high risk

As bleak as the radar looks now, the models are pretty insistent that it will fill in again later this evening. I don't think that most of us will have the the thermals for snow by that point, but I'm at 29 right now, so sleet or freezing rain seems likely here in Howard County.

You ain't kidding. Looks similar to the 12z CMC.

Sleet here in southern Howard County. We knew from the forecast soundings that we had little margin for error with the profiles. Let's give this a little time to see whether rates can cool that pesky warm layer aloft just enough.

Very glad to see the dearth of 10:1 maps! Kuchera is a step up, but those will still be a bit generous in an event with such a sizable depth of the lower atmosphere with temps close to 0. It's also worth noting that the 18z NAM and NAM nest both cut down their QPF a bit, relative to 12z... I'm still going with the NAM nest as my model of choice here, and the Ferrier maps will best account for some accumulation being lost to riming with temps so close to 0 at several levels. DC people will be happy that it wants to whiten things there. For the rest of us north of DC, I think it's best to plan on 1-2 and hope that a few places overperform. Anyone expecting widespread 2-4 will likely be disappointed.

They're not, though. They have support with the HRRR and all 3 hi-res windows.

12z ops and para GFS have trended in a slightly snowier direction too.

I think they'll be making some changes in their next update....

Your thermal profile will certainly support snow, but you'll be fighting reduced QPF that far north. Some snow, though, seems quite likely.

To be clear, I'm saying to discount the heaver NAM12 QPF, and always discount the 10:1 maps in an event with marginal temperature profiles. But I would absolutely not discount the NAM nest, for which the reasonable snow products have 1-2" for areas north of DC (and maybe some snow inside the DC Beltway too for a little while). Look at the other CAMs rolling in this morning: the HRRR and all 3 hi-res windows have snow for a few hours at the start. The NAM idea is not an outlier.

Use the NAM nest, and use the snow depth or Ferrier product for an event like this. Shows the idea of a 1-2" event from northern Loudoun across Montgomery and Howard into Baltimore which matches the HRRR nicely.

exactly. Here is the HRRR QPF during the time when it shows mostly snow for areas north of DC: And the vertical profiles show that temps are just barely cold enough aloft to stay as snow until you get further north in MD. Ugly tradeoff here: You need to go southwest to get higher QPF, but the better profiles will be further north. There *may* be a sweet spot somewhere in the middle (Howard-Montgomery deathband, anyone?) where they get into the slightly higher QPF and have a profile that just barely supports snow

When an upgrade for any model is being prepared, the newer version has to be run in retrospective mode (to generate stats on performance for past years and to recalibrate historical guidance) and then in parallel mode. NCEP simply doesn't have the computer resources to run retrospectives and then a real-time parallel for both the GFS and GEFS at the same time. Ultimately, having the GFS and GEFS not be the exact same model is not ideal at all. Combining them into a single system will eliminate that flaw.

A single global ensemble system. In other words, we right now have a deterministic GFS that is run at a different resolution than its ensembles and can have a very different configuration. In ~2024, they'll be updated together. There will still effectively be a "GFS", but it will be control member of the ensemble.

Yes, the GEFS will not be updated when GFSv16 is implemented. The good news is that the GFS and GEFS will be merged into a single system in 2024.

probably sometime in March now.

Fair point. I think I'm at close to my breaking point because of Weather53's crusade against NWP.... Honestly yes. The NAM is colder than the GFS which is notoriously crappy with low-level temperature profiles, and the NAM (in conjunction with the HRRR and some other meso models) is telling me that if I'm north of the Beltway tomorrow evening, I have a decent shot to see a modest accumulation of snow. I personally rarely look at the 12 km version of the NAM, because the NAM nest is a much better model, and I think that is a good strategy here this evening with the 00z cycle.

Ok, that was my snarky answer. The better answer builds on that a little more. It's still one of the best models for handling shallow cold air masses and things like terrain-induced wind storms in the west and fog. Ultimately, the NAM nest is a way better model, but it's driven by the 12 km NAM, so that's another reason to keep running it. Ultimately, the NAM nest and parent are frozen (no more development is being done on them) and will likely be retired in ~3 years.

Because there is way more to judge it by than whether it's too cold and wet for DC area mixed precip events.

Knock 'yerself out.

Just to add some actual science to justify tossing the parent NAM, rather than dismissing it by "it's the NAM"..... - the NAM parent is on an island right now with heavy precip extending through the DC metro area. All of the other guidance has jumped ship on higher liquid totals, including the NAM nest. - thermal profiles, while still snow in the NAM forecast, have zero margin for error. It's shown nicely in the NAM nest, which also has snow for much of the area, but has more realistic QPF: Multiple levels are pretty much right at 0C, so it wouldn't take much to turn that into sleet. Ultimately, I do think that those of us north of the Beltway have a shot at an inch or maybe two (for some lucky folks), and that is supported by several CAMs this evening.

Fair enough. I'm still hopeful of an inch at the start here in Howard County, but I'm far from confident.

Two notes: 1) Anyone showing a 10:1 map for event like this should have his/her account locked for a month. Sleet and snow are tallied together in the model output field used to make the 10:1 images, so they will automatically inflate when you have a mixed event. Tropical Tidbits has the Ferrier method and the accumulated snow depth change, and Pivotal has the Kuchera. Plausible and reasonable alternatives exist. 2) The 18z NAM nest is noticeably warmer with the 2m temps. It gets the entire area above freezing Monday afternoon, and it barely gets below freezing anywhere (including the Shenandoah Valley) Monday night - not the recipe for a high impact ice storm. The HRRR, though, still suggests a slightly bigger temperature drop for areas well north and west of the cities Monday night.

That map includes a modest contribution from the Monday event.

I don't want to discount the lack of low-level cold air, and I suppose that struggling to cool off at the surface even with a really nice synoptic setup is possible, but the GFS is known for poor handling of thermal profiles in winter events, and this forecast sounding looks really goofy to me:

Nice front end thump on the Euro for those of us north of the DC Beltway, but there is a very sharp cutoff on the precip for the northeastern part of this forum.