Significant snowstorm likely near the coast on Saturday

The rollercoaster with the model runs has continued today, with regards to Saturday’s winter storm. However, for the most part, guidance continues to trend west, and there are some reasons to believe that will continue. There is still a limit as to how far west this can go, though, and the cutoff to the west looks pretty sharp, so this may only be a “major” snowstorm for Eastern Long Island. Still, it’s looking like a snowier Saturday than first thought a few days ago.

Generally speaking over the past few days, the models have become much more impressive with the strength of the initial shortwave that entered the US from the Pacific. It’s now in the southern stream and tapping into a lot of Gulf of Mexico moisture, leading to some strong thunderstorms in the deep South. Sometimes what happens here is that once the models realize that there is plenty of convection going on, the latent heat in the atmosphere increases, which leads to higher heights along the East Coast and thus further northwest shifts. This appears to be happening in today’s model guidance.

Even just looking at this afternoon’s NAM model shows the obvious trends. Look at how much more amplified and consolidated the shortwave in the South was compared to the 12z run. It’s a domino effect/feedback — stronger shortwave = higher heights = more Gulf of Mexico moisture tapped in = more latent heat release = higher heights again.

What’s also very interesting and personally my favorite feature for snow is the big improvement in the strength and orientation at the onset of the storm and during the storm. Whenever you see a massive jet streak to your north, you are usually placed in what we call the “right-entrance” region, which is the right or east side of where the jet begins. In this area there is enhanced upper-level divergence, which is a good indicator of a lot upward vertical motion and lift throughout the column.

The jet streak trending stronger of course would increase the strength of the right-entrance region and thus provide more lift. The expansion of it and its northwest shift also allows more expansion of that lift further northwest, which allows for accumulating snow to make it west of NYC, where it hadn’t before. It’s very important to have this trend occur early as it pertains to snow, because early Saturday morning is when the storm and precipitation are still well to the south. We need to establish reasons to expand the lift and precipitation towards our latitude and longitude before the storm heads too far east. These trends help that a lot and increase the confidence in accumulating snow, or even a significant snow for some.

As we head during the peak of the storm, there is another secondary jet streak that peaks and moves over our area. This one is important for preventing the low pressure from escaping east. If this jet streak is more meridional and stronger, that means there has to be plenty of lift just off the coast, rather than further east. Naturally, the lift lowers surface pressure, which forces a surface low pressure to track towards that right-entrance region, and allow moderate snow to pivot into the area, with heavy snow for coastal regions.

There are still a couple of factors that limit the overall ceiling for this storm. The animation below shows that while the 500mb energy is very amplified, takes a negative tilt, and has a pretty good consolidation, there is also some vorticity that runs out ahead. That leads to some positive vorticity advection offshore, which may lead to lift and lower pressures also getting focused offshore — fortunately for snow lovers the jet streak looks mostly strong enough to combat that somewhat. But the bigger problem is that compared to the impressive 500mb trough, the 700mb trough is much flatter and the shortwave energy is more elongated. That may help slip the surface low a tad east. On the bright side for snow lovers, this 700mb energy has trended a bit sharper and consolidated in the past couple of runs.

So because of the above, we may not truly get into any very heavy snow bands with extremely low visibility. What may happen instead though is because of how strong the jet streak is and how expansive strong lift will be throughout the mid and upper levels of the column, we may get strong lift in a large dendritic snow growth zone. The dendritic growth zone will probably be a large layer between 700mb and 500mb — so above the 700mb layer where we have less shortwave energy. This means we can get a lot of lift and saturation within the dendritic snow growth zone, which will produce ideal snowflake textures and fluffy cotton balls that can accumulate at a 15 or 20:1 snow-to-liquid ratio. But because of most likely less lift at 700mb and below due to the aforementioned less shortwave consolidation there, the lift does not expand for the entire column, so snowfall rates only end up being moderate instead of truly heavy. However, with the ideal snowflake formation, you can still get 1″/hr snowfall rates at times even with only moderate snowfall intensity, but the potential for a truly high-impact widespread major snowfall gets somewhat limited.

Overall, these trends indicate that it’s still very much possible that NYC and immediate suburbs can get a 6″+ snowfall, with the Jersey Shore and Nassau County getting in excess of 8″, and Suffolk County getting in excess of 12″. A good chunk of Connecticut could also still get 6″+. NYC suburbs further to the west and north would get less snow. It’s even possible that the ceiling is even higher than the these amounts, if the 700mb energy can sharped up a bit more which may actually lead to some heavy snowfall rates.

But we are still not necessarily at a point where we’re confident in that. For now, a more realistic scenario could be 3-6″ in NYC, 6-8″ in Nassau County, and most of the Jersey Shore, and 9-15″ in Suffolk County and the SE NJ Coast. Areas further west and north may see 2-4″, but it could drop off pretty sharply west of Morris County.