what: 15.10 inches - a new Minnesota 24-hour rainfall record
where: one mile south of Hokah in Houston County of southeastern Minnesota
who: a National Weather Service Cooperative Observer
when: 8:00 AM Saturday, August 18 through 8:00 AM Sunday, August 19, 2007
The heavy rain episode of August 18-20, 2007 was one of the most extraordinary precipitation events in Minnesota's modern history. During the event, a new Minnesota 24-hour rainfall record was broken. The State Climate Extremes Committee has agreed that the 15.10" total recorded at 8:00 AM on Sunday, August 19, 2007 near Hokah in Houston County is the largest 24-hour rainfall total ever measured at an official National Weather Service observing station in Minnesota.
The Minnesota State Climate Extremes Committee is composed of representatives from the National Weather Service field office with forecast responsibilities for the location in question (in this investigation, La Crosse), the Minnesota State Climatology Office (MN DNR), the Midwestern Regional Climate Center, the National Weather Service Central Region Headquarters, and the National Climatic Data Center. Only meteorological measurements from weather stations recognized by the National Oceanic and Atmospheric Administration (NOAA) are considered for state records.
The record-setting value was measured using a National Weather Service-issued eight-inch diameter stainless steel rain gauge with a holding capacity of 23 inches. A member of the State Climate Extremes Committee visited the Hokah site on Tuesday, August 22, 2007. He was satisfied with the location of the gauge and that proper measuring techniques were followed. The other members of the Committee concurred with his conclusions. All members of the Committee agreed that the 15.10-inch value was consistent with official and unofficial ground-based rainfall measurements made in the vicinity of the Hokah, radar-based precipitation estimates, and the weather conditions in place during the event. The Hokah observer recorded an additional 1.17 inches of rain for the period from 8:00 AM Sunday, August 19 through 8:00 AM Monday, August 20.
The previous 24-hour rainfall record for Minnesota was 10.84 inches at Fort Ripley of Crow Wing County, set July 22, 1972. The old record was observed during one of the most damaging flash floods of the 20th century.
During the August 18-20, 2007 storm, a non-National Weather Service rainfall observer located approximately four miles west of La Crescent (Houston County) reported 17.21 inches for the 24-hour period ending 7:00 AM, Sunday, August 19, 2007. The observer, a volunteer with the Root River Soil and Water Conservation District, used a four-inch diameter plastic rain gauge with a holding capacity of 11 inches. The gauge was dumped several times during the event. The 17.21 inch rainfall total is the largest 24-hour value in the Minnesota State Climatology Office database, which includes data collected by National Weather Service observers and data gathered from Minnesota's long-term high spatial density precipitation monitoring network. The previous 24-hour rainfall record for a non-National Weather Service location was 12.75 inches. This measurement was made in Bloomington (Hennepin County) on July 24, 1987 after the Twin Cities Superstorm.