LAKE-EFFECT RAIN EVENTS

Seven years of autumnal (September-November) precipitation data are ex amined to determine the characteristics of lake-effect precipitation d ownwind of Lake Erie. Atmospheric conditions for each lake-effect even t are compiled and the mean atmospheric environment for rain events is constructed and compared to conditions for lake-effect snow events. I t is found that lake-effect precipitation occurs approximately one out of every five days with a diurnal peak in precipitation intensity dur ing the afternoon and evening. The greatest number of lake-effect days occurs in October followed by November and then September. Comparison of these results to regional precipitation climatologies strongly sug gests that the season of lake enhanced precipitation begins in late su mmer. Precipitation is predominantly rain throughout September and Oct ober and snow after the first week of November. A transition period of both rain and snow occurs in early November. Analysis of thunder even ts for the 7-yr period show a late September to mid-October peak with a decline in frequency by November. The decline in thunder events is d ue to a seasonal decrease in the depth of the conditionally unstable l ayer. As might be expected, the mean atmospheric conditions during rai n events are similar to those found during lake-effect snow events. Th is is particularly true with regard to the overall positions of transi ent synoptic features. Differences are most apparent in the thermodyna mic profile of the lower troposphere. Extreme low-level instabilities typically observed in lake-effect snow events are absent from lake-eff ect rain events. However, in contrast to most snow events, a much deep er layer of conditionally unstable air is usually present during rain events.