The statewide (Iowa) average temperature for May 2015 was 60° which was only 0.1° below normal (See Figure 1). May 2015 ranks as the 69th warmest May out of 143 years of statewide records. The month of May witnessed several big temperature swings with the first third of the month (2nd to 10th) beginning well above normal before an extended period of chilly weather (11th to 22nd) settle into Iowa during the middle of the month. The coldest days were from the 18th to 21st where most stations ranged from 10 to 15 below their respective normal values (See Figure 2). Waterloo on the 20th had an average temperature of 45° which was 17 degrees under its normal. There were a couple additional days at the end of the month when the daily average temperature dipped to near 10 below normal. However, prior to the aforementioned two days, temperatures generally ranged slightly above normal. The coldest temperature at Des Moines and Waterloo was 39° (19th) and 34° (1st) respectively. The coldest temperature in the state throughout May was 31° at Elkader and Lowden on the 1st while the hottest temperature was 89° at Little Sioux on the 3rd. In fact, the last time when temperatures did not reach 90° during the month of May was in 1995.
The statewide average precipitation tallied 5.13 inches or 0.57 inches above normal (See Figures 3 & 4). May 2015 became the 35th wettest May among 143 years of statewide records. The rain fell quite regularly throughout the month as measurable precipitation fell on an average of 16.3 days for any single location. This was 39% more frequent than a typical May. Here’s a list of some of the stations that recorded at least 0.01” or more throughout the month:
Monthly rainfall totals ranged from 2.58 inches in Newton to 9.21 inches near Denison. Des Moines ended up 0.41 inches below normal for month even though the station recorded 16 days of measurable precipitation. Waterloo was 0.34 inches above normal for the month, while Mason City was 1.35 inches below its normal for May. The heaviest rainfall fell over west-central to southwest Iowa where 7 plus inches of rain was fairly common (See Figure 5). From the 23rd to the 27th, a statewide average precipitation of 1.56 inches was the wettest period throughout the month of May. Even with the persistent rainy pattern during the month, Iowa remained below normal precipitation for the year (See Figure 6).
Several statistics were courtesy of Iowa State Climatologist Harry Hillaker:
Blog post by Kenny Podrazik – NWS Des Moines