Long term variability of precipitation mechanisms in the Great Lakes region

Great Lakes water levels play a significant role in a wide variety of areas in the Great Lakes region, such as commerce, hydropower, recreation, and the environment. This study was conducted to gain insight into mechanisms that affect precipitation and water levels in the Great Lakes region. The focus is on the fall season because increasing precipitation in this season has contributed to record high lake levels since the 1960s and because the investigation into the cause for such an increasing trend in precipitation can be synoptically evaluated. Precipitation events during the period 1935-1995 were identified and analyzed from NOAA Daily Weather Maps. A methodology was developed to classify precipitation events according to synoptic or mesoscale forcing mechanisms. Variability in the frequency of precipitation classifications, 24 hour precipitation totals, and the frequency and intensity of precipitation days and events were analyzed and possible connections among these variables were investigated. Analysis of precipitation mechanisms and amounts revealed a long term increase in precipitation over time that was driven by an increase in the amount of precipitation per day and an increase in the frequency of precipitation days. Combined increases in the intensity of low pressure and most frontal mechanisms, the frequency of frontal mechanisms, and the precipitation contribution by warm, stationary, and occluded fronts also played a very important role to this long-term increase. Analysis of precipitation mechanisms and large-scale circulation features revealed that when the Pacific-North American (PNA) pattern is in the positive phase (more meridional flow) the Great Lakes region experiences more cyclones but fewer precipitation days, less precipitation, and a smaller precipitation contribution from frontal mechanisms. The relatively negative phase of the PNA (more zonal flow) is likely responsible, in part, for the increased precipitation over the region despite the fewer number of cyclones.