June 24-25 Raccoon River Flooding Review

Blog post by Kenny Podrazik – NWS Des Moines
SVRDMX-0082

Figure 1

SVRDMX-0098

Figure 2

Two periods of storms affected central Iowa during a 24-hour period from June 24 to June 25, 2015.  The first round of storms occurred during the morning hours on June 24 that affected portions of Iowa along and south of Highway 20. This was the “primer” for significant flash flooding and river flooding that would result from the second round of storms to affect the same area later that evening.  The second round of thunderstorms began to develop over west-central Iowa late in the afternoon and quickly became severe by the early evening. The first Severe Thunderstorm Warning (Figure 1) issued by the National Weather Service in Des Moines was at 6:20 p.m. CDT on June 24 for Guthrie and Dallas Counties.  The final Severe Thunderstorm Warning (Figure 2) expired at 1:50 a.m. CDT on June 25 for Marion and Jasper Counties.  There were multiple reports of large hail that ranged from the size of a quarter (one inch) to hen egg (two inches) throughout the night. Winds to 60 mph caused numerous trees to become damaged and there was even a report of an 80 mph wind gust in Guthrie County early in the evening.  Extremely heavy rain brought widespread flash flooding from west-central Iowa to southeast Iowa, including the Des Moines Metro, late in the evening into the overnight hours Thursday morning. The video below shows the entire second round of storms unfold on radar from the first development of a thunderstorm around 6 p.m. CDT on June 24 to the final drop ending around 2 a.m. CDT on June 25.

Widespread rainfall amounts of three to seven inches were reported after the event, with the highest report of 7.25” near Dawson, Iowa in northwest Dallas County.  From Bagley to Jamaica, Iowa, radar estimated near nine inches of rain fell within this area of the upper reaches of the Raccoon River Basin. The majority of the heaviest rain fell within the Raccoon River Basin (Figure 3).  As a result, major to near record flooding occurred along the Raccoon and Des Moines River Basins, including several tributaries (Figure 4).  In fact, the Walnut Creek recorded its highest crest ever at two river gauge locations. The Clive I-80/35 (CLVI4) and Des Moines 63rd Street (DOSI4) gauges crested at 13.41 feet and 18.82 feet respectively, both breaking the previous records set back on August 9, 2010.  Near record flooding occurred on portions of the Raccoon River, with Van Meter (VNMI4), Des Moines Highway 28 (DMWI4) (Figure 5), and Des Moine Fleur Drive (DEMI4) all cresting at their third highest crest in history. All the flooding on the Des Moines and Raccoon subsided by June 29, 2015.

Storm Total Radar Estimated Precipitation ending 7 am June 25, 2015. The yellow oval received 6-9 inches of rainfall in the headwaters of the Raccoon River Basin.

Figure 3: Storm total radar estimated precipitation ending 7 am June 25, 2015. The yellow oval received 6-9 inches of rainfall in the headwaters of the Raccoon River Basin.

June 2015 crests along central Iowa Rivers.

Figure 4: June 2015 crests along central Iowa Rivers.

Figure 3: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple) on June 26, 2015.

Figure 5: Hydrograph of the Raccoon River at Des Moines Highway 28 shows it crested just above major flood stage (purple horizontal line) on June 26, 2015.

Summer Weather Review

By Craig Cogil, Senior Forecaster

Temperatures:

Temperatures across Iowa were generally favorable this summer with limited bouts of heat and, in most instances, readings at or below normal. Departures were generally one to two degrees below normal with the largest departures along the Highway 30 corridor. The warmest reading in Iowa during the summer was 99 degrees in Sioux City on June 9. This year was the second year in a row that no summertime 100 degree readings were recorded in the state. Otherwise, temperatures were at or below normal for long stretches, especially in early July and again in mid to late August.

 Summer temperature departure from across Iowa – generally slightly cooler than normal

Summer temperature departure from across Iowa – generally slightly cooler than normal.

Precipitation:

Moisture was more than sufficient for much of the state during the summer. The exception was across northeast Iowa where deficiencies were common for much of the growing season. Abnormally dry conditions did creep into the northeast by the second half of the summer but the area still received enough rain to keep widespread drought conditions from developing. Farther south and west, thunderstorms were more common with localized heavy rain from June into August. A couple of events produced flash flooding in and around the Des Moines metro area as well as far south Iowa near the Missouri border. Rainfall frequency did decrease into August, but remained common enough to support crop maturation.

Summer rainfall departures – Drier in the far northeast, quite wet south half.

Summer rainfall departures – Drier in the far northeast, quite wet south half.

SummerTable