release - trof development feedback loop theory can explain or partly explain the AI vs. physics-based model divide over the past few days, you wouldn't expect a gradually shift towards compromise. You would expect the AIs to hold firm and the physics-based models to shift suddenly and significantly at the last minute. Only when their initializations start to capture the effect in real time. That would basically be now as Gulf moisture is just now showing up on radar/satellite.

This shouldn't have been much of a surprise. This was well signaled for days as a short duration, high intensity event. And models were showing near or just below freezing wetbulb temps even to the coast. That's not a terrible antecedent airmass.

Approaching 4" here. Might be the best event of the year locally!

Pretty well advertised quick hard hitting event. Just a wonderful daytime weekend snow event! Trust the models.

I meant the GFS & ECMWF vs. the AI models. Considering the size of the model domain, the camps aren't that far apart. The difference just feels really big considering the local sensible weather outcome. I feel like there have been many 75 mile shifts in precip. shields over the past 20 years with coastal storms within 48 hours.

They (GFS/ECMWF) aren't that far apart at this point, are they? What, maybe 50-75 miles with the heavy stuff offshore? The ECMWF OP in particular has shifted NW with the heavier stuff. As many people have mentioned, these are the kinds of setups that can and sometimes do shift NW in the last 48 hours. The outcome is extremely sensitive to minor changes in the shortwaves near the Gulf and also the Lakes. Presumably the AI models are correcting for those cases somehow. But that sensitivity is a double edged sword. I can also see how the AI models might overcorrect if their training datasets aren't perfectly representative or well matched to the current setup.

It looks like it comes is 2 waves, with the front buckling north and then back south. Most of the predawn stuff should be snow. If precip. shuts off - and esp if the sun comes out briefly, surface temperatures are likely to spike. Whoever get into the banding the longest could get some decent snow.

I'm not sure what you mean. I'm interested - fascinated really - why the AI models have been consistently west of their parent models for the past few days. I have not seen that behavior consistently or persistently earlier this season. And I offered one potential explanation for this specific situation. I reflexively tend to think the least snowy model will be correct, even if that's not scientifically sound.