The average statewide temperature during the month of September was 68.5°F which was 5.3°F above normal (See Figure 1). September 2015 became the 7th warmest September among the 143 years of state climate records. Des Moines’ average monthly temperature was 71.9°F or 6.3°F above normal. The coldest temperature of the month at Des Moines was 47°F on the 30th while the hottest temperature reached 94°F on the 6th. Waterloo was also well above normal for monthly average temperature with an average of 68.3°F or 5.3°F above normal. The coldest temperature at Waterloo in September was 39°F on the 30th while the warmest temperature was 92°F on the 6th. 90 degree temperatures were fairly common during the first week of September. In fact, the first seven days of the month were the warmest week of the year in the state with temperatures averaging 10.6° above normal (See Figure 2).
When temperatures were above normal during September, they were well above normal with some of the warmest stretches occurring from the 3rd to 7th, 15th to 17th, and 22nd to 25th. In fact, Des Moines had more days (13) when the average daily temperature was at least 10°F greater than normal, than it had days when the average temperature dropped below normal (7). Waterloo only had 6 days when the daily average temperatures was below normal and had 11 days when daily average temperature was at least 10°F or greater than normal. A cold front pushed through the state at the end of the month and brought some temperatures in the 30s across northern Iowa. Estherville and Cresco dropped to 33°F on the 30th.
The average statewide precipitation was 4.08 inches or only 0.70 inches above normal (See Figure 3). September 2015 becomes the 50th wettest September among 143 years of statewide records. It was a “hit-or-miss” for precipitation during the month of September with the west to southwest portions of the state receiving the bulk of the rain and the north to east the less amounts (See Figure 4). Typically when it rained in September, it rained in abundance but in isolated areas. Fort Dodge for instance only received 0.90 inches while Glenwood reported 11.63 inches for the month. Des Moines and Waterloo had 5.25 inches and 2.59 inches respectively. Torrential rains fell over portions of central to southwest Iowa during the overnight hours from the 6th to the 7th and caused some significant flash flooding in Carroll, Guthrie, and Greene counties. The towns of Bagley, Coon Rapids, Bayard, Glidden, and Carroll all had significant impacts due to flash flooding (See Figure 5), with major state Highway 141 receiving water over it in multiple locations. Radar estimated rainfall amounts ranged from 3 to 8 inches on the night of the 6th (See Figure 6) with a report of 6.25 inches from a spotter in Bagley. Portions of southeast Iowa also had 2 to 4 inches of rain from the 6th to 7th. On the 23rd, western Iowa received a round of torrential rain and mainly affected the Council Bluffs area. In fact, a storm total of 9.28 inches was reported on the north edge of Council Bluffs. Severe weather was fairly limited across the state during the month of September with a few reports of large hail and damaging winds on the 6th and then again on the 10th to the 11th.