eduggs

We don't know exactly how this event will play out yet. But even if it ends up an interior event, non-elevated NNJ through the lower Hudson Valley haven't had a decent snow event in a while. So this wouldn't really be a repeat scenario in that sense.

The 12z is the 4th cycle in a row that the CMC has shifted south with the vortmax... it had been up in Ohio/Great Lakes more like the GFS. The two models show important differences at day 4 with the 6z ECM kind of a compromise.

This is the kind of storm that I can imagine looking borderline for snow right down to the Bronx until the NAM sends the vortmax to Buffalo and torches 900mb to 8C... signaling the rest of guidance to rain to Albany. That said, synoptically, right now as modeled, it's pretty close for snow even in the City itself. The ICON and GFS have shifted slightly colder/south at 12z and 6z respectively, and ensembles (if you believe they are useful at this range) are south of their parent operational runs. The antecedent cold is marginally supportive. It wouldn't take a huge change in the shortwave structure aloft to lead to a snowier outcome further south. It's not what I would bet on, but there is still some reasonable chance... especially outside of concrete jungles.

The 0z UKMET and 18Z ECM-AI squash the shortwave and yield very little precip. At 4.5 days out this possibility is still plausible.

Just noticed that there are still pretty big differences between the CMC and GFS days 4-5 at 500mb. The CMC actually doesn't look too bad aloft... that's a borderline snowstorm to pretty close N&W. The GFS upper levels look like they would support more rain and less snow than depicted. I feel like the GFS will end up more right... hope I'm wrong.

It's difficult to know how significant it is that the GFS op is significantly wetter and further west compared to the GEFS mean. The GEPS and EPS are also a bit southeast and drier. Obviously the averaging-effect decreases the mean precipitation the more you go out in forecast time. But the majority of the individual ensemble members are also well east of the GFS. The op also appears to be a bit faster than ensemble consensus... maybe in response to the sharp Great Lakes shortwave/vortmax that is at the leading edge of the trof. I suspect the op (maybe higher resolution?) is more accurately resolving the shortwave progression than the ensembles. I expect the ensembles to play catchup, and I don't think we're done trending yet.

The surface features aren't driving this outcome. The position and strength of the surface highs and lows follow the upper levels. The trend over the past 2 days is for the shortwave to not "dig" as far south. The vortmax that passes through the Great Lakes is becoming increasingly prominent. The path of this shortwave and vorticity advection ends up tracking pretty similarly to Sunday's wave that precedes it. That leads to a stronger surface reflection (primary) near Western PA and southeasterly low level flow that quickly changes snow to rain along the coastal plain. The hope is that maybe guidance is keying on the wrong packet of vorticity and that the models will shift back towards a more favorable trof evolution. As of now this is transitioning towards a SWFE-type event that favors the interior, particularly New England. But it's still close to a wintry outcome... and highly trackable.

I'd like to see a sharper trof like last night's GFS and UK. That allows more intense and widespread precipitation and it also anchors the surface high further west in Quebec due to the strongest upper level convergence staying further west. With a more positively tilted trof, the surface convergence shifts east and the surface high goes with it. (upper levels and surface are connected in the sense that converging air aloft tends to sink [surface high pressure] and diverging air aloft tends to rise [surface low pressure])

The GFS lost the big western trof and sharpened a different shortwave in the central US in just a few cycles. This happened in the mid-range, not fantasy range. It just goes to show how variable model output can be in this range.

GEFS, EPS, and GEPS members clustered where we'd want them I think, mostly offshore with the SLP. Hints of a weak primary up the Apps. The bulk of the precipitation along the coastal plain. Surface temperatures for low elevation coastal areas are marginal for snow and the real cold doesn't move in until the storm is departing. But that's a pretty good look overall for a 6 day forecast. The bar for me at this point of the season is a shovelable snowfall. I don't want to get carried away by 10:1 snow maps.

0z GEFS is slightly drier/south compared to 18z. So the GFS and GEFS moved towards each other this run.

The GFS retrogrades the CA ULL west into the Pacific. If that doesn't happen, the longwave flow across the US would be significantly altered. Our downstream weather is extremely sensitive to the modeled evolution of the western trof over this period. Could still end up balls cold or periodically torched days 6-15.

The 0z GFS is an absolutely filthy glutton for snow. I'm going to pretend I didn't see the extended run and keep my personal focus on the mid-range.

The UK is also an excellent run at 0z (both surface and aloft). Supports the GFS - maybe slightly better even.

Can't complain about the 0z GFS. We haven't had a widespread coastal snowstorm like that in a while. Gorgeous ouput! CMC, ICON, ECM, and ensembles close enough for interest. Hopefully no big swings from here on out (doubtful)... Small, incremental improvements would be ideal. We are still almost a week out, so it's still going to be tough to pull this off I think.

The last two GFS op runs are well east of the GEFS mean days 5 through 6 with the shortwave and SLP in the east. The GEFS mean shows precipitation primarily in the lower MS valley while the op GFS shows it along and off the east coast. Discounting the op runs for now though an eastward trend is noted in the other mid-range op models. The GEFS seem to be somewhat of a compromise between the 12z EPS and GEPS. The GEFS would put us right on the edge of both significant precipitation and marginal temperatures for wintry precipitation.

I'm still in the warm/wet or cold/dry camp, for now. I agree that chances are next week won't work out along the coastal plain. But no two snow events are exactly alike. This period could work out, so we track.

So I guess we have our first definitive threat period Dec. 2-3. All mid-range modeling indicating at least the threat of a wintry storm. Still highly variable setup so I'll try to keep my enthusiasm in check. Of note, the GEFS have backed off the past few runs but that brings us closer to a model consensus.

All guidance appears to be "struggling" with the amplitude of shortwaves coming down the ridge from AK to CA. This is an area where minor perturbations grow into mature trofs or deep upper lows. It may or may not also be a data sparse region. This is leading to huge intra-and inter-model variability with respect to west coast trofs, their amplitude, and which is dominant.

Check out this discontinuity between 0z and 06z on the GFS. That's day 5.

Pretty sick GFS run! Wow. The CMC is going to be a little warmer, but close-ish. The key difference is the high pressure nosing down reflecting the slightly more suppressive mid-upper level flow over New England. Here's to hoping the models home in from here instead of wild swings.

I'm rooting for this day 5 shortwave on the CMC. It gets closest to a wintry outcome in the mid-range. I always root for mid-range instead of long-range fantasy-land. There's not much support on the ensembles, unfortunately. The ICON and UKMET might get a few flakes to the area, but everything is north and/or weak. I don't even want to consider day 10+. Too unreliable.

I haven't looked at the UKMET in a while. The 0z is also significantly divergent from the (hideous) GFS as early as day 5 and pretty cold at the end of its 7 day run. It actually tries to get some snow into our area on Sunday with the weak in-between shortwave. Other guidance has this feature very weak or further north. I'm happy with any model run that doesn't send repeated cutoffs to CA. I'd love to see the ECM and EC-AI move away from the southwest cutoff at 0z.

At 12z the CMC and GFS were in decent agreement all the way out to day 7. Now at 0z they suddenly diverge pretty significantly as early as day 5. They've kind of switched places with the GFS suddenly much deeper and further southwest with a mature ULL meandering through CA and the CMC moving away from that look towards a more positively tilted and further east trof. The take home message is that this is a period of high model volatility. It probably doesn't matter much because either way we're too far from the cold air (through the mid-range) until the moisture is gone. I prefer the CMC look, however. It keeps us a little closer to the goods. I despise the deep ULLs burying themselves in CA , which are really hurting our chances.

The 12z ECM and 18z GFS (with support from the EPS/GEFS) show what we fear along the coastal plain. It's a classic cool/rain to cold/dry and then repeat scenario. That's not unexpected or unseasonable for this time of year. But it's been a nuisance for a few years now, including in the heart of winter. So it would be nice to get a complete breakdown of that repetitive sequence inside 7 days. We've had way way too much rain in recent winters. If a more favorable longwave pattern is always out beyond 10 days, it's just a tease.