high risk

That would literally break this board, but the plan to run the NAM in hourly rapid refresh mode was scrapped many months ago.

yes, but the HRRR warming occurs after the precip shuts off edit: and as expected, with that warm afternoon, the temperatures barely drop in time to give us a pity amount of wet snow at the end of the rain

really strange in that it looks like it's snowing pretty good according to the reflectivity, but there is no precip accumulated. Looking at the soundings, it's insanely dry in the low levels, so I guess it's virga, but it seems odd that it would snow decently for a few hours and never reach the ground. It's also surprising that the HRRR takes much of the area into the low 40s during the early afternoon.

this is the type of event in which resolution and explicit convection (as opposed to a convective scheme) will help a lot, so the NAM nest forecast may be legit. yes, but it's so narrow and fast-moving that I'm not thinking in terms of accumulations. It's more of a deal with potential quick reductions in visibility and then freezing of snow on the ground if enough falls to cover surfaces.

Love the line of snow squalls on the arctic front midday Wednesday on the 18z NAM nest. Forecast soundings show impressive low and mid-level lapse rates!

mid-March at the earliest. The final 30 day stability test has not yet been run.

agree totally with this. it's also complicated by the NAM and HRRR showing light precip for the *morning* rush hour too, especially north of DC. Depending on your model of choice, it could be snow, sleet, or freezing rain.

Even if this ends up as only an inch or so of snow, this is going to be a high-impact event for sure. With temps falling into the 20s quickly after the precip ends, that covering of snow will freeze quickly and lead to icy streets and sidewalks. I expect widespread school delays Wednesday morning, with cancellations also possible, especially if the 00z GFS is right with us not getting anywhere near freezing Wednesday. (Earlier runs and the 00z NAM have us getting to near 30 Wednesday just ahead of the push of the real arctic air.)

I'll go back to my earlier post and say that while it's still most likely that this will have no impact (either due to no precip or warm temps), ground temps are extremely cold, so light rain falling at 35 degrees tomorrow could cause some icy surfaces. We've see this happen before.

While we focus on the medium range and an outside shot at a little snow as the rain ends Thursday, there appears to be a very subtle threat for tomorrow midday and afternoon. While the bulk of the rain will fall Wednesday night when temps are plenty warm, the NAM and NAM nest both show the possibility of a few light showers around Wednesday, and temperatures are only slowly going to climb into the mid and upper 30s. More relevant, the ground is insanely cold, so I could see a situation in which light rain is falling at 36 degrees, and the roads and sidewalks become icy due to the skin temps lagging far behind the air temps. I called it a subtle threat, as temperatures will be rising, and there probably won't be much precip around, but an icy surprise for some folks isn't out of the question. EDIT: the nam nest is somewhat on its own with temps staying very cold during the day tomorrow, but I wouldn't write it off

ugh. Yes, I came out with the right "totals" but typed 0.6 where it should have been 0.4. I've edited the original post. People are right that math shouldn't be done at 11pm....

What'a going on, I think (based off the NAM3), is that you have a huge slug of precip occurring during the evening hours - looks like well over an inch between 00z and 06z when the "snow" really piles up. The soundings are really, really iffy in terms of sleet potential, but the model microphysics are allowing for some pingers to be mixed in with the rain. Let's say that we have 1.3" of liquid falling and the model thinks that it's 60% rain and 40% sleet pellets: we know that would not accumulate, but the tallying does 1.3" x 0.4 = 0.52" liquid of "snow (+ sleet) accumulation". TT slaps a 10:1 ratio on that and gets 5.2" of "snow", while the Ferrier method will assign a way lower SLR since it knows that the frozen component is sleet and not snow.

a friendly reminder that when the NAM accumulated snow depth and Ferrier (from the nest) maps show much less than the 10:1 "snow" accumulation map, then the model is not predicting the snowfest that you think it is.....

GFS warms most of DC metro into the upper 30s Saturday afternoon in advance of the precipitation, but we have to remember that the GFS is notoriously terrible at maintaining low-level cold air. I don't think we can discount the NAM idea of the cold air being tough to scour out, especially if we continue to have snow cover.

I realize that the 12z parent NAM got everyone's attention, but it's a common rule that when the NAM parent and nest differ significantly, and the nest is line with other guidance, it shouldn't take anyone more than a half second to toss the parent NAM solution.

interesting how the NAM nest (vs the parent NAM) shut off the precipitation Saturday night for a few hours in southern MD and the EZF area. This, combined with some mixing, puts that area in a minimum for snow totals, while other guidance maximizes in that corridor.

Wrong. The 00/06/12/18z cycles of the operational HRRR are run out to 36 hours.

The GFS medium range scores did drop for sure right after the shutdown started, and a data issue related to the shutdown was suspected. But no such issue has been found.

There are no data issues with the American models. The GFS medium range scores did slip during the end of December / early part of January, but they seem to be recovering.

The differences in the 850 winds Saturday afternoon between the NAM and RGEM explain the differences in the precip shield. Remarkable disagreement.

hi-res guidance has a gusty line of showers on the cold front this evening. Mid-level lapse rates above a low-level inversion will be really impressive.

It was supposed to replace the GFS in early February, but the shutdown is going to ruin that plan.

MD issued - basically no chance of a watch, but we do have some "moderately interesting" convection approaching....

as we enjoy a Friday in the 60s, it looks like a repeat next Friday.

So, no MRGL yet, although based on SPC's Day 1 outlook, they almost pulled the trigger on it. Still think that there is a window for some interesting cells, especially west and northwest of DC during the mid and late afternoon hours, although the forcing mechanism is admittedly unclear. Most guidance also has a forced line of heavy showers along the cold front during the late evening, but the wind fields will have largely relaxed by then.