high risk

My standard word of caution that the 10:1 maps should not be used verbatim in many events, but especially in one with warm ground and marginal temps (at least at the start). We're not going to see 10:1 for most of the event, and while I am a believer in rates overcoming warm ground (especially when it's not the middle of the day), some accumulation could initially be lost. The accumulated snow depth maps tend to cut amounts a bit too much in these scenarios, but a compromise between the 10:1 and snow depth maps can work.

I'm rooting hard for this, not just because I want snow, but also because it will be even more difficult to retire the NAM if it scores a huge win on this event.

great sunset pics! it was definitely a pretty evening:

There is definitely some sneaky wind potential Saturday with a forced line of convection on the front later in the day. I'm tossing the NAM Nest solution which has the line fall apart - it tends to run cool, and I'm not buying its idea that we only get into the low 60s Saturday with the temp dropping as the sun sets. I think we have a good shot to hit 70 and get a dew point up around 60, and it will stay warm until the line arrives. With really strong winds just above the ground, it won't take much to mix down that momentum - that said, we won't have "much". Even at 70/60, instability will be really limited due to warm mid-level temperatures. So I think that we'll get a gusty line of showers, but it seems like little chance of lightning and a low chance of gusts close to SVR. The strongest winds might be later in the evening right behind the front.

Even if we got a dusting, which seems unlikely now, the need for pre-treatment is really, really questionable. You'd have light snow falling during the daylight hours with marginal air temperatures and road temperatures clearly well above freezing.

We already do, for the RAP/HRRR.

You're correct that it's a mesoscale version of the GFS (it's not as simple as just saying it's the GFS run at 3 km). The FV3 is now one of the Hi-Res Windows, along with the ARW and ARW2. (The FV3 replaced the NMMB in the last HiResWindow upgrade.) All 3 of those models, along with the NAM Nest and HRRR, make up the HREF. The hi-res version of the FV3 is an initial step towards replacing all of the hi-res models (the Hi-Res Windows, HRRR, NAM Nest, and the HREF products) with an hourly, high-res FV3 ensemble in a few years.

70! It's glorious.

At least with the clock change this weekend, we can now get screwed by the guidance an hour earlier.

1.61" at BWI in the past 3 hours. Over 2" for the event.

The models have been indicating that some scattered convection would develop in the dry slot, where some weak instability would develop. Fortunately, the good low-level shear is rapidly moving away to the northeast.

There is a large area of heavy rain with convective elements now affecting the areas just east of DC moving north into Howard Co and the Baltimore area, with more redevelopment further south. I'm not saying that it's going to be a widespread 4-5" event or anything like that, but the totals are really going to add up over the next few hours.

But the heaviest convective elements have yet to move through the areas on the east side of the Potomac, and the HRRR (and other CAMs) have been suggesting redevelopment which will keep the rain going for a while, and the latest HRRR runs suggest another 1-2 inches for some. Accordingly, WPC is discussing potential flash flooding in the 2-5pm time frame: https://www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=1115&yr=2021

The big question is whether the line of forced convection that races northeast through the area this afternoon can mix down the stronger winds just above the surface. It may be too stable to do that (especially on the north side of town), and the best low-level winds will be exiting to the northeast as the line arrives, but if we can overlap just a little bit of instability with the stronger wind fields, it could get really ugly.

In addition to the coastal flood threat, areas generally south of DC are in a MRGL risk for damaging convective wind gusts Friday afternoon. Several models are now showing a little bit of sfc-based instability Friday afternoon, which could allow some of the faster wind speeds just above the surface to mix down. I'll note that 1) a few CAMs develop instability even further to the north than currently indicated in the MRGL region implying a more widespread threat but 2) a lot of the stronger wind speeds above the surface will move off to the northeast before the instability develops which implies an overall lesser threat

Good point, and the HRRR is still struggling to initialize them. The only guidance that explicitly showed this was the ARW2. Some of the other guidance has shown cells developing later this afternoon south of DC and moving north, so I think that these solutions correctly captured the northward-moving forcing mechanism but struggled to initiate storms much earlier and much further south.

Dew points have been lowering at all 3 airports over the past few hours (likely due to mixing), which seems to match what the HRRR has been forecasting.

HRRR seems to have a good handle on the environment. It's going for roughly 80/58 by mid-afternoon which seems reasonable. NAM Nest is too cool - it really raises the dew points later this afternoon, and I'm not sure that's going to be right.

Yeah, the signal for this started showing up yesterday, and even the Hi-Res Windows are buying in, although they disagree a bit on location and timing. (The HiResW FV3 initiates the lead cells notably further north.) Seems like several cells will race from south to north out ahead of the more organized line rolling east. Still unclear how much instability will be available: the HRRR is much warmer (but drier), while the NAM Nest is more moist (but cooler). Perhaps there is a temperature/moisture "sweet spot" in there that could increase the potential for some hail and wind.

seems to be some growing potential for the Tuesday deep offshore low to loop back to the south/southwest, which opens the door to rain rotating back into our area during the afternoon, likely accompanied by some stiff winds. This is after the likelihood of heavy convective rains Monday evening. And with the potential for a big soaker Friday/Saturday, this will be a much-needed wet week.

This post didn't age well, as the timing now appears to be much faster. If we toss the anemic, outlier NAM Nest solution, a large area of convection should be approaching from the southwest in the early evening with perhaps a few lead cells out ahead racing south to north. Deep layer shear is good but is mainly due to very fast winds at the higher levels; the combination of weaker shear in lower levels and limited instability will likely mitigate the threat in the DC/Baltimore area, although an earlier arrival would open the door to a slightly higher threat.

SLGT for the southwest part of the area and MRGL for the rest. The question will be how warm it can stay after dark. Clouds and southerly winds should keep temps up through the evening and overnight hours, but it will still be cooling, and instability will be limited. The better threat is definitely well west and southwest of our area, but a line of storms (or heavy showers) well after dark with some wind potential is probably worth the threats SPC has assigned.

Nothing has really changed. Instability is going to limit the severe threat, but a forced line of convection will sweep across the area during the early to mid afternoon hours on Saturday. Lightning threat is limited, but I would still not be surprised if some healthy gusts accompany the line.

Certainly possible. Looks likely that there will be a line of forced convection along the cold front in the late afternoon. Deep layer shear will be robust, but as you note, the instability is the question mark.

Fabulous lightning show to my north-northwest. It's not frequent, but it's super bright and looks amazing in the drier air with stars out.