Harry Hillaker, state climatologist with the Iowa Department of Agriculture and Land Stewardship, has released a preliminary summary of the weather for June and the first half of 2008.
The full report is below, but it shows that 2008 has had the wettest first six months of any year among 136 years of records. The statewide average precipitation was 24.30 inches. The previous record was 1951 when Iowa received 23.55 inches.
A map of the precipitation surplus in each county is available on the department's Web site at www.IowaAgriculture.gov.
June 2008 was second-wettest in 136 years
The preliminary June 2008 statewide average precipitation total is 8.86 inches for June. However, there are numerous locations across Iowa that recorded more than one foot of rain in June. The greatest preliminary totals are 15.13 inches at Dorchester in far northeast Iowa and 15.05 inches at Massena in the southwest.
This ranks as the second wettest June among 136 years of records. Only June 1947 was wetter at 10.33 inches. This is the fifth top four finish among the monthly rankings since last August. August 2007 was the wettest August on record, October 2007 ranked 4th wettest among Octobers and December 2007 ranked second wettest among Decembers. April 2008 preliminarily ranks second wettest April.
Iowa Ag Secretary Bill Northey made this statement regarding the weather summary: "Everyone knows intuitively that the rain and severe weather we have received so far this year is unprecedented, but these numbers show it with even more clarity. Iowans have responded heroically to the challenges that have come upon our state, now let's hope the weather becomes more agreeable."
IOWA PRELIMINARY MONTHLY WEATHER SUMMARY – JUNE 2008
From Harry Hillaker, State Climatologist
General Summary: June temperatures averaged 69.7º or 0.1º below normal while precipitation totaled 8.86 inches or 4.22 inches above normal. This ranks as the 2nd wettest and 66th warmest June among 136 years of state records. Only five calendar months have been wetter since statewide averages were first calculated in 1873: July 1993 with 10.50 inches; June 1947 (10.33); August 2007 (9.78); September 1926 (9.65) and September 1965 (9.65).
Temperatures: Iowa recorded an exceptionally small range of temperatures during June. Daytime highs reached into the 90's on only three dates (7th, 8th and 25th) with a statewide maximum of 92º recorded on the 7th at Onawa and Little Sioux. A lower June maximum temperature has not been recorded since 1928. Meanwhile the month's lowest temperatures were 45º readings at Cresco on the 2nd and at Chariton and Cresco on the 17th.
A higher June minimum temperature has not been recorded since 1932. The overall monthly range in extreme temperatures was only 47º, the smallest June temperature range on record (previous record was a 51º range in 1932, from 48º to 99º). The first half of June averaged 2.1º warmer than normal and it appeared that Iowa would break its string of six consecutive colder than normal months. However, the second half of the month averaged 2.2º cooler than normal, allowing the cold weather streak to extend to seven consecutive months.
Precipitation: Iowa endured an exceptionally wet period from May 29 through June 12 when a statewide average of 8.99 inches of rain fell (normal for the period is 2.45 inches). Daily statewide average rain statistics are not available very far back in time but it is doubtful that a larger amount of rain has been recorded in Iowa in only 15 days.
Record flooding occurred along the entire length of the Cedar River, the Iowa River below Marshalltown, the Mississippi River along Louisa and Des Moines counties and a portion of the Des Moines River between Fort Dodge and Des Moines. Many other smaller rivers and streams also recorded major or record flooding. The most exceptional flooding occurred at Cedar Rapids where the Cedar River crested on June 13 at 19.12 feet above flood stage and 11.12 feet higher than the previous record crests recorded in 1929 and 1851.
Monthly precipitation totals reported thus far vary from 3.35 inches at Sioux City (0.26 inches below normal) to 15.05 inches at Massena and 15.13 inches at Dorchester (10.42 inches above normal). It is expected that higher totals occurred but have yet to be reported.
Severe Weather: Flooding was obviously the primary severe weather element of the month. However, other forms of severe weather were very frequent. A tornado resulted in four fatalities at a Boy Scout camp near Little Sioux on the evening of the 11th. Iowa has now recorded the most tornado related fatalities (12) since 1968. Non-flood related severe weather was reported from 97 of Iowa's 99 counties during June (Audubon and Pocahontas were the exceptions). Severe storms were reported somewhere in Iowa on 21 days during the month with Benton County alone reporting severe weather on ten dates.
Outlook: The latest National Weather Service 30-day outlook favors cooler-than- normal weather for Iowa during July with near normal precipitation. Normal July precipitation is 4.25 inches. If this amount is realized this would be Iowa's driest month since March. Localized downpours are almost a given in Iowa during the summer months. Thus it is likely that flash flooding will occur somewhere in the state. May and June are typically Iowa's most active months for severe thunderstorms. July is still part of Iowa's severe weather season but the month usually brings about one-half the amount of severe storms as May or June.
First One-Half of 2008: The first six months of 2008 were the wettest recorded since statewide records began in 1873. The statewide average precipitation was 24.30 inches or 7.96 inches above normal. The previous wettest January through June period came in 1951 with 23.55 inches of precipitation. Meanwhile, Iowa also recorded the coolest start to the year since 1982. Statewide temperatures have averaged 39.8º or 3.0º below normal. This ranks as the 17th coolest January through June period among 136 years of state records. The temperature has failed to exceed 92º anywhere in Iowa so far this year. This is Iowa's lowest year-to-date maximum temperature since 1877 (also 92º that year).
Factors Leading to the Historic Flooding of 2008: As in 1993, there have been several factors that have contributed to the severe flooding in 2008. First, the state was already saturated with moisture before the year even started. Last year was Iowa's fourth wettest year among 135 years of state records. Second, this past winter was unusually cold and wet. Record or near-record snowfall was reported from parts of eastern Iowa while overall Iowa recorded its 21st coldest and 8th wettest winter among 135 years of data.
Next, the spring was also unusually cool and wet, ranking 29th coldest and 12th wettest spring among 136 years of data. The lower temperatures are significant in that they result in lower evaporation rates, thus slowing the rate of drying. The magnitude of the rains in the May 29 to June 12 period were such that very severe flooding would have resulted anyway, but these antecedent conditions definitely made things worse. Virtually the same combination of factors were present in 1993 as well.