May 8th is the last average date for the last spring frost with the latest observed on June 12, 1972. October 12th is the average date of the first fall frost with the earliest observed on August 28, 1982. Fifty-six percent of the annual precipitation falls during the spring and summer. The greatest monthly total precipitation was 12.00 inches in November 1985, and the highest daily rain was 3.85 inches on October 16, 1954. The return period for 24 hour rains on the order of 4 inches is about 25 years.
Thunderstorms occur most frequently in the summer but have been observed each month of the year.
With a strong dependence upon showers for the moisture supply in the summer and fall, there is always a threat of drought. Prolonged periods of drought are seldom experienced however, and the longest expected period without measurable rainfall is on the order of 25 to 30 days. Damages due to wind and hail accompanying severe thunderstorms are recorded almost every year in Butler County. Since 1854 when records were first kept on such storms only 2 tornadoes have been sighted in Butler County, neither occurred near the Slippery Rock area nor caused death or injury although considerable property damage resulted. Edited with data current through December 1994.