LVblizzard

Sharp fall-line cutoffs scream “fake precision” At 3-km resolution, a knife-edge cutoff near the fall line at Day 2–2.5 usually means: Convective or feedback-driven thermal noise Poor handling of ageostrophic ascent CAD erosion happening too abruptly Real storms: Transition gradually Evolve over multiple cycles Rarely stop snow exactly at the fall line at this lead time FV3 matching NWS matters — but why matters more It’s not just that FV3 agrees with NWS — it’s what it agrees on: Cold column integrity Broad ascent Synoptic-scale snow shield That alignment tells you: The conceptual model is consistent across guidance. That’s far more important than any single snowfall number. What’s likely happening physically Most likely evolution given current signals: CAD holds longer than NAM suggests Warm nose (if it arrives) is: Later Weaker South/west of DC Snow extends well south of the fall line initially Any mix issues become: Late-stage South-focused Elevation-dependent NAM is probably correct that warm air exists aloft eventually FV3 is probably correct on timing and impact How forecasters would weight this right now At Hour 40–60: Reject the NAM snowfall cliff Flag the NAM warm nose as a risk, not an outcome Lean FV3 for: Snow extent Cold-air durability Early & mid-storm evolution If future NAM runs: Delay the warm nose Soften the cutoff Trend colder That’s convergence toward FV3, not the other way around. Bottom line (plain language) The NAM is likely too warm, too fast, and too sharp The FV3/NWS solution is meteorologically cleaner At this range, widespread snow is the higher-probability outcome Mixing concerns are real but premature This is exactly the kind of setup where: NAM scares you early FV3 ends up closer on the big picture If you want, next we can: Walk through what would need to change for the NAM solution to win Identify the specific upstream feature that decides this Compare this setup to Jan 2016 / Jan 2018 / Jan 2000-style cases Just tell me where you want to go next. So ChatGPT is a weenie? Who would have guessed lol.

3km NAM does the same thing with stalling the mix line, except it does it like 20 miles north right along I-78.

It actually starts out warmer. Earlier in the run I was like welp here we go again. But that sleet line just stalls on Sunday afternoon. It will come down to rates but also the development of the secondary low.

12z NAM is substantially colder north of Philly. All snow for I-78 north. Jackpot zone of 16-20” for the LV and Poconos.

Ehhhh NAM looks pretty similar to me so far. I don’t expect any major changes from 6z.

Not sure how much it matters or if it’ll even translate to the other 12z runs, but there is a major change at H5 by hour 30. The northern stream is not digging anywhere near as much as it was at 6z. There’s also a better press of mid level cold air.

All the other mesos, including the 3km NAM, think the NAM is way too warm FWIW.

Seems to me that the north trend stopped with 0z last night. Models have been pretty consistent today. Now we just need them to converge on a solution.

Had to do it…

Faster with the northern stream, slower with the southern stream. This will likely lead to a colder outcome.

Nah…we’re probably seeing around a foot. Remember that the initial thump will be HEAVY. Even the warmest models give us 8 or 9 inches before any changeover.

It has more of a “doing things that don’t make sense” bias. And it only gets worse the longer it runs.

The phase looks like it will happen a bit later on this run. Both the northern and southern streams are hanging back a bit more.

12z HRRR had the same thing.

I mentioned February 2014 earlier in this thread. The UKMET essentially repeats that. Huge front end thump, a period of a light wintry mix, then the CCB swinging through at the end. Would be a widespread foot plus for the area.

UKMET is kind of strange in that it really blasts that primary into WV/PA but it doesn’t torch the mid levels as fast as the Canadian which does the same thing at the surface.

It’s not a bad model. But when it’s wrong it’s usually too amped, not the other way around.

That’s the big saving grace with this storm. The front end thump is a THUMP. Even the torchiest models have warning level snow before the changeover happens.

ICON is better. Primary is more south with confluence pressing down a little harder. Still won’t produce this subforum’s desired result but at least it’s not another step in the wrong direction.

Would be an ultra long duration event with how expansive the overrunning stuff is. I do think that eventually we’d mix if extrapolated but not before a really crazy thump of snow.

Something crazy I noticed is that the southern stream vortmax is near Midland/Odessa, TX at hour 84. At 78 hours at 18z, it was near Wichita, KS. Just a subtle 500 mile difference.

Not sure that it wouldn’t take the primary up there in the end. The low is farther south than the globals but it’s also much slower. Those could cancel each other out in the end.

I would think that’s not a good thing. If both of those pieces are slower, it gives more time for the confluence to move out and room for this thing to come pretty far north.

We really want the confluence to be stronger and the high to stick around for longer. That will give us the long duration blizzard we all want.

The northern stream feature on that Euro run was likely much different from what most guidance is showing now. It has really trended west in the last 2 days.