On January 11-12, a potent storm system brought heavy sleet and snow to parts of the southeastern United States. Preliminary reports from the National Weather Service offices in Memphis, and Paducah indicated snowfall totals of between 1 and 4 inches with some locations receiving up to between 6 and 7 inches across northwest Tennessee, and west Kentucky. The meteorological term, deformation zone, is solely responsible for the excessive snowfall totals. These deformation zones are common across parts of the southeastern United States, and can ultimately produce large snowfall totals.
A deformation zone is an area of converging air, whereby atmospheric stretching occurs. This stretching is often a result of multiple air streams flowing toward each other, then subsequently diverging, or spreading out after they converge. Under the deformation band, heavy precipitation can fall resulting in heavy snowfall rates and totals. The enhanced precipitation is a result of increasing divergence aloft, and an increase in the temperature gradient, a process referred to by meteorologists as frontogenesis. The deformation zone is usually found to the west or northwest of the area of surface low-pressure. They are usually found with an orientation of north-to-south or northeast-to-southwest. An important challenge to meteorologists is forecasting the location of these deformation bands, and associated snowfall amounts within them.
In the early morning hours of January 12, an area of surface low-pressure developed under favorable upper level dynamics across central Mississippi. This area of low-pressure moved northeast into eastern Tennessee and Kentucky before crossing the Appalachian Mountains. As the surface low was in the processes of forming, precipitation was ongoing across parts of the southeastern United States. As the colder air mass moved into the area, a graduate transition to freezing rain and sleet occurred. In the mid morning hours on Friday, an eventual transition to all snow occurred as the surface low to the east slowly deepened (i.e., increased in intensity with respect to its minimum central pressure) with time.
As the surface low moved to the northeast, a favorable area of heavy snowfall was becoming increasingly likely over western Tennessee and on over towards west Kentucky as the deformation zone formed. As mentioned above, these deformation bands can produce excessive snowfall rates, and eventual snow totals. That is exactly what happened, as snowfall totals ranged from 4 to 7 inches in the deformation zone. Although, 1 to 2 inches of those storm totals could be in the form of sleet, it goes to show the power of the deformation zone as well as and the associated atmospheric dynamics which are involved.
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©2018 Meteorologist Allan Diegan