Monthly Archives: May 2012

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.

Fort Armstrong Weather Records (1820-1836)

This semester a University of Illinois Department of Atmospheric Science student, Lauren Graham, worked to make available scanned images of the weather records for Fort Armstrong that cover the period 1820 to 1836. Fort Armstrong was located at the present-day Rock Island Arsenal. The original records were maintained and scanned by staff at the Rock Island Arsenal Museum.

These are the oldest official weather records that I have been able to find for Illinois. The records contain daily temperature readings taken at 7 am, 2 pm, and 9 pm, as well as comments on the weather. My favorite is a note in the first month about a “violent hurricane” on July 21 1820 (see image below). I’m sure it was not really a hurricane but either a tornado or severe thunderstorm with high winds.

Here is the press release of the story.

Here is the Fort Armstrong page containing the images and preliminary analysis.

The plans are to introduce more analysis of these data over the summer.

Fort Armstrong, July 1820. Note the comment on July 21 about the “violent hurricane”. Click to enlarge.

Warm and Dry for May in Illinois

May 2012

A glance at the forecast for Memorial Weekend shows near record-breaking temperatures in Illinois. Already the statewide average temperature for May 1-22 is 66.1 degrees, and 3.7 degrees above average. It is now in 19th place as far as the warmest May on record. We will see how that changes over the weekend and into next week.

Monthly Temperatures Since 2011

Below are the temperature departures in Illinois since the beginning of 2011. The statewide average temperature in every month in 2012 has been above average in Illinois. As a result, we are now at 48.4 degrees for the period of January – May 22. That makes this January-May the warmest on record. It beats second place by a full 1.1 degrees (47.3 degrees set in 1921). Statewide records go back to 1895. By the way, the summer of 1921 was about 3 degrees above average with average precipitation.

The streak of warmer than average temperatures extends back to October 2011. Also, eight of the 12 months in 2011 had above average temperatures.

Statewide monthly temperature departures for Illinois in 2011 and 2012. Blue bars mean cooler than average. Red bars mean warmer than average. Average is the 1971-2000 statewide average. Illinois State Water Survey 2012. Click to enlarge.

Latest Seasonal Outlook

NOAA’s Climate Prediction Center released their June and June-August outlook yesterday (figure below). On the left hand side of the figure, they have far southern Illinois with an increased chance of above normal temperatures for June. For June-August, that area of increased chances expands and covers an area roughly south of I-70.  The rest of the state has “EC” or equal chances of above, below, or near-normal temperatures. In other words, they don’t see anything to push temperatures one way or another.

It’s the same story with precipitation – equal chances of above, below, or near-normal precipitation across the state for June and June-August. I think its fair to say that our current skill is pretty low in forecasting summer-time precipitation months in advance.

NOAA’s Climate Prediction Center outlook for this summer. Click to enlarge.

Soil Moisture in Illinois

Observed soil moisture data are collected at 19 sites in Illinois by the Illinois State Water Survey’s Water and Atmospheric Monitoring program. The soil moisture is measured at 5 cm (2 inches), 10 cm (4 inches), 20 cm (8 inches), and 50 cm (20 inches). All amounts are reported as the fraction of water by volume of the soil. For example, a value of 0.35 means that 35% of the soil is occupied by water.

Silty clay loam soils, common in Illinois, are considered saturated at values of 0.45 to 0.50. Field capacity is in the neighborhood of 0.35. Values below about 0.20 would be at the permanent wilting point. As the name implies, that is the level where plants wilt and do not recover. Plant roots are unable to recover any water below this level because it is tightly bound to the soil particles. Of course, soil moisture is very dynamic with layers drying or wetting at different rates so it would take low levels of soil moisture at all layers in the root zone to kill a plant.

In general, soil moisture is highest in spring and slowly declines in the summer months because of the heavy demands from evaporation and from transpiration of growing vegetation. In the fall, cooler temperatures and the end of the growing season allow soils to recover quickly.

The map below shows the May 14, 2012, soil moisture status at 10 cm (4 inches). Values in the 0.2 and 0.3 range are widespread. The noticeable exception is in Mason County where the site is located on a very sandy soil. As a result, the moisture drains right out of the soil after a rainstorm.