Relationships between large-scale circulation anomaly indices, such as the North Atlantic Oscillation (NAO), Pacific-North American (PNA) pattern, and Southern Oscillation Index (SOI), and precipitation anomalies over the northeastern United States are hypothesized to exist. To test this hypothesis, we have accumulated daily time series of these large-scale circulation anomaly indices. Daily, rather than monthly, values of the large-scale circulation anomalies are used to obtain a better understanding of the evolution of individual cyclone structure and life cycles in relation to changing large-scale circulation regimes.

Daily NAO and PNA indices were calculated for 1951 to 2000. A five-day running mean was applied to the daily values in order to remove high-frequency variations. The differences of domain averaged 500 hPa heights between an Iceland domain (55°-70°N, 10 °-70°W) and an Azores domain (35°-45°N, 10°-70°W) were used to determine the daily NAO index. A similar method using four domains: Hawaiian domain (15 °-25°N, 140°-180°W), Alaskan domain (40°-50°N, 140°-180°W), Pacific Northwest domain (45°-60°N, 105°-125°W), and Southeast US domain (25°-35°N, 70°-90°W) was used to calculate the daily PNA index.

To relate individual storms to the indices, a list of significant northeast United States storms was constructed from earlier lists compiled by Uccellini and Kocin, Junker, and Novak. Relationships between these indices and planetary-scale flow signatures associated with cool-season heavy precipitation events and extratropical cyclones in the northeastern United States are currently being studied. Results to date have shown that major cyclogenesis and the associated rearrangement of the large-scale flow patterns occur in conjunction with phase shifts and sign changes in the NAO and PNA indices.