18z Model Guidance

[eduggs]

Is this model run even possible? The low literally swings across the mountains to the coast, I don't even see a secondary!

As other have said. Not likely to happen. In very general terms, the likely scenarios are a wrapped up inland solution or a weaker, squashed low sliding NE along the coast. When you average clusters of those polarized solutions you get something like what the ensemble mean shows.

The center of low pressure forms and tracks wherever the surface convergence/upper level divergence is strongest. The terrain features and coastline play some role in this as well. But the center is not likely to jump to the coast unless the energy digs much further S and E or the longwave trof remains flat and positively tilted for a longer duration.

[earthlight]

I definitely thought about this whole de-amplification we saw with the last 2 events, and it shouldn't be ignored. Granted that doesn't necessarily mean anything for us in terms of snow, but it is what is probably preventing this (if it indeeds occurs more like the 18z GFS says) from amplifying, phasing with the PV and giving chicago a blizzard. earthlight, is there anything in particular on the 18z GFS that shows why the s/w digs and then kinda meanders east before finally climbing the shoreline? (esp wrt to the 500mb setup because i have been watching the trends and that low between greenland and labrador/newfoundland has been trending further south and stronger)

The two main factors that seem to be favoring the de-amplification on the 18z GFS are the weaker ridge on the west coast, and the Polar Vortex being further north in Central Canada. That being said--the Polar Vortex remains in a very poor position for anything frozen-precipitation wise on the East Coast. Also, the model run just seems weaker and less amplified with the shortwave over the Central US, which leads me to believe there could be some "classic" 18z GFS bias coming into play here. We will have to see, but if you ask me it's absolutely asinine how people are declaring this system to be completely dead as far as potential goes in our area at 130+hours away. The fundamental pieces that cause the system to develop and track the way it does aren't even in a good RAOB area yet. The ensemble agreement and pattern recognition suggests an inland track is more likely, that's about all we can say at this point.

[Billabong]

here is the 18z dgex accum 24hr precip from hr 144 on when the cold front and freeze line is over eastern half of nj, this is all frozen precip from atleast del river west

Accumulation maps have 3-4" for Philly and Trenton, 4-5" for Allentown, and NYC is on the 2" line. Jackpot is from Scranton north and east into New York state... which has the scale maxed out at 18"+.

[eduggs]

Look at how far south the surface low tracks on the P004 GFS ensemble member before ultimately taking the perfect track to hammer the metro corridor. It moves through the deep south! That's very unlikely to happen. I just don't see how we end up on the cold side of this with the antecedent flow out of the south at most of the critical levels.

Just a frustrating setup.

[tombo82685]

Look at how far south the surface low tracks on the P004 GFS ensemble member before ultimately taking the perfect track to hammer the metro corridor. It moves through the deep south! That's very unlikely to happen. I just don't see how we end up on the cold side of this with the antecedent flow out of the south at most of the critical levels.

Just a frustrating setup.

i dont see it that far south either, but a low over ark through tenn is plausible. Its all coming down to when its phases in my mind. Also in a lot of those snowier solutions the charge of cold air south is stronger after the clipper passes which im not sure how.

[tombo82685]

here is the 18z snowfall map post storm. You can obviously pick out lake effect precipitation. Basically, it shows for the cities to get snow you need about anther 150 mile shift east to aquire some snow from this

[eduggs]

i dont see it that far south either, but a low over ark through tenn is plausible. Its all coming down to when its phases in my mind. Also in a lot of those snowier solutions the charge of cold air south is stronger after the clipper passes which im not sure how.

Yeah, if the "clipper energy" were further south that would help. For example if the strongest vorticity and sharpest kink in the flow were associated with a more southerly geopotential height line, the associated surface low could gain more easterly longitude. Then the whole thermal gradient might not retreat north so quickly and we might benefit from the high building south with backside CAA. But I don't see any of this happening. The clipper provides no help whatsoever.

The other chance is if the developing trof develops a closed circulation much further south. Most of the guidance shows a fairly healthy and elongating trof approaching the Ms valley, but the center of mid-level circulation is too far NW. If we could form a pronounced 2nd circulation down closer to the base of the trof as it goes negative near the Atlantic coast, cold air could be pulled back into the mix. The trof does show signs of amplifying and pinching southward over time, but the primary center is stubbornly entrenched in a bad spot for us.

[tmagan]

Could see a 'flash freeze' if all of this comes to fruition early next week.

[SnoSki14]

The best we can hope for is some wraparound snows after the storm passes. It's too bad we don't have a strong high pressure system in place, then maybe it wouldn't warm up so quickly or the storm would transfer to the coast. The final solution doesn't look so clear cut though and it's still far enough in the future for things to change. The biggest change I've seen, however, is the power of the cold air coming in after the storm.

If we take the 18z gfs and narrow it down, from Sunday night into early Monday morning, we'd probably have temps close to 50F, temperatures would fall during the day and end up around 30-32 degrees during the peak of the afternoon hours with wind chills probably in the high teens, low 20s. On Tuesday morning, we would see lows at least in the mid teens for the city (maybe even lower), low teens, if not single digits for the western suburbs, and wind chills below zero everywhere. NW NJ, the Poconos and a bit further NW could see lows near 0 to up to -5 below zero on Tuesday morning with frigid wind chills.

The highs on Tuesday could only hold in the mid, to possibly low 20s even for the city with wind chills ranging from 0 to 5 above. The lows would be a little bit warmer for Wednesday morning, but still very frigid.

And just look at the south. We'd be talking about a record cold air mass, I could see lows down to the mid 30s for parts of Miami-Dade County, upper 30s in Miami itself. Everyone north of that is pretty much at freezing or below except the immediate coasts. Highs for most of Florida on Tuesday would be in the low to mid 40s, even some upper 30s in northern Florida and the panhandle. Highs in the upper 40s to around 50 for Miami on Tuesday. This is what the 18z gfs pretty much shows, will it be right, who knows, but it will certainly get a lot colder.

This will be much colder than I had first thought it would be, possibly the coldest temperatures I've seen in December for a while.

[Kaner587]

doubt anyone sees 50 at any point this weekend (NYC and PHL metro and suburbs) and potentially all december (obv except for Dec 1st lol) but agree with the rest of the post. Could def see mid 40's and rain but if the apps cutter track verifies we wont make it to 50 because there isn't as much of a warm surge.