1987 Superstorm still No. 1

It rained. And rained. And rained.

Thirty-two years ago today, the Twin Cities Superstorm dumped 10 inches of rain on the south and west metro. It still stands as the biggest rainfall event in the Twin Cities’ history. More than two month’s worth of rainfall fell in a few hours.

The total amount of rainfall that night that fell on the Twin Cities was enough to fill Lake Bde Maka Ska 80 times.

Here’s a clip from local news media on the storm.

Stalled front: ‘Training’ storms

The setup for a flash flood that night was already in place. Heavy rains had already soaked soils on July 20. A stalled front set up across the Twin Cities on July 23. Storms that erupted along the front “trained” over the same areas like boxcars on a passing train.

The heaviest rainfall axis laid out across the western and southern Twin Cities.

Here’s more analysis from the Minnesota DNR Climatology Working Group:

The 23-24 July storm formed during the late afternoon of 23 July. At that time, extremely high dew points 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 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.

A thousand-year storm

Rainfall totals exceeded 6 inches in just a few hours across a large area. The rapid rainfall rates lead the Minnesota State Climatology Office to comment that this storm generated a 1-in-1000-plus year recurrence interval.

The flood event damaged about 7,000 homes in the Twin Cities. Damage costs tallied near $30 million.