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 remembers 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.
Last modified: June 26, 2015


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