Studies of selected precipitation cases from METROMEX. [Urban factors leading to summer precipitation]

The primary goal of these case studies was to focus the entire METROMEX field and research effort by attempting to define better the various urban factors leading to measurable changes in rainfall and severe weather in the St. Louis area. Nine cases (rain periods lasting usually 12 to 24 hours) were chosen to represent a variety of summer precipitation conditions in the St. Louis area. This report is organized to present the cases in their chronological order. Five of the cases are from a 7-day period in August 1973 and include a case with relatively small air mass showers and a very complex day with a relative severe squall line system followed by a cold frontal rain system. Other cases were chosen from August 1972 and July 1973. Since the majority of the summer precipitation in the St. Louis area is associated with squall lines, cold fronts, and squall areas, it is not surprising that the frequency of these synoptic events dominates the case studies. Two air mass situations are reported, one dealing with a post cold-frontal thunderstorm and the other with warm sector showers. Six squall lines are studied, two of which were associated with frontal systems and four in which no front impacted on the storm situation. Studies of a cold frontal passage and its associated precipitation and of a squall area with no front in the vicinity are also included.