Midwest Weekly Highlights - May 25-31, 2008
Cold Weather Persists
The last week of May was characteristic of the rest of the month - cool and generally wet. Signs of a change appeared the last two days of the month as the warmest weather of the season and humid conditions fueled severe thunderstorms across the Midwest.
Much of the Midwest north of a line from St. Joseph, MO to Louisville, KY was below normal the last week of May, ranging from near normal along the aforementioned line to 6°F to more than 8°F below normal in northern Minnesota (Figure 1). South of this line temperatures were 2°F to 4°F above normal.
Rainfall was generally above normal across the western two-thirds of the region, as high as three to four times normal across central Iowa (Figure 2). Less than 25 percent of normal rainfall was received in northern Kentucky, southern Ohio, and northeastern Ohio. The continued wet weather across the heart of the corn and soybean belt continued to have an impact on agriculture. Planting was able to progress in some areas, but in other areas crops will have to be replanted due to excessively wet conditions (Figure 3). Some dryness continued to be depicted on the May 27 U.S. Drought Monitor in Minnesota and Wisconsin (Figure 4), but precipitation occurring late in the period should help improve conditions.
Vicious Storms Hit Iowa and Minnesota
Most of the Midwest enjoyed a sunny but cool start to the long memorial Day weekend. However, a strong low pressure system in the Central Plains funneled warm, humid air northward through Missouri and into Iowa and Minnesota (Figure 5). By mid to late afternoon thunderstorms erupted in Kansas, Missouri, Iowa, and Minnesota, and by the morning of May 26 severe weather reports were received from southern Missouri to northern Minnesota and Wisconsin, and into western Illinois.
Tornadoes first hit eastern Minnesota/western Wisconsin during the afternoon. At 4:38 p.m. a tornado touched down near Coon Rapids, MN (Anoka County), causing moderate tree and roof damage along its six-mile path. At 4:55 p.m., a second and more damaging twister touched down near Lino Lakes, MN (Anoka County) and struck the town of Hugo, MN (Washington County). This tornado, rated an EF3, destroyed at least 50 homes and seriously damaging 100 others. The tornado resulted in the death of a 2-year-old child in Hugo, and seriously injured eight other people. Two other weaker tornadoes were also confirmed in eastern Minnesota with the thunderstorms. In addition to the tornadoes, hail 2.0 to 3.5 inches in diameter was reported with the severe thunderstorms in Minnesota and Wisconsin. A storm summary, radar images, and storm damage photos can be seen at the NWS Chanhassen, MN web site.
Two hundred miles to the south, a supercell thunderstorm spawned a large tornado at 4:48 p.m. in northern Grundy County, IA (Figure 6). The tornado struck Parkersburg, IA (Butler County) at 4:59 p.m., destroying the southern half of the town. The tornado, rated EF5, ranged from one-half to three-quarters of a mile wide and was on the ground for more than an hour traveling 43 miles. The storm resulted in seven fatalities and injured more than 70 people. A preliminary estimate by the Federal Emergency Management Agency and the Iowa Homeland Security and Emergency Management Agency attributes $5.8 million in damages to the storm, most in Butler County. Iowa Governor Chet Culver declared Butler, Buchanan, and Black Hawk Counties disaster areas. The last time an EF5 tornado occurred in Iowa on June 13, 1976. For more information on this storm, including maps of the tornado track, radar images, and damage photos, visit the Des Moines, IA NWS web site.
In addition to the tornadoes in Iowa and Minnesota, two brief EF0 tornadoes touched down in Perry and Jackson Counties in Illinois.
Much Colder, then Warmer
The cold front slowly pushed through the Midwest on May 26 and May 27. After highs in the low 80s on May 26 (Figure 7), temperatures plummeted behind the front, and high temperatures across the northern half of the region were reached at midnight in the northern half of the region. Easterly winds, cloudy skies, and occasional rain kept daytime temperatures on May 27 in the low to mid 50s as far south as central Illinois (Figure 8), more than 20°F below normal. The high pressure system responsible for the cool weather moved east by May 29, and warmer weather began to return as winds turned into the south. The stage was also being set for more severe weather across the region (Figure 9).
Torrential Rains and Severe Weather Close out the Month
A mesoscale convective system (MCS) moved across Iowa overnight May 29 through the morning of May 30 (Figure 10). A number of these thunderstorms were severe during the evening and nighttime hours of May 29, and there were at least four tornadoes reported in Iowa. More information on the tornadoes can be found at the NWS Des Moines, IA web site. In addition to tornadoes and damaging winds, thunderstorms in this complex dropped 4.50 to 5.50 inches of rain in central Iowa, causing widespread flash flooding and causing many rivers and streams in Iowa to remain near or above flood stage. The hardest hit area was from Greene County east through Story County (Figure 11).
The MCS continued eastward into northern Illinois during the morning and a Tornado Watch was posted for western Illinois. The storms gradually weakened through the morning, but the decaying storms led to a wake low developing in northern Illinois. A wake low is a small-scale low pressure area that often develops behind a squall line, and they often are accompanied by strong non-thunderstorm winds due to the strong pressure gradient that is produced. This low resulted in sustained winds from 45 to 55 mph with gusts to 65 mph for up to 30 minutes across northern Illinois. The winds did result in damage to trees and power lines. A 37-foot long charter boat with seven people on board capsized in Lake Michigan near Waukegan. All those on board were rescued by the Coast Guard a half hour later. More information on this unusual event can be found on the NWS Chicago (Romeoville) web site.
After a break of a few hours, storms rapidly developed in the afternoon ahead and along the leading edge of the cold pool of air left from the overnight MCS. The thunderstorms became severe from Missouri through Illinois, Indiana, and Ohio as the afternoon and evening progressed. The were a number of reports of hail great than two inches in Missouri, Illinois, including a report of softball-size hail in Champaign County, IL. There were tornadoes reported from Missouri through Ohio, most resulting in minor damage. However, one tornado in Hancock County, OH badly damaged 10 homes producing EF2 damage. The Illinois legislature had to take cover in the basement and pedestrian tunnels in the capitol when a tornado warning was issued for Springfield (Sangamon County) shortly before 7:00 p.m.
Many of these thunderstorms also produced very heavy rain. Milford, IL (Iroquois County) received 6.00 inches of rain in two hours, and flooding from the rain still had roads closed on May 31. Rainfall in excess of three inches was common from central Illinois through Indiana. Some corn and soybeans will have to be replanted due to the continued wet weather and ponding in fields.
Scattered severe storms occurred again on May 31 from Minnesota southeast into northwestern Illinois, and in Missouri into southern Illinois and western Kentucky. In Mahnomen County, MN hail one inch in diameter was reported to have continued for 20 minutes, covering the ground.