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Lake-effect snowstorms are important parts of the climate of the U.S. Upper Midwest, with significant economic and societal impacts on communities close to the Great Lakes. Some impacts, particularly those on air and ground transportation, depend critically on the time of day that lake-effect precipitation occurs. This study utilizes hourly precipitation data collected near Lakes Superior and Michigan to determine the diurnal behavior of lake-effect precipitation frequency. Precipitation data from approximately 200 lake-effect days during 1988–93, identified by a previous study based on visible satellite data, are examined. A distinct morning maximum and afternoon/evening minimum in lake-effect precipitation frequency was observed, with the largest variations at sites within the snowbelt regions. The relative importance of several factors known to influence lake-effect precipitation development was examined to gain insight into the physical mechanisms controlling the diurnal evolution of lake-effect precipitation. [ABSTRACT FROM AUTHOR]

This paper is the first of two papers that examines the organization of the precipitation field in central U.S. cold-season cyclones involving inverted troughs. The first portion of the study examines the varying precipitation distribution that occurred during a 6-yr synoptic climatology of inverted trough cases. The definition of inverted trough cases has been expanded from the groundbreaking work by Keshishian et al. by 1) not requiring a closed cyclonic isobar along the frontal wave along the conventional surface front and 2) not requiring a surface thermal gradient to be present along the inverted trough. Only 8.5% of the expanded dataset produced the precipitation primarily occurring to the west of the inverted trough ("behind" cases) as seen in Keshishian et al. The largest group of cases, comprising about 40% of the cases, produced precipitation that primarily occurred between the inverted trough and the conventional warm front ("ahead" cases). A composite study compared a subset of the ahead cases with a subset of the behind cases. The ahead cases tended to be more progressive with a stronger jet stream located over the center of the parent low. Broad warm-air advection and frontogenesis in the lower troposphere were observed between the inverted trough and the surface warm front. Cold-air advection to the west of the inverted trough precluded the development of "wraparound precipitation." In contrast, the behind cases had a stronger low-latitude wave couplet with a trough upstream of the surface low and a ridge downstream. The region of warm-air advection and frontogenesis were displaced to the west of the inverted trough and surface cyclone. In addition, the entrance region of a southwest-northeast-oriented jet streak aided the development of ascent to the west of the inverted trough while precluding the development of precipitation to the north of the conventional warm front. Thus, the inverted trough tended to act like a warm front in behind cases,... [ABSTRACT FROM AUTHOR]