Cool-season moderate precipitation events in the Northeastern United States

Cool-season (1 October-30 April) moderate precipitation events contribute to a significant percentage of the total cool-season precipitation in the Northeast United States. It is important to investigate the structure and causes of these moderate precipitation events because 1) they are relatively common, 2) they tend to occur in relatively weak synoptic-scale forcing regimes, and 3) forecasting these events can be challenging. The purpose of this presentation is to provide a 10-year climatology of cool-season moderate precipitation events across the Northeast and to examine the synoptic-scale forcing regimes that govern when and where these events occur.

A moderate precipitation event in the Northeast was defined as having a liquid equivalent of 0.6-1.3 cm (±0.1 cm) or a snow total of between 6.4 and 19.0 cm. Daily precipitation amounts were obtained from the National Oceanic and Atmospheric Administration's Local Climatological Data available through the National Climatic Data Center from 1994 through 2004 for 35 first-order National Weather Service stations across the Northeast. The station domain was bounded on the west by Detroit, MI, and on the south by Charleston, WV. For each station, every day between 1 October and 30 April was examined to see if a moderate precipitation event occurred. Hourly precipitation data were used to refine the event selection process in potentially ambiguous cases such as back-to-back days where moderate precipitation occurred, but the event total was above the moderate criterion. Based on the hourly rainfall data, if there was less than a 6-h gap in the precipitation, the precipitation was considered to come from one event. Histograms were produced from these data to show the distribution of cool-season moderate precipitation events across the Northeast by city, state, and geographic region.

The results from the histogram analysis indicate that more cool-season moderate precipitation events occurred in the Great Lakes and northern New England, with fewer events farther south and east. Another peak in moderate events occurred in elevated terrain. Erie, PA, received the most cool season moderate precipitation events over the past 10 years (197), while Baltimore, MD, received the least (95). To help illustrate the importance of weak synoptic-scale forcing on the evolution and structure of moderate precipitation events, the results from a couple of brief case studies will be presented. The case studies will focus on identifying lifting mechanisms and moisture sources, along with the modulating role of atmospheric stability, on the distribution of observed precipitation. A classification scheme is being developed to help forecasters identify different synoptic regimes that lead to moderate events.