high risk

Way more SREF information than anyone ever wanted or needed, but in addition to initial condition differences, the members are comprised of two different cores, and there are some other physics differences too. And one of those model cores is a system that has mostly been retired, so as noted, it's very much an outdated system.

I'm really surprised by what I've seen from the guidance so far today, as while the GFS is documented to do a poor job with shallow cold air masses, it's usually *slow* to bring them in. Seeing the GFS be faster than other guidance with the arrival of the cold into our area is not what I expected to see. The bottom line is that I need to see another model show the same idea before I'm going to get anything close to being on board.

I lean towards lower totals in the jackpot zone too, but as some of the cooler heads (or major weenies) have noted in that discussion, there was a storm in 2015 (I think) where the models kept the huge snows out of southeast New England despite an amazing look at h5, and they ended up getting crushed.

H5 looks super, but several models show a bit of a strung-out mess at the surface. The New England forum is freaking out.

edit: and "looks great" should have been "looks improved with the front end snow" and of course, the Nest doesn't look as good.

NAM parent looks great with the snow, but temps are above freezing until early evening, so it might struggle to accumulate during the daylight hours.

As we try to sort through all of the disagreement between the models, one thing that stands out to me is how much the GFS and NAM Nest agree on the precip field. I have no idea how this is going to play out, when the NAM and GFS disagree, but the NAM Nest is close to the GFS, I take notice.

or not.....

18Z GFS looks much different than the 18Z NAM that had us throwing ourselves over the cliff last hour. Stronger, more phased wave and higher heights out front. Not saying that this will put the coastal low in a favorable spot for us, but it should hopefully retain our Friday evening/night snow here.

Perhaps, but guidance has us getting to the upper 30s during Friday and only falling back to around freezing for the evening rush. Moderate snow at rush hour will have some impact for sure, but that's not the standard recipe for rush hour disaster here.

I'm thoroughly confused, as Pivotal 10:1 has almost 3" on the ground around DCA by 12Z, while TT 10:1 has a small fraction of that. The final totals then of course end up with a big discrepancy. There is a single snowfall field in the GFS (it's a liquid equivalent), so they should in theory be using the same field, applying the same 10:1 SLR, and getting the same result. One of them is doing something funky, and my money is on Pivotal being the goofball.

I'm always a big fan of the NAM Nest for this type of event, but it's worth noting that it's a bit later with the changeover than other CAMs, and it's also less aggressive with the temperature crash during and just after the event. Actually, the HiRes Window FV3 is also a bit slower with the change to snow and is a bit closer to the NAM Nest temperatures (although it still drops lower as the event gets cranking). I'm going to watch these details in later CAM runs today.

The Kuchera ratios seem a bit generous for an event with marginal temps through a decent depth above the surface. For now, I like a blend of the 10:1 maps with the positive snow depth change; a 1-3 along and northwest of I-95 and 2-4 well north and northwest might work well.

Some of that is rain at the start. The HRRR in particular has a couple of rounds of showers ahead of the snow.

I would bet my left nut that DC goes above freezing before we do.

The NAM for Monday morning shows this: and it's easy to look at that and say "some patchy light snow" in the DC metro area, but the 500 map shows an impressive vort pass right through the area at that time, so this is likely a representation of some heavy convective snow showers.

also worth noting that the GFS is considerably warmer at the start of Sunday than most other guidance.

The GFS usually underdoes the amount of sleet and freezing rain in the transition zone. It mostly shows rain or snow here in the evening hours, and I don't buy that at all.

This is true, but when the GFS and NAM have large synoptic differences at longer ranges, it's not often that the NAM will be correct. It's simply a lot to ask of *any* regional model to nail synoptic details beyond day 2. One the snyoptics align, one should absolutely use the NAM (and preferably the NAM Nest) for temperatures and important mesoscale details.

Sort of. I said that the GFS was upgraded early in 2021, while the GEFS was not. So we can therefore not treat the ops GFS as a true control run for the GEFS. As for whether to buy the inland GFS track or the more coastal track of the GEFS mean, I have no idea.

NAM has low teens here Saturday night with dew points around 0. GFS keeps a slight northerly component to the sfc winds ahead of the sfc low on Sunday and has a known bias for scouring out low-level cold air too quickly. I don't disagree at all that we'll be able to torch above the ground, taking the current track verbatim, but the surface cold air is not going to give up easily at all.

I can't argue with this. It's certainly possible that higher resolution is the key to a further west solution. I'm just trying to point out that resolution isn't the only significant difference between the GFS and GEFS. Really wish that the GFS and GEFS could be upgraded simultaneously.....

This is how it *should* be set up, but it's unfortunately not the case right now with the GEFS. The operational GFS underwent a significant upgrade this past March, including changes to the PBL scheme, radiation, and data assimilation. Those changes were not made to the GEFS. So, even the GEFS control run cannot be considered only a lower-resolution version of the operational GFS. Does that fact that the GEFS members and GFS have larger differences than just resolution explain why no GEFS member looks like the GFS solution? Impossible to say....

The ensemble members are run at lower resolution than the operational GFS. And likely more importantly, the GFS had an upgrade early in 2021 that has not been applied to the GEFS.

To be clear, this isn't a big deal in any way, but it's not a TT issue. They're using the p-type information directly out of the GFS (and do that for all of the models) and not are computing it on their own. In lighter precip, the GFS can sometimes overmix the lower levels and warm them up a bit too much, and that appears to be what is happening in that example.