Midwest Overview - September, 2004

---

Another Unusual Month, But the Opposite of the Last One

September was a very unusual month in the Midwest. Flood and drought both visited the region, with severe flash floods in the far northwestern and southeastern Midwest, and very little rain in the bulk of the area in between. The potent remnants of two hurricanes entered the eastern part of the region during the month, while a strong ridge steered weather systems through Minnesota into Canada. In addition to the lack of rain in the central Midwest, well above normal temperatures were maintained almost continuously until the 28th of the month, especially in the northern half of the region. Chicago set a new all-time record for sunshine during September: 90% of possible sunshine reached the ground in 2004, eclipsing the previous record of 86% set in 1895. St. Louis reach 88% of possible sunshine, and a station in between, Lincoln, IL, received 85%, so a very large area had very little daytime cloud during September. Detroit set a new record by having 23 consecutive days with no measurable rain through October 1. Louisville, KY, received only 0.09 inches of rain in September at the airport (2nd driest all-time), while several places only 100 miles to the east received more than 10 inches of rain over the same period from Frances and Ivan. Despite the hurricanes, the Midwest had its 34th driest and 17th warmest September in 2004.

The pattern of precipitation shown in the MRCC September precipitation map (Figure 1) is a bit smoothed compared to the actually sharper gradient from wet to dry to wet that exists from the northwestern to southeastern Midwest. The percent of mean precipitation map shows this relationship more starkly (Figure 2), where precipitation totals drop from 200% of normal to less than 50% of normal in less than 100 miles. It was the dominance of the eastern U.S. mid-level ridge (Figure 3) combined with strong high pressure off the East Coast that steered the storms approaching from the west through Minnesota and into Canada, while forcing Frances and Ivan to travel into the Midwest. The stationary ridge also allowed abundant amounts of warm air to ride northward into the region, leaving the northern Midwest 3-7°F above normal for the month (Figure 4). A number of cities in the northern Midwest had average temperatures for September 2004 that were higher than August 2004 and even June 2004. One of these cities, Marquette, MI, set a new monthly temperature record by a huge 1.9°F margin over the previous record, and even came with 0.1°F of tying the July 2004 temperature. The combination of high temperatures, low precipitation, and ubiquitous sunshine brought large sections of the Midwest from a fairly moist state (August was cool and wet) to an abnormally dry state in one month (Figure 5, National Drought Mitigation Center). However, the dryness and warmth were most beneficial to farmers, allowing for some northern crops to catch-up on desperately needed growing degree days. The conditions also allowed a rapid dry down of corn and soy beans and a timely harvest with few interruptions in the Corn Belt.

Disastrous floods occurred along the Ohio River after Hurricane Ivan dropped huge amounts of rain on a drainage basin already saturated by Hurricane Frances. The hardest hit city was Marietta, OH, where 400 businesses and about as many homes were flooded on September 19 when the Ohio River reached 9 feet over flood stage. Twenty counties in Ohio were included in a federal disaster declaration. Damage to public infrastructure in the area exceeded $52 million dollars, and more than $6 million in individual grants had been given to flood victims by early October. Private losses have not been fully tallied, but will be counted in the tens of millions of insured and uninsured losses