September 2015 Iowa Climate Review

Temperatures

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 7^th 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 30^th while the hottest temperature reached 94°F on the 6^th. 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 30^th while the warmest temperature was 92°F on the 6^th. 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 3^rd to 7^th, 15^th to 17^th, and 22^nd to 25^th. 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 30^th.

Figure 1: Average temperature departure from normal for the month of September 2015. Map is courtesy of MRCC.

Figure 2: Average temperature departure from normal during the first week of September 2015. Map is courtesy of MRCC.

Precipitation

The average statewide precipitation was 4.08 inches or only 0.70 inches above normal (See Figure 3). September 2015 becomes the 50^th 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 6^th to the 7^th 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 6^th (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 6^th to 7^th. On the 23^rd, 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 6^th and then again on the 10^th to the 11^th.

Figure 3: September 2015 total precipitation for the month across Iowa shows the west to southwest received the most significant amounts with the far eastern corners of the state the lowest amounts.

Figure 4: Average precipitation departure from normal for the month of September 2015. Map is courtesy of MRCC.

Figure 5a: Flash flooding photos from Carroll on September 6, 2015. Photos are courtesy of KCCI and Iowa Storm Chasing Network.

Figure 5b: Flash flooding photos from Carroll on September 6, 2015. Photos are courtesy of KCCI and Iowa Storm Chasing Network.

Figure 6: Radar estimated rainfall amounts from September 6-7, 2015 show a very heavy swath of rain from Coon Rapids to Bayard to Rippey Iowa.

Blog post by Kenny Podrazik – NWS Des Moines