Rainfall Over the Last 2 Weeks in Illinois

 

Here are the amounts for the last week, as of this morning. The areas in purple in far southern Illinois have received between 10 and 15 inches. Areas in shades of red have received between 5 and 10 inches. The areas in orange and yellow have received between 2 and 5 inches. Areas in green in northwest IL have received between  0.5 and 2 inches.

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Here are the rainfall totals for the past two weeks. Same color scheme as before. Notice how the areas with 10 to 15 inches have expanded across much of southern IL south of Interstate 64 with a few areas just to the north.

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NWS Cooperative Observer Network rainfall totals in Illinois for the period of April 26 to May 5, 2017, ranked from high to low. Totals that have exceeded the expected 10-day, 100-year rainfall amounts for that area are in red. Continue reading

Wet April and 2nd Warmest January-April on Record for Illinois

Precipitation: The statewide average precipitation for April in Illinois was 7.01 inches, 3.23 inches above normal and the 2nd wettest April on record. Here are the top five wettest Aprils. Notice a pattern? Three out of the five have been since 2011.

  1. April 2011 – 7.62 inches
  2. April 2017 – 7.01 inches
  3. April 1957 – 6.99 inches
  4. April 2013 – 6.93 inches
  5. April 1927 – 6.87 inches

The largest monthly total for April in Illinois was Carbondale with 14.41 inches. Several other sites in southern Illinois had similar amounts including Bush (Williamson County) with 13.63 inches, West Frankfort (Franklin County) with 13.35 inches, Kaskaskia (Randolph County) with 13.34 inches, and Murphysboro (Jackson County) with 13.02 inches.

Temperature: The statewide average temperature for April was 56.4 degrees, 3.8 degrees above average and the 11th warmest April on record. The warmest reading for April was 89 degrees at Kaskaskia on April 20.  The coldest reading for the month was 19 degrees at Morrison on April 10.

Statewide records of temperature and precipitation go back to 1895.

Graphs

Here are the temperature departures for 2017. As you can see, every month in 2017 has been well above normal. So far this January-April is the 2nd warmest such period on record with a statewide average temperature of 43.2 degrees, 5.4 degrees above normal. Only 2012 was warmer at 44.5 degrees, 6.7 degrees above normal. [corrected from an earlier version]

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Widespread, heavy rains possible over next week in Illinois

As of April 25, the statewide average precipitation for Illinois is 2.8 inches, which is 94% of normal. However, we have several opportunities for widespread rains this week and into the weekend, according to the NWS precipitation forecast.

The first round of rain on Wednesday and Thursday has potential rainfall amounts of 1 to 2 inches across most of Illinois, along with the chance for severe weather. Continue reading

National Climate Assessment for Illinois

Earlier this year the National Climate Assessment for Illinois was released, tracking the historical changes in temperatures and precipitation in Illinois. You can find the full report for Illinois and other states at the State Climate Summaries website.

Here is Key Message #1

Average annual temperature has increased by about 1°F since the beginning of the 20th century. There has been seasonal variation in this warming, with average spring temperature increasing by about 2°F and average summer
temperature increasing very little. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.

Key Message #2

Precipitation in spring and summer has generally been above average over the past two decades, affecting agriculture in both positive (adequate soil moisture) and negative (delays in spring planting) ways. Precipitation in winter and spring is projected to increase, which poses a continuing risk of spring planting delays.

Key Message #3

Severe flooding and drought have occurred periodically in recent years. Future increases in extreme precipitation events and in evaporation rates may increase the intensity of both floods and droughts.

You may notice that two out of the three messages relate to increases in precipitation and especially heavy rainfall events. The graphic I found most compelling was this one for the increasing number of 2-inch rain events with time. The caption explains how this graph was created. The increased frequency in 2-inch rain events seen since the early 1980s lines up very well with my experience and research over the years in Illinois.

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Figure 5: The observed number of days with extreme precipitation events (annual number of days with precipitation greater than 2 inches) for 1900–2014, averaged over 5-year periods; these values are averages from 43 available long-term reporting stations. A typical station experiences 1–2 such events each year. The number of extreme precipitation events has been above average since the 1990s. During the most recent 5-year period (2010–2014), Illinois experienced a record high number of events when stations averaged more than 2 events annually. The dark horizontal line is the long-term average (1900–2014) of 1.62 days per year. Source: CICS-NC and NOAA NCEI.

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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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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