Average temperatures for the month of May ranged from the lower to upper 50s across Minnesota, Wisconsin, northwest Iowa, and the northern half of Michigan, and gradually warmed to the 60s across southeast Iowa, southern Michigan, Missouri, Illinois, Indiana, and Ohio (Figure 1). Southeast Missouri, roughly the southern halves of Illinois, Indiana, and Ohio, and all of Kentucky observed average temperatures in the upper 60s. Average temperatures were predominantly 1-5°F above average east of the Mississippi River, and 2-3°F below normal west from the Mississippi River (Figure 2). Northeast Ohio observed average temperatures for the month were 6°F above normal. The monthly average maximum temperatures for the region followed a similar distribution to the average observed temperature, with warmer temperatures in the eastern half of the region (Figure 3). Monthly average minimum temperatures were 2-5°F warmer than normal across much of the region, with western Iowa and Minnesota experiencing average minimum temperatures for the month of May (Figure 4). Overall, the region experience its 53rd warmest May on record (1895-2015), only 0.1°F above normal (59.4°F). The warmest state was Ohio with an average temperature 3°F above normal, making May 2015 the 21st warmest May on record. The coldest state, with a departure of 2.3°F below normal, was Minnesota — ranking 43rd coldest on record. A warm spell over the eastern half of the region broke many daily maximum high temperature records when high temperatures climbed into the mid and upper 80s May 7th - 11th. A cold spell over the region May 19th - 22nd broke many daily minimum low temperature records and brought frost to northern latitudes of the region.
Seasonal Temperature Summary
Meteorological spring (March 1 through May 31, 2015) in the Midwest experienced average temperatures in the upper 30s across the arrowhead of Minnesota and the Upper Peninsula of Michigan, gradually warming into the upper 40s heading south into central Iowa, northern Illinois, northern Indiana, and northern Ohio. South of these areas, average temperatures for March-May 2015 were in the low to mid 50s, with the warmest temperatures in the upper 50s across southern Missouri, southern Illinois, far southern Indiana, and much of Kentucky (Figure 5). In regards to normality, the region was near to slightly above normal (0.7°F departure) ranking 22nd warmest on record (1895-2015) (Figure 6). All states were above normal for the season; however, only Minnesota experienced warmth high enough for 2015 to be placed in the top 20 warmest springs. It tied for the 18th warmest spring on record.
Drought Relief in the Northwest, Dryness in the Southeast
May brought over 5 inches of precipitation across much of Minnesota, central Wisconsin, and portions of Iowa, Illinois, northwest Indiana, and all of Missouri (Figure 7). Western and southwest Missouri recorded 7.5 or higher inches of precipitation. Other locations in the region observed between 2-5 inches of rain, with the driest areas being southern Ohio and eastern Kentucky where only 1-2 inches of rain fell. This pattern provided 100 percent of rainfall to much of the region except for the southern half of Indiana, the southern two-thirds of Ohio, and the eastern two-thirds of Kentucky (Figure 8). These portions only received 25-75 percent of normal rainfall. Therefore, the southeast portion of the Midwest region saw rainfall 0-4 inches below normal, while the remaining part of the region, except for some isolated areas in Iowa, southeast Wisconsin, and the thumb of Michigan, saw rainfall 2-4 inches above normal (Figure 9). The Midwest region was 0.14 inches below normal for May, having observed 4.06 inches of precipitation. The wettest state as a departure from normal was Minnesota, with May 2015 ranking 15th wettest on record for the state. Minnesota observed 4.8 inches of precipitation, with May’s normal precipitation being 3.19 inches. The driest state, experiencing close to a 3-inch deficit of rainfall, was Kentucky. May 2015 experienced only 2.39 inches of rain, 2.9 inches below normal. It was Kentucky’s 13th driest May on record.
Seasonal Precipitation Summary
Spring precipitation across the Midwestern region was primarily concentrated along the Ohio River Valley. Seasonally, 15+ inches fell across much of Kentucky, southern Indiana, southern Illinois, and southern Missouri, with 20+ inches being observed in the bootheel of Missouri (Figure 10). As an entire region, precipitation was 0.4 inches above normal, ranking the 41st wettest meteorological spring on record. Overall, the region saw 100% or greater seasonally normal precipitation along the Ohio River Valley, with 75% of seasonal precipitation observed across the central corridor (Figure 11). Central Wisconsin and northwest Minnesota received 100% of seasonal precipitation with other, smaller regions in Minnesota and Wisconsin observing 75-99% of normal rainfall. The Ohio River Valley had precipitation values 3-6 inches above normal with parts of Minnesota, Wisconsin, and Michigan observing 0-3 inches above normal precipitation (Figure 12). The driest state in the region was Iowa, having experienced 88% of its normal precipitation. Kentucky was the wettest state, with a departure of 3.56 inches above normal. The March-May 2015 time frame was Kentucky’s 15th wettest meteorological spring on record. For the region, March-May 2015 was the 41st wettest meteorological spring on record.
May Crop Progress
A split weather pattern of drier and warmer weather in the northwest part of the Midwest region, and wetter weather across the southeast portion of the Midwest region in late April and early May, allowed many farmers across the northwest states to get into the fields earlier than average and complete planting. Much of the region was above the 5-year average for corn planting. Minnesota and Wisconsin were the states to benefit most from the drier and warmer weather pattern before heavier rain events came later in May. Those two states had 100 percent and 85 percent of the crop planted across the states, respectively, by the week ending on May 17th, 2015. This was roughly 60-70 percent greater than the 5-year average. The wetter conditions in the southeast portion of the region delayed planting some, but all states remained ahead of the 5-year average planting dates. Soybean planting in Minnesota was also much ahead of schedule, with 88 percent of the crop planted, with the 5-year average normally at 59 percent by this time of year. Remaining states had 60-76 percent of soybeans planted by May 26th, with Missouri only at 20 percent planted.
Severe Weather Events
May was an active severe weather month across the Midwest region. Tornado and wind events were the predominant reports. On May 10th -11th, wind damage was reported in southern Illinois and southeastern Missouri, and a confirmed EF-1 tornado moved through Lake City and Rockwell City, IA. The system progressed eastward May 11th, resulting in winds of 50-70 miles-per-hour across eastern parts of Indiana and Kentucky as well as most of Ohio. Northeastern Ohio was hit particularly hard with reports of mobile homes sustaining major damage. May 15th -17th was another active period. Strong thunderstorms formed on May 15th across Missouri into central Illinois and then across eastern Indiana as the system progressed eastward. The most severe storms developed on May 16th as the main low pressure system organized in the eastern Dakotas. Wind damage was reported from Iowa through Missouri with embedded EF-0 & EF1 tornadoes in storms that fired along the cold front. Closer to the main low, tornadoes formed in central and western Minnesota. Preliminary reports from the National Weather Service-Chanhassen identified 15 separate tornadoes. May 17th brought Wisconsin its first tornado of the year. Additional severe weather events with wind and tornado reports occurred on May 24th (Iowa, Missouri, and Illinois), May 26th (Wisconsin, Illinois, and Ohio), and May 30th (Indiana).
A wet period from May 8th -21st brought some much needed rainfall to Minnesota, downgrading moderate drought conditions to abnormally dry conditions in much of Minnesota and Wisconsin, according to the U.S. Drought Monitor (Figure 13).