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 “100-Year Storm Strikes Illinois State Fair”

Heavy Rains on August 12-15

As forecasted by the National Weather Service, heavy rains have fallen across Illinois since Friday. Here are the totals by day and for the three days combined. More rain is expected today and tomorrow (last map). Friday’s rainfall is shown on the 24-hour totals for the morning of August 13, etc. We have had a stationary front parked over Illinois since Friday, which is usually a key ingredient for getting significant rainfall amount.
Here is the set of maps showing the rainfall totals for each day, using rain-gage calibrated rainfall totals. It provides a higher level of details with fairly good accuracy. The downside is that the color scale changes slightly from map to map.

Rainfall totals for the morning of August 13, 2016

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Morning of August 13, 2016.

Rainfall totals for the morning of August 14, 2016

Continue reading “Heavy Rains on August 12-15”

Flooding – Deadly Risk in Illinois with 28 Deaths in 19 Years

Right now the attention is on the widespread flooding in Texas and Oklahoma, with the number of deaths sitting at 21 and climbing. Illinois has been faced with deadly flooding in the past. An examination of the national storm database reveals that Illinois has had 28 flood-related deaths in the last 19 years*.

Here are the numbers by year in Illinois. Some years like 2000, 2008, 2009, and 2013 were especially bad with 4 to 5 deaths each. A few years had no deaths, including 2004-07, 2012, and 2014. While not part of this database, six deaths were reported in Illinois during the 1993 flood, according the ISWS report.

Flood-related deaths in Illinois from 1996 to 2014, based on data from the National Climatic Data Center.
Flood-related deaths in Illinois from 1996 to 2014, based on data from the National Climatic Data Center.

Continue reading “Flooding – Deadly Risk in Illinois with 28 Deaths in 19 Years”