Flash flooding in southern Wisconsin

August 20th, 2018 |

GOES-16 “Red” Visible (0.64 µm) images, with hourly plots of surface reports [click to play MP4 animation]

1-minute Mesoscale Domain Sector GOES-16 “Red” Visible (0.64 µm) images (above) showed multiple clusters of convection which developed across far southern Wisconsin during the late afternoon and early evening hours on 20 August 2018, producing very heavy rainfall and flash flooding (with at least one fatality) that was focused in western Dane County (CoCoRaHS | AHPS). As much as 15.33 inches of rain was reported in Cross Plains (Local Storm Reports). which set a new record for 24-hour precipitation in the state of Wisconsin (the old record was 11.72 inches at Mellen in northern Wisconsin on 24 June 1946). Animations of radar base reflectivity and storm total precipitation (courtesy of Pete Pokrandt, UW-AOS) showed that the combination of slow overall motion — and a pivoting of precipitation bands, due to weak flow aloft within a deformation zone (300 hPa analysis) —  and cell mergers played a role in producing the heavy rainfall. There was also an EF-0 tornado at Delavan (NWS Milwaukee summary).

The corresponding 1-minute GOES-16 “Clean” Infrared Window (10.3 µm) imagery (below) showed that cloud-top brightness temperatures were generally in the -50º to -60ºC range with these initial areas of convection.

GOES-16 Infrared images [click to play animation]

GOES-16 “Clean” Infrared Window (10.3 µm) images, with hourly plots of surface reports [click to play animation]

A longer Infrared animation (below) with a different color enhancement (adapted for winter convection) better emphasized the colder cloud tops as convective development persisted into the subsequent overnight hours. Note the absence surface observations from Middleton KC29 after 03 UTC or 10 pm CDT — this was due to an extended power outage to that area and other parts of western Dane County.

GOES-16

GOES-16 “Clean” Infrared Window (10.3 µm) images, with hourly plots of surface reports [click to play MP4 animation]

GOES-16 Mid-level Water Vapor (6.9 µm) images (below) revealed the large circulation associated with an occluded low (surface analyses) over the lower Missouri River valley.

GOES-16 Mid-level Water Vapor (6.9 µm) images [click to play MP4 animation]

GOES-16 Mid-level Water Vapor (6.9 µm) images [click to play MP4 animation]

The GOES-16 Total Precipitable Water derived product (below) showed that values of 1.3 to 1.5 inches were being advected northward toward the area.

Composite of GOES-16 Water Vapor (6.9 µm) imagery and Total Precipitable Water product [click to play MP4 animation]

Composite of GOES-16 Water Vapor (6.9 µm) imagery and Total Precipitable Water derived product [click to play MP4 animation]

With widespread cloudiness prevailing across much of the Upper Midwest, the CIMSS All-Sky Total Precipitable Water product (below) was helpful to better track the transport of moisture into the region — TPW values of 40-43 mm (1.6-1.7 inches) were seen feeding into southern Wisconsin within a TROWAL airstream around the northern edge of the occluded low pressure system (WPC discussion). The All-Sky products blend GOES ABI clear-sky retrievals with GFS background fields in cloudy regions; these products have been evaluated by the NWS Hazardous Weather Testbed (see here).

GOES-16 AllSky Total Precipitable Water product [click to play animation | MP4]

GOES-16 All-Sky Total Precipitable Water product [click to play animation | MP4]

The Aqua MODIS Total Precipitable Water product at 1943 UTC (below) showed TPW values of 40-45 mm (1.6-1.8 inch) on either side of the frontal boundary in northern Illinois.

Aqua MODIS Total Precipitable Water product [click to enlarge]

Aqua MODIS Total Precipitable Water product [click to enlarge]

One example of the hydrologic impact of the heavy rain was seen at the Pheasant Branch Creek USGS gauge (map), where nearly 11 inches of rainfall were recorded. A dramatic time-lapse video showed the rise of the normally-small creek as it inundated the adjacent multi-use path on 21 August.

Pheasant Branch Creek flows into the northwest corner of Lake Mendota, which crested at 852.3 feet on the morning of 22 August. This was the third highest lake elevation on record — and the highest level on record for so late in the calendar year. Portions of the University of Wisconsin – Madison campus adjacent to the lake experienced some impacts due to the high water, as shown on the map below. There were also several road closures in Madison due to high water.

Map of flood impacts for portions of the UW-Madison campus adjacent to the southwestern shoreline of Lake Mendota [click to enlarge]

Map of flood impacts for portions of the UW-Madison campus adjacent to the southwestern shoreline of Lake Mendota [click to enlarge]

Farther downstream on the Yahara River chain of lakes, Lake Waubesa reached its 100-year flood level on 22 August.

 

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