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.

May precipitation. Click to enlarge.

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


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.


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”.



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.



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.


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.

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.

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


Warmest Spring on Record for Illinois

Temperature This Spring

In climatology we use the three whole months of March, April, and May as spring. It makes more sense than the astronomical definition of the March 20 – June 20. As this year demonstrates, summer-like conditions arrive long before the summer solstice. This year the statewide average temperature for spring in Illinois was 59.1 degrees. That makes it the warmest spring on record for Illinois. The statewide records go back to 1895. The five warmest springs in Illinois were:

  1. 2012 with 59.1 °F
  2. 1977 with 57.3 °F
  3. 1921 with 56.0 °F
  4. 1991 with 56.0 °F
  5. 2010 with 55.4 °F


The statewide average temperature for May in Illinois was 68.1 degrees. That is 5.7 degrees above normal and the 5th warmest May on record, based on preliminary data. The five warmest month of May were:

  1. 1962 with 69.5 °F
  2. 1977 with 68.9 °F
  3. 1896 with 68.8 °F
  4. 1991 with 68.5 °F
  5. 2012 with 68.1 °F

Year to date

It was the warmest January-May on record in Illinois with an average temperature of 48.8 °F. What’s interesting is that out of the top five warmest January-May periods, three have occurred in the last 15 years.

  1. 2012 with 48.8 °F
  2. 1921 with 47.3 °F
  3. 1998 with 46.3 °F
  4. 2006 with 45.8 °F
  5. 1938 with 45.6 °F


The statewide average precipitation for May in Illinois was 2.4 inches. That is 1.8 inches below normal and 57 percent of normal. That makes it the 21st driest May on record. The driest May was 1934 with 1.03 inches.

The statewide average precipitation for spring in Illinois was 7.71 inches. That is 3.65 inches below normal and the 18th driest spring on record. The driest spring on record was 1934 with 5.16 inches. In case you are wondering, 1988 was the eleventh driest spring at 6.87 inches.

The May 29 US Drought Monitor has several areas in Illinois in “abnormally dry” conditions and far southern Illinois in moderate drought. The good news is that significant rains fell in northwestern Illinois after the map was released so some of those abnormally dry conditions in that part of the state have been washed away.

May in Illinois – Above Average Rainfall


It should come as no surprise that the May 2011 statewide average rainfall was 5.60 inches, and 1.34 inches above the 1971-2000 average. In the first half of May, the rainfall distribution was lopsided with southern Illinois getting 3-5 inches of rain while much of the rest of the state had less than 2 inches. But the rest of the state caught up in the second half of the month.

For May, the two sites with the heaviest rainfall totals in the NWS cooperative observer network were near the Quad Cities: Woodhull with 9.92 inches and Altona with 9.28 inches. From the CoCoRaHS network, the two sites with the heaviest rainfall totals were on opposite ends of  the state: Anna with 10.04 inches and Algonquin with 9.12 inches.

Here is an amazing thing about the statewide May rainfall – they are almost the same in the last four years:

  • 2011 – 5.60 inches
  • 2010 – 5.59 inches
  • 2009 – 5.65 inches
  • 2008 – 5.39 inches
That’s a spread of only 0.26 inches in four years!


The statewide average temperature for May was 62.1 degrees, 0.8 degrees below average. Temperatures ranged from a high of 95 degrees in Prairie City, Fisher, Little Red School House (Willow Springs), Princeville, and Trimble to a low of 24 in Elizabeth.
May 2011 rainfall in Illinois.
May 2011 rainfall in Illinois (click to enlarge).