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105 reporting hourly accumulations of 10.01 inches and 20.02 inches. These values are clearly unreasonably large and, due to a repetitive pattern, can be identified and eliminated. Somewhat less obvious are extremely large reports (e.g., 15.55 inches in an hour), but the extreme value still makes it possible to eliminate them automatically. Another set of errors involves the recording of hundredths of an inch values in the tenths of an inch column (e.g., 7.80 inches in one hour, followed by 6.50 inches with no precipitation on either side of those two hours). These are problematic, as are other isolated instances of large precipitation (~4 incheslhour), because there is no objective way to determine whether they are the result of "bad" data or extremely large "good" data. Indeed, this is a fundamental problem in using any dataset about Brooks, Stensrud, and Mitchell any rare, extreme events. We individually checked every hourly report of greater than 4.5 inches (approximately 360 reports) with reports from meteorological and climato- logical journals, and found that only a few were likely to be good reports; the rest fell into the kinds of errors mentioned above. The real difficulty comes in attempting to hand check the much larger number of reports at smaller values. The distinction between obviously bad and good data becomes blurred, and the volume of work becomes prohibitive. Observed Frequency of Hourly Precipitation One of our purposes in investigating the frequency of heavy precipitation is to estimate the number of times operational weather forecasters will have to deal with this problem and to understand its implications for a national forecast center. Hence, we confine ourselves to looking at the average number of events in the contiguous United States and do not consider the lack of spatial uniformity. We touch only on the highlights of the precipitation record, focusing on hourly accumulations of an inch or more and, particu- larly, on the average number of events in July. Longer accumulation times and larger amounts are referenced from the July hourly base values. We organized data in Ih-inch aggregates (e.g., 1 inch to 11/2 inches, Ilh inch to 2 inches, etc.) for ease of analysis and to increase the number of samples at higher values. We found that the annual cycle of heavy precipita- tion peaks in July and is symmetric about that month (see Figure 1). In January, the month in which heavy precipitation is least frequent, there are about 7 % of the number of events as in July. In total, on average, there are approximately 2,400 reports of 1 inch to Ilh inches hourly rainfalls in the HPD each year and 3,200 reports of 1 inch or greater. Twenty percent of all observations occur in July and more than 50% occur in June through August.