Midwest Weekly Highlights - August 15-21, 2004

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Cold August Weather Continues - More Records Tumble

The unusually cold weather that settled in over the Midwest continued during the third full week of August, with temperature records set almost every day.  Much of the Midwest also received rain this week, with the most widespread rain in the central and southern portions of the region.

Temperatures this week ranged from 8F to 10F below normal in Minnesota and Wisconsin to 3F to 6F below normal across Ohio (Figure 1).  Precipitation for the week was near to above normal across parts of Missouri, most of Illinois, Indiana, and the northern two-thirds of Ohio (Figure 2).  Much of the northern half of the Midwest was dry, although portions of southern Minnesota did receive some rain ahead of an advancing cold front.

Most of the temperature records this week were record low temperatures as clear skies and dry air allowed for maximum radiational cooling at night. Ample sunshine during the day allowed temperatures to reover into the 60s and 70s over the northern Midwest, and into the 70s and even low 80s in the remainder of the region. On August 15, 16, and 17 most of the records were in Missouri, Illinois, Indiana, and Kentucky as high pressure drifted over the eastern Midwest (Figure 3).  August 18 was the only day this week with no temperature records set as southerly winds brought in warmer and more humid air ahead of yet another cold front.  This front stalled over the central Midwest on late on the 18th and on August 19 as a wave of low pressure developed and moved along the boundary.  This low produced a large shield of precpitation that brought much needed rainfall to the central and southern portions of the Midwest. Despite this rain, August precpitation over much of the Midwest remains below normal, with the notable exceptions of central Iowa into central Missouri and northeastern Indiana through central Ohio (Figure 4). In these areas August rainfall  ranges from 100 to 150 percent of normal.

Low temperature records were set across Minnesota and Wisconsin on August 19 as these areas were already under the influence of the cold air behind the front.  By August 21 high pressure dominated most of the Midwest, and numerous low temperature records were set from Minnesota south through  Indiana and Missouri (Figure 5). Embarrass, MN dropped to a frigid 20F.