August 8-14, 2007
Midwest Weekly Highlights - August 8-14, 2007
Hot, Humid Weather Holds On
The southern half of the region continued to swelter this week as a string of 90°F to 100°F days continued without a break. The northern half of the region did get relief from two incursions of drier air which only slightly lowered air temperatures but provided welcome relief from the very humid conditions. Temperatures this week ranged from only 1°F above normal in northwestern Minnesota to 9°F above normal from east-central Missouri into the Ohio Valley (Figure 1).
Almost all of the rainfall this week was in the northern half of the region where several weak cold fronts interacted with the southern extent of the jet stream. Little rain fell under the upper level ridge of high pressure (Figure 2). The rainfall this week reflected the path of several thunderstorm complexes (Figure 3) which produced not only heavy rain but severe weather. The rain did not affect the overall pattern of drought in the Midwest, as the driest areas missed out. The August 14 U.S. Drought Monitor (Figure 4) shows drought continued to intensify and expand in the southern half of the region, and also intensified over parts of Minnesota. All of Kentucky is now included in the Severe Drought category, and a small portion of southeast Kentucky is in Extreme Drought. Drought diminished across northern Missouri in response to the rainfall of the last two weeks.
Southern Half of Midwest Bakes in 100°F Heat
Record temperatures were set every day this week in the southern half of the region as the upper level ridge held fast. Several weak cold fronts were unable to push very far south. While the northern portions of the region enjoyed some relief from the high dew points, maximum temperatures continued to reach or exceed 90°F on most days except for Wisconsin and lower Michigan. Missouri has taken the brunt of the heat, with much of southern Missouri experiencing maximum temperatures at or above 90°F every day this month. Joplin, MO had high temperatures of 100°F or above each day from August 10 through August 14. The heat was most intense on August 12, when much of Missouri, southern Illinois, and the western one quarter of Kentucky experienced high temperatures of 100°F or above (Figure 5). At least one or more temperatures records were set in the southern half of the region each day this week. There were four heat-related deaths reported, one in the Kansas City, MO area, and three in Illinois in the St. Louis metro east area. Most high temperature records for this region date back to the Dust Bowl or the drought of the early 1950s and still stand, although several were tied or broken this week. Daily temperature records can be seen in Table 1.
Crops SufferThe continued dry weather was having an effect on corn and soybeans in the southern Midwest and crop conditions continued to deteriorate in the southern half of the region. The biggest declines in the past two week were in Indiana, where the percentage of corn in good to excellent condition has dropped to 43 percent compared to 60 percent two weeks ago. In Kentucky soybeans declined from 69 percent good to excellent on July 29 to 46 percent on August 12. Stress from the hot dry weather has left soybeans vulnerable to spider mites which have started to colonize soybean fields. Table 2 shows the percent of corn and soybeans in good to excellent condition.
Waves of Thunderstorms Bring Much Needed Rain, but also Severe Weather
Several thunderstorm complexes developed on the periphery of the upper level ridge, bringing much needed rain to previously dry portions of the Midwest. Unfortunately, almost all the storms developed to severe levels and produced wind damage to trees and power lines.
The first of these thunderstorm complexes moved through northwestern Missouri on August 8. A line of storms began to develop during the early evening hours from southwestern Kansas into southwestern Iowa (Figure 6). The most of intense of these were in northwestern Missouri. By 9:00 p.m. the line of storms had developed into a bow echo configuration, indicative of damaging winds, and a gust front out of ahead of the main storms is evident on radar (Figure 7). There were numerous reports of winds of 60 to 70 mph and one report of a tornado near Fairfax, MO (Atchison County).
A second wave of severe storms marched through much of Ohio on August 9. A line of thunderstorms developed in the mid-afternoon and raced through the state (Figure 8). There were numerous reports of wind damage and three confirmed reports of tornadoes from preliminary damage surveys. An EF0 tornado occurred in northwest Richland County, and EF1 tornadoes were confirmed in Ashland County and Medina County. In Medina County there was also damage from downburst winds estimated at 100 mph and significant crop damage due to hail.
A third thunderstorm complex began to develop over southeastern Minnesota and western Wisconsin during the late evening on August 11. By midnight these thunderstorms had evolved into a bow echo and were moving southeast across southern Wisconsin (Figure 9). They moved through northern Illinois and the Chicago area in the pre-dawn hours, and then into northern Indiana. A swath of wind damage reports from southeastern Minnesota to northeastern Illinois remained in the wake of the storms. A measured wind gust of 74 mph was reported in Rochester, MN (Olmsted County) from a storm, and there were numerous measured gusts in excess of 60 mph. Most of the damage was to trees and power lines, and more than 250,000 customers of Xcel Energy customers in southeastern Minnesota were without power due to the storms. Lightning ignited a few grass fires in western Wisconsin.
A fourth thunderstorm complex developed over eastern Nebraska during the afternoon of August 12 and propagated southeastward into Missouri. As the thunderstorms entered northwestern Missouri about 8:00 pm CDT they were beginning to exhibit a bow echo configuration, indicative of damaging winds (Figure 10). By 9:40 p.m. CDT the severe storms were pushing through northern Missouri and warnings were in effect for counties ahead of the advancing line (Figure 11). The storms continued on southeast during the night eventually moving into southwestern Illinois and southern Indiana before weakening during the morning of August 13. There were many reports of wind damage with these storms from southern Iowa through northeastern Missouri and into southwestern Illinois. There were widespread power outages in Missouri along the path of the storms.
The focus of the fifth waves of storms shifted north again to Minnesota and Wisconsin. The midday outlook from the Storm Prediction Center had much of southern Minnesota, northern Iowa, and western Wisconsin in a Moderate Risk for severe weather (Figure 12). The first of the severe storms dropped hail to one inch in northwestern Minnesota during the late afternoon of August 13, but the main event began to take shape over central Minnesota during the early evening (Figure 13). There were many reports of large hail with the early storms, with hail reaching 2.50 inches near White Bear Lake (Ramsey County). The storm become more organized during the early evening over east-central Minnesota, and a number of severe thunderstorm watches were issued from covering an area from southern Minnesota to northern Illinois. There was one unofficial report of 5 inch hail near Eden Prairie, MN (Hennepin County). This thunderstorm complex also developed into a bow echo (Figure 14), producing wind damage all along its path. The storms knocked out power to more than 55,000 Xcel Energy customers Minneapolis-St. Paul metro area. The storms pushed into central Illinois around dawn on August 14, bringing the first significant rain of the month to many locations. By early afternoon the last of the storms were weakening over central Indiana.
As the week came to a close a sixth thunderstorm complex developed over eastern Nebraska on the afternoon of August 14. This system moved across Iowa during the evening and overnight. However, this system was much more benign than the previous five systems. A report of 0.88 inch hail in south-central Iowa was the only severe report during the afternoon and evening hours.
SDH