Long-Duration Freezing Rain Events over North America: Regional Climatology and Maintenance Mechanisms

Freezing rain can have devastating impacts, particularly when it persists for many hours. Predicting the precise temperature stratification necessary for freezing rain events remains an important forecast challenge. Latent heat released when rain freezes can warm the near-surface air to 0°C, eventually causing precipitation to transition from freezing rain to rain. Conversely, the extraction of latent heat when snowflakes melt in the above-freezing warm layer aloft can cool the warm layer and potentially change freezing rain to ice pellets or snow. Therefore, compensating mechanisms (e.g., temperature advections) must generally be present to maintain the above-freezing warm layer and below-freezing cold layer.

Past studies have identified regional variations in the conditions supporting freezing rain, but have typically relied on data from a limited number of stations resulting in small sample sizes. To better elucidate the conditions supporting severe events, we concentrate on surface and upper-air observations of long-duration (6 or more hours) freezing rain events throughout North America from 1979-2016. While freezing rain is most frequently observed in the northeastern United States (NEUS) and southeastern Canada (SECA), long-duration events have affected areas as far south as the Gulf Coast. Notably, a disproportionately large number of very long-duration (18 or more hours) events have occurred in the Southern Plains states relative to the climatological annual frequency of freezing rain there.

Our analysis of observations concurrent with events reveals coherent regional structures in their characteristics. For example, freezing rain events over the NEUS and SECA tend to occur ahead of a warm front, with warm air overrunning cold air already in place at the surface and producing relatively modest warm layers on the order of +1-3°C. Event duration is closely linked with the temperature at event onset, as surface warming tends to occur during the events. Over the Southern Plains and southeastern U.S., freezing rain more often occurs behind an Arctic cold front, with surface cold air undercutting warm air in place aloft and surface temperatures remaining nearly steady or cooling during events. In these regions, warm layers tend to be much warmer and deeper than over the NEUS/SECA. In this presentation, we discuss these regional variations as well as differences between short- and long-duration events. We also introduce common synoptic patterns leading to these events in each region. An improved understanding of the conditions leading to freezing rain events is important both for forecasters who hope to predict them in the short and medium ranges, as well as for researchers concerned with how these events are expected to change in a warming climate.