General Snow Forecasting

Snow is a precipitation type that results from the build-up of ice deposition upon condensation nuclei. The snowflake size is amplified as ice crystals join together as the crystals move in the wind. A common theme in ice crystal development is the 6-sized structure of the ice crystals. The chemical bonding of ice molecules produces the common 6-sided structure. It can be heard that no two snowflakes are alike. This is because the crystals combine together in a variety of ways. However, upon close examination, the individual dendritic ice crystals will generally be 6-sided. Each snowflake is an aggregate of the 6-sided crystals joining together.

Snow is perhaps the most interesting precipitation type. It slowly falls to the ground and can accumulate on the ground if the ground is cold enough. Snow that accumulates on the ground will produce a variety of aesthetic wonders and can also cause all sorts of headaches, especially to travelers.  Who is not fascinated by the snow day or at least is cognizant of the white layer that blankets the ground? This article focuses on what meteorologists look for when developing a snow forecast and the physical processes that lead to the development and accumulation of snow.

Historic snowstorm in North Texas on 2/12/2010. DFW recorded its greatest 24-hour snowfall of all time (12.5").
Historic snowstorm in North Texas on 2/12/2010. DFW recorded its greatest 24-hour snowfall of all time (12.5″).

Three ingredients need to be in place in order for snow to occur and they are lifting, moisture and a temperature profile that supports the snow reaching the ground surface. Lifting is the gradual rising of air that is produced from lifting mechanisms such as a low level convergence axis or the evacuation of air aloft. Rising air in conducive to cloud developing since it causes the air to cool which in turn increases the relative humidity of the air. Once the air rises enough, the air will become saturated and then further lifting will cause the water vapor in the air to change state. For the case of snow development, the water vapor will ultimately produce ice crystals.

Moisture is a general term meaning there is preexisting water vapor in the air that is dense enough that even gentle lifting will be able to saturate the air and produce precipitation. When the air is referred to as dry, meteorologists are often referring to the lifting not being significant enough to be able to saturate the air. The drier the air, the stronger the lift needs to be in order to saturate the air. The presence of moisture is often noted as significant dewpoints that are close to the temperature and air that has a high relative humidity. Moisture and lift go hand in hand in producing precipitation. If either one is weak, the other will need to be very significant in order to produce precipitation.

The third ingredient is the temperature profile. This factor can range from easier to diagnose to incredibly challenging. The easier cases are when the temperature profile is well below freezing throughout the troposphere. This will lead to snow when precipitation is generated. Even in this easier case, there are complicating factors such as the temperatures within the snow growth region, the snow to liquid equivalent forecast and the expected intensity/duration of the snow that all add complexity. The more challenging cases occur when layer(s) of the troposphere are near freezing. This adds the additional complexity of having to determine the precipitation type. The chance of an accumulating snow can be lost if the temperature structure aloft does not support it or when the ground temperature is too warm. Next, we will take a look at snow types and the temperature structure that leads to them. This is important since it has a significant impact on what is experienced at the surface when snow occurs.

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Snow Temperature Profile

The characteristics of snow experienced at the surface will depend on the intensity of uplift and the temperature within the snow growth region. Three of the general structures that snow crystals can have are plates, columns and dendrites. Dendrites are optimum for accumulating snow since they interlock so well creating air pockets within the snow crystals. The optimum temperature range for dendritic snow growth is between -12 C and -18 C (approximately between 10 F and 0 F). Plates and columns do not interlock as well which reduces accumulation when temperatures are warmer or colder than the optimum range. Forecasters look for the region in the troposphere with the greatest uplift since this will help determine the characteristics of the snow.

The greatest uplift is typically found aloft in the middle levels. An uplift mechanism promotes low level convergence (air piling together) and upper level divergence (air spreading apart). The middle levels is the transition zone between the air coming together and the air spreading out and thus the vertical component of air flow tends to be greatest here. The importance of this is that the temperature profile near the surface and the temperature profile in the upper levels will not greatly influence the snow characteristics. An exception to this is melting or partial melting of snow that can occur as snow falls near the surface when temperatures there are above freezing. A forecast sounding combined with model data can show a forecaster the general region aloft with the greatest uplift and the location of the range of temperatures for optimum dendritic snow growth. Strong uplift within the optimum dendritic snow growth region with subfreezing temperature throughout the lower levels will support significant snow accumulations.

When snow is a possibility, a forecaster will ask several questions related to what could go wrong. Important questions to ask include: Could the uplift not be strong enough to support snow? Could the best uplift not be in the optimum snow growth region? Is the temperature structure too warm to support snow (melting layers aloft)? Is the ground temperature warm enough to allow significant melting of the snow? Since there are several variables to consider, it is important for a forecaster to consider everything that could go wrong when it comes to a snow event not materializing as well as it could have. There is much more confidence that a snow event will occur when the forecast is a short term forecast (2 days out or less) and the answers to all the questions combined support a significant snowfall.

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