Abstract
Understanding precipitation change is made difficult by the complexity of precipitation processes, which vary by region but also within region. Improved understanding may require refined analysis based upon stratification of precipitation by cause. As an example, the cool season (November – April) of the North American Great Lakes region involves precipitation resulting from two broadly different mechanisms – synoptic-scale dynamical forcing and mesoscale lake-effects. This paper shows that station data from within a distinct lake-effect region indicate that precipitation from the contrasting causes trended in opposite directions over a recent 47-year period. These changes are masked within total precipitation series, for which trends are not statistically evident. An increasing amount of synoptically forced precipitation was countered by a decreasing amount and frequency of lake-effect precipitation. Increases in the frequencies of light, heavy, and very heavy synoptic precipitation days are not evident for lake-effect precipitation, such that when aggregating to total precipitation, a frequency increase is only evident for very heavy precipitation days. Increasing volatility in daily synoptic precipitation amount is opposite decreasing volatility for lake-effect precipitation. The binary stratification of precipitation by basic cause explains a misalignment with precipitation change for the broader region of the northeastern United States. The changes in synoptically forced precipitation match those of total precipitation for neighboring areas outside the lake-effect region. This highlights the value of distinguishing precipitation by process prior to analysis of hydroclimatic change.