Snowfall NYC subforum Jan 6 and OBS if needed

[Allsnow]

1 minute ago, Stormlover74 said:

Still early in the run

Snowing in Chicago this run 

[Allsnow]

Way north. Going to look like icon 

[RU848789]

8 hours ago, RU848789 said:

Interesting that the NBM snowfall forecast bumped up for 07Z vs 01Z last night and yet the NWS-Philly is clearly downplaying the event potential north of Philly, as per the part I bolded below, when they often follow the NBM for snow events.  Maybe they're going by the WPC precip forecast and not the NBM snowfall forecast...

@MGorse- any comments on this apparent disconnect?  Just trying to figure out what you guys are thinking, thanks...

National Weather Service Mount Holly NJ

401 AM EST Thu Jan 2 2025

LONG TERM /SUNDAY THROUGH WEDNESDAY/...

Guidance from a synoptic standpoint is depicting a Miller B
nor`easter to develop with an upper level trough swinging a
surface low from west to east across the KY/TN area before
redeveloping as a low pressure system offshore Monday. Ensemble
guidance is split into 2 different clusters with the variance
generally explained by how strong the upper level ridge will be
over Quebec. This boils down into two scenarios, one with a more
northern track bringing snow towards the RDG/TTN/PHL, and one
with a more southern track keeping the snow more over the DC
area. The more northern stream track relies on a weaker high and
thus develops a bit more cyclogensis over our region thus
lifting the band of 2-4 inches of snow further north, whereas in
the southern track, the high is stronger and displaces the
surface low to the south more leading to most of the area seeing
little in the way of snow.

01Z/02 NBM generally is taking a blend of the two scenarios thus
leads to probabilities similar to the 13z/01 NBM with snowfall
greater than 1 inch is 60 to 80 percent across southern New
Jersey, the Delaware Valley including Philadelphia, and the
eastern shores of Maryland and Delaware while the probability of
snowfall greater than 3 inches is 40 to 50 percent. North of
Philadelphia, probabilities lower to 30 to 50 percent for
greater than 1 inch of snow and 20 to 30 percent for greater
than 3 inches of snow.

For the time behind will continue to show a blend of the two
tracks but anticipate that the gradient of snowfall will sharpen
quickly somewhere north of Philadelphia over the coming runs
depending on how guidance handles the upper level ridge. While
it is still too soon to tell how much snow will fall, the trend
looks to be for advisory level snowfall for Delmarva, sub-
advisory amounts for southeast New Jersey and the Delaware
Valley, and minimal amounts north of Philadelphia.

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.

 

[LVblizzard]

Just now, Allsnow said:

Way north. Going to look like icon 

I wouldn’t say *way* north but it is a pretty decent sized jump.

[Allsnow]

1 minute ago, LVblizzard said:

I wouldn’t say *way* north but it is a pretty decent sized jump.

Sweet spot now between nyc and phl 

[EastonSN+]

Seems we need the primary to hang on longer

[Allsnow]

[Stormlover74]

[Allsnow]

[brooklynwx99]

i think we know where this is going

[RU848789]

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...

[ForestHillWx]

A 996 headed for the benchmark? Wasn’t this snowing in Georgia last night?

[WestBabylonWeather]

Just now, brooklynwx99 said:

i think we know where this is going

Boston? 

[EastonSN+]

Going to be an interesting Euro run

[EastonSN+]

3 minutes ago, WestBabylonWeather said:

Boston? 

LOL uh oh we started the north trend a day too early.

[SnoSki14]

Just now, EastonSN+ said:

LOL uh oh we started the north trend a day too early.

There's a limit to how far north this can go, confluence is very strong 

[EastonSN+]

Just now, SnoSki14 said:

There's a limit to how far north this can go, confluence is very strong 

Yeah I was joking on that one. Will be interesting to see what the ensembles say

[bluewave]

All we need is a slightly stronger transient Southeast Ridge separating the two systems on Sunday for NYC to go from a C-2” to 2-4”type event.

New run 

Old run

[jm1220]

Good step in the right direction. 

[Allsnow]

Just now, jm1220 said:

Good step in the right direction. 

You just said it was over 

[SnowGoose69]

Just now, bluewave said:

All we need is a slightly stronger transient Southeast Ridge separating the two systems on Sunday for NYC to go from a C-2” to 2-4”type event.

New run 

 

Old run

 

As forky pointed out the other day too, there is going to be convection for sure with this in TN Valley which might have a downstream impact on track too.  It likely also will totally shaft someone on QPF across IL/IN/OH somewhere.  Thankfully we never have that problem here

[Allsnow]

4 minutes ago, bluewave said:

All we need is a slightly stronger transient Southeast Ridge separating the two systems on Sunday for NYC to go from a C-2” to 2-4”type event.

New run 

Old run

This is why I said a few days ago a southeast ridge would help us in this setup 

[jm1220]

2 minutes ago, Allsnow said:

You just said it was over 

Without significant changes and unless models other than the Icon jump on board.  

[Dark Star]

9 minutes ago, EastonSN+ said:

LOL uh oh we started the north trend a day too early.

 

8 minutes ago, SnoSki14 said:

There's a limit to how far north this can go, confluence is very strong 

That's a good point.  I didn't realize how narrow the band of precipitation is supposed to be.  It would require NYC to be in the bullseye or any type of accumulation in this region?

[Franklin0529]

8 minutes ago, jm1220 said:

Without significant changes and unless models other than the Icon jump on board.  

GFS also came North 

[EastonSN+]

Think the gefs came a bit north 

[SnoSki14]

20 minutes ago, bluewave said:

All we need is a slightly stronger transient Southeast Ridge separating the two systems on Sunday for NYC to go from a C-2” to 2-4”type event.

New run 

Old run

Rare instance where SE ridge actually helps us

[winterwx21]

The trend is our friend, despite that slightly disappointing Euro run. 

[EastonSN+]

1 minute ago, winterwx21 said:

The trend is our friend, despite that slightly disappointing Euro run. 

The 18z euro is out?

[MJO812]

Gefs has some really amped members