Assessment of Operationally-Issued Long Range Precipitation Outlooks for Illinois

Abstract

As a result of a 1980?81 drought, statistically derived outlooks of monthly and seasonal precipitation began to be issued to Illinois officials who were making management decisions relating to water supplies and agricultural activities. Outlooks of above, near or below normal precipitation have subsequently been issued operationally over a 3-year period for four areas of Illinois. They are assessed here as to their skill and major uses. This assessment shows that 56% of the seasonal outlooks were correct as opposed to 33% expected by chance, and 30% were correct when only persistence is used to forecast the coming season. The seasonal outlooks were correct most often in fall (67%) and least often in winter (42%). Monthly operational outlooks were correct 52% of the time. The skill levels in the monthly outlooks during the operational period were very similar to those in earlier experimental tests, being 53% correct in monthly tests of 1940?79. However, the seasonal tests using 1970?80 showed 41% accuracy compared with 56% in the 3-year operational period with the high value a result of sampling vagaries. The monthly outlooks were correct in detecting the occurrence of ending of extreme conditions, including the wet month that ended the 1980-81 drought, the lack of above normal rain during potential flooding conditions in the spring of 1982, and the deficient rainfall in summer 1983. The magnitude of the conditional probability value associated with the most likely monthly outlook value was well related to its correctness. When the maximum probability value obtained was 50% or more, 64% of the monthly outlooks (which specified that category) were correct, but when it was 35 to 45%, only 39% of the outlooks were correct.