high risk

Perhaps the smallest amount of sfc-based instability for that cell to work with down by Culpeper, where sfc temps are 5-6 degrees warmer than around the DC Beltway and points north and east.

Agreed. We'll have no problems mixing behind the front.

Several of the morning CAMs show the same thing now. Seems like some showers break out ahead of the line in those runs and ruin the convergence.

This is a very good point! But that event had some actual sfc-based instability.

I still don't understand why SPC is now even considering an upgrade to SLGT, but there is no argument that if sfc-based parcels can find a way to be buoyant, the wind fields are amazing. A faster evolution of the event might allow the line to take advantage of daytime warming for the western part of the outlook, so I understand the outlook expanding west. I'm setting my goal for getting a torrential downpour and maybe hearing thunder. Seeing several consecutive HRRR cycles wanting to break up the line as it moves east is giving me some pause, though......

For sure. The reflectivity looks so impressive! But it's not surfaced-based. That said, there is some elevated cape, so while there wouldn't be any severe threat with that (assuming that the NAM nest is correct with the stable sfc layer), there would definitely be some chance of lightning. I would call that a win!

LWX is pretty bullish in their afternoon AFD, and as Yoda mentioned, the MRGL risk now comes into the DC metro area. I still, however, haven't seen guidance showing surface-based instability here. I do see good mid-level lapse rates, so there will likely be some elevated cape, and I certainly believe that some lightning is possible. Until, however, I see some sfc-based instability being progged, I can't buy in to SVR potential until you get well south of the DC Beltway.

I'm always interested in high shear low CAPE setups, but it could be a high shear NO CAPE setup. I tend to trust the NAM nest when it keeps us cool at the sfc in these events, but if the HRRR is right, it would be a SLGT risk day for sure.

So we're in to the range of the NAM nest now, and it has been emphatic for several consecutive cycles now that a strongly forced convective line will move through the area early Wednesday evening. But it's also been emphatic that the line will not be surface-based, and most of the guidance I've seen agrees with that, so the chances of taking advantage of the excellent shear profiles seems low. That said, the GFS is trying harder to find a tiny bit of sac-based instability, so I won't totally close the door, even though I'm really hesitant to consider the GFS it wants to scour out the low-level cold air.

Great looking 500 map for sure. Right now, most of the guidance I've seen keeps the warm front south of the DC metro area, but it's obviously still 5 days out.....

While the main synoptic precip shield is obviously going to be way south of here, there are hints in several of the hi-res runs for a band of snow showers moving southeast across the DC metro area this evening.

classic event in which SPC takes the approach of "we didn't call for it, so we're not going to box it and hope it goes away quickly"

Thank you. I should quit posting for the year now, as I likely won't make a better call......

Some folks are thinking about snow showers in northern MD tomorrow, and that is on the table, but I'm more interested in the "warm sector" along the I-95 corridor tomorrow morning ahead of the occluded front. A lot of guidance briefly puts this area into the low-mid 50's, and there should be a band of convection along the front. Some of the CAMs develop a little bit of instability, and it's really cold aloft with decent lapse rates, so while I'm not ready to call for thunder, I'm wondering if some places might get a burst of graupel or small hail.

10:1 NAM maps have some "accumulation" for some of the metro area Wednesday evening. That's because the model microphysics have a mix of rain and sleet - definitely supported by the soundings which have a decent layer below freezing above temps around 34 at the sfc. Definitely could see some pingers around during those initial hours of rain early Wednesday evening before the column warms.

So yeah, the sfc temps look a tad high along the i-95 corridor, but even if it's 34, that's a wet snow sounding with such a shallow layer of "warmth" at the ground. Add in the fact that the GFS has been running warm at the sfc in recent winter events here and the fact that it's at night, and I'll be very happy with what will be falling if we can get some decent precip.

FWIW, the 18z GFS trended way better with the weekend shortwave. Edit: LOL. Looked nice at 132 but then got flattened 6h later.....

While obviously acknowledging that it's not worth parsing details 84 hours out, that NAM solution verbatim at 12z Thursday has surface temps a few degrees below freezing in the local area, and even though the sfc high is shifting east across New England, surface winds in the mid-Atlantic are progged weak and from the northeast, suggesting a damming scenario that could persist well into Thursday.

roads are ok here in southern Howard County, but there is a pretty glaze on the trees.

This is a legit concern. Most of the 00z hi-res guidance increases the lull time.

This is a good catch, but the idea of that initial band having some sleet in it is an outlier solution, and the associated forecast soundings have a warm layer that is just barely above freezing. Something to watch for in the evening runs, though.

because the snow falling tomorrow morning will fall at temperatures in the 20s which always causes problems

Fantastic agreement among the hi-res models for the initial band of snow at some point during the 7-10am or so time frame (earlier for those further west) followed by a break for a few hours before the main batch of precip arrives with snow changing to sleet to (maybe a period of freezing rain) to rain through the mid/late afternoon hours (with the temps taking extra time to crack the freezing mark north of I-70). I still believe that the morning snow band is going to cause a lot of road problems with the temps in the 20s.

It's worth noting that there is really good agreement that there will be a band of snow pushing across the area during the morning tomorrow. It's narrow, and it won't last long (and it's not clear how far south it will extend), and accumulations won't be great. BUT, it will fall with temps in the mid 20s, so it will likely cause some travel troubles. It's also clear that we then get a break before the primary precip band arrives from the west. How much we can warm before it arrives will of course determine the impacts from that.

No doubts at all about the shear. NAM nest showed a strongly forced line of convection moving through very early Sunday. As Yoda notes, instability is pretty meager, but temps will be rising through the 60s Saturday night, so being able to mix down some momentum to the sfc isn't a total fantasy.