high risk

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.

Remember that any freezing rain accumulation map is simply tallying rain falling with a sfc temp at or below 32; it is NOT a measure of how much ice will accrete on surfaces. Looking at the NAM and NAM nest, the temperature during Monday stays at or just above freezing before the precip arrives and then drops to or just below freezing after the precip arrives. This usually is not a recipe for significant ice accumulation on roads, sidewalks, etc.... although it can accumulate on trees in that scenario. The only place in the NAM where sfc temps fall a few degrees below freezing is far northern MD.

Maybe? If, say, the GFS badly initialized the wave off of the west coast at 18z yesterday, it *could* take a few cycles to undo the damage. For the 00z cycle, it would start with the specification of the wave from the 18z 6h forecast (the first guess). If there is new data where that feature resides at 00z, it will take that into account, but it will probably only adjust the first guess partially towards the new obs there. Then the 06z cycle will adjust further. It depends how much data is available where the feature is and how different it is from the first guess. A famous example is with the NWP failures with the 2000 blizzard. On the morning before the blizzard, the Peachtree City, GA raob had big wind speeds up at jet level, as a huge jet streak rotated through the trough and helped it go negative tilt. The first guess value of the jet level wind speed over GA was about 40 (I can't remember the number) knots lower than the ob, and the models all treated the ob as garbage and pretty much didn't adjust the wind speed there at all.

Both are true. Yes, the GFS ingests new data for every cycle, but you have to use something as the first guess, and that is the 6-hr forecast from the previous GFS cycle. You can't just insert obs onto a blank slate - there have to be initial values for all prognostic fields at all grid point at all levels, and you adjust those initial values based on the observations to get a solution that is balanced. You can't just use blunt force with a data observation and try to just perform straight interpolation of its value in the horizontal and vertical.

FWIW, one of the findings during the GFSv16 evaluation was that it handled cold air damming better than v15, possibly due to having more vertical levels. It still won't be nearly as good as the NAM nest, which in my opinion handles damming the best of the models (and sometimes even overdoes it), but once synoptic disagreement is removed, I'd favor the v16 profiles over v15.

done! NAM3 in particular looks much better for later Saturday afternoon.

That's a good catch. It's not as blatant on the NAM3 and the Hi-Res Windows, but even the older cycles of those models clearly show a bit of a southerly component to the winds tomorrow that is absent in the 12z cycle.

12z guidance pretty much all took a pretty big step back in terms of coverage of convective rain/snow/graupel showers Saturday afternoon. It's not obvious why, but they all seemed to pick up on *something* that's hostile to the potential.

You're too kind. Easy to make a call on this when you have a strong upper trough axis swinging through with lapse rates like these:

Nice signal in the NAM nest for some heavy rain/snow/grapuel showers later Saturday as the trough axis swings through with steep lapse rates.

I don't have enough fingers to count the number of dead-wrong statements in this post.

There are two machines: one in Orlando and one in Reston. One of them is the production machine, and one is the development machine, but they switch back and forth.

https://www.emc.ncep.noaa.gov/users/meg/gfsv16/realtime/gfsximages.html It looks like the original post was indeed referring to an older run, but the latest run does have a coastal around the 4th (although it's not a snowstorm, verbatim.)

Yep. Big upgrade in early December. https://www.emc.ncep.noaa.gov/users/meg/rapv5_hrrrv4/

Yeah, that's fair. It just feels like LWX likes to warn on these things a lot, and the false alarm rate is super high. For this event, I'd rather wait for a report to come in before pulling the trigger, although it's easy for me to say that without my neck on the line.

I'm struggling to understand the severe warning, as while that's an impressive feature on radar, it shouldn't be surface-based and therefore unable to get the winds to the ground. But I'm prepared to be wrong....

Definitely thunder. We had a severe thunderstorm watch that afternoon.

I lost track trying to count the number of incorrect details in this post.

While it's a bit unclear how much precip will linger immediately behind the front (for a brief period of sleet or snow early Christmas morning in the DC metro area), there may be more hope for a period of snow showers during Christmas Day as the upper trough axis swings through, and low-level lapse rates steepen. There is a very weak signal in the NAM nest, but there is a smack-you-in-the-face-with-a-small-snowball signal in the 18z HRRR.

You're absolutely correct about the very strong winds above the surface and the intense forcing that will be present. But that sounding is not one that would promote strong winds mixing to the ground. There is the slightest hint of mixing in a very shallow layer at the ground, but you do have an inversion above that, and the box below the sounding explicitly shows 0 sfc-based CAPE. The reason you see some impressive convective signatures in the guidance is the strong lift with the PVA and the front. There is also a little bit of elevated instability (the most unstable CAPE is listed as around 430) to assist. To be clear, though, if we can end up a few degrees higher on both the temperature and dew point, we might then have a legitimate threat to have sfc-based storms that are capable of strong wind gusts.

It's definitely worth watching Christmas Eve with likely intense shear profiles and forcing, but right now, it just doesn't look like we're going to be able to squeeze out any sfc-based instability.

This band moving through Montgomery County and the northern part of DC is the last hurrah.