eduggs

The RGEM has several hours of rain on the eastern half of LI overnight Sun - especially south shore. Anybody buy that?

The 18z RGEM is the best run I've seen in a while from any model. Very impressive thump with several hours of 0.1"+ liquid and it shifted the sleet line meaningfully south! Unfortunately when the 3rd party graphics for the RGEM and NAM differ with northern extent of sleet, it's usually best to go with the NAM. But a shift south is still welcomed.

Unfortunately the ECMWF-AIFS treats all frozen precip. (e.g., sleet) as snow in its 10:1 total, so that map isn't going to tell us much. Same is true for the UKMET and a few others.

Right now the model consensus has the dividing line between big snows and more modest snow accumulations plus sleet from extreme northern Morris County through northern Passaic, Rockland, northern Westchester, and northern Fairfield. Right now I would take the under on double digit amounts south of this line. Hopefully the last two model runs and then nowcasting shift that line south a bit.

Don't tell everybody Don't make them wise. Lots of money to be made of these hyped events

RRFS looks pretty solid for Orange and Putnam - keeps it mostly snow and longer duration. Many more hours of snow for whoever can stay north of the sleet line.

Friendly reminder that 700mb is not the warmest level.

18z NAM looks slower or south with the sleet line through VA and MD initially. Good sign for far!

I think the 3rd party graphics showing the snow-sleet line on the RGEM are probably too far south. The NAM usually wins on this when it is insistent and all guidance has moved towards it. I'm not really sure about the actual thermals on the model - that would be difficult to diagnose without balloon soundings. But even if sleet mixes in to Sussex, Orange, and Putnam I think it will be a snow-sleet mix during the steady precip. as opposed to a clean flip to all sleet.

Anybody else notice the cool upslope-downslope couplet on strong easterly flow in CT and the Hudson Valley visible at like 1z Sunday night on the RGEM? I don't see that signature on any other model.

The RGEM is a strong thump with several hours of legitimately heavy snow. But it still brings sleet in late afternoon. The LHV on north stays mostly snow and continues lightly through Mon morning.

Recent runs of the RGEM, HRRR, UKMET, RRFS etc bring sleet into the area as early as 2-3pm. I suspect that's probably correct but I think there will be sleet & snow mixed or oscillating ptypes for several hours.

It's warmer than 12z, sorry.

I'll be watching obs from VA late tonight with interest. The NAM/RRFS bring sleet rapidly through S and C VA overnight while the colder models (RGEM/HRRR/RAP) keep it snow. That should give us a good heads up regarding how quickly we are likely to mix or change over on Sunday.

As I mentioned earlier, the NAM doesn't forecast snow. It forecasts QPF and temperature across multiple vertical levels. Other organizations have developed algorithms to translate that data into accumulated snowfall. And in marginal cases where a warm nose is very close to freezing (like Little Rock this morning), it's possible those algorithms incorrectly count snow as sleet. When QPF is significant, this graphical error can add up quickly. In these cases, the fault lies with the algorithms, not the model itself. We would need this morning's sounding from Little Rock airport to try to diagnose any model errors. But based on the NAM forecast sounding from the past 4 runs at Little Rock, I suspect this was the case. Unfortunately the NAM is not showing a marginal sounding for NYC tomorrow afternoon. But NWNJ and the LHV could benefit from a similar clown map error.

Careful about extrapolating a borderline snow sounding in Arkansas to our area. On one hand, we could also benefit from snow and sleet mixed being treated as non-snow on the clown maps. Sometimes rimed flakes aggregate into parachutes that accumulate fairly well (~12:1). Unfortunately the NAM- and RRPS-modeled soundings have a much more pronounced warm layer for our area tomorrow afternoon than for Little Rock this morning. It's several degrees C above freezing in the warmest layer. We need that to be completely wrong. In the Little Rock case the shallow warm layer is near freezing.

That raises a good point about 3rd party vendor snowfall graphics. Weather models don't forecast snowfall explicitly. They generate QPF and temperature at numerous vertical pressure levels at each horizontal grid point. 3rd party algorithms translate that information into a ptype forecast. But sometimes more than one ptype is present at the same time. Snowfall can be under-counted in cases where the warm layer is shallow and not-pronounced because the algorithm may lump it all as sleet when it's actually a mix of ptypes or rimed snowflakes. See the Little Rock NAM sounding below.

The steady precip. should be done by midnight if not sooner, so theoretically there's plenty of time to clear snow and treat roads to open schools and businesses Monday morning. But just based on the hype I expect most things to be closed on Monday.

The lead in forecast evolution for tomorrow's storm reminds me a lot of the December 2020 across our area (differences of course especially SNE and mid-Atl). The big snows for that one slipped away too but the impact was still significant locally.

FWIW I'm pretty sure the 10:1 counts sleet as snow on the Pivotal graphics for the RRFS. In this case the Kuchera algorithm (while not great) is better for visualizing snow accumulations.

It also shows half the QPF as sleet (or rain - SNJ).

Synoptically it tracks with the GFS. So usually too cold. Too bad though because it's a snowy solution... right into Mon. morning.

God that's an ugly sounding for NYC. I wonder if the warm layer being between 700mb and 850mb makes it less likely to show up on 3rd party graphics for other models... maybe other models don't generate and distribute temperature data in between the more common pressure heights.

This reminds me a lot of December 16/17 2020. I don't think the public freakout was as over-the-top for that one... but then again, people didn't have the apple weather app yet I don't think.

We go through this so often. And people cover their eyes and ears beforehand and pretend it didn't happen afterwards. When the NAM shows sleet it sleets. And the extent is usually north of where the 3rd party vendor graphics show it.