2015 Climate by the Numbers in Illinois

Here is a quick update on the statewide monthly temperature and precipitation departures from average for 2015 in Illinois.

February 2015 stands out as a painful reminder of how cold it was late in the winter. However, both April and May have been warmer than average. All together, the average temperature of the first five months of 2015 stands at 40.5 degrees, 2.3 degrees below average.

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May Was Warm, Wet for Illinois

Despite cool weather at times, May was 2.3 degrees above average on temperature and one inch above average on rainfall with 5.6 inches.

The statewide average rainfall was 5.6 inches, which was 1 inch above average and the 22nd wettest May on record. Some of the heaviest amounts were seen in northwest Illinois and just southeast of St. Louis (orange and red areas on the map). The highest reported monthly rainfall total was 9.41 inches at Red Bud (Randolph County). The next highest total was 8.77 inches at Mt. Carroll.

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May precipitation. Click to enlarge.

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May in Illinois: Warmer Than Average, Dry in the West

Temperature

Based on preliminary data, the statewide average temperature for May in Illinois was 63.9 degrees. That is 1.2 degrees above average and the first month to be above average in Illinois since October 2013.

Precipitation

The statewide average precipitation for May in Illinois was 4.26 inches, just 0.34 inches below average. Below is a map of precipitation throughout the state. This is a radar-based product that is adjusted with rain gauges, resulting in higher resolution than a rain gauge network and more accuracy than a radar-only precipitation measurement. Sometimes hail can mislead the radar into calculating higher rainfall rates. That may have been the case in southern Champaign County, for example.

Some of the heaviest rainfall totals from the CoCoRaHS network for May occurred in Cook County, including Burnham-Hegewisch (IL-CK-82) with 7.64 inches and Homewood (IL-CK-64) with 7.58 inches.

The area of concern for May was the large section of blue across western and central Illinois, representing rainfall totals of only 1 to 3 inches. There are some smaller patches of blue in southern Illinois and far northwestern Illinois as well. One of the drier locations in west-central Illinois was Roseville (IL-WR-2) with only 1.32 inches with all 31 days reported. The US Drought Monitor list parts of western Illinois as “abnormally dry”.

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Plots of Temperature and Precipitation

Here are the time series plots for temperature and precipitation departures for each month of 2013 and 2014. In the first plot, one can clearly see the string of very cold months from November 2013 to April 2014. On the second plot, the dryness of late last summer shows up. While March of this year was dry, it was counterbalanced by a wetter than average April.

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First Half of May in Illinois – Warm, Wet, And Then Snow

According to preliminary records, the first half of May was both warmer and wetter than average for many locations in Illinois. The statewide average temperature was 61.2 degrees, about 1.4 degrees above average. Meanwhile, this morning there are reports of snow falling in northern Illinois. Talk about weather extremes. This was after last weekend when we saw widespread reports of highs in the upper 80s and low 90s.

The statewide average precipitation was 2.43 inches, 18 percent above average. Here is a screenshot of the last 14 days showing the widespread and heavy rainfall in much of the northeast, east-central, and southern parts of Illinois with many sites reporting between 3 to 6 inches of rain. Parts of western and central Illinois have not been as wet with amounts in the range of 1 to 3 inches of rain.
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Snow in May? Read more on the Chicago NWS page. It looks like Rockford set a new record for the latest report of snowfall in the season. The Chicago record still stands at June 2, 1910.

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Map courtesy of the Chicago NWS office.

May Precipitation In Illinois – Wet in the West, Dry in the East

Here is the latest map of May precipitation in Illinois through the morning of May 16, showing the generally wetter conditions in western Illinois and parts of southern Illinois. Amounts were on the order of 3 to 6 inches with a few heavier amounts around White Hall. Areas in the southeast, east-central and much of northern Illinois were drier with amounts of 0.5 to 2 inches common.

There was a strong relationship between temperature and precipitation this month (second figure). Areas with heavier rainfall amounts (western and southwestern IL) had temperatures 2 to 4 degrees below average. Meanwhile areas in east-central and northern Illinois had temperatures near to slightly above average.

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May 1-16, 2013 precipitation.
May 1-16, 2013, temperature departures from average.
May 1-16, 2013, temperature departures from average.