A deeper look into the upcoming storm

Although confidence has increased in a coastal storm developing and giving the area plenty of rain and wind from late tonight through Thursday, there are still a lot of details that need to be ironed out. This is because the storm system is not truly a Nor’Easter, it will remain primarily disorganized, and the weather pattern aloft does not necessarily favor a strong storm system. Instead, we have a system that is forming during the climatological peak of the hurricane season, and it will initially be sitting in an area allowing for some tropical development. No, this will not be a tropical storm, but this will be a weak storm system with some tropical characteristics, rather than a true Nor’Easter. That will create a big headache for the computer models, since they tend to be better at forecasting Nor’Easters.

Last night’s NAM Model showed plenty of rain for most of the region, but the storm system itself is very weak.

Note how in the image above, there are surface pressures of 1030mb + sliding offshore of New England, but the storm system approaching is not very impressive. The “Low” Pressure is barely below 1022mb — these pressures in some weather patterns would be a high pressure! Areas of high and low pressure on a weather map are denominated relatively, not absolutely — meaning the lowest pressure in a certain area is the “low”, where counterclockwise motion forms and inclement weather is generally associated, and the highest pressure in a certain area is the opposite. But the fact that our “Low” is so high in pressure is a good indicator that the storm system itself is just not that impressive.

Another indicator going against true storm development is the overall weather pattern in the middle of the atmosphere. Normally, a Nor’Easter forms on the eastern side of a large trough — the trough provides a nice cold air source and plenty of energy to run into the warm, moist air of the storm system in the Atlantic Ocean, creating the battleground that a storm loves. But if we take a look at the 500mb weather pattern, there is no large-scale trough to be seen, at all.

Last night’s NAM Model valid for 8am on Thursday does not show an overall impressive weather pattern for storm development.

The shaded colors do show some vorticity (counterclockwise spin in the atmosphere), but the only large-scale troughing is in the immediate West Coast. So, wait, are we actually getting a storm? Because a weak storm system in an unimpressive weather pattern certainly does not scream one.

This is where we have to dive a bit deeper into the meteorology. As alluded to before, there will be some tropical characteristics with this system, so the moisture content will be quite high. There will actually be some Gulf Stream interaction with this system, which further augments some of the tropical characteristics, and it also leads to the storm system — at least initially — having a lot more convection than pure synoptic-scale forcing. Models tend to really struggle when it comes to convection, as there has been very little continuity with how much rain is forecast, as well as where it will fall. Convection tends to keep lifting for precipitation a lot more localized, but in order for the atmosphere to balance out, there is often an area of subsidence right near-by. Areas of strong lift right next to areas of strong subsidence will create a large forecasting headache — in a classic Nor’Easter, the precipitation tends to be a lot more uniform.

A weak, but developing storm system, some tropical moisture helping to lead to convection, and a strong high pressure system to the north are all we have right now. But the strong high pressures that are moving east will still be able to create an impressive pressure gradient with our weak storm. Furthermore, since the high will be to our north and east, its clockwise flow around it will combine with the counterclockwise flow from the low to the south to help to create a strong low-level jet, pushing bouts of moisture from the Atlantic into our area. Had the high pressure been located much further west, this same combination would not have been possible.

Last night’s NAM Model valid at 8am on Thursday morning shows some strong 850mb winds pushing from the ocean into our area, as well as strong frontogenic forcing.

At 850mb, we see that strong onshore winds will be developing, which will help to push moisture into our area. Additionally, since the source region of these winds is a warm, moist airmass, running into a much cooler airmass, the temperature gradient increases. This essentially acts as a frontal system on a small scale — we call this frontogenesis. Frontogenesis is often an important trigger for lifting and precipitation, and may serve to make up for the lack of a strong storm system.

An important feature that may help to understand the role of convection is the shading, representing omega, which essentially represents upward vertical motion at 850mb. There are strong areas of omega in Southern New Jersey as well as off the coast, but areas of subsidence (blue colors) and potential lulls in precipitation activity. Another area of lift is found again to the north of the the subsidence. This shows the lift –> subsidence –> lift balance that often occurs to a large extent when there is convection involved. Where exactly this occurs will have a major impact as to who seems locally flooding rains, and who sees only a quarter to a half inch of rain.

An even better illustrative example can be seen by upper-level divergence maps. If there is divergence (air spreading out) at the upper levels of the atmosphere, it is usually the result of upward vertical motion beneath it. Thus, large upper-level divergence is a good indicator of potential clouds and precipitation.

Last night’s NAM Model shows extremely impressive upper-level divergence, which is ideal for heavy rain. But look at the gap in the divergence in New Jersey — that will create a forecasting headache.

The above map is early in the storm’s development, but it proves the point well. There is plenty of upper-level divergence, particularly in Maryland and Pennsylvania, which is poised to move into New Jersey. There is also another area of upper level divergence just offshore. But look at the gap in the middle! This illustrates the classic lift –> subsidence –> lift balance once again. How exactly this manifests itself is extremely crucial, but also quite difficult to forecast.

Nonetheless, the fact that the upper-level divergence, frontogenesis, and low-level jet are this impressive does certainly point to the potential for some very heavy rain. Additionally, the strong pressure gradient will allow for persistent, strong east and northeasterly winds gusting to 30mph, potentially creating minor coastal flooding. That being said, since the storm is so weak and is reliant on factors that are hard to forecast, we are uncertain as to quite exactly how consolidated the areas of precipitation will be, who will see the heaviest rain, and how much rain will fall. The potential for a heavy rain and wind maker is certainly there; but for some areas, it might be a lot more on-and-off in nature — as is often the case when some convection is involved — with bouts of heavy rain and then just drizzle, as opposed to prolonged, steady rain.

Because of this, one potential scenario is that the heaviest hits PA and W & S NJ, giving them 2-3″ of rain, but subsidence lingers in N NJ, NYC, and LI, giving them only around a half inch of rain, and the secondary area of strong rain misses everyone offshore. Perhaps the gap between the strongest areas of lift will be smaller, so the further east bout of precipitation could still hit most of the area. The strong areas of rain and the gaps where there is little rain could still easily shift — or perhaps the storm may become a bit more consolidated — we just do not quite know yet.