Flash Flood — Dane, Wisconsin

2007-08-18 to 2007-08-19 · near Mazomanie, Dane, Wisconsin

Event narrative

Although general flooding affected all of Dane County, significant flash flooding affected the community areas of Mt. Horeb, Oregon, Sun Prairie, and Dane. At least 798 homes in Dane County sustained water damage to basement contents and/or basement walls, with damage totals of about $3.96 million. Thirteen homes had major water damage and two were destroyed, while 783 had minor damage. At least 10 businesses reported minor water damage totaling about $380,000. Crop losses were an estimated $5 million. Livestock losses were about $200,000, and farm building losses were about $500,000. Additionally, road damages were about $538,000, water control facility damage was about $320,000, public building damage was about $288,000, public utility damage was about $124,000, and other public property damage was about $37,000. Not included in property or crop losses were debris clearance costs of $197,000 and protective measures of $258,000. A bridge on CTH Y just north of Mazomanie was undermined and damaged by the flood waters and will need to be replaced. Water depths on STH 69 south of Verona reached 3 to 4 feet. Several gravel shoulder washouts occurred and crops across the county were damaged. Soil erosion and crop damage was reported. The water level on Lake Mendota rose to within 1 foot of it's record maximum level. The water level on Lake Monona came within 0.02 feet of it's record maxmium level. Rainfall rates were impressive - a spotter near Oregon measured 2.1' of rain between 2000 and 2100CST on August 18th. In Morrisonville, a spotter measured 7.07' overnight through 1700CST on August 19th. Overnight through 0245CST, 4.40' was measured by a coop observer 3NNE Sun Prairie with water overflowing most roads in that area. Also, water was reported in many basements in the Sun Prairie area. A spotter measured 6.39' overnight in DeForest. At a location 1SE Verona, 5.59' fell overnight through 0122CST. The Stoughton coop observer had 6.03' overnight. In Oregon a spotter measured 5.45' overnight. For comparison, locations in Dane County normally receive about 4 to 4.25 inches of rain in the entire month of August.

Wider weather episode

Significant flash flooding affected parts of south-central and southeast Wisconsin during the overnight hours of August 18th into the pre-dawn hours of August 19th, in a roughly 75-mile wide band from the Iowa-Sauk County border through Dane County to Racine and Kenosha Counties. The flash flooding was the result of very heavy overnight rains from August 18th into August 19th, rangning from 4 to 7 inches (roughly one or two summer month's worth of rain in 24-hours). However, WSR-88D Doppler radar rainfall estimates ranged up to 8 to 9 inches in parts of Sauk, Dane, and Iowa County. These rainfall amounts in such a short period of time only occur once in 100 to 200 years. A graphic of WFO MKX's WSR-88D Doppler radar rainfall estimate is attached to the Flash Flood event for Dane County. Many rivers and streams exceeded flood stage by 1 to 5 feet, and some would remain above flood stage through the end of August, 2007, through the first week of September, 2007. Synoptically, a stalled surface frontal boundary stretched from northern Iowa through northern Illinois, while warm, moist air flowed north up and over the boundary. Just north of the surface boundary was a nearly stationary, thunderstorm initiation zone near the Minnesota/Iowa border. The resultant rounds of storms moved southeast across southern Wisconsin and repeatidly traversed over the same areas. Additional severe flash flooding extended northwest into Vernon, Crawford, and La Crosse Counties and northeast portions of the state of Iowa. Refer to StormData events from those areas for related information.

View location on OpenStreetMap → (43.1700, -89.7800)

Source: NOAA Storm Events Database, event_id 56509. Narrative written by the NWS forecast office that issued the report.