high risk

Back in town now, and while this is nowhere as good of a setup as it looked like it might be a couple of days ago, it's not dead. The instability is of course the big question, but with strong forcing on the front and pretty good shear, there will likely be a line of convection ahead of the front. (It is clearly getting organized now). If we get up near 70 over the next few hours, there is probably some wind damage potential. The better chances are certainly further south, but I'm not totally writing it off north of town.

I fully agree. The shear on all guidance is incredibly impressive - I'm even seeing big, looping hodographs and PDS TOR on some forecast soundings. Still some questions about instability: the parent NAM by far looks the best. GFS keeps showers and clouds around for much of the day, limiting heating. NAM nest seems to mix out the low level moisture a bit during the afternoon, although even that model has an impressive QLCS approaching late in the day.

I'm totally out on this one. Honestly, I'm not sure that discussion makes sense unless they're talking about the very southern part of the CWA. We do not get anything close to SVR here from elevated storms above a stable boundary layer. I'll consider it a big win if I hear thunder Thursday evening.

It's my day!!! I'm humbled, but I can't help but think that a high-end MDT would have worked fine for today..... In other news, this may have been covered elsewhere, but the NCEP "critical weather day" means that GFSv16 can't be implemented today (or tomorrow with the MDT). As a result, it has to wait until next Monday.

We’re getting close, but implementation is actually on Wednesday morning.

all 3 major airports already getting close to 70!

big flakes mixing in here in southern Howard County

This product from the NAM nest (based off what the model microphysics thinks about how "pure" the snow is as it reaches the surface) has worked really well in some recent events, and it looks pretty reasonable to me for tomorrow (1-2 inches in northern MD). IF the NAM nest is too slow with the arrival of the precip (a common bias, but a lot of the other guidance doesn't look *too* much faster), then we can potentially take more advantage of the colder profiles in place at the start of the morning.

@MillvilleWx likely has a way better feel for this than I do, but the largest drift is going to be at the upper levels where the winds are stronger, and some drift has already taken place during the ascent. The amount of drift by the time the sonde reaches 850 is much less.

It absolutely accounts for balloon drift. The GPS sensors in the sonde relay lat/lon info at each level, and the data is assimilated at the actual location.

two things to note from the 6z NAM nest: 1) it still has the heaviest precip falling during the mid to late morning hours. It's not going to be a snowfest, and we all thought that the precip shield would come in with a much greater thump, but the current scattered nature of the steadier precip doesn't warrant all of the 'busted QPF' calls. 2) it now has a period of (modestly) accumulating snow for a lot of us Friday morning

If we're going to post the HRRR maps every hour, please use the Pivotal Weather Total Accumulated Snowfall (not the 10:1 or the Kuchera). This is the true snow output from the model with an SLR that it computes based on low-level temps. You'll see clear differences between it and the other two.

the GFS has a remarkable amount of precip fall after 18z tomorrow along and east of I-95 (including a crazy amount of ice), but it's alone on an island. The NAM is at the other end of the extremes with perhaps the GFS para offering a more realistic middle ground (and more of a light sleet to light snow for most of us scenario, instead of freezing rain)

I've learned to not ebb and flow too much with the hourly HRRR runs in winter precip. There always seems to be lots of run-to-run fluctuation with precip amounts.

Besides the NAM nest (and the parent NAM, but I'm focusing on hi-res), the HiResW ARW and ARW2 this evening both also favor the earlier transition to sleet. The HRRR and the HiResW NMMB are both snowier, but they both tend to be too snowy in these setups. I won't abandon hope since the forecasted soundings are walking such a tightrope with the warm layers, but I don't like what I've seen so far from the 00z suite for a healthy snow accumulation in most of the area.

I remember that storm really well. That was an incredible thump - the precip arrived like a wall of white.

Still some pretty decent difference between the GFS/GFSpara and the NAM for the late day/overnight precip. GFS is a big hit of ice during that period for most of the metro, but it still seems to be an outlier. Even the wetter para keeps its ice threat further southeast.

I'll agree with that, but while I might delay its timeline for the transition by an hour, I don't know that I'd bet against it for much longer. If it is an hour, though, I hope that we get 2"/hour rates during that period.....

I think that scenario would be "heavily-rimed snow". It will still accumulate but at a reduced ratio. I think the idea of less-than-pure-snow is what the NAM nest has in mind, as the Ferrier accumulation on TT shows really, really low amounts. It does that when the rime factor (accounting for supercooled water building on falling snowflakes in an environment that promotes partial melting) of the falling precip is pretty high. I think that the amounts being shown on these maps today is too low, but it is telling me that we're going to be walking a fine line between precip types.

The NAM nest would probably cut those totals down a bit due to an earlier changeover to sleet. Not saying that the nest will be right, but that would be how we bust low. Hoping that enough liquid comes in at the start so that we can still pile up several inches, even with early mixing.

I'm really blown away by the lingering discrepancies with precip later in the day Thursday into the overnight. The NAM (and NAM nest) continue to have only light precip over much of the area with perhaps a bit more for eastern sections (where some significant ice may fall over southern MD and parts of the Eastern Shore) area The GFS, however is so much wetter and has some big ice accumulations for a lot of folks: The GFS para looks much like the ops does. The CAMs overall favor drier solutions, although the NMMB is a bit wetter. I'm struggling to sort this out....

yes - all of the high-resolution models.

really good agreement among most of the CAMs (NAM nest, WRF NMMB, WRF ARW, WRF NSSL ARW) of ice storm warning criteria being met in southern MD.

Yeah, the GFS para seems to be somewhat on its own. You can see it in the 700 winds, which are much weaker up along and south of I-66. I haven't look into why v16 is weaker, but the much reduced convergence early Thursday over northern VA and the DC area is very clear.

Interesting that the GFS para again focuses the initial thump further south. The northern areas score, but it's actually during the afternoon. It's also more aggressive than the ops with moving the changeover line further north. Verbatim, it hits southern areas with a lot of snow in the morning and hits northern areas with a lot of snow in the afternoon - there is an impressive screw zone running along I-50 from DC to Annapolis.