wdrag

6Z.HRRR. then 00z/excessive HRRR, and the 7Z/12 NBM. Definitely concerned about what happens south of I80. I just dont know... too much uncertainty because of sfc temps which I doubt drop below 32 or 33. AND no guarantees on area best lift. Model variability still looks large to me. Back at 10A.

From my perspective through 00z/12 HRRR NYC-LI... looking very good for 2"+, maybe 7" wet snow somewhere n LI. Thinking power outages whereever the boundary of 6" between I80-I78 where 33F wet snow. NYC streets slushy and slow. Back in the morning. Will be surprised if 01z NBM is under 2" NYC.

It's all played on paper right now... the game on the snowboard starts at midnight Tuesday morning. mPing are you ready?

19z NBM is only 0.9". I sort of doubt the 18z NAM is in this, nor the 18z RDPS, and so the 01z/12 NBM should come up two inches on LI, I would guess... IF not, then I'm at a loss. If I was sitting in the seat I'd add 2" to N LI but there could be multiple reasons to remain as a minor event NYC. Not sure...its been a pretty bumpy variable ride near I80-LI from the start.

18z NAM cold and snowy NYC like 18z HRRR. Wow! I dont have an answer. NWS does I guess.

Saw the 18z HRRR..astounding. Here's the 19z NWS ensemble stats for this event. Not the 18z HRRR look.

Warnings out in pink.

If others had not posted... this is the northern USA winter temps so far. Many Great Lakes areas heading for record warmest.

Now that we have a moment: Pivotal weather users may have noticed the HRRR now has Model snowfall!! Total Snowfall: Model Ratio (in). That is a big deal. Please read how snowfall is produced. Model Ratio, Kuchera and Positive snowdepth change are the recommended with 10 to 1 last. It's a long but worthy read while we wait for models. I just took a snapshot but Kuchera is described as well, though not appended here. ----- On the other hand, the snow depth variable does attempt to account for melting, compacting, and sublimation on a representative ground surface, and is even able to take advantage of minute-to-minute changes in the soil model state while doing so. So, in that regard, it can be more useful for estimating the ground accumulation at the end of a snowstorm than our 10:1 and Kuchera snowfall products. Still, this benefit is offset by the substantial pitfalls of using very imprecise SLRs and typically treating sleet as snow. Conceptually, users should realize the snow depth variable is just a byproduct of internal model considerations around surface fluxes; this is a domain of physics where the precise snow depth may not be quite as crucial as the total mass of frozen precipitation covering the ground. As such, using model snow depth to forecast snowfall is subject to caveats and errors that are of similar magnitude to 10:1 or Kuchera, and it may perform even worse in some situations! Explicit model snowfall The HRRRv4 and RAPv5 (implemented at NCEP in December 2020) began providing explicit forecasts of snowfall; to our knowledge, this is a first in mainstream operational NWP. These forecasts can be found on Pivotal Weather as the “Total Accumulated Snowfall” parameter. Although the HRRR/RAP use a very simplistic SLR that is a strict function of 2-m AGL temperature, they are able to assess SLR and melting at every model time step, which affords far greater temporal precision than our post-processed products (e.g., Kuchera). In addition, the NWS National Blend of Models applies relatively sophisticated SLRs to each input model’s QPF. Although NWS NBM precipitation forecasts are essentially post-processed ensemble means, and therefore may tend toward smoothing out maxima in forecasts beyond the first 12-24 hours, the NBM’s SLR approach is more advanced than any individual NWP model on Pivotal Weather. Summary and practical recommendations Our snowfall products generally attempt to forecast the snow that falls to the surface; not necessarily the snow pile you see on the grass, interstate, your rooftop, or anywhere else after a long storm. There are some caveats with Kuchera (penalizes warm temperatures in part to account for on-ground melting) and accumulated positive depth change (explicitly accounts for melting, albeit with model data file frequency as a confounding factor) — but none of these products will consistently provide an accurate forecast of final ruler-measured snow depth, even if the model’s QPF and vertical profile are spot on! We recommend using the Kuchera snowfall products in most situations. Kuchera snowfall is certainly imperfect, but from our perspective, it is the least flawed practical option on the table right now for most models. Model snow depth can also be quite useful if your main forecast problem is the final ruler measurement on a natural surface, especially for events where melting is a major concern. This product sometimes includes sleet, however, and the SLRs it uses implicitly are probably less accurate than Kuchera much of the time. The 10:1 ratio snowfall products exist primarily as a very conservative estimate for legacy purposes, and because they are painless to compute. For certain datasets, we may not be able to compute Kuchera, leaving 10:1 as the only practical option. In the future, we may consider removing some or all 10:1 products, but they still may have value to some users as a baseline approach that is easy to compare between all datasets. In our view, the best path forward toward more accurate and less confusing NWP snow forecasts is for modeling centers to track snowfall internally during integration, rather than just liquid equivalent frozen hydrometeors. The current situation leaves it to end users like us to apply SLRs (and, in some cases, infer precipitation type) based on limited and temporally sparse data. Even a simplistic in-model SLR algorithm estimating the expected crystal type and riming (with the benefit of full-grid data and microphysics parameters), applied much more frequently during integration than publicly available data files, may yield drastically better snow forecasts than today’s. ADDENDUM: the RAP and HRRR have started providing explicit snowfall forecasts as of December 2020, so hopefully more models will follow suit in the near future!

model resolution.

NWS ensemble guidance probs for 1,4,8", the NWS overall 5AM deterministic forecast which went into the swath of watches, and the 07z/11 NBM which looks like a reasonable baseline for me and where I started with a forecast elsewhere. Please be careful about relying on 10 to 1 snowfall on the warm side of the max axis... it's fraught with ptype-qpf-ice pellet conversion issues. I'm even concerned about getting 5" of snow up here in Wantage of extreme nw NJ. I'll check back late today...

No solid insight on when modeling goes into the NBM, except when this runs (19z,01z etc), I dont think it has the latest instantaneous modeling... example 19z cycle. I doubt that it has the 18z model run, nor the 18z ensemble. I was stunned by the sudden upward shift in the 19z NBM yesterday as well as the post of the NWS probs for 4" etc. It didnt make sense. I can only think it incorporated some of the SREF which was broader snowfall for NJ. This is where we have to take this stuff with a grain of salt since I don't know exactly the percentages of modeling and statistical temp guidance that drops into each NBM cycle. I'll bet most on here don't know about LAMP stats. So... I think something was wrong with the 19-22Z/10 NWS probs for 1,4" and NBM amount guidance since it briefly radically shifted. My confidence in the EPS snowfall forecast on the warm side gradient of the event continues below average... This event or non event will tell us more. I definitely would not use 10 to 1 ratios on the warm side of the axis... it doesn't recognize ptype or sweeps it over a 3 -6 hr qpf. I see the GEFS +snowdepth change continues meager... if this verifies again, then I think everyone has to show more appreciation for this guidance. Event has yet to occur so I won't badger on my concerns but I'm a Canadian has to have it (not the 10-1 excess on the warm side gradient) and the GEFS +snow depth change has to be used as strong consideration of what we expect.

No solid insight, except when this runs (19z,01z etc), I dont think it has the latest instantaneous modeling... example 19z cycle. I doubt that it has the 18z model run, nor the 18z ensemble. I was stunned by the sudden upward shift in the 19z NBM yesterday as well as the post of the NWS probs for 4" etc. It didnt make sense. I can only think it incorporated some of the SREF which was broader snowfall for NJ. This is where we have to take this stuff with a grain of salt since I don't know exactly the percentages of modeling and statistical temp guidance that drops into each NBM cycle. I'll bet most on here don't know about LAMP stats. So... I think something was wrong with the 19-22Z/10 NWS probs for 1,4" and NBM amount guidance since it radically shifted. My confidence in the EPS snowfall forecast on the warm side gradient of the event continues below average... This event or non event will tell us more. I see the GEFS +snowdepth change continues meager... if this verified again, then I think everyone has to show more appreciation for this guidance. Event has yet to occur so I won't badger on my concerns but I'm a Canadian has to have it (not the 10-1 exes on the warm side gradient) and the GEFS +snow depth change has to be used as strong consideration of what we expect.

This should perk you up... NWS Probs for 2,4,12" These are all pretty large for 48-72 hours. Disconnect between published snowfall-Tomer Burg 18z GEFS snow probs 1", and the 2,4,12" probs attached plus the 19z attached robust NBM. I can only think the NWS is latching onto the colder EPS values? I saw the18z GEFS predominant ptype for 12z Tue and s of I80 its rain (NYC) as well. Close call I80 for sure. So I'm not saying what will happen but will revert back to these varying stats and see what results. mPing will be helpful midnight ish -8AM Tue. fwiw we just had about .06" rain here in Wantage NJ the past hour.

Noticed WPC pops for snow hadn't updated in a while... (07z). I think they were busy collaborating with the WFO's. First watch is out for central PA.

I'll add the new WPC low chance of 3+" NYC after 12z Wed by a very good WPC forecaster-collaborator. So there is hope - it isnt over. Need to watch the trends but allow for noise 20 mi fluctuations until 24 hours out.

I dont think there is nearly enough respect for Positive snow depth change, ESPECIALLY the gradient edge where temps are a possible accum factor. Gradient is where the largest errors occur. You can go 10 to 1 in colder thickness below 540 but not above. That's why the NWS uses ratios in small increments to match temp/type/vertical thermal profile. It's not the old days... we now have better techniques to attempt the reality. Referencing 10 to 1 in warmer side of the gradient is not my choice. I've seen comment about big snowfall rates overcoming but what if that very strong lift ends up north of NYC? Then it's harder to accum. I may learn my lesson, but for now 12z/10 Canadian is pathetic, ensembles not too good I80 south except the more robust EPS and so prior reputation for the Euro warrants keeping that in mind. I've lost faith in the Euro and notice that even though the GES is not a great flag for events...it's GEFS Positive Snow Depth Change has been better than the Euro in my opinion. NBM is still up for 2-3" so not all is lost. I will keep an eye on GEFS positive snow depth change and if it grows... NYC good, but otherwise we need the Canadian on board. I can only think the Canadian will move its solutions north and snowier on the edge in the next couple of cycles. I'll check back tomorrow morning.

Couldn't agree more... however, once the cat is out of the bag... This system is not gospel today once way or the other. Just need to ride out the variability. Very sensitive interactions.

The 353AM attached.Ihavent checked for an update. In my opinion as I do my own non AMWX posts... once/day is enough... that steadies out all the model fluctuations and tends to keep my group on message whether its trending up or down.

Not all the storm. Their map through 7p resides on their winter wx link

Don't know but they apply, probably differing snow ratios with applicable qpf for 1 or 3 hour periods during the event along with multiple ensemble surface temps. Complex but good baseline. I'm still very careful about snow south of I80. GFS already has above freezing temps aloft at LGA and I have privately mentioned a possible sleet mix to Sussex County for an hour or two. I dont want to get into debates about snowfall rates overcoming above surface freezing temps. Potential exists but I'd wait til 36 hours our before great enthusiasm. Canadian still all over the place which tells you sensitivity problems to the upper air interactions. I could be wrong but right now I'm conservative still 1/2-2" CP. Blessings if it triples that and a hard won model victory for the NYC participants. 3 days out. The prob map for 2" or more is only through sunrise Tue.

I'm in agreement about caution: All modeling except the previous storm overzealous EPS has less than 1" accum NYC. Ensemble temps are above freezing NYC metro midday Tuesday. Even sign of near freezing temps aloft which means mixing. I think caution warranted... The Canadian especially all over the place. Maybe it will coalesce into a nice event, but for now I think northern PA, I84 corridor is a best fit with a probable climo snow on the back side for NYC of 1/2-2". Could be wrong but that's what I'm publishing privately for a D4 outlook.

For the enthused, a thread was started for the event and you can shift comments there uf you wish. I'm still reticent as written via comment in the thread.

19z/8 NBM 1.6" NYC, 12z/8 GEFS less than 1" NYC, 12z/8 CMCE less than 1" and the EPS while around 2" NYC Positive snow depth growth, it busted pretty bad 1/28-29 on being too far south. So, we'll see how this plays. WPC has all chances of 3" snow north of I80. D5-6 opportunity: will want to verify against these data that I'll append shortly and we'll see what might have been best at this distant time. These graphics added at 539PM. All identified. Positive snow depth is my base amount that I use, then double in the axis, or even use Kuchera or 10 to 1 if cold enough thicknesses for TOP of the range. For now, the positive snow depth change is good start for me.

I was going to wait til at least 6A Friday to further review since there is much uncertainty including sfc temps, but someone else started the thread.