Wettest July-August, Second Wettest August in Illinois

Illinois is already experiencing its wettest July-August and second wettest August on record with a few days left in August. [updated through August 30]

August

Based on rainfall through August 30, the statewide average rainfall for August is 6.70 inches, 3.11 inches above normal and the 2nd wettest August on record. The wettest August was 1977 with 6.86 inches. Continue reading

Wet August in Illinois

As of yesterday, the statewide average rainfall was 4.47 inches. That is already 0.9 inches above normal for the month and the 27th wettest August on record (since 1895). The wettest August on record was 1977 with 6.86 inches. The latest NWS forecast is showing the potential for another inch of rain to fall in the next 7 days (last map). That would put us in the top ten wettest Augusts on record.

The highest rainfall total in the state for August is Waltonville (Jefferson County, in southern Illinois) with 13.43 inches. A CoCoRaHS station just 3 miles away reported 12.94 inches (IL-JF-2).

Here are the rainfall totals through the morning of August 19 (left) and the departure from normal (right). The heaviest amounts have been in southern Illinois with 6 to 10 inches pretty common. Most of the state is well above normal on rainfall except for a spot in western Illinois and several counties in northeast Illinois. Click to enlarge.

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Fall and Winter Forecast for Illinois

Summary: The NWS Climate Prediction Center has issued their forecasts for September, September-November (Fall), and December-February (Winter). Illinois has an increased chance of being warmer than normal this fall, and wetter than normal this winter.

According to the Climate Prediction Center, the main factors in the forecast are the recent warming trends and the expected La Niña. While the conditions in the Pacific are in the neutral stage between El Niño and La Niña, there is a 55-60 percent chance of a weak La Niña during fall and winter.

Fall

The September forecast (top row) has Illinois and the Midwest with equal chances of being above, below, and near-normal on both temperature and precipitation. I call this a neutral forecast since there are no indications that we will be significantly cooler, warmer, wetter, or drier.

The September-November forecast (bottom row) has Illinois and the US with an increased chance of being warmer than normal. They are neutral on the precipitation forecast.

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Winter

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5-Day Rainfall in Illinois and Midwest

Here are the 5-day accumulated rainfall totals for Illinois and the Midwest. Rainfall was heaviest south of Interstate 70 where amounts of 4 to 8 inches were common (lighter shades of blue). The largest 5-day rainfall total was from a CoCoRaHS station at Waltonville (IL-JF-2) with 10.79 inches. CoCoRaHS is a national network of trained volunteer precipitation observers, learn more at cocorahs.org

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100-Year Storm Strikes Illinois State Fair

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Radar-estimated rainfall from August 12 storm.

The 5.59 inches of rain reported at the Springfield Airport on Friday night fell in 6 hours. This represents the 100-year storm for that duration in central Illinois, according to Bulletin 70. The results were dramatic, as reported by the Springfield Journal Register.

The concept of the 100 year storm is commonly used by engineers for assessing the risk of heavy rainfall. The 100-year storm is more completely described as the storm expected to have a return period of once every 100 years on average. The phrase “on average” being key. It does not mean the storms are exactly 100 years apart. Instead it means that if you look at rainfall statistics long enough the average frequency of such a storm would be 100 years. Unfortunately, we don’t have hundreds of years of rainfall data. Instead we estimate the values based on fitting a statistical model to the observed data.

While the phrase “100-year storm” is eye catching, it does not do a good job conveying the risk of such an event. A better way of describing it would be the “1% chance storm”, Continue reading