A Technical Discussion on Tuesday’s Snow

We have another November snowfall event on the way for tomorrow – which is pretty unprecedented. It should not be a major snow event, but at the very least it should whiten the colder surfaces in most locations.

Forecast models have been showing a pretty weak disturbance traversing across the country from west to east, projected to pass south of Long Island. There won’t be much in the way of wind with this system because the storm itself will not be strong enough to create much of a pressure gradient with the high pressure to the north. However, despite the weak storm, precipitation should spread throughout the New York Metro region via overrunning – warm air to the south is colliding with cold air to the north, creating lift and precipitation.

18z NAM Precipitation Forecast shows an area of .50 to .75″ of liquid equivalent precipitation. Other models are not as impressive, however.

For the most part, the models are not too terribly impressive with the precipitation amounts due to the fact that this is a weak storm. The 18z NAM, however, was pretty potent with its precipitation, as it shows an area of .50″ to .75″ of liquid equivalent precipitation from the Lehigh Valley, traversing east northeastward through Central NJ, NYC, and Long Island. Other forecast models show about half of this amount of precipitation, meaning the NAM is probably a bit too robust, but they have been trending towards showing more precipitation.

Notice the strong cutoff in precipitation just to the north. This represents an important atmospheric phenomenon that would help to generate lift and precipitation: frontogenesis. This is quite important because without a good source of frontogenesis, there will not be enough lift to support an area of heavy precipitation. We’ll first take a look at a few forecast soundings, and then analyze the frontogenesis.

18z GFS Sounding for Newark, valid for 2pm tomorrow. It does not show enough precipitation to cool off the temperatures enough to support accumulating snow.

Here is the 18z GFS forecast sounding at EWR, valid for 2pm tomorrow. As you can see, temperatures in the mid levels of the atmosphere are clearly cold enough for snow, with even plenty of wiggle room in that department. However, if you take a look at the temperatures closer to the surface, they jump above 0 degrees Celsius, which would make it hard to support accumulating snow. That being said, this does not seem to be a rain sounding, unless precipitation is very light, since the mid level temperatures are quite cold. This would support non accumulating snow, mixed with some rain.

So, why the warmth at the surface? If you take a look at the dotted black line, which is the dewpoint, you can see that there is a bit of separation between the dewpoint and the temperature, indicating that the atmosphere at the surface is not completely saturated. This is indicative of lighter precipitation rates. The heavy precipitation rates are important because they would support evaporational cooling of the atmosphere, as the temperature would fall towards the dewpoint in the presence of heavy precipitation.

Also notice the dotted black lines higher up in the atmosphere. It appears to be quite dry around 550mb and higher, which would seem to be concerning. However, snow flakes grow most efficiently in the layer that is between -12 degrees Celsius and -18 degrees Celsius — saturation in this layer supports the growth of snowflakes called dendrites, and dendrites can accumulate quite efficiently. The -12 to -18 degree Celsius layer actually appears to be pretty saturated, though not completely saturated. It would still suffice for decent snowflake growth — maybe not all dendrites, but still good snowflakes that would accumulate pretty readily assuming they could reach the ground.

18z NAM Sounding for Newark, valid 2pm tomorrow. It shows temperatures cooling off to around freezing, supporting accumulating snow.

Now let’s take a look at the 18z NAM sounding for Newark, valid 2pm tomorrow. Once again, the temperatures in the mid levels of the atmosphere clearly support snow. However, taking a look at the surface paints a much different picture from the GFS. As you can see, the temperature and dewpoint contours are overlapping through the surface, indicating a completely saturated column. This is an indicator of heavy precipitation, which helps to evaporationally cool the temperature to at or just below freezing. This would support accumulating snow.

Also, the aforementioned -12 to -18 degree Celsius layer — while not completely saturated — is pretty close to being saturated and would support good snowflake growth, as well. Ideally, you’d like to see this layer completely saturated, but as said before, this would certainly suffice for pretty good snowflake growth.

So obviously, it seems pretty apparent that the rate of precipitation will help to determine if the snowfall we see would accumulate. The November 7th nor’easter was quite a strong storm which helped create heavy bands of precipitation. Unfortunately for snow lovers, we do not have that same luxury. We are going to have to rely on an area of frontogenesis developing to promote the lift necessary for moderate to heavy precipitation.

Frontogenesis represents an increasing temperature gradient over time. If at any given level or layer, you have warm air advection coming from south to north, and there is cold air advection coming from north to south, those areas of advection will “collide” at that spot, helping to increase the temperature gradient. This increasing temperature gradient promotes atmospheric responses that lead to enhanced lift. It is these areas of frontogenesis where the heaviest precipitation is expected.

18z NAM at 850mb shows cold air advection to the north and warm air advection to the south, which supports frontogenesis.

Taking a look at the 850mb map valid for 2pm tomorrow, with temperatures, heights, and winds, you can see the process pretty well. There is plenty of cold air to our north, and winds at 850mb in these areas are blowing northeast, which helps to advect that colder air towards our area. However, you can see that to our south, the winds at 850mb are more southerly, which helps to advect warmer air towards our area. When the wind barbs are perpendicular to the isotherms, that is when the most efficient advection is taking place. It is not quite parallel to the north, though it is not parallel, either, so cold air advection is moderate. But the wind barbs are perpendicular to the isotherms to the south, which supports sufficient warm air advection, thus an area of frontogensis when combined with the cold air advection.

18z NAM Model Frontogenesis Forecast, valid for 3pm tomorrow.

Now, let’s take a look at the visual frontogenesis forecast. This 18z NAM forecast valid for 3pm tomorrow shows a sufficient — though not off the charts — area of frontogenesis. And not coincidentally, it is located in the exact same areas that the heaviest precipitation is being forecast! If this were to come to fruition, then snowfall amounts could be maximized in the 3-6″ range, even near the coast. The ocean will have some moderating effects to the temperatures, but not as much as people might think. This is because since the storm system is so weak, the winds are quite weak, meaning there is no strong onshore flow that would warm the coastal regions too much. Thus, as said before, the main determining factor in the temperatures will be the precipitation rates.

The big question of course is whether the NAM is overdoing the effects of frontogenesis, considering how weak the storm is to begin with. However, models with lower resolution like the GFS might not see the frontogensis fully, meaning it could be too conservative with its precipitation rates and amounts.

Another good indication of the lift over our area is the fact that there is little frontogenesis just to our north. When there is a sharp cutoff in precipitation and lift just to the north, that is usually an indicator of an area of subsidence – or downward vertical motion. The atmosphere tends to like to balance that out with an area of enhanced lift near-by. The balancing area of lift happens to be located in the NY Metro area, which also partially explains the lift sufficient for moderate to heavy precipitation in our area. This phenomenon is called frontogenic banding, and it is often very prevalent in overrunning storm systems.

An additional thing snow lovers have to be concerned about is if the weak flow takes on more of an easterly component. If the winds have more of a northerly component, the amount of fetch over the water is quite low, meaning the marine layer tint would be low. However, a slightly stronger system that will be to our southwest and south for a time would promote a ENE flow, providing a longer fetch of Ocean and Sound water for coastal regions.

We’ll have a better idea as to what to expect after tonight’s model runs come out, so stay tuned.