25th Anniversary of the 1987 Twin Cities Superstorm:
July 23-24, 1987
Any resident over the age of 30 who lived in the Twin Cities remember where they were on July 23, 1987. The largest rainfall event in Twin Cities' history began in the late evening and within six hours, ten inches of rain fell at the Twin Cities International Airport, causing massive flooding especially in the southern and western parts of the metro.
An analysis was made of the event by the State Climatology Office in 1987 along with event details.
A synopsis of the event was chronicled by climate historian Tom St. Martin and is reproduced below.
The heaviest rainfall ever officially recorded at a Twin Cities weather station fell between about 1800 hours CDT on 23 July and about 0200 hours CDT on 24 July 1987. During this eight hour interval, observers at the Twin Cities International airport station measured an even ten inches of rain (9.15 inches of which fell in a five hour period). And, although it escaped the worst of the storm, most parts of St. Paul received totals in the five to seven inch range, including 5.47 inches at the St. Paul NWS cooperative station; 5.30 inches at the North St. Paul NWS cooperative station; and 6.03 inches at the compiler's St. Paul Battle Creek area station. In addition to the heavy rainfall, the 23-24 July storm spawned a tornado which first touched down at about 1900 hours CDT near Goose Lake in the northwestern corner of the Twin Cities area. The funnel then moved in a southeasterly direction, causing extensive damage in the Twin Cities suburbs of Maple Grove and Brooklyn Park. Damage in other areas (including St. Paul) was extensive, largely the result of flooded homes and businesses, ruptured storm sewers, and washed out or inundated streets and highways. Two flood related deaths were reported and property damage was estimated to be in excess of $30 million (by any calculation, one of the greatest weather related losses ever to occur in Minnesota). The 23-24 July storms occurred along a frontal boundary which, during the preceding week, had separated extremely warm, moist air to the south and east and much cooler, drier air immediately to the north and west. The interaction of these air masses produced intense thunderstorms with extremely heavy rainfall over the southwestern portion of the Twin Cities on 20-21 July 1987, two days prior to the 23-24 July outbreak. Rainfall amounts during this event included 3.83 inches at the Twin Cities airport station, 9.75 inches near Shakopee and 7.83 inches at the neighboring community of Chaska. The St. Paul station recorded only 1.47 inches. The 23-24 July storm formed during the late afternoon of 23 July. At that time, extremely high dewpoints prevailed over most of southern Minnesota and a strong upper level jet stream provided an ample supply of cold air aloft, creating a strong west to east frontal flow. The resulting instability produced the first storm cells just north and west of the Twin Cities. The system moved south but, by mid-evening, became stalled on a east-west axis over the southern part of the Twin Cities area. Successive thunderstorm cells formed along the front throughout the evening and early morning hours, bringing nearly eight hours of heavy rain. The 23-24 and 20-21 July storms, together with the rainfall produced by thunderstorms earlier and later in the month, brought unprecedented July rainfall to the Twin Cities area. The International airport station recorded 17.91 inches, approximately six times the July normal. A monthly total of 19.27 inches was recorded in Bloomington (several miles west of the airport station) by an observer in the Minnesota state climatology office's "backyard" rain gauge program. The St. Paul station recorded 11.56 inches of rain during July 1987, breaking the previous St. Paul July record of 10.00 inches. Other monthly totals included 12.82 inches at the New Hope Robbinsdale station; 12.98 inches at the North St. Paul station and 12.08 inches at the compiler's St. Paul Battle Creek station. Ironically, July 1987's excessive rainfall came in the middle of a prolonged period of subnormal precipitation. Precipitation had been below normal for every month from October 1986 through June 1987 and, following about six weeks of wet weather in July-August 1987, the drought returned. Extreme dryness prevailed during much of the ensuing year with a near record dry June and record warmth during the summer of 1988.