RU848789

wow, even Philly gets screwed with essentially nada

it's probably over for more than 2" for CNJ, but I'd still be happy with 1-2". There's still time for minor shifts back north to make that happen, as we're not that far off. CMC still has 1-2", GFS/RGEM have an inch, and NAM/ICON have an inch or so for the southern half of CNJ, plus there's the 2-4" on the SREFs. If we hadn't seen snowier solutions over the past few days and were magically transported to where we are now with no knowledge of the past, we might look at where we are now and say, hey, some minor to moderate changes get CNJ and maybe even NYC into at least a couple of inches of snow. The models don't "remember" previous runs and are independent events with new data every time (as far as I know).

Did you see the key part of the Mt. Holly AFD where they talk about their reasoning for going bullish with the snow for CNJ/NNJ? Wish they'd come on one of these boards and clearly show what they mean by "looking at the overall atmosphere aloft including jet dynamics and mesoscale features." The biggest challenge that still persists at this time is with regard to the track of the surface low and where it will be as it crosses through the area. While the majority of ensemble guidance has shown a southern shift over the past 12-24 hours with QPF, looking at the overall atmosphere aloft including jet dynamics and mesoscale features, it seems that the overall consensus of the track seems to be too far south. Depending on the strength of the system as it approaches and confluence that lingers over the Northeast US, will simply determine how far north the snow shield reaches. Have started to see some waffling back and forth with the 12Z guidance, so don`t completely buy the entire southern shift. Unfortunately, there is still a lot of uncertainty amongst global guidance, where the greatest potential for higher-level impacts remains over the southern-half of the forecast area.

Surprised at this, but more hopeful now, but also a bit skeptical...we'll see soon...see the part in bold, in particular for amounts, but to summarize here: 1-2" N of 80, 2-4" from 80 down to 276/195, 4-6" south of 276/195 with lollipops to 6-8 where the best banding sets up in SEPA/SNJ. I could certainly live with this. Edit: here's the NWS map for NWS-Philly and parts of adjacent offices. Nice. Area Forecast Discussion National Weather Service Mount Holly NJ 335 PM EST Fri Jan 3 2025 .SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/... Our attention then turns to the winter storm potential that will impact the Mid-Atlantic region late Sunday night through Monday, possibly lingering into Monday night. A potent shortwave ejecting out of the Rocky Mountain region will develop into a closed low as it slides across the Central Plains. This closed low will undergo cyclogenesis and lead to a surface low developing while tracking through the Central US on Sunday night. The surface low will continue to track due east crossing somewhere between the southern Delmarva and eastern North Carolina by Monday afternoon. The biggest challenge that still persists at this time is with regard to the track of the surface low and where it will be as it crosses through the area. While the majority of ensemble guidance has shown a southern shift over the past 12-24 hours with QPF, looking at the overall atmosphere aloft including jet dynamics and mesoscale features, it seems that the overall consensus of the track seems to be too far south. Depending on the strength of the system as it approaches and confluence that lingers over the Northeast US, will simply determine how far north the snow shield reaches. Have started to see some waffling back and forth with the 12Z guidance, so don`t completely buy the entire southern shift. Unfortunately, there is still a lot of uncertainty amongst global guidance, where the greatest potential for higher-level impacts remains over the southern-half of the forecast area. With all this in mind, the forecast this afternoon features a solution that incorporates warning level snowfall for areas around Philadelphia and points south. The highest snowfall amounts will end up being located within a frontogenetic band where SLRs will be locally higher under an area of enhanced snowfall. The confidence is rather low on where this FGEN band will set-up over the region though. Based on latest 700-850 mb frontogenesis progs, the greatest likelihood will be somewhere between the Philly metro and central Delaware. In fact, some of the hi-res guidance that has come in this afternoon shows a warm nose between 850-925 mb that creeps northward during the day on Monday into portions of the Eastern Shore of Maryland and southern Delaware. This would result in some potential mixing issues in the form of sleet and/or freezing rain which will drastically undercut snow totals, with the heaviest snow totals located just north of the mixing line. Upon discussion with neighboring offices and given the overall uncertainty, have opted to not issue any Winter Weather headlines with this forecast package. In terms of snowfall amounts, our initial forecast is for a 4-6 inch snowfall event for areas on a line from I-76 to I-195 and points south, including the Philadelphia metro. Within this area, will be a localized maxima of 6-8+ inches depending on where the frontogenetic banding sets up. Between the I-80 and I-76/I-195 corridor, expecting totals in the 2-4 inch range with totals less than 2 inches to the north of I-80. There will be a sharp northern gradient to the snowfall due to confluence and dry air off to the north, so some of these totals may be slightly too high.

Thanks!! I tried looking through that website before and just now and maybe I'm just dense, but I couldn't find anything on the time lag nature of the inputs into the NBM, but I'll certainly take your word for it. A time lag definitely explains how the NBM output at a particular time can look more like the output from the models from 6 hours earlier rather than the models that run somewhat concurrently with that time, assuming that the NBM is not including those concurrent runs. And yes I do realize the shorter term/high res guidance does start getting incorporated as one nears an event.

Just a minor shift south over the last 18 hours for the ICON.

A couple of folks on 33andrain, including one met, have said that the NBM is time lagged, i.e., the 07Z NBM would only include model inputs through 0z and not the 06Z models. That would at least explain why the NBM snowfall for 07Z is greater than the 01Z NBM for areas N of Philly and much greater than the 06Z models would suggest, since they're generally much lower than the 0Z models were. To me, that makes the NBM a much less useful tool, if that's correct, as it would always be 6 hours behind. @MGorse- sorry to tag you again, but can you clarify if this is the case, i.e., what time model inputs go into a particular NBM run? I'd also be curious (see the quoted post) if the 12Z WPC QPF output is simply the 07Z NBM QPF field (they're identical), which is based on 0Z model data - that seems to be very old data, then.

Would love to know how the NBM increased snowfall north of 276/195 up through 80 and even 84 at 07Z vs 01Z when every single model I have access to (GFS, Euro, ICON, and the ensembles for these) showed a significant decrease in snowfall for this area at 06Z vs 12Z. Only one model at 6Z, the ICON, has NYC at >1" (and the 0Z UK and CMC also had NYC at <1"), yet the NBM has NYC going from 2.4" to 3.5" from 01Z to 07Z. @wdrag? I know we had a conversation yesterday morning about inconsistencies with the NBM (between its high snowfall vs. the WPC low precip at the time), but this seems even worse. Edit: well, the 07Z NBM clearly thinks there will be 0.2-0.3" QPF for NNJ/NYC, as its QPF estimate is identical to the latest WPC QPF from 12Z today, below...

Ya never know. My guess is 75% of the area under advisories gets <1", but that a few locations that get sustained banding get up to 2", but those are very hard to predict.

Wasn't sure where to post this HRRR comparison of 18Z and 0Z for tomorrow's little storm in SNJ/SEPA/DE, but in the vein of model mayhem, here's reason #993 to not trust the HRRR beyond 24 hours, lol...

Every pro here and on 33andrain and elsewhere is scratching their heads over what to make of all of this now that the Euro joined the UK as misses for our forum, meaning I have no shot of figuring anything useful out at this time. Even using the NBM Model Blend is probably a bad idea when there are two very distinct camps like this. One can blend them and get a moderate snowfall for our area, but the outcome is just as likely now to be one camp or the other. FWIW, the NBM is about the same as it was at 18Z. I don't envy forecasters for this one. At this point I would probably just punt and wait for better model consensus and use the NBM which still says 1-3"/2-4" for CNJ (less north and more south), but I'd note that <1" (or even nada) and 3-6" are still on the table. Crazy.

I still like 1-3/2-4" for CNJ, which is what we're still seeing for the NBM, which actually got a little snowier for CNJ at 01Z vs. 19Z. Of course, a blend will miss if one of the outliers is on to something and can be terribly wrong, but major outliers rarely verify, so it'll usually at least be in the ballpark, unless we see some huge shift by all the models, which has never happened before 3+ days before an event, lol.

Reason #993 to not trust the HRRR beyond 24 hours, lol..

Just when I thought we were going to have consensus on a snow party with the ICON/GFS, the CMC brings us back to reality and then the UK impersonates Dr. No.

Holy crap, we just got NAM-ed by the 0Z GFS, lol. Stepped away for 30 min and boom!

Why? It was an outlier at 18Z north, so coming back a little bit south is no surprise; it's still likely going to be the furthest north of the 0Z models.

I'll take that, thanks for guaranteeing that outcome, lol.

11.25" - not a top 10 storm, accumulation-wise, but a top 5 storm, experience-wise, partly because of the surprise element and partly because of the incredible 3-4" per hour rates for most of 2 hours. Different setup and I doubt we'll see that kind of gradient on both sides of a very narrow band.

Very nice for our area to see the 18Z GFS, ICON and Euro continuing to move the snow/precip shield further north putting CNJ potentially in line for 2-4" and NYC for 1-3" (or possibly more...or less)...and to see the NBM do a further increase for northern areas. Still 3+ days to sweat it though and we know what can happen in 3 days. Here's the Euro and NBM at 12/18Z.

While the ICON is just nuts, gotta like the snow shield shift north again on the 18Z GFS, with a 3" line along 78 now and 2" in NYC...

Just closing the loop on this. Thankfully, the NWS-Philly has finally come around to the reality of what the models are showing, as I figured they had to - and they're quoting the NBM (National Blend of Models) and noted the northward shift of the snow shield/accumulations mentioning that further northward shifts are possible; full discussion on Monday's storm is below. Too early for snowfall maps yet (they'll come Friday night), but their comments are fairly well in line with the NBM (and what I posted earlier): they have their 4" line from Philly to Toms River (the NBM is closer to 276/195) and they say more is likely south of that line and then snowfall amounts will decrease north of that line, with areas N of 78 likely well below advisory criterion (which is 3", so reading between the lines, maybe 1-2" along 78 and 2-3" for most of CNJ. Area Forecast Discussion National Weather Service Mount Holly NJ 304 PM EST Thu Jan 2 2025 .LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/... The synoptic pattern starts out with a flattening ridge shifting offshore Sunday night, followed by a flattening trough approaching from the west on Monday. A low pressure system associated with the trough will approach from the west beginning Sunday night while weakening as it crosses the Appalachians. The low will then cross the Mid-Atlantic and strengthen again as it passes offshore. Following the wintry system, conditions will turn colder and drier as a phased deep trough across the eastern CONUS supports shots of reinforcement for the arctic airmass persist through the end of the week. Confidence is increasing that widespread snowfall accumulations will impact the region Sunday night into Monday, with potential for significant amounts within a relatively narrow band roughly 30-50 miles wide. The exact track of the low pressure system and frontogenetic forcing will dictate where exactly this heavier snowfall band develops. This will depend on the strength of the high to the north and the evolution of the surface low, which will likely wobble a bit in the guidance from run to run and from model to model for the next 36-48 hours or so. Current indications from a consensus of deterministic guidance places this band somewhere between central Delaware and adjacent areas of eastern Maryland and far southern New Jersey, to potentially as far north as the Philadelphia metro area. There has been a slight north trend evident in the latest guidance alone, however I`m a little worried even this may not be quite far enough north. There will almost certainly be frontogenetic forcing and other mesoscale factors involved, which models have progged to be maximized somewhere between a line from DC to Delaware Bay, to perhaps as far north as northern Delaware near I-95. This would put the heaviest QPF/snow axis potentially as far north as the Philly metro. The 850 mb low center looks to cross the Appalachians around WV and east into northern VA and MD into Delmarva. The derterminisitc guidance seems a little too far south with the surface low given the jet dynamics aloft. This along with the warm advection aloft could easily spell mixing issues in the far south of the forecast area, such as southern DE and Cape May. This mixing area could include a mix with sleet, freezing rain, or even all rain for a time. PoPs were increased slightly across the board given the growing confidence in a potentially significant system. Current NBM probabilities support a 60-70% chance of snowfall exceeding 4" across much of Delmarva and southern New Jersey south of Philadelphia. Around Philadelphia proper and along a line east and west of there, the probability of 4" or more is near 50%, then lessens quickly farther to the north. The exact totals remain unclear, and we do not currently have an official snowfall forecast for this event yet. That won`t come until Friday afternoon`s update, but initial probabilities suggest *at least* advisory level snowfall (2-4") from around the Philadelphia metro and south, potentially low end warning snowfall (5"+) where ever the heavier band of snow sets up. Farther north of Philly, amounts are likely to lessen considerably to below advisory levels, especially along and north of I-78.

the latest from the NWS on tomorrow's little shortwave that could...advisories are actually up for 1-2" from Philly metro to about AC and SW of there in PA/DE/MD/NJ.

I know this is the 1/6 thread, but didn't know where to put the latest on tomorrow's little shortwave that could...advisories are actually up for 1-2" from Philly metro to about AC and SW of there in PA/DE/MD/NJ.

I get the confluence concerns, but I think you're underdoing it for CNJ. I would be surprised if the 1" line isn't at least as far north as 78, meaning most of CNJ south of there would get 1-3" with ~3" being down towards Trenton to Belmar. Most models are showing that and the NBM is showing more, including 1-2" for NYC.

3" line moved ~30 miles N and the ~1" line moved 10-15 miles N. Progress.