August 8-14, 2004
Midwest Weekly Highlights - August 8-14, 2004
August Cold Air Outbreak - A Broken Record
of Broken Records
The big weather story the second week of August was the unusually cold
air which spilled into the Midwest. Temperatures this week
were much below normal, ranging from 6F to 7F below normal in far
eastern Ohio and northeast Michigan, to more than 12F below normal in
central Iowa through central Missouri (Figure 1). This was also a very dry week
across most of the region. Precipitation
was generally limited to some showers and thunderstorms accompanying a
cold front on August 9-10 in the central Midwest. Later in the
week showers and thunderstorms moved through northern Minnesota and
Wisconsin as a strong upper level low spun over the Great Lakes
(Figure 2), and this
was the only part of the region with above normal rainfall for the
week. Rainfall for the first two weeks of August has been
generally below normal except for Iowa and Kentucky (Figure 3).
The first two days of the period were generally warm and somewhat
humid, but that changed on August 10 as a strong cold front moved
through the Midwest (Figure 4). For the next four days numerous minimum temperature and
low maxmimum temperature records were set as the cold air spread
southward. The upper level wind pattern was more characteristic
of January than August with a strong trough of low pressure over the
central United States (Figure 5).
On August 10 many locations in Iowa, Minnesota, Wisconsin, and
a few in western Illinois set new records for the lowest maximum
temperature for the date (Figure 6). In addition, a number of locations in Minnesota and
Wisconsin also set new records for the coldest maximum temperature for
the month of August. Records continued to be set on August
11, but now also included locations throughout Missouri, Illinois, and
Michigan (Figure 7). The
consecutive days of cold weather were not only due to the unusually cold air
mass, but also from lack of sunshine from cloud cover
associated with the strong upper level low (Figure 8). Temperatures in areas that did
receive some sun were very cool, but not at record setting levels.
Records continues to tumble on August 12, mostly in the central and
southern portions of the Midwest. New record low
temperatures were set throughout Iowa, Illinois, Indiana, and Kentucky (Figure 9) as clearer
nighttime conditions allowed for some radiational cooling. Tower
and Embarrass, MN reported low temperatures of 28F and 29F,
respectively. Clouds formed again during the afternoon in the
cool but unstable air (Figure 10), and the limited sunshine resulted in more record low maximum
temperatures in Illinois, Indiana, and Kentucky (Figure 11). The
unstable air mass also produced cold air funnels over southwestern
Michigan during the afternoon.
Frost advisories were posted across northern Wisconsin for the early
morning of August 13, and tempratures did fall to near freezing in
parts of northern Minnesota and Wisconsin by sunrise. Tower and Embarrass, MN both
reported a low of 27F, the second consecutive morning of lows in the upper
20s. More record low temperatures were set across the
central Midwest on (Figure 12), with only a few scattered record low maximum temperatures in
Wisconsin, Kentucky, and Missouri.
By August 14 the center of high pressure was parked over the Midwest (Figure 13), and this
remarkable week concluded with another round of record low temperatures
mostly in Illinois, Kentucky, and southern Missouri (Figure 14).
For the period August 8-14, there were 55 low temperature records
and 85 record low maximum temperatures reported for the primary
observation locations in the nine-state Midwest region. These
counts are based on preliminary data and will likely increase as more
data becomes available.