The Influence of Large-Scale Flow on Fall Precipitation Systems in the Great Lakes Basin

Abstract

A synoptic climatology is presented of the precipitation mechanisms that affect the Great Lakes Basin. The focus is on fall because increasing precipitation in this season has contributed to record high lake levels since the 1960s and because the causes can be synoptically evaluated. Precipitation events were identified for the period 1935–95 from NOAA Daily Weather Maps. Precipitation days were classified as one of nine types. Trends in the precipitation classifications, 24-h precipitation totals, and the frequency and intensity of precipitation days and events were analyzed. It was found that the precipitation increased 15% over the basin and 35% at Grand Rapids, Michigan, from 1935–65 to 1966–95. The increased precipitation was driven by an increase in the amount of precipitation per day (from low pressure systems and warm, stationary, and occluded fronts) and an increase in the frequency of precipitation days (from troughs and cold, warm, stationary, and occluded fronts). All classifications except for isolated convection contributed to the increase. Increases from warm, stationary, and occluded fronts contributed the most. Analysis of precipitation mechanisms and large-scale circulation features for two 10-yr periods from 1950 to 1959 and from 1980 to 1989 revealed that higher precipitation amounts were associated with a more zonal flow pattern that existed over the United States during 1980–89. This pattern was accompanied by more baroclinicity and moisture over the Rockies, a stronger upper-troposphere subtropical jet, and stronger low-level flow from the Gulf of Mexico. These features allowed a greater number of southern systems with more moisture to influence the region. Specifically, the increased frequency of low pressure systems approaching from the south(west) and their associated more rapid deepening rates allowed more precipitation from warm, stationary, and occluded fronts. The similarities in the synoptic precipitation classifications and precipitation amounts between the two 10-yr periods and the two 30-yr periods examined suggest that more meridional flow was present for much of the 1935–65 period and that more zonal flow was present for much of the 1966–95 period.