August 2007
Midwest Overview - August 2007
The temperature and rainfall pattern this month reflected the generally stagnant nature of the overall weather pattern in the Midwest in August. The mean location of the jet stream over the north-central Midwest/southern Great Lakes (Figure 1) allowed a quasi-stationary front to set up across the central Midwest. This front provided the focus for heavy rain and severe weather as it oscillated north and south through the region. Sandwiched between two areas of severe drought, the central third of Midwest received record amounts of rainfall. Precipitation was two to three times normal in a wide band across the central Midwest, falling off to only 10 to 25 percent of normal in southwestern Kentucky and less than 25 percent of normal in the Michigan Upper Peninsula (Figure 2). August rainfall in the central Midwest ranged from less than an inch across the Michigan Upper Peninsula and southern Kentucky to more than 20 inches in southwestern Wisconsin (Figure 3). Temperatures in August ranged from 3°F below normal in northern Minnesota to 8°F above normal in southern Kentucky (Figure 4).
Southern Midwest Swelters with Record Heat
Maximum temperatures reached or exceeded 100°F on a number of days this month from Missouri eastward through southern Illinois, southern Indiana, and Kentucky. Residents in southern Missouri and Kentucky suffered the most as the few pushes of cooler, drier air during the month typically did not make it much further south than the Ohio River. The peak of the heat wave in Missouri was in the second week of the month. The maximum temperature at Vichy/Rolla reached or exceeded 100°F on nine days between August 7 and August 17, and new high temperature records were set on eight of those days. The highest temperature recorded was 107°F on August 15. This was the warmest August on record for Vichy/Rolla, with nine high temperature records set during the month.
Temperature records piled up in Kentucky during August. This was the warmest month ever at both Louisville and Bowling Green. Louisville's average temperature for the month was 85.0°F, 8.0°F above normal. The average temperature at Bowling Green was also 85.0°F, 8.2°F above normal. The previous records for both locations were set in July 1901. In Paducah, this was the warmest August on record with an average temperature of 83.6°F, breaking the old record of 82.9°F in 1980. It was also the warmest month on record at London, KY with an average temperature of 81.4°F. A high temperature of 102°F on August 16 was the new all-time record high temperature for London. The first 20 days of August reached a temperature 90°F or higher, breaking the old record of 13 consecutive days in August 1933. London experienced 28 days with temperatures of 90°F or higher during August. The NWS cooperative observer in Hopkinsville, KY reported high temperatures of 100°F or more on 20 of 26 days from August 6 to August 31. A high of 109°F was reported on August 17, two degrees shy of the all-time highest temperature of 111°F recorded on August 9, 1930.
Cincinnati, OH set a new August temperature record with an average temperature of 81.6°F, breaking the old record of 80.2°F set in 1942. There were 25 days at or above 90°F, a new record for days in any month (old record 24 in July 1934 and July 1901). Fifteen of those days were consecutive. There were five days at or above 100°F, a new record for the month of August. The previous record of four days was set in 1948, 1936, and 1930.
Severe Drought in Northern and Southern Midwest
Drought conditions continued to expand and intensify in parts of the Midwest during August. In the southern third of the region, August rainfall was less than 25 percent of normal from the Missouri bootheel into western Kentucky. The drought status in Kentucky intensified from Severe Drought over the southern half of the state on the August 7 U.S. Drought Monitor to Extreme across most of Kentucky on the August 28 Drought Monitor.
In the far northern Midwest, Extreme Drought was found across the western half of the Michigan Upper Peninsula and the Minnesota Arrowhead. Marquette, MI received only 0.85 inches of rain in August. This was the driest May-August on record in Marquette, with only 7.20 inches of rain. The previous record was 8.28 inches in May-August 1998. Water levels in Lake Superior reached a new record low for August as a result of the combination of warmer than normal weather and below normal precipitation over the last 20 months over the Lake Superior drainage basin. The average Lake Superior water level at the end of August 2007 was 600.4 feet above mean sea level, a foot lower than a year ago and two inches lower than the previous record of 600.5 feet in 1926. The all-time record low water level for Lake Superior is 599.5 feet in March and April 1926. Water levels in Lakes Michigan and Huron are about two feet below normal level, but still about nine inches above the record low water levels.
Flooding Rains in Between
Waves of thunderstorms periodically developed and moved across the central Midwest between the areas of drought. The thunderstorms produced major flooding in southeastern Minnesota, southwestern Wisconsin,northern Illinois, northern Indiana, and central Ohio. A number of locations set new records for the all-time wettest calendar month, while many others set records for August precipitation. A partial listing of the monthly precipitation records can be viewed in Table 1. For daily records, please see the weekly Climate Watch narratives for August 2007.