One of the best things about this time of year is the fall color. I suppose I’m hopelessly Midwestern because I even enjoy the change in color of the corn fields.
The question I often get is “How will the weather affect this year’s color?” It is hard to answer. Here are some comments I made in a press release a few years ago about the recipe for good fall color.
“Trees and shrubs need to go into the fall without a lot of stress from disease or severe drought. Chilly, not frigid nights and cool, sunny days enhance the changing leaf colors. Detrimental conditions include extended periods of rain or cloudiness that mute colors, high winds that blow leaves off trees, and hard freezes that stop color changes entirely… October and early November in Illinois should put on quite a show. Take some time to go out and enjoy it,” concludes Angel.
The remains of Tropical Storm Hermine have reached Illinois this morning. Rainfall amounts of 1 to 3 inches are possible and flash flood warnings have been issued. Check the National Weather Service as this event unfolds.
While we don’t often use the words “tropical” and “Illinois” in the same sentence, the remains of tropical storms and hurricanes have reached us before. Most of these systems came on shore in Texas and Louisiana and weakened considerably as they moved northward. Usually, the severe weather is gone by the time they reach Illinois but they can produce large amounts of rain than can lead to flooding.
The passage of four tropical systems alleviated drought impacts, particularly in southern and central Illinois during the 2005 growing season. The four systems were Tropical Storm Arlene, and Hurricanes Dennis, Katrina, and Rita. An article in the Illinois State Academy of Science by me described that situation in more detail.
More recently, in September 2008 the remains of Hurricanes Ike and Gustav produced heavy rains and flooding in Illinois. More on that particular situation can be found on my website.
This summer was one of the warmest and wettest in Illinois history, based on preliminary data. The average statewide temperature for this summer (June-August) was 76.4 degrees, 2.7 degrees above normal and the seventh warmest summer on record. The average statewide rainfall was 16.7 inches, 5.2 inches above normal and the sixth wettest summer on record. Statewide records for Illinois extend back to 1895.
The average statewide temperature for August was 76.8 degrees, 3.2 degrees above normal. That puts it at the 13th warmest August on record. August was on track to being even warmer but a late-month cool spell knocked it down a few notches in the ranking. August rainfall has been close to normal with a statewide average of 3.4 inches, just 0.3 inches below normal.
[This is an update of a post earlier in August, now removed to avoid confusion]
After a wet start to the 2010 growing season, two areas of dryness have persisted in the last 60 days (see map below). The first area is far southern Illinois, where dry conditions extend back to June. The second, more recent area is in east-central Illinois. The hardest hit area appears to include parts of the following counties: McLean, Ford, Livingston, Iroquois, and Kankakee. Rainfall in that area has been between 25 and 75 percent of normal. Meanwhile, temperatures during that same period have run about 2.5 degrees above normal.
As expected, topsoil moisture in east-central Illinois has declined rapidly in recent weeks. According to the latest USDA NASS report, the “East” Crop Reporting District (CRD) in the heart of the dry area is showing that 35% of the fields sampled had “very short” and 48% of the fields had “short” topsoil moisture.
In southern Illinois, the percentages of fields with “very short” to “short” topsoil moisture are as follows:
East Southeast CRD: 16 % “very short” and 43% “short”
As of August 16, cooling degree days (CDD) are well above normal this year in Illinois. The statewide average is 1051 CDDs, which is 268 CDDs above normal or 134 percent of normal. Last year the statewide average through this date was only 640 CDDs.
Cooling Degree Day accumulations at selected cities.
Departure from Normal
Percent of Normal
Cooling degree days are calculated by subtracting the mean daily temperature by 65 degrees. Results above zero are accumulated over time. So if the mean temperature for the day was 75 degrees, then 75 – 65 = 10 cooling degree days. Accumulated over a season and compared to normal gives a relative idea of potential cooling costs. The higher the number, the higher the cooling costs.
Other factors that can influence cooling costs are humidity levels and solar radiation. Personally, I ran the A/C at home pretty much every day since mid-May. Last year I ran it for about two weeks the entire summer.
Here are the thumbnail maps (click to enlarge) of July temperature departures, and July rainfall and rainfall departures. It was warmer than normal across much of eastern Corn Belt. It was much wetter than normal across much of the western Corn Belt.
The average temperature for July in Illinois was 77.7 degrees, 1.9 degrees above normal. That’s warm but not record-breaking by any means. By contrast, the average temperature of last July was 70.2 degrees. Not only was this year warmer but it was considerably more humid as well.
The average rainfall for July in Illinois was 5.6 inches, 1.8 inches above normal. BTW, the wettest July on record was 1958 with 8.03 inches.
The largest rainfall totals occurred in western and far northern Illinois as well as an area along I-70. Amounts of 8 to 12 inches were common in these areas.
Southern Illinois was much drier in July with amounts of only 1 to 3 inches in many locations. In fact, the US Drought Monitor categorized southern Illinois as being “abnormally dry” by. Another area of dryness is through central Illinois in an area bound by Moline, Kankakee, Danville, Springfield, and Peoria.
The statewide average temperature for July 1-22, 2010, in Illinois is 77.2 degrees, 1.1 degrees above normal for this time period. The state average precipitation for the same period is 3.46 inches, 0.70 inches above normal.
Northern and eastern Illinois are a little drier on average. However, even those drier areas contain locations with very impressive rainfall amounts.
Table 1. Temperature and precipitation amounts and departures from normal by climate division in Illinois for July 1-22, 2010. A map of the climate divisions can be seen below the table.
Here are the June 2010 rainfall totals for sites in Illinois, ranked from highest to lowest. The numbers are impressive with 35 sites scattered throughout northern and central Illinois reporting 10 or more inches of rain.
These numbers are preliminary and have not been quality-controlled. In some cases data may be missing that could lead to totals that are too low. Due to the localized nature of some of the heavier storms, one may see large differences over short distances. These sites are part of the NWS cooperative observer program – volunteers with standard equipment and observing practices.
Illinois has experienced the 2nd wettest June on record, based on preliminary records through June 30. The statewide average rainfall was 7.8 inches, 3.7 inches above normal. The wettest June on record was 1902 with 8.37 inches. The rains over the weekend, especially north of I-80 and along I-70 caused this June to move up from fourth to second wettest June on record. Statewide records extend back to 1895. This number is provisional and may change slightly as more data comes in.
The larger rainfall totals occurred in the northern two-thirds of the state where amounts of 7 to 12 inches were common. Meanwhile far southern Illinois remains closer to normal with amounts ranging from 3 to 6 inches.