A Climatology of 500-hPa Closed Lows and Associated Precipitation in the Northeastern United States

Temporal trends in precipitation associated with 500-hPa low pressure systems are assessed for consistency with overall precipitation trends in the northeastern United States. Increases in precipitation associated with closed upper lows can be attributed to an increase in the number of closed lows and/or an increase in the average precipitation occurring in association with closed lows. A climatological description of 500-hPa closed lows was developed, and precipitation was analyzed using data from the U.S. Cooperative Observer Program. Closed 500-hPa lows were identified within a rectangular geographic region bounded by 35° and 50°N and 65° and 90°W. Precipitation observations within 1.25° of latitude and 1.25° of longitude of the closed-low center were attributed to the closed low. Statistical testing procedures were conducted to evaluate whether a long-term trend existed in the closed-low frequency, the average precipitation occurring near closed lows, or the percentage of >2.54-cm precipitation observations associated with the lows. Regional trends (1948–2007) in the frequency of closed lows were evident, and a statistically significant increase in average precipitation near closed lows was found. Likewise, the percentage of precipitation totals in excess of 2.54 cm (and separately 5.08 cm) showed a statistically significant increase with time. In both cases, recent (2007) values were nearly 50% higher than the comparable value in 1948. Increases were particularly evident during the winter months. These trends are consistent with increases in tropospheric water vapor due to increased global mean temperature.