In mid-August, a cold front moving through the Midwest stalled and transitioned to a stationary front, causing significant rainfall and subsequent flooding to occur. The cold front can be seen in the 00 Z (7pm CDT) Weather Prediction Center’s (WPC) satellite overlayed surface analysis. Another prevalent feature in the Midwest at this time were squall lines centered parallel and to the east of the cold front. The lines of slow moving thunderstorms were the driving factor for this event, as they dropped heavy rain over the same areas for an extended period of time.
ussatsfc2015081700

The National Weather Service’s (NWS) local storm report, shown below, depicts all reports received and all watches/warnings issued with a radar overlay of the rainband that caused this event. Local NWS offices issued several flash flood and flood warnings in the area and received dozens of local storm reports of heavy rain and a few reports of floods spanning from the 16th through the 17th. One report had just under 6″ of rain fall in under a 5 hour period from 6pm CDT through 10:30pm on the 16th.

NWSchat

 

The 48-hour maximum precipitation for this event, based on the MetStorm™ analysis, was 8.56″. There were Two major areas of heavy precipitation, one located north-northwest of Fort Dodge, IA and the other located at, and to the northeast of Sioux City, IA.

P_allsites_201529_googlemap

For this event, an ARI for a 6-hour duration yielded a maximum of ~151 years. In frequency terms this means a 6-hour maximum precipitation of this magnitude in this area has a 0.6% chance of occurring in any given year.

ARI_metstorm201529_6hr_max_ppt

Looking at the temporal distribution using the mass curve plot at the center of the storm, there were two distinct periods of intense precipitation. The first occurred at about 2UTC on the 17th (9pm CDT on the 16th) and the second at about 16UTC on the 17th (11am CDT on the 17th).

mass_curve_43.0938-95.4737_201529_Zone1

Overall, this storm which dropped a maximum of 8.56″ of rain in two major pulses in a 48-hour period was determined to be a rare event for northwestern Iowa with a ~151-year ARI. The resulting flood warnings and reports exemplify the consequences of a storm of this magnitude occurring in this location.

Please note that the maps presented here are preliminary and will be updated when new data become available.  If you are interested in this product, or any other product from our MetStorm™ Precipitation Analysis tool, please contact us at media@metstat.com or through our contacts page at http://metstat.com/contact-us/.

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-MetStat Team