Flooding in Iowa, June 9 to 15, 1999

The first two weeks of June saw a continuation of the pattern that persisted through much of May with heavy rains across most of the Des Moines HSA during the period. These rains were primarily due to meso scale forcing in the very humid airmass that persisted across the central Plains during the first half of the month. Heavy rainfall during the afternoon and evening of June 8 fell over northeast parts of the HSA as a very moist and unstable airmass combined with several weak outflow boundaries to generate widespread thunderstorms. Radar estimated rainfall of up to 6 inches was indicated over northwest Grundy County, and Flash Flood warnings were issued for Black Hawk and Grundy Counties. The highest rain gage report came from Steamboat Rock on the Iowa River with 2.65 inches. The heaviest rains fell in the Iowa and Cedar River basins, especially along the Beaver Creek. A Flood Warning was subsequently issued for the beaver Creek at New Hartford as the runoff from these storms entered the river, and Flood Statements were issued for several other forecast Points. The night of June 9 saw a repeat of the previous evenings convective activity, but fortunately centered over a different river basin. The heaviest rains were observed over Sac and Calhoun Counties in the extreme upper portions of the Raccoon River basin. Spotter reports from this area ranged from 2 to 3.5 inches, while radar indicated that up to 5 inches fell in some locations. Rains were still significant over parts of the Iowa and Cedar basins. Steamboat Rock on the Iowa River picked up an additional 2.12 inches to add to the 2.65 inches the night before. Williams in the northeast part of Hamilton County received 2.31 inches. An Urban/Small stream Flood Advisory was issued for the overnight activity in Sac, Calhoun, and Webster Counties. Convective rainfall was widespread across the HSA on the 10th for the third night in a row, with the heaviest amounts over the central and southern parts of the area. Two to 3 inches of rain fell over parts of the lower Des Moines River basin with 1 to 2 inches in the upper Skunk basin. Winterset, located in the North river basin, measured 1.95 inches and Story City in the Skunk basin, and Ames 3S exceeded flood stage by the evening of the 11th. Renewed convection developed over the counties south and east of Des Moines during the evening hours of the 11th, prompting the issuance of Urban and Small Stream Flood Advisories for several counties. Most of the rain barely missed the Raccoon River basin, where the river was already in flood due to rains on previous days. Rains continued to plague southern portions of the HSA late on the 12th and early on the 13th, as weak short waves, relatively strong jet stream dynamics, and relatively high moisture combined to generate slow moving thunderstorms. Again, the heaviest rain fell over basins which previous escaped heavy rains. This time the East Fork of the 102 river in Taylor County rose quickly to just below flood stage as a result of 2 to 4 inch rains and a Small Stream Advisory was also issued for Decatur County. For the most part, flooding was restricted to agricultural lands during this extended event. Crops were lost and some were able to be replanted. Scattered damage occurred along many of the states rivers, however most areas affected were areas that routinely receive high water, thus damage was limited.