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.

August

Based on rainfall through yesterday, the statewide average rainfall for August is 6.34 inches, 2.75 inches above normal and the 2nd wettest August on record. The wettest August was 1977 with 6.86 inches.

July-August

Combine a very wet July with a very wet August, and you have the wettest July-August on record. The rainfall total of July-August was 13.19 inches, which is 5.52 inches above normal. It beat out the old record for July-August of 12.83 inches, set back in 1915. Here are the rainfall departures from normal for July-August so far.

Here are the rainfall departures from normal for July-August so far. Almost the entire state is green, meaning rainfall was above normal. The darkest shade of green in southern  Illinois, south of Chicago, and northeast of St. Louis were 9 to 12 inches above normal and the area in blue in southern Illinois was 12 to 15 inches above normal.

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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

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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