May 1-10, 2010
Midwest Weekly Highlights - May 1-10, 2010
A Roller Coaster Start to May
The first ten days of May ranged from mild to very warm to unusually cool, and from dry to occasionally wet. When all was said and done, most of the region received good rainfall, and temperatures ranged from much cooler than normal in the west to warmer than normal in the east. May snow fell in parts of Minnesota and Wisconsin, while flooding rains affected Kentucky.
Temperatures for the ten-day period ranged from 8°F below normal in western Minnesota to 3°F to 4°F above normal in Ohio, with the dividing line between above and below normal roughly along the Mississippi River (Figure 1). There were numerous record temperatures set during this period, with record highs and record high minimum temperatures set during the first few days of the period, and record lows and record low maximum temperatures during the last three days.
Much of the Midwest received above normal rainfall this period, with Kentucky accumulating 200 to 400 percent of normal (Figure 2). In contrast, southwestern Minnesota and the western half of Iowa were very dry, with precipitation 10 to 25 percent of normal. A band from central Missouri into central Illinois recorded less than 75 percent of normal rainfall. There was little change in the May 4 U.S. Drought Monitor this week in the Midwest (Figure 3), but much of the precipitation in the drought areas occurred after the map was issued.
Frequent Weather Systems
The first ten days of May were characterized by the frequent passage of weather systems through the Midwest. The month got off to an auspicious start as a front stalled in a north-south orientation in the central Midwest (Figure 4). This was the focus of three days of torrential rains in the lower Ohio Valley. While Tennessee was the most seriously affected by the rain, western and central Kentucky received five to more than ten inches of rain. Based on preliminary reports, the highest amount reported for this event was 10.14 inches at a Kentucky Mesonet station in Warren County. The heavy rain resulted in serious and widespread impacts including flash flooding, river flooding, dam failures, road closures, evacuations, and water rescues. Two deaths were reported as a result of the flash flooding. Many of the rivers in Kentucky crested at one in the top ten highest levels on record. Louisville (Jefferson County) received 1.32 inches of rain on May 1, resulting in a sloppy, muddy track for the running of the Kentucky Derby. The wettest derby on record was in 1918, when 2.31 inches of rain fell.
This system also produced an outbreak of severe weather on May 1 that was largely concentrated in the northern Gulf States. However, tornadoes touched down in southeastern Missouri and in western Kentucky.
Once the system responsible for the heavy rain moved to the east, two cold front passages ushered in dry and cooler air. The second of these produced severe weather in southeastern Minnesota and in central Wisconsin, including one tornado that caused damage in Winnebago County,WI on the evening of May 4. After this second front, dewpoints were in the 30s and 40s across the region (Figure 5), indicative of very dry air and unusual for May. The dry weather allowed planting activities to continue in full swing except in the areas affected by the heavy rain early in the week. By May 9, 84 percent of the corn was planted in the nine Midwestern states, compared to 36 percent last year and the five year average of 60 percent.
Snow Returns to Upper Midwest
After an uncharacteristically snowless April across much of the upper Midwest, and storm system developing in the Central Plains on May 6 brought several inches of snow to parts of Minnesota, Wisconsin, and Michigan (Figure 6). Some of the snow melted as it fell, but snowfall was heavy enough to disrupt travel. Snowfall in northern Wisconsin accumulated 0.5 to 2.0 inches, but amounts from five to seven inches fell in the Michigan Upper Peninsula. Forty locations set new daily snowfall records during the storm, with the highest amount 7.6 inches in Herman, MI (Baraga County) on May 8.
The snow and cold weather devastated the ginseng crop in northern Wisconsin. Wisconsin produces 95 percent of the U.S. crop of ginseng and is a major exporter to China. The winter-like weather hit before growers could take steps to protect their crops. Mature plants that froze will go dormant and lose a year of growth, while seedlings were killed or severely weakened. The crop is worth from $60,000 to $80,000 an acre. The majority of ginseng, about 1,400 acres, is grown in Marathon County. Initial estimates are that 80 to 90 percent of the crop in Marathon County was damaged by the snow and cold.
Thunderstorms fired to the south of the frontal boundary in the warmer air, and severe weather occurred across northern Missouri in Illinois on May 6 and then to the east in Indiana and northern Ohio on May 7. Most of the severe weather was high winds and hail, but one tornado did touch down near Bowling Green, OH (Wood County) causing damage to several homes.
While snow is not typical in the upper Midwest in May, it does occur from time to time. On May 27-29, 1947, a major snowstorm hit parts of Minnesota, Iowa, and Wisconsin, leaving behind a band of more than eight inches of snow.
Freezing Temperatures
The storm responsible for the snow in the upper Midwest moved east and was followed by a strong push of unseasonably cool air. The center of high pressure settled in over the Midwest (Figure 7), and on the morning of May 9 temperatures plummeted to near freezing across the northern two-thirds Midwest under clear skies and calm air (Figure 8). temperatures dropped into the 20s across parts of Iowa, Minnesota, and Wisconsin. In Iowa, minimum temperatures of 32°F or lower were reported from 71 percent of the state. Freeze coverage was fairly complete over all but the southwest and extreme southeast corners, plus a few scattered spots elsewhere. A more widespread freeze has not occurred so late in the season since May 13-16, 1997. Although it is too early to assess freeze impacts on recently emerged corn, it is expected that permanent damage to corn will be minimal (corn’s growth point is not above the surface yet; thus the portion of the corn plant above the ground surface may be burned off, but the plant will grow back with no loss of yield potential).
-SDH-