A flash flood event occurred in the Duluth area during the late evening of September 11th into the early morning hours of the 12th. An area of thunderstorms developed over Duluth during the evening hours and continued to backbuild over the hillside for a few hours before another area of rainfall pushed in from the west and helped push the backbuilding storms out over the lake. Rainfall totals of 3 to 6 inches fell over a small area at the top of the Duluth hillside in the Chester Creek basin in the area around College of St. Scholastica. This sent a surge of water downstream on Chester Creek down the Duluth hillside just to the east of downtown. The creek is generally underground for most of its run down the hill to Lake Superior, but the volume of water overwhelmed the tunnels and led to fast-flowing water on the surface. This resulted in a a few rescues by the fire department as the water cut-off access to an apartment building. At the bottom of the hill, Interstate 35 runs through a series of tunnels and the runoff from Chester Creek collected in the tunnels leading to a closure of the highway for a few hours until the water subsided.

Hail in CASS, MINNESOTA
From 11-SEP-23 13:55:00 to 11-SEP-23 13:56:00 (CST-6)
Location: MILDRED
Magnitude: 0.88 
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Trained Spotter

Flash Flood in ST. LOUIS, MINNESOTA
From 11-SEP-23 21:00:00 to 12-SEP-23 03:00:00 (CST-6)
Location: DULUTH
Flood cause: Heavy Rain
Property damage: 100.00K
Crop damage: 0.00K
Reported by: Broadcast Media

Heavy rainfall from backbuilding thunderstorms over Duluth led to 2 to 5 inches of rainfall in 2-3 hours. As the water rushed down the hillside to Lake Superior, it scoured out gravel alleys and sent around 1,000 cubic yards of construction debris, including sand, dirt, rock and other materials, onto Superior Street at the base of the hill. Superior Street at First Avenue East was closed for much of the 12th as crews worked to remove the piles of debris using heavy machinery.

Flash Flood in ST. LOUIS, MINNESOTA
From 11-SEP-23 21:00:00 to 12-SEP-23 03:00:00 (CST-6)
Location: HUNTERS PARK
Flood cause: Heavy Rain
Property damage: 250.00K
Crop damage: 0.00K
Reported by: Fire Department/Rescue

Thunderstorms over parts of Duluth dropped upwards of 3 to 6 inches of rain over a localized area focused on the Chester Creek basin over a period of 2 to 3 hours. The highest rainfall totals were located at the top of the hill in the College of St. Scholastica area. Flash flooding was reported upstream from this location along Arrowhead Road where 3 to 6 inches of water was flowing over the road where Chester Creek passed under the road. As the surge of water flowed downstream, it damaged walkways along the creek and sent 2 to 5 inches of water onto side streets and Kenwood Avenue. Chester Creek runs through tunnels for much of its trip down the Duluth hillside and this volume of water overwhelmed the tunnels sending a large stream of rapidly flowing water 3 to 10 inches deep down the hill. This affected a few apartment buildings in the vicinity of 7th Avenue and 3rd Street which required a few rescues/evacuations of lower level residents and pets in the buildings by the fire department. Interstate 35 runs through tunnels underground at the base of the hill and the water surging down the hillside began to collect in the tunnels. Standing and flowing water in the tunnels resulted in the closure of the tunnels for a period. Water of at least 4 to 6 inches in depth was covering the southbound lanes in spots. The water receded during the early morning hours of the 12th with cleanup occurring over the remainder of the week.