high risk

Ah, so it's a Kuchera-derived product. And I missed that you were referring to areas well west of DC Metro. Thanks!

I'll bite. Where are you getting that the NAM ratios are 11-12:1???

I like what I've seen from the Kuchera plots. I'll admit that it drives me nuts when it's going to be an all sleet or a white rain event, and people post the 10:1 maps screaming "The GFS/NAM/whatever is predicting 14" here. LOL." The models are NOT doing that. Any sleet or snow gets tallied together into a liquid amount, and then the web sites slap on a generic ratio that makes a product with already severe limitations 5x worse.

I'd respectfully argue that it's way better than 10:1 maps in any sort of sleet or mixed events or in a daytime event with lousy rates or super warm ground. It fails in the events with it knows that that the temps are marginal and cuts amounts that can stick, but in the real world the rates overcome the marginal temps.

Nope. Still going strong....

This isn't true. It absolutely still is an operational system.

NAM actually takes most of the area up to 40 tomorrow ahead of the precip (low to mid 40s in southern MD) but has strong cooling as the precip arrives to bring most areas down into the lower 30s.

It's always worth remembering in these marginal events that the 10:1 maps do NOT represent what the model thinks will be on the ground. There is a total snowfall liquid equivalent parameter that reflects how much liquid is falling as snow out of the model. It makes no attempt to answer what the SLR will be or whether it will stick; those maps are TT throwing a 10:1 ratio on that liquid total. The accumulated snow depth plots DO reflect what the model thinks will actually accumulate on the ground.

THIS. We've had many instances over the years with temperature well below freezing leading up to an event, and even a small amount of snow turns the roads into a disaster. Tonight's event will feature temps well above freezing slowly falling to maybe around freezing. Any place that gets under a significant band could certainly see some slick spots, but it's hard to imagine a scenario with widespread travel problems (until you get further north and northwest from the 95 corridor).

Guidance had been pretty consistent on weakening this snow band and confining the remnants to northern MD, but the 00z NAM and the Hi-Res Windows now bring a dusting all the way south to Montgomery/Howard Counties (and some even into northern part of DC/PG County). Hoping to wake up to a coating here in southern Howard.

Mesonalysis shows 500-1000 j/kg sfc-based cape in our area, and the trends function shows it persisting or even increasing slightly ahead of the line. (No risk for temps dropping after sunset.) I'd still like to see the special Dulles raob to see how the instability is distributed within the column, but based on those values alone, I'm more ready to buy in to the idea of a lot of wind damage reports and a few QLCS TORs.

The HRRR does initiate a few supercells out ahead of the main forcing, so I understand SPC's thinking. But they form in western VA and move north-northeast into PA, consistent with where the box will be issued. Still think that the chances of discrete supercells in the DC metro area is really low.

the PDS TOR stuff is fun and all, and I see why the soundings are spitting it out, but that would only be realized in a discrete supercell which will definitely not be the mode this evening. As you noted, all guidance is consistent with a strongly-forced QLCS feature. Any tornado threat would be limited to mesovortices within the line due to the intense low-level shear.

I would believe so... is that correct @high risk? well, yes, there is potential to mix down significant momentum to the surface with those wind speeds just above the ground, IF you have some instability in the lower levels. I was talking up this event yesterday, but I feel the need to put the brakes on it a bit today. There is a little bit of instability in the soundings, but it's not much, and it's shallow. It might be enough to get SVR gusts down to the surface, but we fail in these setups a lot. To be clear, an intense line will cross the region tomorrow evening, but I can't help but feel that a SLGT would have been sufficient. I'm not even sure if we'll hear thunder. But the potential for a significant event across the area is certainly still on the table.

and all 3 of the Hi-Res Windows also show a 2-3z arrival in DC Metro too. That HRRR run showing an earlier arrival is an outlier.....at least for now.

Timing isn't really relevant for SVR chances. Most guidance shows temperatures remaining steady right up until FROPA due to clouds and strong warm advection.

not totally surprised given that last night's runs added in a little bit of sfc-based cape. As I mentioned yesterday, if the guidance is correct about a long break between the final round of showers (early afternoon) and this squall line (late evening), we can actually achieve some (modest) low-level lapse rates and have a shot to mix some of the impressive winds just above the ground to the surface. And as noted by @Eskimo Joe, while these low instability/intense shear QLCS setups rarely work around here, I agree with him at least a little bit that the threat of a few TORs within the line is non-zero, especially further south.

Yes, while the deep south was getting destroyed, we did indeed have close to 24 consecutive hours of tornado watches here which is just remarkable. There were a few low end tornadoes in the local area during the afternoon and evening, and there was a long-track EF2 in the Shenandoah Valley after midnight.

as I wrote a few posts back, I think this one has more potential than other similar-ish setups (although I'm certainly not "in" yet).

Right now, a lot of guidance does show a warm, windy, and dry period for trick-or-treating.

I'm coming around a little on this event. Timing looks to be very late evening, but even if it's a few hours later, with the clouds and strong warm advection, temps won't fall much at all after sunset. (I'm seeing the NAM keep temps in the 70s until fropa.) And a lot of the guidance (for now) shows a break in the rain for several hours ahead of the front, which would be awesome for trick or treaters and allow low-level lapse rates to steepen a bit, and some of the forecast soundings do suggest that any frontal band will be surface-based. I think that a gusty line of intense showers is quite likely, with thunder and severe winds still a low threat but well above a zero threat.

it's definitely on the table, as the wind fields ahead of the fropa will be fantastic. As with all of our fall systems, though, it's a question of instability. Should be plenty warm and moist ahead of the front (Thursday night ?), but we may again deal with showers being too widespread to allow for any decent low level lapse rates.

The low-level shear will be fantastic. It's just a question of whether the storms can become surface-based. Right now, it appears that they won't, but it's not impossible. Having the line arrive as late as possible, with no showers out ahead, to allow for heating in advance is the scenario that would make things extra interesting around here.

Interesting Sunday morning for sure. All guidance has some sort of squall line feature approaching DC Metro around mid-morning. The HRRR sweeps it through most of the region, but the 00z NAM nest basically rips it apart as it arrives. Even in the "worst case" HRRR scenario, it looks to be slightly elevated which would limit the wind damage threat, but it's close to being surface-based, so the MRGL risk is probably a fair call for now.

GGEM has made some improvements, but it's still behind the ECMWF and UKMET, although it's been closer to the GFS for medium range synoptic performance in recent weeks....