Average Temperature Departure 5/1-5/7
Average Temperature Departure 5/8-5/10
Total Precipitation
Ohio River Gage at Cairo, IL

Midwest Weekly Highlights - May 1-10, 2011

Cool Start, Hot End

The first ten days of May were cooler than normal across the region, although at the end of the period summer-like warmth surged northward, bringing western portions of the region to near normal. Average daily temperatures rainged from 3°F to 4°F below normal from central Minnesota to central Wisconsin, to 1°F above normal in extreme western Iowa (Figure 1). An upper level trough over Hudson Bay dominated the flow the first week of the month, maintaining a flow of cool air into the Midwest (Figure 2). Temperatures during this period ranged from 3°F to 7°F below normal across much of the Midwest (Figure 3). The pattern began to transistion on May 7th as a trough moved into the western U.S. and ridging developed in the east. During the last three days of the period this trough/ridge pattern produced strong southwesterly flow across the central U.S. (Figure 4), resulting in mid-summer like conditions for much of the region. Temperatures for the period May 8-10 ranged from near normal in northern Lower Michigan to 16°F above normal in the extreme west (Figure 5). Maximum temperatures on May 10th were well into the 90s from northern Missouri north to southern Minnesota and east to northern Illinois (Figure 6). In Iowa the first official 100°F reading of the season was recorded in Jefferson, IA (Greene County) on May 10th. The last 100°F reading in Iowa during May came on May 15, 2001 at Sioux City.   The only earlier 100 degree readings were April 22, 1980, May 6, 1934 and May 8, 1934.

During the first six days of the month numerous record low minimum and record low maximum temperatures were set across the the region, including a number of records for any date in the month. On May 9-10 numerous record high minimum and record high maximum temperatures were set or tied, espcially across the western half of the region.

Rain: Feast or Famine

Precipitation during the first ten days varied greatly across the region (Figure 7). It was very wet in the Ohio Valley and in northwestern Minnesota, but very dry from Iowa northeast to southern lower Michigan. On the first two days of the month some snow was reported in far northern Minnesota, with 1.3 inches measured in Warren (Marshall County). Rainfall in the Ohio Valley was 200 to 300 percent of normal, with most of that rain falling in the first two days of the month. Rainfall was less than 25 percent of normal in an west to east band from western Iowa to lower Michigan. The dry weather across Iowa allowed corn planting to move rapidly, and by May 9th 69 percent of the corn crop was planted, which is the five-year average and the highest in the region as of this date. On the other side of the coin, record rainfall in April and major river flooding in the Mississippi and Ohio River Valleys has significantly impacted planting progress there, while cool and wet weather has delayed planting in the upper Midwest.

Planting Progress as of May 9, 2011

Corn Planted
5-year average



Deluge in the Mississippi and Ohio Valleys

Heavy rain continued over the southern Midwest the first three days of May, with much of the rain accumulating May 1-2 (Figure 8). Three-day totals were generally from 3.00 to 6.50 inches, further exacerbating the serious flooding on the Mississippi and Ohio Rivers and tributaries. On the night of May 2nd the U.S. Army Corps of Engineers blew an 11,000 foot hole in the Bird's Point, MO (Mississippi County) levee to release water into the New Madrid Floodway and relieve pressure on the flood walls at Cairo, IL (Alexander County). The levee breach flooded more than 130,000 acres of farmland and required the evacuation of more than 300 homes. Within 24 hours of the breach the river at Cairo had dropped more than a foot and a half (Figure 9). The Ohio River reached a new record flood stage of 61.72 feet at Cairo on 10:00 p.m. CDT on May 2nd. The top of the flood wall at Cairo is 64 feet.

Before and after photos of the Bird's Point levee breach on May 2, 2011 (NASA Earth Observatory)

At the end of this period, minor to moderate flooding was still occurring on the Mississippi River and some tributaries from New Boston to Cape Girardeau, MO (Figure 10). Moderate to major flooding was occurring on the Ohio River from Mt. Vernon, IN to the confluence of the Ohio and Mississippi Rivers at Cairo, IL (Figure 11).

Scattered Severe Weather

There was no severe weather reported in the Midwest the first five days of the month and then scattered severe storms May 6-7. Thunderstorms produced several weak tornadoes in east-central IL on May 7. A photographer in Champaign County, IL took this photo of a funnel cloud/possible tornado northeast of Urbana, IL (used with permission). No damage was reported. Winds from thunderstorms on May 9th in west-central and northern Wisconsin knocked over trees and power lines, cutting power to 15,000 customers. On May 10th thunderstorms in Ohio produced hail to 2.00 inches in diameter (Figure 12). Heavy rain from the storms produced intense flash flooding in Jackson County, OH, necessitating the evacuation of 100 people from an apartment complex and leading to a number of water rescues. An estimated 5.50 inches of rain fell in just a few hours (Figure 13). Storms in Minnesota spun up an EF-1 tornado near St. Michael, MN (Wright County).

The Iowa State State Office and Minnesota State Climate Office also contributed to this report.

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