IOWA PRELIMINARY MONTHLY WEATHER SUMMARY – MAY 2013
General Summary. Temperatures averaged 59.0° or 1.1° degrees below normal while precipitation totaled 8.84 inches or 4.28 inches above normal. This ranks as the wettest May (old record 8.48 inches in 1903) among 141 years of records and 51st coolest. This was also the snowiest May among 126 years of records with a statewide average snowfall of 3.4 inches (old record 1.2 inches in 1947).
Temperatures. Several inches of snow fell on northwest Iowa on the 1st keeping daytime highs in the 30s while southeastern Iowa basked in the 80s. At Iowa City a high of 80° on the 1st was followed by snow by the next evening. Daytime highs in many areas remained in the 30s on the 2nd and 3rd while more snow fell. Temperatures climbed above normal from the 6th to the 9th and then well below normal again from the 10th through the 13th. The month’s lowest temperatures came on the morning of the 12th with Sibley dipping to 23° with some eastern Iowa locations setting daily record lows the next day. A remarkable warm-up began in western Iowa on the 13th and spread statewide on the 14th. Sioux City reported the state’s first 90 degree reading of the year on the 13th and then soared to a local monthly record 106° on the 14th. While not an Iowa record for May, Sioux City’s 106° reading came a full 15 days earlier than the previous record early date for that threshold (set in 1934). Clinton went from a daily record low of 33° on the 13th to a daily record high of 93° the next day. This 60 degree rise tied for the largest warm-up from one day to the next at any time of the year among 121 years of record at that location.
Heating Degree Day Totals. Heating degree day totals, an estimate of home heating requirements, averaged 163% greater than last May and 17% greater than normal. The 2012-2013 heating season (since July 1, 2012) has averaged 26% greater than last season and 2% more than normal.
Precipitation. The month began with a May record snow event. Snow began in northwest Iowa early on the morning of the 1st with six inches reported at Sibley. Heavy snow shifted to north central Iowa on the night of the 1st into the morning of the 2nd with Forest City and Britt setting state records for one day May snowfall of 11.0 inches (old record 10.0 inches at Le Mars on May 28, 1947). The heavier snow began to shift southward on the night of the 2nd into the morning of the 3rd but not before Osage set a state record for May total snowfall of 13.0 inches. Southern Iowa snow totals included 7.5 inches at Indianola by the morning of the 3rd and 8.0 inches near Promise City in Wayne County. The winter storm system slowly moved back to the west with a few snow flurries persisting into the afternoon of the 4th. Slightly higher temperatures kept precipitation from this storm completely in the form of rain over far eastern Iowa with two to four inches falling in southeast and east central areas. Precipitation continued to be frequent, but not as heavy, through the 9th. A welcome period of mostly dry weather began on the 10th and continued through the 18th. This dry period finally provided an extended period favorable for spring planting and in hindsight allowed river levels to subside and better accommodate much heavier rains to come. Rain occurred somewhere in the state ever day from the 19th through the end of the month. Widespread rain fell on the 19th-20th with heavy rain in Mitchell County where Saint Ansgar reported 5.34 inches. However, the wettest weather in most areas came over the Memorial Day weekend. Heaviest rains were centered over northwest Iowa on the 25th-26th with 8.10 inches at Sioux Center, 8.83 inches near Cleghorn and 8.25 inches at Cherokee. Another area of torrential rain occurred over central Iowa with 8.17 inches at Zearing, 7.44 inches at Marshalltown and 8.04 inches at Grinnell. Flooding was widespread over the state and numerous records for rainfall were set. Rain totals for the month ranged from 3.34 inches at Clinton (0.59 inches below normal) to 16.63 inches at Zearing. Two locations set records for wettest calendar month: Grinnell 16.16 inches this May (old record 14.17 inches in June 1924 among 130 years of record) and Toledo with 14.89 inches (old record 14.73 inches in August 1977 among 119 years of record).
Severe Weather. The very quiet start to Iowa’s severe weather season ended on May 19 when severe thunderstorms were reported from 44 counties. A record streak of 359 consecutive days without a tornado in Iowa, which began on May 25, 2012, finally ended with several brief tornado occurrences on the 19th (old record 355 days from May 7, 1955 through April 25, 1956).
Spring Summary. Iowa temperatures averaged 43.7° or 4.6° below normal while precipitation totaled 17.66 inches or 7.44 inches above normal. This ranks as the wettest and fifth coolest spring among 141 years of records. The previous wettest spring was 1892 with 15.36 inches and a cooler spring has not been recorded since 1960. The other cooler springs were all in the 19th Century (1875, 1882 and 1888). This has also been the wettest year-to-date with 19.93 inches of precipitation through the end of May. The old record was 17.72 inches set the first five months of 1973.
Harry J. Hillaker, State Climatologist
Iowa Dept. of Agriculture & Land Stewardship
Wallace State Office Bldg., Des Moines, IA 50319
Telephone: (515) 281-8981; E-mail: Harry.Hillaker@IowaAgriculture.gov