A Winter Weather Climatology for the Southeastern United States
Christopher Fuhrmann and Charles E. Konrad, II
Department of Geography
The University of North Carolina at Chapel Hill
Deepening events are defined by the occurrence of a precipitation type transition indicative of a thickening wedge of cold air near the surface. Examples of these precipitation transitions include rain to snow, rain to freezing rain, sleet to snow, etc. Deepening events are most common along the windward slopes of the Appalachian Mountains and the adjacent plateau (greater than 36% of all events) and least common across central portions of the region (less than 20%). Deepening events along the windward slopes may be tied to adiabatic cooling due to orographic lift that occurs during the latter portions of storms as winds shift to the northwest and advect cold air into the area. Roughly 90% of these events involve a rain to snow transition with minimal freezing rain and sleet accumulations. Their infrequent occurrence immediately east of the mountains may be related to the downslope warming of this cold air or the slow movement of the dense, colder air across the mountains.
Shallowing events, defined by the occurrence of a precipitation type transition indicative of an eroding cold air wedge, are less frequent across the region (ranging from 10 to 20%) and most often involve a transition from snow to rain. They occur with the greatest frequency along the coastal plain where an onshore flow of relatively warmer air often develops later in the event, eroding the existing cold wedge. The shallowing event maximum in central Virginia is tied to a relatively large number of events that begin as snow and then transition to freezing rain. Frozen precipitation is more likely in this area because the cold wedge is typically the thickest. It is unclear what is responsible for the relative maximum across South Carolina, although a thicker cold wedge may be responsible near the mountains.
Each map below shows the most common precipitation sequences and their percentage out of all possible sequences observed at six sampled stations across the Southeast.