Through this winter one of the major impacts that has significantly impacted snow fall totals is the development of mesoscale snow banding within the precipitation shield. The impact of such banding has produced staggering snowfall gradients throughout the region and has created significant impacts on many locations this winter.
Mesoscale banding can have several different causes within a winter storm. In some causes, the interaction of moisture advection and the geographic position of mountain ranges can cause strong orographic lifting. In other cases, the structure of the temperature gradient at the mid levels down to the surface can also cause areas of stronger rising and sinking air. Sometimes the structure of atmosphere from the surface up through the 500 MB can also support rapidly rising air in a localized air, which can produce such impacts as thunderstorms within the snow storms.
The process of orographic lifting is basically the interaction of moisture being driven from the Atlantic into the coastal plain and lifting by the Appalachian Mountain chain, specifically the Pocono Mountains. When I look for the potential influence of orographic lifting, I want to keep an eye on two key features. The first is the position of the 700 MB and 850 MB low roughly 100 to 150 miles east of southern New Jersey. The position of the mid level upper lows at this location supports strong moisture advection from the Atlantic right into northeastern Pennsylvania. As the moisture is driven westward into Pennsylvania, the mountains force a natural lifting motion of the low level moisture, thus creating a heavier band of snow. Now, this type of influence can be easily negated by stronger mid and upper level atmospheric motions, so I want to make sure that such a process is not impacted by other mesoscale banding features over New Jersey. I should note that although the Kittatinny Mountains in New Jersey can support orographic lifting at times, the mountain range is usually not steep enough to support mesoscale banding on its own, but can act as an enhancement of snow bands via other processes.
Mid Level Thermal Gradient
We all know that when a surface warm front moves through that low level lifting creates precipitation ahead, over, and at times behind the frontal passage. When a surface low intensifies along the Mid Atlantic coast, the thermal gradient at the mid levels usually tightens as the 850 MB. This tightening of the 850 MB gradient acts like a warm front and creates additional lifting. This tightening of the thermal gradient supports the overall coma shaped precipitation shield. However, it is not the thermal gradient along that creates the mesoscale banding, but the interaction of that thermal gradient with low level jet streams, seen as the green arrow to the left. The low level jet streams drives warmer air and moisture over the progressively colder air mass to the west. This advection, seen to the left, produces a jet of additional moisture and lifting along the coastal plain and thus leads to areas of banding of precipitation. These low level jet streaks can occur anywhere from 950 MB to 750 MB, which can lead to a variety of locations being impacted by these mesoscale bands. Further, the orientation of the jet streaks at various levels also has an influence not only or the intensity of the mesoscale bands but also the orientation of those bands. This nature of such jet streaks are rather unpredictable and thus the need for NOW-Casting.
Atmospheric instability is when the air mass is warm at the surface as oppose to the mid and upper levels. Now, most of the time, the atmosphere is progressively colder as one increases in height. However, it is when this nature of the atmosphere is substantial in intensity that an increase in lifting is present. When I look for atmospheric instability, I want to focus from the surface to 500 MB. In this area of the atmosphere I look for areas where there is a strong vertical thermal gradient that supports drastically colder air aloft compared to the atmospheric temperatures just below. There are various forms of this instability depending on the location of the warmer air. The cases where convective precipitation is the most powerful is when the boundary layer is very cold, supporting a sounding for frozen precipitation, followed by warmer temperatures closer to freezing between 900 MB and 850 MB, and then much colder temperatures above 850 MB. This type of set up is usually found in and around the 850 MB low level jet stream where warmer air is being driven in from the Atlantic and at times Tropical Atlantic. The development of convection over the Atlantic at times can be a key indicator of this process developing as latent heat is released and aids in the development of the 850 MB low level jet stream. The development of such a sounding is driven by what meteorologist call a TROWAL or trough of warm air aloft.
The unstable nature of such a sounding can create intense rising motion in the atmosphere, which leads to precipitation rates of 1 to 4 inches per hour. In some extreme cases, snow rates of 5 to 6 inches per hour have been observed. Significant instability in the atmosphere can also enhance electrical discharges which can produce thunder and lightning within these snow bands.
Deformation zones are very complicated processes in the atmosphere, usually at the edge of an extratropical low pressure system. Deformation zones are areas where the atmosphere is being stretched and sheared in different directions. The way the deformation develops can either enhance frontogenesis or enhance the thermal gradient at the mid levels or can cause divergence at the mid levels and thus sinking air. The process and impacts of deformation in the atmosphere is very complicated and will not be covered here, but an excellent source of information on the process can be found here.
Deformation zones usually form in a position of northwest to southeast or north to south. They can produce very heavy snowfall for a long period of time and can extend the influence of a winter storm for several hours. One of the best cases of deformation zone impacts was from the February 13, 2006 blizzard seen to the left. Note the banded feature from north to south that drove from western New Jersey through New York City, producing snowfall rates of 2 to 4 inches per hour. That is a deformation zone!
Note that as drier air is working in from the west, the low pressure system is driving moisture in from the Atlantic. The low and mid level convergence of the moisture combined with the divergence aloft of the 700 MB and 500 MB pattern leads to the formation of the deformation band and thus the heavy snowfall.
Impacts of all mesoscale bands
All mesoscale bands feature a rising and sinking motion in the atmosphere. While forecasting the impacts of the mesoscale bands is pretty easy, forecasting the exact location of such bands is nearly impossible beyond a few hours ahead of the actual development at best. The impact of these bands can have a significant impact on snowfall totals. The image to the left illustrates the rising motion of the atmosphere under the band and the sinking motion of air surround the band. If air is going to rise rapidly, there must be a sinking motion of the air at another place to keep a balance in the atmosphere or basically a conservation of mass within the atmosphere. This sinking air can create “dry slots” in the precipitation shield as sinking air inhibits the precipitation process. Mesoscale banding is best observed on the radar as rapidly increasing areas of radar reflection or DBZ in one location is followed by rapidly decreasing areas of DBZ in another area relatively near by. The intensity of the lifting in one area is balanced out by the strength of sinking air at another location. This is why in some snow storms one locations may receive a foot or more of snow while an area just down the road may only have a few inches on the ground.
Mesoscale banding of snowfall is a complicated process that is very difficult to forecast for. It is usually a process that is misunderstood by the general public and can lead to some thinking that meteorologist have no clue what the storm is going to do. The best way to approach the forecasting of such environments is to try to explain the threat and the impact of such processes. In convective snowfall there will always be an area that gets significantly less than forecasted, but that is the very nature of the storm itself.