May 2000
Temperature and Precipitation Anomalies.
May was considerably wetter than the past two spring months in the Midwest. A large swath from southwestern Minnesota, through northeast Iowa, northern Illinois, and southern Wisconsin, into lower Michigan recorded 150% or more of normal precipitation (Figure 1). Michigan experienced its 9th wettest May in 106 years. In addition, an area centered on east-central Missouri also received a tremendous amount of precipitation, most of which arrived during a severe event on May 6-7. Despite these two high precipitation sub-regions, the Midwest had only its 37th wettest May on record. Most of Iowa and northern and western Missouri received well under the normal precipitation amount, as did northern Minnesota, northern Wisconsin, and the upper peninsula of Michigan. The southern Ohio River Valley was also somewhat dry compared to normal. Therefore, while the northern and eastern edges of drought were reduced to a more normal status, the central core of the drought remained and even intensified in Iowa and Missouri.
The Midwest states individually averaged between 1.5-3.0 Deg F warmer than normal, with only isolated pockets below normal in the Midwest (Figure 2). May 2000 ranked as 25th warmest for the Midwest, because all states in the region were at least somewhat above normal in temperature. However, no individual state ranked greater than 23rd warmest (Kentucky), so the general pattern is only modestly warmer than normal.
The extent of severe drought in the Midwest has contracted since the beginning of May. Only western and southern Iowa, eastern Nebraska, much of Missouri, and west-central Illinois are still classified as having severe drought (Figure 3, National Drought Mitigation Center). The northern tier of the drought has been dramatically reduced by substantial rains in northeastern Iowa, northern Illinois, and locations further north and east. Only two Midwest states report top soil to be short or very short for a large segment of their agricultural land: Iowa (40%) and Missouri (60%). Kansas and Nebraska in the HPCC region are also quite dry.
The condition of corn and soybean crops in the Midwest is very good to excellent in most locations. The May rains have been quite favorable to corn that was planted earlier than normal due to the dry and warm spring. However, some soybeans have had too much rain and cool conditions during the second half of May, and are not in as good condition as the corn. Of the drought-affected states, only Missouri has crop conditions deteriorating, with only 52% of corn and 46% of soybeans rated as in good or excellent condition. It appears that Iowa will be the next to deteriorate, if timely rains do not reach the southern half of the state. Much of Iowa, Missouri, Illinois, and Indiana lack sufficient subsoil moisture to weather a summer dry period without crop damage (Figure 4, Climate Prediction Center). On the other hand, there are farmers in southern Minnesota, northeastern Iowa, southern Wisconsin, northern Illinois, and a small region south of St. Louis, Missouri that have had fields flooded by torrential rains or damaged by severe winds and hail, thus requiring replanting as soon as the soil is dry enough to allow access to farm equipment.
The first two weeks of May heralded a return to more seasonably wet conditions in most of the Midwest. A compact low traveled through the area on May 2, leaving 0.5 to 1.5 inches of rain in a band from northeastern Illinois to Ohio. Conditions were warm and stagnant for several days following, with little rain except for a very unusual event on May 6-7, when the core of an upper level low triggered the formation of a mesoscale convective system in eastern Missouri. A small area just southwest of St. Louis received between 9 and 14 inches of rain in 24-hours, leading to major flooding along many rivers and creeks in that limited area; a wider area around St. Louis received 2 to 6 inches of rain (Figure 5). Three cooperative observers reported more than 9 inches of rain for this event: Union, 13.40 inches; Washington, 10.48 inches; and Rosebud, 9.47 inches. All three stations exceeded the 100 year return interval for a 24-hour precipitation total, and it appears that most of the rain actually fell within a 9 hour period. This would correspond potentially to a 400-year event. However, the state of Missouri one-day record remains the 18.18 inches of rain that fell in Edgerton on July 20, 1965.
The May 6-7 storm affected an area of about 4,000 square miles, with over 1,900 square miles receiving more than 6 inches of precipitation. This area experienced a 100-year rain event. The storm created major flash flooding along creeks and rivers. The Bourbeuse River rose over 16.5 feet in a few hours, the Big River rose to 20 feet, and the Meramec River rose to 22 feet, which is 9.5 feet above flood stage. Franklin and Jefferson Counties have reported two deaths in flooded cars, about 125 structures destroyed or severely damaged, and another 400 structures and many elements of infrastructure seriously damaged. The rapid rise of local creeks and rivers created walls of water that swept away soils, cattle, houses, trailers, business structures, bridges, trees, and crops. Preliminary damage estimates by Missouri officials reached $100 million. In Jefferson County alone, approximately 100,000 acres of corn, wheat, and soybeans were damaged, causing an estimated loss of $19 million. Franklin and Jefferson counties were declared federal disaster areas, and along with eight adjacent counties are now eligible for federal disaster assistance (Figure 6, from FEMA). All of these events occurred in the midst of a severe drought in this region.
During the second week of May, significant rain occurred on almost every day somewhere in the Midwest. A slow moving cold front entered the region late on May 7th, and took three days to clear the eastern borders of Ohio. More than 1 inch of rain fell in Minnesota on the 8th, a wide band from southwest Missouri to northeast Lower Michigan on the 9th, and much of the central and eastern Midwest on the 10th. About 235,000 customers in the Chicago area lost power in the windy storms that passed through on the evening of the 8th, with more than 30,000 without power for more than 12 hours. Light rain returned to Minnesota and Wisconsin with a warm front on the 11th, and dumped 0.5 to 1.0 inches of rain in Minnesota, Wisconsin, and Michigan on the 12th as a strong cold front and low with damaging storms passed through. Finally, the cold front left the Midwest on the 13th and cleared out the remaining precipitation.
The storms on the 11th and 12th caused significant injuries and damage in Iowa and Wisconsin, where tornadoes and straight-line winds exceeding 100 mph occurred. The town of Dunkerton, IA, had its second major disaster in a year, having been severely flooded last May. A nursing home there was damaged by a tornado, resulting in six injuries and the evacuation of 139 residents. A Cedar Falls women lost an arm when her trailer home was destroyed. A number of others were injured in the Iowa tornadoes. In Wisconsin, a straight-line wind storm with winds of up to 110 mph and baseball sized hail hit east-central portions of the state, injuring 31 people. The village of St. Nazianz was most severely affected, with more than twenty injured when several dozen trailer homes were tossed off their foundations, and even more sturdy houses and businesses suffered serious damage.
On the 18th, tremendous hail storms resulted from a frontal system associated with a strong upper-level jet stream moving across Iowa, southern Wisconsin, Illinois, and Indiana (Figure 7, Storm Prediction Center). In the warm sector, a series of supercell thunderstorms formed in east-central Illinois, spawning a few tornadoes and very large hail measured at up to 4.5 inches in diameter. Hail of this size destroyed a large greenhouse, and caused considerable roof and automobile damage. Emerging crops in this location were also damaged. Many of the hail storms in the four states produced damaging 2 inch hail, an extremely infrequent event in the Midwest. Along the front itself, more tornadoes were spawned in Iowa and northern Illinois, and both hail and strong winds caused considerable damage, especially in the suburban areas of northeastern Illinois. Winds exceeding 85 mph in the Chicago suburbs brought down trees and power lines. Two persons were killed during these storms, and 250,000 customers lost power for up to 24 hours. More than 100 flights were canceled at O'Hare Airport. Insured losses in Illinois presently stand at $250 million for this event.
On May 31st, a substantial number of communities and highways were affected by flash flooding and river flooding in southern Minnesota and Wisconsin. Areas in southwestern Wisconsin received more than 6 inches of rain in bands of training thunderstorms following a stationary front draped across the region. Near-record rainfall totals for the month of May have been recorded in several communities in southern Wisconsin. A substantial number of rivers are near record flow for the time of year. Further reports on this situation will be forthcoming in the June report.