August 1-7, 2007
Midwest Weekly Highlights - August 1-7, 2007
The Three "H"s
Normally three "H"s refers to hazy, hot, and humid weather. This week, however, they refer to hot, humid, and heavy rain. Hot and sultry conditions spread over most of the Midwest this week on the heels of a cool July. Only Minnesota, western Wisconsin, and northern Iowa enjoyed relief from the heat as a cold front sagged south over the Great Lakes on August 3 and then became stationary (Figure 1), only oscillating slowly back north and then south as upper level disturbances crossed the Midwest. Rainfall this week was concentrated along the frontal boundary, with two significant rain events in southern Wisconsin and northern Illinois.
There was a large gradient in average temperature departures from normal this week (Figure 2). Temperatures ranged from 1°F below normal to 3°F above normal in the northwestern third of the region, which remained north of the frontal boundary most of the week. South and east of this area, temperatures ranged from 5°F to 7°F above normal. The core of the warmest weather extended from western Missouri eastward into central Ohio. High temperatures reached 90°F or higher most of the week across the southeastern half of the region. By August 7 maximum temperatures topped 100°F over the southern portions of the Midwest (Figure 3) and a few records were set.
Rainfall was very heavy along the mean position of the front, with many locations receiving in a couple of days twice the normal rainfall for the entire month (Figure 4). Thunderstorms developed each day along the front or the outflow boundaries from the prior storms. Unfortunately, the rain tended to fall repeatedly over the same areas, and the portions of the Midwest which have been dry continued, for the most part, to remain dry. There was some diminishing of drought conditions over western Iowa and along the heavy rain band as depicted on the August 7 edition of the U.S. Drought Monitor (Figure 5). The biggest change occurred in the Michigan Upper Peninsula where the northwestern portion was placed in the Extreme Drought category this week.
Wisconsin Deluge
Beginning the evening of August 4 and continuing overnight into the early morning hours of August 5 training thunderstorms dumped more than10 inches of rain on portions of southwestern Wisconsin. The regional radar image at about 1:00 a.m. CDT (Figure 6) shows the heaviest storms extending in an arc from near La Crosse, WI through Lone Rock to Monroe, WI. The image from the Milwaukee/Sullivan, WI radar (Figure 7) shows the storm in more detail. Unofficial rainfall amounts ranged from more than 10 inches west of Barnveld, WI to around 5 inches near Monticello, WI. The heavy rain resulted mudslides, toppled trees, and flooded buildings and roads. The flooding would have been worse had it not been for the dry conditions for most of the summer.
Northern Illinois Receives the Second Punch
On August 6 the front had slipped south across northern Illinois. Thunderstorms began firing along the boundary during the afternoon and evening. Persistent and heavy storms dropped torrential rains on Stephenson, Boone, Winnebago, and McHenry Counties in Illinois, as well as Rock and Green Counties in Wisconsin (Figure 8). Rain continued through the night, with rainfall rates exceeding 2 inches per hour for three hours or more. The heaviest rain generally fell between 11:00 p.m. and 3:00 a.m. CDT. CoCoRaHS observers in Freeport (Stephenson County) reported 24-hour rainfall amounts 6.70 to 7.00 inches of rain (Figure 9), while observers in southeastern Winnebago County reported 6.20 to 6.84 inches of rain (Figure 10). Rockford, IL (Winnebago County) set a new daily record rainfall of 5.15 inches, totally submerging the old record of 1.49 inches in 1924. The radar estimated precipitation shows the extent of the heavy rain from this event (Figure 11), but underestimated the maximum amounts by one to two inches. The resultant flooding stranded many people in their homes and in vehicles, and closed roads for several hours. The heavy rain affected the same areas that were hit by severe flooding on September 4, 2006. Many residents who had just completed or were in the process of completing rebuilding from last year's storm were hit hard again. Winnebago County was declared a state disaster area by the Governor for the second time in less than a year because of the severe flooding from this storm.
Severe Weather
Severe thunderstorms occurred in all states but Indiana this week.The greatest number of reports was on August 7 in association with the thunderstorms that developed along the stationary front. Severe storms were reported in Missouri, Iowa, Illinois, Michigan, and Ohio. Most of the damage was minor from thunderstorm winds, while the flooding from the heavy rain caused more serious problems. Out of four severe weather reports in the Midwest on August 5, three were for tornadoes. There were two reports of tornadoes in Ohio, and one in northern Illinois near the community of Bolingbrook (Will County). Damage from the tornadoes was generally minor. On August 6 the public reported a possible tornado northeast of Beloit, WI (Rock County), but there were no reports of damage.
Records - Hot, Cold, and Wet
There were just a few temperature records set this week, mostly at the end of the week as the hot weather began to build over the southern portions of the region. The hot weather accompanied by dew points in the mid to upper 70s pushed heat indicies over 100°F over the southern half of the Midwest (Figure 12). However, Rochester, MN set a record low maximum temperature of 65°F on August 4 as a cool air mass settled over the Great Lakes. A number of locations set new daily rainfall records in the storms of August 6-7. Temperature and precipitation records can be seen in Table 1.