wdrag

Ensembles (10-1 SLR) will be attached for the anticipated results of a western Canadian short wave digging into the northeast USA Saturday morning. Latitude of the leading low pressure system will assist determining the axis of best snowfall which for the most part on 2/14 is near PHL. The SREF of 15z/ 14 is closest to I80-LI. Amounts with subfreezing surface temps and accumulation on all untreated surfaces could range roughly anywhere 1/2 to 4". A conservative approach right now from the modeling, is to expect less than 2 inches I80-LI northward and a bit more to the south of I80. There will be modest lift in the DGZ (GFS/NAM not shown) and what seems to be a nice 3 hour period of decent snowfall near sunrise as seen in the NAM Banding tool (not shown). A sample of the GFS model SLR for 12z Saturday leads the imagery and reflect higher ratios north of I80 and about 9 to 1 for LI. Hopefully we can enjoy more snow. Added the 12z/14 GEFS snowfall chart at 441PM. This weekend we can look back at reality vs modeling. Corrected date 18th to 17th at 828PM Thu the 15th. No other changes needed at this time. Max snowfall axis continues near the Mason Dixon line where watches and/or advisories are already posted for NJ/PA/MD. Some modeling have the max axis a little further north near I78. 18z/15 GFS op has increased snowfall ratio for LI 11 to 1 for early Saturday. Title simplified 704PM Fri Feb 16. Verification added at 1PM/18 via NOHRSC and CoCoRaHS with the infamous banding near I78 that dumped 6-13" from s LI beaches west to Allentown. DCA 0.1, BWI 2.2, PHL 2.6, ABE 5.7, NYC CP 2.0, BDL 0.7, ORH 0.5w, and BOS T. looks to me like GEFS did best using the initial ensembles within this thread.

I plan to get the thread started at530 PM with multi ens and broad wording. It wlll have “refresher” and “obs” in the title attempting to limit title changes. especially only 72 hours out. Have not been using UKMET thos year. Also these events are worthy of model perception perspective. Only imo, EC is no longer the constant King.

Caution on tossing the towel before 2/25. I do think we will warmup big time the first few days of March, if not around 2/28-29. BUT... weeklies seem optimistic about multiple qpf opportunities in March. We'll know I think at 4PM Friday when the updated March monthly posts from CPC. In the meantime, minor minor Thu night. Will re review at 530PM for a 1-4" refresher thread NJ-LI portion of the forum, so snow starve. Looks too me like a nice colder consistency Saturday morning. 2/22-24 is on... no thread but pretty sure large winter storm then. Probably a little too warm here, but maybe not, so keep monitoring the ensembles/ops reading up.

Don good... myself shaky. I wasn't going to thread for another day or 2 cause this was north of NYC event. However, the NAM eventually had me concerned about NYC-PHL and to an extent the EPS trying to reclose the 500MB ow as it crossed the mid Atlantic to our south but that consistent NAM got my attention-concern for NYC. Anyone who wants to go back to p1 of this thread to check the ensembles... including WPC... you can see how far in error they were at D5! EPS was closest but all were north up near I84. Am concerned that we maneuver our forecasts too much to the EPS. I do not believe the EC is KING of the hill anymore (as a friend referenced)... recent past couple years have been shaky, especially yesterdays 12/12 version which forced too much northern retraction of snowfall. Anyone notice the power outages near I78 e central PA and w central NJ today. Probably near 33F wet snow 6+? I'll add NOHRSC analyzed snowfall to p1 when its ready.

Aside from the minor brief snow showers Thu night, and a possibly nice wet snow that will melt a fair amount on pavement Saturday; the 22nd-24th is ensemble supported with a potential coastal event per CMCE, GEFS, EPS all showing an inch or so, 7-9 days in advance. Seems minor but it's something and it may incorporate a southern stream short wave.

Thread was good-worked out with all the model variability. Eyeball idea of max snow axis attached. Very sharp northern cutoff N of I84. Click for clarity. Little doubt in mind that Postitive snow depth change is an excellent bottom amount of expectations-it's conservative and can temper enthusiasm, then you handle the upside range as you wish via 10 to1, Kuchera , HREF.

CP snowfall today?

Wantage NJ, this part anyway a solid 11 inch event with a solid com-acted SD of 9.5-10" everywhere. Season just over 30", heaviest since the 20-21 season.

Wantage NJ (this southern part) 8.5" as of 830AM with one snow board clearing 630A. 3.5" past 2 hours. SOLID 8" on the ground due to wind compacting. 28.8F.

Wantage NJ this southern part. 5" solid snow cover. 28.8F. snow light-moderate rates.

Wantage NJ (this southern part): 4" so far. Wind causing some blowing and drifting but a solid 4 out there. Will clear snow boards at 7A. btw: 1.8" past 85 minutes. Walt

Wantage NJ 345AM. 2.2" and moderate snow (yellow on radar scope). 29.5F

Sensitive interactions from the northerly NAM/GFS/HRRR to the southerly models (EC) that do not match the just issued WPC 17z/12 D1 QPF. Also, the WPC snow amount probs are quite high N of I80. So, as someone said, spooky-unsettling but we 've got to go with as smart a forecast we can. How about the SPC HREF: Yikes. Huge. One thing I see is that banding should be pretty large Poconos through nw NJ se NYS-CT-BOS near and just of I84 per 12z/12 NAM HRRR and HREF. Again though its your modeling of choice.

fwiw... f you're interested in hourly snow ratio's it's on the RAP. I just sampled nw NJ at 6z and 11z as the vertical column cools...going from 7 to 1 at the start to 11 to 1 by 11z. NYC prior to 12z probably 7 or 8 to 1.

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...