Cataloguing Severe Winter Cold Snaps in the Eastern and Midwestern United States for Medium-Range Predictability

Severe winter weather can be extremely disruptive to daily life and the economy. Accurate forecasts of cold outbreaks are crucial for health and safety, as well as for emergency planning by governments and the transportation and energy sectors. Short-range winter storm warnings have become much more accurate in recent years, however forecasts in the medium-range on the order of two-weeks to a month remain a challenge. To improve probabilistic forecasts, in both accuracy and lead-time, a historical catalog is prepared to describe the evolution of each winter cold outbreak from 1948, including relevant weather-climate connections. A severe cold index (SCI) for the eastern/Midwestern U.S. is constructed for the 1948/49-2008/09 winter periods, where the SCI is a local daily measure of threshold exceedance, taken as the 5th percentile of daily temperatures. The duration and spatial extent of cold spells are also explicitly considered and an “event set” of discrete cold outbreaks is designed based on the historic data. Principal components analysis is applied to surface and atmospheric observations (NCEP Reanalysis and other sources) to identify the predominant weather regimes in each season, and the evolution of these regimes. The SCI is then evaluated to identify consistent precursors. Relationships with relevant climate modes such as ENSO, NAO, PDO, MJO, etc. are used together with the annually-isolated weather regimes to develop statistical models and qualitative rules-of-thumb for use in medium-range to seasonal forecasts. The products are currently designed as a tool for operational meteorologists in forecasting energy demand, but could be functional for other end-users.