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The preliminary statewide average August temperature was 75.6 degrees, 2 degrees above the 1991–2020 average and tied for the 20^th warmest on record going back to 1895. The preliminary statewide average total August precipitation was 3.76 inches, 0.20 inches above the 1991–2020 average and the 50^th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

August Heat & Humidity

Apart from the first few days of the month, August temperatures were largely above normal across Illinois. As Figure 1 shows, all but 5 days in August had above normal temperatures in Elgin, and several days in the last week of August exhibited temperatures that were 10 to 15 degrees above normal.

Overall, the August average temperature ranged from the low 70s in northern and north-central Illinois to the high 70s in southern Illinois. Last month was 2 to 4 degrees warmer than normal in northern Illinois and within 1 degree of normal in southern Illinois. However, as the maps in the bottom panel of Figure 2 show, daily August minimum temperatures had larger deviations from normal than daily maximum temperatures. In fact, in far southeast Illinois August minimum temperatures were 2 to 3 degrees above normal, while maximum temperatures were 1 to 2 degrees below normal.

Unusually high nighttime low temperatures last month were partly caused by persistently high humidity. One useful measure of humidity is the dewpoint temperature, which is the air temperature at which the air is saturated. Higher dewpoint temperatures indicate higher humidity, and humans begin to feel uncomfortable as dewpoints exceed 65 degrees.

The figure below shows the hourly frequency of dewpoint temperature observations exceeding 65 degrees at Champaign last month. Frequencies from last month (orange bars) are compared with average frequencies over the past 40 years (blue bars). August dewpoint temperatures exceeded 65 degrees more frequently than usual in each of the 24 hours of the day in Champaign, but the largest anomalies were in the early morning between 1 and 5 a.m. For example, well over half of the 3 a.m. dewpoints exceeded 65 degrees in Champaign last month, compared to around 30 percent of days on average. August in Champaign had the 2^nd highest frequency of 3 a.m. dewpoint temperature observations.

Overall, the preliminary statewide average August temperature was 75.6 degrees, 2 degrees above the 1991–2020 average and tied for 20^th warmest on record going back to 1895.

Climatological Summer Temperatures

August brings a wrap-up of climatological summer. Summer average temperatures ranged from the high 60s in southern Illinois to the mid- to high 70s in southern Illinois. This summer was 1 to 3 degrees warmer than average in the northern one-third of the state, and within 1 degree of normal south of Interstate 74. Similar to August, we saw large differences between minimum and maximum temperature departures. Summer minimum temperatures were 1 to 4 degrees above normal statewide, while summer maximum temperatures were more than 1 degree above normal only north of Interstate 80.

Wet, Active Weather in Northern Illinois

August precipitation totals ranged from less than an inch in parts of northwest and western Illinois to over 11 inches in north-central Illinois. Last month was 1 to 3 inches drier than normal throughout most of the northern two-thirds of the state and was between 1 and 5 inches wetter than normal in southern Illinois.

Because of the heat and humidity, August precipitation was accompanied by frequent severe weather events including several tornadoes, hail, and strong winds. Seven confirmed tornadoes touched down in northern Illinois on August 9 alone, including three EF-1 tornadoes. The five National Weather Service offices serving Illinois issued a combined 18 tornado warnings last month, the second most for August on record going back to 1986. Additionally, there were 93 severe thunderstorm warnings issued in Illinois last month, tied for the fourth most on record for August.

On August 12, a series of thunderstorms moved over the same area of north-central Illinois for hours, a phenomenon called training. The storms produced 4 to 5 inches of rain in just 6 hours across parts of McLean, Champaign, and Ford Counties in central Illinois. The epicenter of the heavy rainfall was Gibson City in Ford County, which received 10 to 12 inches in less than 6 hours on August 12. Consequently, Gibson City suffered from destructive urban flooding that displaced residents and flooded roads and buildings. 

Overall, the preliminary statewide average total August precipitation was 3.76 inches, 0.20 inches above the 1991–2020 average and the 50^th wettest on record going back to 1895.  

August ended a mostly wetter than normal summer for Illinois. Total summer precipitation ranged from around 5 inches in northwest Illinois to over 25 inches in parts of southeast and central Illinois. Most of the northwest corner of the state was 3 to 5 inches drier than normal this summer, while parts of central and south-central Illinois were 9 to 12 inches wetter than normal.

Outlooks

The fall season has begun with cooler and less humid weather across Illinois, much to the relief of us all. As crops continue to mature and we inch toward sweaters and pumpkin-flavored drinks, we look to what the season has in store.

Climate Prediction Center outlooks for the second week of September show strongly elevated odds of below normal temperatures sticking around, with slightly elevated chances of below normal precipitation statewide.

Although outlooks for the entire month of September do not show any expectations as to wetter, drier, hotter, or colder than normal, the 3-month outlooks for September through November lean toward warmer than average conditions across the Midwest.

The preliminary statewide average July temperature was 74.6 degrees, 0.8 degrees below the 1991–2020 average and tied for the 45^th coldest on record going back to 1895. The preliminary statewide average total July precipitation was 5.37 inches, 1.31 inches above the 1991–2020 average and tied for the 18^th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Cool but Humid July Weather

Apart from a late-month heat wave, temperatures in July were mostly below the 1991–2020 normal. Figure 1 shows daily average July temperature departures in Springfield. Most places observed a single-digit number of days with a high temperature above 90 degrees. For example, Carbondale observed only 9 days in July with a high temperature above 90 degrees, the 10^th fewest on record there going back to the early 1900s. Comparatively, Carbondale recorded 29 days with a high above 90 degrees in 2012.  

Most of central Illinois was between 1 and 2 degrees below normal for the month of July, while northern and southern Illinois were less than 1 degree below normal for the month (Fig. 2). Last month, 45 daily low maximum temperature records were broken across the state, including a 67-degree high temperature on July 11 in Kewanee. Additionally, three daily low minimum temperature records were broken in July. Rock Island was the lone station with a broken high maximum temperature record in July, with a 99-degree high on July 28, and Barrington was the only station with a broken high minimum temperature record last month, with a 72-degree low on July 24.

The cooler July temperatures maintained growing degree day (GDD) accumulation departures from the end of June. The maps below in Figure 3 show total GDD accumulation and departures from the 1991–2020 normal since April 1. As of the start of August, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.

Overall, the preliminary statewide average July temperature was 74.6 degrees, 0.8 degrees below the 1991–2020 average and tied for the 45^th coldest on record going back to 1895.

July Humidity

Continental-scale atmospheric circulation and the high pressure over the north-central Atlantic Ocean maintained the movement of moisture and humidity out of the Gulf of Mexico into the Midwest all month. The result was a very humid July, even by recent climate standards. Stations in both Jacksonville and Taylorville recorded dew point temperatures of 86 degrees in the afternoon of July 28, which was the highest dew point observed at both stations on their 15+ year records.

Another way of representing humidity is the vapor pressure deficit (VPD). The potential amount of humidity in the air is dependent on temperature, and VPD is a measure of how much humidity is in the air relative to how much could potentially be in the air at that temperature. Therefore, a low VPD indicates a humidity level that is very close to the maximum potential humidity at that temperature, and when VPD nears zero, the air approaches saturation. The plot below (Fig. 4) shows last month’s average VPD at Scott Air Force Base near Belleville was the lowest (i.e., most humid) on record going back to 1938.

“Rinse and Repeat” Rainfall Patterns in July

High humidity and southerly flow off the Gulf of Mexico helped continue to fuel scattered thunderstorms across the state in July. Combined with a handful of mesoscale systems that moved across the Midwest last month, most of Illinois experienced a near to wetter than average July.

The maps in Fig. 5 show total July precipitation ranged from less than 2 inches in the northeast part of the state to over 10 inches in south-central Illinois. Long-running precipitation deficits in northern Illinois grew as July rainfall was 2 to 3 inches below normal there, within 1 inch of normal in most of central Illinois, and 2 to 5 inches above normal in southern Illinois.

Last month was the second wettest on record in Salem in Marion County (9.01 inches) going back to 1915, and the seventh wettest on record in Du Quoin in Perry County (7.78 inches) going back to 1942. In contrast, Chicago observed only 1.9 inches in July, which was the 27^th driest on record there going back to 1871. Chicago has recorded 15.63 inches of rain since the start of the year, making the first 7 months of 2021 the 20^th driest on record and the driest since 2005.

Drought Continues in Northern Illinois

Below normal temperatures throughout July helped alleviate stress from high evaporation rates in drought-stricken northern Illinois. However, most of northeast Illinois was unable to make progress reducing long-running precipitation deficits last month. The latest version of the U.S. Drought Monitor released on July 29 showed most of northern Illinois from Jo Daviess County to Cook County remained in moderate drought, and parts or most of Boone, McHenry, and Lake Counties were in severe drought (Fig. 6).

Lake and McHenry Counties in particular have been in at least severe drought for 11 consecutive weeks, going back to late May. Soil moisture and streamflow in this area remain well below average for this time of the year, and several reports of ecological and agricultural impacts–including trees dropping leaves and corn showing signs of drought stress–have come from the area.

Overall, the preliminary statewide average total July precipitation was 5.37 inches, 1.31 inches above the 1991–2020 average and tied for the 18^th wettest on record going back to 1895.  

Outlooks

Moderate temperatures and dry weather ended July and started the month of August. However, outlooks from the Climate Prediction Center indicate the potential for a transition to more summer-like conditions. Outlooks show strongly elevated odds of warmer than normal conditions and elevated odds of wetter than normal conditions in the second week of August (Fig. 7).   

June took a page out of March’s playbook—came in like a lamb and went out like a lion. Overall, the preliminary statewide average June temperature was 73.8 degrees, 1.6 degrees above the 1991–2020 average and tied for the 25^th warmest on record going back to 1895. The preliminary statewide average total June precipitation was 5.33 inches, 0.68 inches above the 1991–2020 average and tied for the 30^th wettest on record going back to 1895.  

Data are provisional and may change slightly over time

Hot Start, Cooler End to June

Drier soils in the northern two-thirds of the state helped elevate temperatures in the first half of June. Figure 1 shows daily temperature departures and daily average temperature this past month in Joliet. Most of northern and central Illinois experienced temperatures that were between 5 and 15 degrees warmer than normal in the first half of June. Rockford recorded 13 days with a high temperature at or above 90 degrees,  the fourth most for any June on record there back to 1893.

While northern Illinois stayed hot and dry in the first two weeks of June, temperatures in southern Illinois remained near to slightly cooler than normal (Figure 2). Temperatures in the latter half of the month were within 1 degree of normal north of Interstate 64, and between 1 and 2 degrees above normal in southern Illinois.

The warm start to June broke 31 daily high maximum temperature records and 15 daily high minimum temperature records. The subsequent cooldown in the third week of June broke 12 daily low minimum temperature records across the state.

Following a slow start to the growing season from cooler April and May weather, June temperatures helped accumulate growing degree days (GDD). The maps below in Figure 3 show total GDD accumulation and departures from the 1991–2020 normal since April 1. As of the start of July, growing degree day accumulation is ahead of normal in northern Illinois and slightly to much below normal in southern Illinois.

Overall, June average temperatures ranged from the low 70s in northern and central Illinois to the mid- to high-70s in southern Illinois, between 1 and 5 degrees warmer than normal across the state (Figure 4). The preliminary statewide average June temperature was 73.8 degrees, 1.6 degrees above the 1991–2020 average and tied for 25^th warmest on record going back to 1895.

Wild End to Otherwise Dry June

The first two-thirds of June was very dry across Illinois (Figure 5). In fact, the period between June 1 and 20 was the sixth and seventh driest on record in the northwest and northeast Illinois climate divisions, respectively. As the large atmospheric ridge established over the Pacific Northwest, most of Illinois found itself on the edge of a stationary front that produced several rounds of heavy rain from the St. Louis Metro East to Chicagoland. Due to the very wet last 7 to 10 days of the month, June ended wetter than average in all but the northwest and southwest climate divisions, and it was the sixth wettest on record in the east-central division.

Rain in the final week of June was particularly heavy along the Interstate 55 corridor between Bloomington-Normal and Chicago. Areas of McLean and Livingston Counties observed 8 to 10 inches of rain in just 4 days (Figure 6), resulting in serious flooding in Bloomington, flooding on and the temporary shutdown of Interstates 55 and 74, and standing water in fields across central and northeast Illinois.

CoCoRaHS observers in both south Bloomington and Heyworth in McLean County observed over 10 inches of rain in just 3 days between June 25 and 27. According to the recently published Bulletin 75, these totals exceeded the 1% annual probability or “100-year” rainfall event by 1.5 inches.

Total June rainfall ranged from less than 3 inches in far northwest and southeast Illinois to over 12 inches in central and east-central Illinois (Figure 7). Most areas between the St. Louis Metro East and Chicago areas were between 1 to 5 inches wetter than normal, while southern and northwest Illinois were 1 to 3 inches drier than normal. Overall, the preliminary statewide average total June precipitation was 5.33 inches, 0.68 inches above the 1991–2020 average and tied for the 30th wettest on record going back to 1895.

Drought Continues in Northern Illinois

The dry, hot start to June culminated in the U.S. Drought Monitor introducing extreme drought in northeast Illinois in mid-June in response to very dry soils, low streamflow in major streams, and increasingly concerning crop stress. Persistent heavy rains in the Chicagoland area in late June were crucial to improving drought conditions and alleviating crop and water stress across the region. The most recent (July 1) edition of the U.S. Drought Monitor showed most of the southern and central Chicagoland metro area remained abnormally dry to near normal, which represented a significant improvement since mid-month (Figure 8). However, areas north of Interstate 88 and west of Interstate 39 did not receive as much rain in the final week of June, and drought conditions have worsened, particularly from Winnebago to Jo Daviess Counties. In fact, June was the fifth driest on record in Stockton in Jo Daviess County back to 1944, and the driest since 2012.

We will continue to monitor evolving conditions across Illinois and hope to see continued improvement in hydrology and ecology throughout northeast Illinois. Concurrently, the northwest corner continues to deteriorate as precipitation deficits increase. July is the climatologically warmest month in Illinois, and high temperatures can intensify dry conditions. Therefore, the northwest corner of the state will need near to above normal rainfall in July to curb worsening drought conditions.

Outlooks

We’re in a bit of a transition in weather patterns to start July. Climate Prediction Center outlooks for the second week of the month show elevated odds of warmer and wetter than normal conditions statewide (Figure 9). Meanwhile, longer-term outlooks for the entire month of July do not show much of a strong signal in either temperature or precipitation (Figure 10). Illinois appears to be on a boundary between warm, dry conditions to our north and northwest and cooler, wetter conditions to our south and southeast. Climate prediction more than two weeks out in the summer is very difficult here in the Midwest, so we will have to see how shorter-term outlooks and forecasts evolve as we move deeper into the summer.

A warm and dry first half of June has persisted or worsened drought conditions in northern Illinois. Here is an update of current conditions and a look ahead into this summer.

U.S. Drought Monitor

The latest version of the National Drought Mitigation Center’s U.S. Drought Monitor, which was current as of June 15^th, showed an expansion of both moderate drought (D1) and severe drought (D2) across northern Illinois. The Drought Monitor also introduced extreme drought (D3) in all or parts of Lake, McHenry, Boone, DeKalb, Kane, and Cook Counties in northeast Illinois as well as in parts of southern Wisconsin. The last time the Drought Monitor showed extreme drought in any part of Illinois was October 2012. Currently, over 9% of the state is in at least moderate drought, over 6% of the state is suffering from severe drought, and nearly 3% of the state is now in extreme drought according to the U.S. Drought Monitor.

Precipitation Deficits

May wrapped up an extremely dry spring. The period March to May this year was the 3^rd driest spring on record in Chicago and the driest since 1934. The maps below show spring precipitation in northern Illinois was 2 to 6 inches below the 1991-2020 normal, with the largest deficits in far northeast Illinois.

The dry weather pattern continued throughout the first half of June. The northern half of the state received between 0.5 inches and 2.5 inches below normal precipitation in the first half of June, between 10% and 50% of the 1991-2020 normal. The combination of spring and the first half of June has left northeast Illinois with an 8 to 10 inch precipitation deficit since March 1^st, and northwest Illinois with a 4 to 6 inch precipitation deficit since March 1^st (Figure 3).

June Heat Not Helping

Extensive precipitation deficits have left parched soils and reduced flow in both small and large streams throughout northern Illinois. These impacts have been exacerbated by the extreme high temperatures in the region during the first half of June. Average temperatures in northern Illinois between June 1^st and June 15^th were between 6 and 10 degrees above the 1991 to 2020 normal. The first half of June was the warmest on record in Chicago and the third warmest on record in both Rockford and Moline; all three of these stations’ records stretch back to the 1870s. Very warm weather has increased evaporative demand on already dry soils and stressed vegetation and crops, worsening the drought conditions and associated agriculture, water resource, and ecological impacts.

Impacts So Far

The timing of this drought is different than the “droughts of record” of 1988 and 2012 such that the “peak” of those past droughts was later in the summer in July and August. The relatively early onset of this year’s drought has likely limited impacts to agriculture, water resources, and ecosystems so far. However, reports of impacts to crops in northern Illinois include corn plants curling leaves, early firing, and stunted growth in both corn and soybeans. Gardeners and horticulturalists across northern Illinois report having to water gardens and specialty crops much more frequently, and lawns, trees, and shrubs are reportedly showing significant drought stress in residential areas. 

Outlook

The June 17^th National Weather Service precipitation forecast indicates the potential for widespread 1 to 2 inches accumulation across northern Illinois over the 7 days. It is understandable if folks are skeptical of the forecast, given how frequently wet forecasts have not come to fruition in northern Illinois over the past three months; however, this is the best chance for drought alleviation since mid-May. If the forecast does verify it will not eliminate drought in northern Illinois but will go a long way to improve soil moisture and reduce stress in crops and ecosystems.

Meanwhile, Climate Prediction Center outlooks indicate a tendency for cooler and wetter than normal conditions statewide in the third week of June. Both outlooks are potentially good news for drought affected areas, and we could see real improvement with soaking rains combined with below average evaporative demand from the cooler weather. However, many more weeks of wetter weather are needed to ameliorate drought in northern Illinois and stave off further impacts as we move into the heart of summer.