Climatological aspects of freezing rain events in the eastern United States

Ice storms are among the most hazardous, disruptive, and costly meteorological phenomena that occur in the eastern United States. Freezing rain creates dangerous travel conditions, produces numerous power outages, and adversely affects local and regional economies. Due to the combined influence of synoptic, mesoscale, and microphysical processes, freezing rain presents a major forecast challenge. Moreover, empirical evidence suggests that operational models and forecasters often underestimate the extent and duration of freezing rain for events featuring frozen, mixed, and liquid precipitation types. In consideration of these societal impacts and forecasting issues, we are developing a long-term (1975–2010) climatology of cool-season (Oct–Apr) freezing rain for first-order stations within the eastern U.S. The results of this climatology will be applied toward: 1) determining the synoptic and mesoscale environments conducive to significant freezing rain events in the eastern U.S., and 2) developing forecast methodologies to improve the prediction of major ice storms.

The freezing rain climatology is constructed from NCDC's Integrated Surface Database (hourly surface observations) and NCEP/NCAR reanalysis data (upper-air analyses). First, we evaluate the monthly and seasonal distributions of freezing rain variability for first-order stations east of the Mississippi River, excluding those in Florida and in Illinois and Wisconsin (freezing rain events in the Midwest are generally unrelated to freezing rain events in the NWS Eastern Region). Geographic subregions are then defined based on spatial patterns in the seasonal distributions of freezing rain. Next, we examine the dynamic and thermodynamic fields associated with freezing rain events in each subregion. By identifying predominant and consistent features among these fields, we can infer how large-scale circulations, thermal boundaries, moisture transport, and quasi-geostrophic forcing establish synoptic environments favorable for freezing rain. Although freezing rain typically occurs under particular synoptic settings, mesoscale processes such as terrain–flow interactions, cold-air damming, and frontogenesis ultimately determine the location, evolution, and duration of freezing rain events. Therefore, substantial emphasis is placed on diagnosing prominent mesoscale features and evaluating how they modify local environments during freezing rain events. The anticipated findings will build upon existing conceptual models and provide operational forecasters with increased awareness of the synoptic and mesoscale processes that influence the initiation and evolution of freezing rain events in the eastern U.S.