eduggs

IF the latent heat release - trof development feedback loop theory can explain or partly explain the AI vs. physics-based model divide over the past few days, you wouldn't expect a gradually shift towards compromise. You would expect the AIs to hold firm and the physics-based models to shift suddenly and significantly at the last minute. Only when their initializations start to capture the effect in real time. That would basically be now as Gulf moisture is just now showing up on radar/satellite.

This shouldn't have been much of a surprise. This was well signaled for days as a short duration, high intensity event. And models were showing near or just below freezing wetbulb temps even to the coast. That's not a terrible antecedent airmass.

Approaching 4" here. Might be the best event of the year locally!

Pretty well advertised quick hard hitting event. Just a wonderful daytime weekend snow event! Trust the models.

I meant the GFS & ECMWF vs. the AI models. Considering the size of the model domain, the camps aren't that far apart. The difference just feels really big considering the local sensible weather outcome. I feel like there have been many 75 mile shifts in precip. shields over the past 20 years with coastal storms within 48 hours.

They (GFS/ECMWF) aren't that far apart at this point, are they? What, maybe 50-75 miles with the heavy stuff offshore? The ECMWF OP in particular has shifted NW with the heavier stuff. As many people have mentioned, these are the kinds of setups that can and sometimes do shift NW in the last 48 hours. The outcome is extremely sensitive to minor changes in the shortwaves near the Gulf and also the Lakes. Presumably the AI models are correcting for those cases somehow. But that sensitivity is a double edged sword. I can also see how the AI models might overcorrect if their training datasets aren't perfectly representative or well matched to the current setup.

It looks like it comes is 2 waves, with the front buckling north and then back south. Most of the predawn stuff should be snow. If precip. shuts off - and esp if the sun comes out briefly, surface temperatures are likely to spike. Whoever get into the banding the longest could get some decent snow.

I'm not sure what you mean. I'm interested - fascinated really - why the AI models have been consistently west of their parent models for the past few days. I have not seen that behavior consistently or persistently earlier this season. And I offered one potential explanation for this specific situation. I reflexively tend to think the least snowy model will be correct, even if that's not scientifically sound.

A theory to explain the AI vs GFS/ECMWF battle could be the latent heat release - wave development feedback loop that Eric Webb mentioned a day or two ago. Shortwaves that tap Gulf moisture are susceptible to this effect. If this is biasing the global models too flat aloft, it could explain why the AIs are further west since they can bias-correct based on historical outcomes.

I agree with most of this. And I don't expect much if any accumulation in the asphalted area of NYC. But I don't think ensembles have the resolution to capture this partly mesoscale feature. The high QPF - short duration signature is prevalent across multiple models, so I'm inclined to think it's real. The question is where does it set up. Right now it looks north or west of NYC. But if the banding materializes and it impacts NYC, with a little dynamic cooling, I think there could be accumulations even in parts of the City. I really enjoy these low expectation localized events because they make for fun nowcasting.

Rates (precipitation per hour) are actually modeled to be fairly high. Modeled FGEN is significant across several models. But total precipitation is relatively light due to the short duration. Several models including the GFS deliver over 0.3" liquid over 4-6 hours in or near our region. A few models even have greater than 0.5" liquid nearby. That's a fairly significant QPF signal for short-duration frontogenic banding. The forecast difficulty is that the modeled placement of this banded feature (CT, LHV, CNJ) is highly variable. Forecast temperatures range from the upper 20s to mid 30s across our region with NYC roughly 31F to 35F depending on model during precipitation. Wetbulb temperatures are a little lower and modeled surface temperatures dropping during precipitation. Notably the HRRR is warmest, particularly after precipitation ends Saturday afternoon. NYC will probably record a minimum temperature below 33F on Saturday.

Saturday only (10:1)

Don't sleep on Sat. It's basically a mesoscale event. Could be nothing or a fast 3". Pretty significant FGEN values across multiple models. And it could come in multiple bursts predawn or daytime. My guess is CT or the HV is the winner, but a few runs have targeted CNJ to NYC. Could be a fun nowcast with low expectations. Contrary to what some say, this would have no trouble accumulating with 0.1" liquid per hour at 33F. Most will probably miss, but if you have the rates it will accumulate.

It's wild how good the AIGFS looks at 500mb. NYC is again over 0.5 liquid from just the Sunday event and the precip. shield ticked slightly NW.

0.4" liquid in 6 hours showing up on globals and mesos. It's very isolated and variable in placement. But someone could get a surprise with some intense frontogenic banding on Saturday!