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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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

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

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Wet Weekend in Store for Illinois

One thing I love about the weather of Illinois it that it’s always changing. I was just getting ready to write a blog post about how dry it was getting in eastern Illinois. However, I soon realized that the dryness was the least of our worries right now. The NWS precipitation forecast for the next 5 days shows widespread, heavy rainfall across Illinois. The potential rainfall totals are smallest in the north and get progressively larger moving south. The potential totals are 1.5 to 3 inches in northern IL, 3 to 7 inches in central IL, and 7 to 10 inches in southern IL. The National Weather Service has issued Flash Flood Watches for northeast and southern Illinois.

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NWS 5-Day Quantitative Precipitation Forecast (QPF) for the period of Friday morning to Wednesday morning.

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2016 So Far – 7th Warmest in Illinois

The statewide average temperature for January-July was 53.4 degrees, 1.8 degrees above normal and the 7th warmest January-July on record for Illinois. The warmest January-July was 2012 when we reached 57.2 degrees, thanks to the 2012 drought. In an earlier post, it was noted that January-June was the 7th warmest as well.

Here are the Illinois statewide monthly temperature and precipitation departures from normal for 2016. In the first graph, it is clear that we have been running warm for 2016 with 6 out of the 7 months above normal. Only May was below normal.

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August Forecast – Wet for Illinois

After the third wettest July on record in Illinois, the NOAA Climate Prediction Center released their latest August outlook on Saturday. They have the northern two-thirds of Illinois in this region with an increased chance of above-normal rainfall in August (first map, green shading). This is part of a larger area with increased chance of above-normal rainfall that extends into Iowa, northern Missouri, southern Minnesota, Nebraska, and South Dakota. While not part of the official forecast, many of the dynamic forecast models are showing that above-normal rainfall is more likely in southern Illinois as well.

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