'Composite Analysis of Snow-Water Equivalence Ratios in Great Plains Mid-Latitude Winter Cyclones'

Great Plains mid-latitude winter cyclones encompass a wide range of hazards, most notably the potential for heavy snowfall. This potential is partially dependent on the snow-water equivalence ratios of a cyclone. For example, a low snow-water ratios would result in a heavy, wet snow which could down power lines and tree branches creating difficult travel conditions. In contrast, a high snow-water ratio results in a light, dry snow which could be blown around and cause blizzard conditions. This study utilizes 6-hourly snowfall data from 10 different mid-latitude winter cyclones across the Great Plains from 2012 to 2016 to illustrate how snow-water ratio variations take place during the maturation of a cyclone. Snowfall data are typically taken every 24 hours, with the long period being the basis for snow-water equivalence during a cyclone. In utilizing 6-hourly snowfall data, a more accurate representation of snow-water equivalence ratios throughout a storm’s evolution can be made, including the monitoring of when the snow-water equivalence ratios shift moisture content. This study is one of a few of its kind to utilize the 6-hour snowfall data in order to track how snow-water ratios change as a cyclone matures.

The majority of the low pressure systems would be classified as Colorado lows moving across the Great Plains. A standardized, composite cyclone based on the location of the center of the low pressure system shows that snow-water equivalence ratios are lower in the genesis phase of the system and increase until the decay of the system. The program locates the center of the cyclone based on pressure values and centers the lowest pressure of each cyclone over a single data point in the center of the plot. The snow-water equivalences are then plotted around this center point and averaged between all the cyclones, making for an easy visualization of the composite cyclone. Other factors are also plotted, such as temperatures and moisture content, to determine when the cyclone enters the mature and occluded phases for the analyzation of snow-water equivalences at these times.

Snow-water equivalence ratios for locations relative to the center of the low pressure system are also analyzed, with the cool sector of the Norwegian Cyclone Model having the lower snow-water equivalence ratios and the cold sector of the Norwegian Cyclone Model having the higher snow-water ratios. In both situations, the snow-water ratios indicate that as the system matures and moves through a region the snow-water ratios also change, becoming higher and producing a drier snowfall through the precipitation process. Understanding where high and low snow-water equivalence ratios are relative to the center of the system is necessary to anticipate potential hazards as the cyclone progresses.