Effects of 19th Century Records on Precipitation Trends for Select Midwestern Cities

Newly available data from the Climate Database Modernization Program (CDMP) Forts project has allowed the reconstruction of precipitation records extending back into the mid-19th century. An analysis of five major Midwestern United States cities: Minneapolis, Milwaukee, Chicago, Peoria, and St. Louis, indicates a tendency for larger amounts of both extreme and normal precipitation during the mid to late 19th century. Trends established in the pre-1895 values generally showed a tendency for precipitation totals to decrease during the latter portion of the 19th century. In addition, frequencies of heavy precipitation events greater than 1” were higher in the late 1800's than in the following decades of the early 20th century, and the precipitation from these heavy events was of a similar magnitude or greater than that found in the late 20th century.

Extensive quality control and the development of a metadata trail for each station were undertaken so as to ensure confidence in the quality of the data. An analysis of an “undercount bias” for each of the five stations showed that precipitation observers often did not record smaller precipitation totals (values ≤ 0.10”). Additionally, third party records were used to support the high precipitation totals early on in the precipitation observations, in particular for St. Louis, MO.