high risk

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.

I've been bullish on this event, but I can't argue with what everyone else has observed in the guidance this afternoon and evening: while I think it's unwise to completely dismiss the threat further south, the highest chances of seeing precip tomorrow morning are going to be well north of DC.

The big takeaway from the morning model runs is that a lot of the guidance delays the arrival of the precip and allows an opportunity to warm up before arrival. That said: 1) The HRRR still has a lead band of precip (out ahead of the main frontal band) moving through the northern half of the area around 12-13Z. As said previously, even a few hundredths would create huge problems with temperatures likely still in the upper 20s then 2) Even if you consider a solution like the NAM Nest which delays precip until 16Z or so, it's about 32 or 33 degrees when it arrives. Given how cold surfaces will be after a very cold day and night, you can absolutely still get icing on roads and sidewalks in that scenario. The biggest threat is certainly for areas a good distance north and northwest of DC, but I do believe that there is a still a decent threat for everyone along and north of I-66 in VA and Rt 50 in MD, and a slight threat likely extends a little bit south of there if the HRRR solution is correct.

Spot on. It's pretty clearly going to be well down into the 20s Saturday night before the temperature rises later at late. All road surfaces will be super cold, so just getting to 32 or 33 Sunday morning won't end the icing threat. It's also worth noting that the NAM Nest appears to be doing its usual thing of being slow with precip arrival, and the earlier start shown by some of the other guidance certainly suggests a fairly disruptive (and likely a high-impact) icing event. The only thing keeping it from being an extreme high-impact event will be that it's on a Sunday morning.

Correct. It's initialized with the previous GFS cycle and has a fairly similar overall configuration (with higher resolution).

One quick note: maps like these for any models except RAP/HRRR are completely derived by the site that displays them, and they involve a lot of estimating and interpolating. Only the RAP/HRRR have explicit totals by type. That said, most of the guidance now shows at most a very brief period of rain at the start.

one hell of a band in the NAM3. Looks like a freakin' squall line.... edit: Ninja'ed by @Deck Pic

While I don't get too worked up over HRRR snow details 36 hours out, the 18z forecast is consistent with the idea that has been discussed about good amounts to the west with the shortwave dynamics and then a secondary max to the southeast as the coastal low spins up. Unpleasant screw zone in between, although its totals there would be acceptable, at least to me. And the band of heavier totals with the coastal low isn't far southeast of I-95 at all....

True, although temps should rise quickly later tonight as the southerly winds and clouds increase. But you make a good point that temps here in the early evening have plummeted, so all surfaces will be really cold in the early morning, and any very light rain will freeze on the cold surfaces even if the air temp warms to near 32.